PC 2018-01-16_ 8b DCA 18-001 Post Construction Requirements
MEMORANDUM
TO: PLANNING COMMISSION
FROM: TERESA MCCLISH, COMMUNITY DEVELOPMENT DIRECTOR
BY: ROBIN DICKERSON, CITY ENGINEER
SUBJECT: CONSIDERATION OF DEVELOPMENT CODE AMENDMENT 18-001;
DESIGN REQUIREMENTS FOR POST CONSTRUCTION
STORMWATER COMPLIANCE; LOCATION – CITYWIDE; APPLICANT
– CITY OF ARROYO GRANDE
DATE: JANUARY 16, 2018
SUMMARY OF ACTION:
Approval of the Development Code Amendment would modify the Design Requirements
for Post Construction Stormwater Compliance consistent with the Regional Water
Quality Control Board (RWQCB).
IMPACT ON FINANCIAL AND PERSONNEL RESOURCES:
None.
RECOMMENDATION:
It is recommended that the Planning Commission adopt a Resolution recommending
that the City Council approve Development Code Amendment 18-001.
BACKGROUND:
The City is required to adopt regulations in order to comply with the requirements of the
Federal Clean Water Act, the implementing regulations for the National Pollutant
Discharge Elimination System (NPDES), and the California Porter -Cologne Water
Quality Control Act. These requirements provide for the regulation and reduction of
pollutants discharged to waters of the State and United States by extending NPDES
requirements to storm water and urban runoff discharges to and from municipal storm
drain systems.
The City implements these requirements through the Storm W ater Management
Program (SWMP) that was originally approved by City Council Resolution 3708 in 2003.
The SWMP requires programs and regulations to manage storm water discharges
within the City and has been updated periodically to stay current with State
requirements. The SWMP has since been revised in accordance with State
requirements and is now referred to as the St orm W ater Guidance Document (SWGD).
PLANNING COMMISSION
CONSIDERATION OF DEVELOPMENT CODE AMENDMENT 18-001
JANUARY 16, 2018
PAGE 2
The SWGD is a requirement of the California Environmental Protection Agency State
Water Resources Control Board Phase II Small Municipal Separate Storm Sewer
System (MS4) Permit. The SWGD includes a series of steps to ensure compliance,
including requirements to update regulations in the City’s Municipal Code for post
construction management of storm water.
ANALYSIS OF ISSUES:
The proposed Development Code Amendment would amend the Design Requirements
for Post Construction Stormwater Compliance, which are incorporated into the Arroyo
Grande Municipal Code (AGMC) by reference only Sections 16.08.010.T. and
16.68.030.D.
Sections 16.08 and 16.68 Post Construction Stormwater Requirements
The California Regional Water Quality Control Board for the Central Coast Region (the
“Water Board”) adopted Post Construction Requirements in July 2013 and issued Water
Quality Order No. 2013-0001-DWQ to incorporate post-construction controls for MS4
storm water discharges, establishing March 6, 2014 as the date of implementation.
Staff developed design requirements for post construction storm water compliance that
address the following:
Performance requirements, based on the development project area, addressing
site design, water quality, and runoff retention;
Low Impact Development (LID) design standards;
Hydromodification measures;
Post-construction best management practice condition assessment;
Planning and development review process; and
Alternative compliance.
On February 25, 2014, City Council approved the Design Requirements for Post
Construction Storm Water Compliance and passed Ord. No. 660, which incorporated
the design requirements into the AGMC by reference.
The only significant change proposed to the Design Requirements for Post Construction
Storm Water Compliance is the retraction of language, and the associated flow chart
(Figure 2), regarding the exemption of projects from the post construction requirements
if the project retains 100% of stormwater on site for all storms. The final Water Board
Resolution contains no exemptions for post construction requirements. Regulated
projects are projects which create over 2,500 square feet of impervious surface
collectively over the entire project site and are required to meet the post construction
storm water compliance. There are some projects that are not included as regulated
projects such as road and parking lot maintenance, crack sealing, and others as listed
in the Water Board’s Resolution No. R3-2013-0032, Section B, Post Construction
Requirements 1 (Attachment 1). Language clarification pertaining to regulated projects
appears to have been made after publication of the draft Water Board Resolution.
PLANNING COMMISSION
CONSIDERATION OF DEVELOPMENT CODE AMENDMENT 18-001
JANUARY 16, 2018
PAGE 3
The County of San Luis Obispo had similar language in their post construction storm
water requirements and has received a violation from the Water Board stating that the
County provided too many exemptions and were not in compliance with the post
construction requirements as outlined in the final Water Board Resolution. The County
has recently revised their Post Construction Storm Water Requirements to eliminate the
use of exemptions.
ALTERNATIVES:
The following alternatives are provided for the Commission’s consideration:
1. Adopt the attached Resolution recommending the City Council approve
Development Code Amendment 18-001;
2. Modify and adopt the attached Resolution recommending the City Council
approve Development Code Amendment 18-001;
3. Do not adopt the attached Resolution, provide specific findings, and direct staff to
return with an appropriate Resolution recommending the City Council deny
Development Code Amendment 18-001; or
4. Provide direction to staff.
ADVANTAGES:
The City will implement requirements for post construction stormwater management per
the Storm Water Guidance Manual and comply with State requirements.
DISADVANTAGES:
Some requirements, primarily the post construction requirements, will place burdens on
and increase costs for developers, as well as require additional staff resources to
monitor, inspect and ensure compliance. Howe ver, the City is regulated under the
Water Board NPDES General Permit for MS4 dischargers and is thus required to adopt
and implement these requirements.
ENVIRONMENTAL REVIEW:
In accordance with the California Environmental Quality Act (CEQA) Guidelines, it has
been determined that the project is Categorically Exempt per Section 15308 of the
CEQA Guidelines, which governs actions by regulatory agencies for the protection of
the environment.
PUBLIC NOTIFICATION AND COMMENTS:
A notice of public hearing was published in The Tribune and posted at City Hall and on
the City’s Website on Friday, January 5, 2018. The Agenda was posted at City Hall and
on the City’s website in accordance with Government Code Section 54954.2. At the
time of report publication, no comments have been received.
Attachments:
1. Resolution No. R3-2013-0032
RESOLUTION NO.
A RESOLUTION OF THE PLANNING COMMISSION OF THE CITY OF
ARROYO GRANDE RECOMMENDING THE CITY COUNCIL APPROVE
DEVELOPMENT CODE AMENDMENT 18-001; DESIGN
REQUIREMENTS FOR POST CONSTRUCTION STORMWATER
COMPLIANCE APPROVED BY ORDINANCE 660
WHEREAS, the City is required to adopt regulations in order to comply with the
requirements of the Federal Clean Water Act, the implementing regulations for the
National Pollutant Discharge Elimination System, and the California Porter-Cologne
Water Quality Control Act; and
WHEREAS, the City implements these requirements through the Storm Water
Management Program approved by City Council Resolution 3708 ; and
WHEREAS, the City Council adopted Ordinance 660, which incorporated Design
Requirements for Post Construction Stormwater Compliance into the Arroyo Grande
Municipal Code, by reference; and
WHEREAS, project exemptions for post construction requirements are not permitted by
the California Regional Water Quality Control Board Central Coast Region Resolution
No. R3-2013-0032; and
WHEREAS, an amendment to the City’s Design Requirements for Post Construction
Stormwater Compliance is required to remove the allowance for exemptions; and
WHEREAS, the Planning Commission has reviewed the project in compliance with the
California Environmental Quality Act (CEQA), the State CEQA Guidelines, and the
Arroyo Grande Rules and Procedures for the Implementation of CEQA and determined
that the project is Categorically Exempt per Section 15308 of the CEQA Guidelines
regarding actions by regulatory agencies for the protection of the environment ; and
WHEREAS, the Planning Commission has considered Development Code Amendment
18-001 at a duly noticed public hearing on January 16, 2018; and
WHEREAS, the Planning Commission finds, after due study, deliberation, and public
hearing, that the following circumstances exist:
Development Code Amendment Findings:
1. The proposed revision to this title is consistent with the goals, objectives,
policies and programs of the general plan, and is necessary and desirable
to implement the provisions of the general plan.
The City is required to adopt regulations in order to comply with the
requirements of the Federal Clean Water Act, the implementing
regulations for the National Pollutant Discharge Elimination System, and
the California Porter-Cologne Water Quality Control Act and the proposed
RESOLUTION NO.
PAGE 2
revisions to the Design Requirements for Post Construction Stormwater
Compliance are consistent with the requirements of the California
Regional Water Quality Control Board Central Coast Region Resolution
No. R3-2013-0032 regarding post-construction controls for MS4
stormwater discharges, and does not create an inconsistency with the
City’s General Plan.
2. The proposed revision to this title will not adversely affect the public
health, safety, and welfare or result in an illogical land use pattern.
The proposed revisions to the Design Requirements for Post Construction
Stormwater Compliance are consistent with the requirements of the
California Regional Water Quality Control Board Central Coast Region
Resolution No. R3-2013-0032 regarding post-construction controls for
MS4 stormwater discharges, and will not result in an illogical land use
pattern.
3. The proposed revision to this title is consistent with the purpose and intent
of the title.
The proposed revisions to Design Requirements for Post Construction
Stormwater Compliance remove the identification of exemptions from post
construction requirements, are consistent with the requirements of the
California Regional Water Quality Control Board Central Coast Region
Resolution No. R3-2013-0032 regarding post-construction controls for
MS4 stormwater discharges, and are consistent with the purpose and
intent of Title 16.
4. The potential environmental impacts of the proposed revision to this title
are insignificant, or there are overriding considerations that outweigh the
potential impacts.
The proposed revisions to this title that require projects to comply with the
Design Requirements for Post Construction Stormwater Compliance, are
Categorically Exempt from CEQA review per Section 15308 of the CEQA
Guidelines, and therefore the potential environmental impacts of the
proposed revisions to Title 16 are insignificant.
NOW, THEREFORE, BE IT RESOLVED that the Planning Commission of the City of
Arroyo Grande hereby recommends that the City Council adopt a Resolution approving
Development Code Amendment 18-001, amending the Design Requirements for Post
Construction Stormwater Compliance as set forth in Exhibit ‘A’, attached hereto and
incorporated herein by this reference.
On a motion by Commissioner ________, seconded by Commissioner _________ and by
the following roll call vote to wit:
RESOLUTION NO.
PAGE 3
AYES:
NOES:
ABSENT:
the foregoing Resolution was adopted this 16th day of January 2018.
RESOLUTION NO.
PAGE 4
___________________________
GLENN MARTIN, CHAIR
ATTEST:
______________________________
DEBBIE WEICHINGER
SECRETARY TO THE COMMISSION
AS TO CONTENT:
_____________________________
TERESA McCLISH
DIRECTOR OF COMMUNITY DEVELOPMENT
Exhibit “A”
City of Arroyo Grande
Design Requirements for
Post Construction Stormwater Compliance
Purpose
The purpose of this document is to provide guidance and direction on how to comply with Post-
Construction Stormwater Requirements (PCR's) within the City of Arroyo Grande. This document is
intended to be used by developers, contractors, builders, designers, engineers, architects, planners,
homeowners, and all others interested in learning how to address stormwater quality during the planning,
design, construction and maintenance phases of a project. It is meant to be used as an aid for both onsite
and public improvement projects.
Project applicants should use this document to:
• Determine which performance requirement apply
• Understand how each performance requirement will be applied within the City
• Obtain direction regarding means of demonstrating compliance
Impact of Development on Water Resources
Undeveloped natural landscape areas such as forests and grasslands act like sponges for rainfall. When
natural landscape areas are covered with impervious (nonporous) surfaces like roads, parking lots, and
roofs, this “sponge-like” function is lost and the amount of rainfall that can be absorbed is considerably
reduced. The percentage of impervious surface area of a site influences how much stormwater is
infiltrated into the ground, evapotranspirated back into the atmosphere, or leaves the site as stormwater
runoff. See figure 1.
When the amount of impervious surface area of a site is altered the normal watershed processes is also
altered. This can lead to a significant host of problems including:
Increased loads of chemical pollutants;
Increased toxicity;
Changes to flow magnitude, frequency, and seasonality of various discharges;
Physical changes to stream, lake, or wetland habitats;
Changes in the energy dynamics of food webs, sunlight, and temperature;
Changes in biotic interactions between native and exotic species.
Changes in impervious surface area can also alter the amount and quality of stormwater that infiltrates
and recharges our groundwater aquifers.
Figure 1. The relationship between impervious surface area, runoff,
infiltration and evapotranspiration.
(Stream Corridor Restoration: Principles, Processes, & Practices, FISWG 1998)
The loss of infiltration from urbanization has also resulted in significant groundwater changes. As more
surface area becomes covered with impervious surfaces, less water is able to seep back into the ground.
Reduced groundwater recharge rates may result in lower base flows during dry weather as less
groundwater is available to move through the soil and into stream channels and aquifers. See Chapter
13.05 of the City's Municipal Code for more on City's regulations regarding Water Conservation.
To protect surface water quality and groundwater resources, new development and significant
redevelopment projects should be designed, constructed, and maintained to minimize the interruption of
natural watershed processes and to treat storm water as a resource and an asset, instead of a waste
product.
Post Construction Requirements
The Central Coast Regional Water Quality Control Board recognizes that it is necessary to protect
watershed processes so that beneficial uses of receiving wate rs are maintained and, were applicable,
restored. The primary objective of these Post-Construction Stormwater Management Requirements is to
ensure reduction of pollutant discharges to the Maximum Extent Practicable and preventing stormwater
discharges from causing or contribution to a violation of receiving water quality standards in all
applicable development projects that require approvals and/or permit is issued under the planning,
building or other comparable authority. To address the impacts of development on water quality, the
National Pollutant Discharge Elimination System (NPDES) Small Municipal Separate Storm Sewer
System (MS4) General Permit requires the City of Arroyo Grande to develop and implement a Guidance
Document that includes specific Best Management Practices (BMPs).
Figure 2. Performance Requirement Flow Chart
(Source: County of San Luis Obispo)
Regulated projects include all new development or redevelopment projects that create and/or replace
greater than or equal to 2,500 square feet of impervious surface (collectively over the entire project site).
A list of projects impacted by the post construction storm water requirements can be found Resolution
No. R3-2013-0032 Section B Post Construction Requirements 1.Using the flow chart Figure 2, determine
if the project is:
EXEMPT or
SUBJECT TO ONE OR MORE PERFORMANCE REQURIEMENT (i.e. the project is
regulated),
The project is exempt if the project retains 100% of stormwater on site for all storms or if the project
consist of only one of the items listed in Draft Resolution No. R3-2013-0032, Section B. Post -
Construction Requirements 1b. Typically, maintenance and utility projects are also considered "exempt"
from post-construction requirements specified by the Regional Board. A list of projects not included as
Regulated Projects can be found in Resolution No. R3-2013-0032 Section B Post Construction
Requirements 1b. Examples of exempt maintenance non-regulated projects include routine overlays and
slurry seals., Examples of utility projects that are "exempt" include the installation of an ADA ramps, or
solar panels on rooftops, or repair of roads or aerial utilities.
If the project meets the criteria to be considered “Exempt”, project applicants Non-Regulated Projects are
still encouraged to implement practices that will reduce stormwater impacts associated with their
proposed development.
Performance Requirements
If the project is considered to be a Regulated project, the regulated project will be required to meet
Performance Requirement 1 (PR 1). Many regulated projects are required to meet additional performance
requirements. To determine if the project must meet additional requirements, you must determine the Net
Impervious Area.
The Net Impervious Area is the total (including new and replaced) post-construction impervious areas,
minus any reduction in total imperviousness from the pre-project to post-project conditions. Permeable
pavements may only be excluded from the impervious surface area calculation if they are designed to
infiltrate.
Net
impervious
area
=
New and replaced
impervious area
-
Pre-project to post-project
reduction in impervious area, if
any
Pre-project impervious areas consists of all hard, non-vegetated surface area that prevents or
significantly limits the entry of water into the soil mantle, as would occur under natural conditions prior
to development. Common impervious surfaces include, but are not limited to, roof tops, walkways, patios,
driveways, parking lots or storage areas, concrete or asphalt paving, surfaces which impede the natural
infiltration of stormwater. Open, uncovered retention/detention facilities shall not be considered as
impervious surfaces for purposes of determining whether the thresholds for application of Performance
Requirements are exceeded. However, for modeling purposes, open, uncovered facilities that retain/detain
water (e.g., retention ponds, pools) shall be considered impervious surfaces.
Total Project Site Area is the area defined by the legal boundaries of a parcel or parcels of land which
the new development or redevelopment takes place.
Post-project Impervious surface areas is the total (including new and replaced) impervious areas on the
development site once the project is completely constructed. They include, but are not limited to: roof
tops, walkways, patios, driveways, parking lots or storage areas, concrete or asphalt paving, surfaces
which impede the natural infiltration of stormwater. It does not include open, uncovered
retention/detention facilities.
New (%) is the portion of the post-project impervious area that did replace an existing pervious surface
area.
Replaced (%) is the portion of the post-project impervious area that replaced an existing impervious
surface area.
Total Disturbed Area is the portion of the total project site area that will be subject to construction, used
as a staging or storage area, or whose vegetation and soils are otherwise directly impacted by the project.
An applicant will be required to provide supporting documentation regarding permeable paver (or
alternate surface) areas excluded from impervious area calculations.
The five Post-Construction Requirements (PCRs) developed by the State Regional Water Quality Control
Board are summarized below:
Performance Requirement 1: Site Design and Runoff Reduction
All regulated projects, projects creating and/or replace ≥ 2,500 square feet of impervious surface
(collectively over the entire project site), including detached single-family home projects, are subject to
PR 1 and must minimally incorporate the following Low Impact Development (LID) design strategies
into the project.
1. Limit disturbance of creeks and natural drainage features
2. Minimize compaction of highly permeable soils
3. Limit clearing and grading of native vegetation at the site to the minimum area needed to build
the project, allow access, and provide fire protection
4. Minimize impervious surfaces by concentrating improvements on the least sensitive portions of
the site, while leaving the remaining land in a natural undisturbed state
5. Minimize stormwater runoff by implementing one or more of the following site design measures:
i. Direct roof runoff into cisterns or rain barrels for reuse
ii. Direct roof runoff onto vegetated areas safely away from building foundations and
footings, consistent with California building code
iii. Direct runoff from sidewalks, walkways, and/or patios onto vegetated areas safely
away from building foundations and footings, consistent with California building
code
iv. Direct runoff from driveways and/or uncovered parking lots onto vegetated areas
safely away from building foundations and footings, consistent with California
building code
v. Construct bike lanes, driveways, uncovered parking lots, sidewalks, walkways, and
patios with permeable surfaces
The developer shall provide appropriate documentation certifying that project complies with
Site Design and Runoff Reduction Performance Requirements. Certification Statement shall
clearly identify the level of compliance with each of the applicable performance requirements the
project is subject to. All certification shall be signed and stamped by the engineer of Record and shall
include qualify and quantify identified deficiencies, per performance requirements.
Performance Requirement 2: Water Quality Treatment
Projects subject to PR2 are projects with a Net Impervious Area ≥ 5,000 square feet , that are not single
family homes, and detached single family homes ≥15,000 square feet of Net Impervious Area. These
projects shall treat stormwater runoff to reduce pollutant loads and concentrations using physical,
biological and chemical removal. Developer shall provide and include net impervious area calculations.
Regulated projects subject to PR2 must treat a defined minimum volume or maximum surface loading
rate of runoff using onsite measures. Allowable onsite measures are listed in the order of preference
(highest to lowest):
Low Impact Development - implement harvesting and use infiltration, and evapotranspiration
measures that collectively retain stormwater runoff equal to the volume of runoff generated by
the 85th percentile 24-hour storm event, based on local rainfall data.
Biofiltration Treatment Systems - implement biofiltration systems using facilities that must be
demonstrated to be at least as effective as a biofiltration treatment system that meets the
following parameters:
i. a max surface loading rate of 5" per hour based on a rain intensity of 0.2 inches or two
times the 85th percentile hourly rainfall intensity for the applicable area,
ii. Surface minimum reservoir volume of equal to the biofiltration area times a depth of 6
inches,
iii. Proper plan selection with a minimum depth of 24 inches and an infiltration rate of 5
in/hr,
iv. Subsurface drain/storage layer with an area equal to the biofitration surface area and
having a minimum depth of 12 inches.
Non-Retention Based Treatment Systems - implement stormwater control measure that achieve
at least one of the following:
i. a volume capacity designed to treat stormwater runoff equal to the runoff generated by
the 85th percentile 24-hour storm event
ii. a flow capacity sized equal to at least two times the 85th percentile hourly rainfall
intensity for the applicable area and equal to at least 0.2 in/hr intensity.
Design alternatives will only be considered if the applicant can demonstrate that ALL of the following
measures of equivalent effectiveness are demonstrated:
1. Equal or greater amount of runoff infiltrated or evapotranspired;
2. Equal or lower pollutant concentrations in runoff that is discharged after bio-filtration;
3. Equal or greater protection against shock loadings and spills; and
4. Equal or greater accessibility and ease of inspection and maintenance.
Technical guidance for designing bio-retention facilities is available from the Central Coast LID Initiative
web site:
http://www.centralcoastlidi.org/Central_Coast_LIDI/LID_Structural_BMPs.html
The guidance includes design specifications and standard plans. It also includes plant lists appropriate for
installation of LID and Bio-filtration Systems on the Central Coast.
Project subject to PR2 shall include a Stormwater Control Plan (SWCP) containing the following:
1. Project type and address and name of applicant
2. Calculations for total new, replaced and net impervious area pre/post development
3. Applicable requirement for both PR 1 and PR 2
4. Description of all post construction structural stormwater control measures including calculations
5. Documentation certifying that the selection, sizing and design of measures meet requirements
Performance Requirement 3: Runoff Retention
Performance Requirement 3 shall apply to projects, except detached single family homes, that create
and/or replace ≥ 15,000 square feet of impervious surface (collectively over the entire project site), and
detached single family homes ≥ 15,000 square feet of net impervious area, WMZs 1, 2.
Regulated projects subject to PR3 must retain a designated design storm volume. Applicants of regulated
projects subject to this requirement must:
1. Watershed Management Zone 1:
a) Retain 95th Percentile Rainfall Event – Prevent offsite discharge from events up to the 95th
percentile 24-hour rainfall event as determined from local rainfall data.4
b) Compliance must be achieved via infiltration
2. Watershed Management Zone 2:
a) Retain 95th Percentile Rainfall Event – Prevent offsite discharge from events up to the 95th
percentile 24-hour rainfall event as determined from local rainfall data.
b) Compliance must be achieved via storage, rainwater harvesting, infiltration, and/or
evapotranspiration.
PR3 regulated projects shall use the following LID development standards:
1. Site Assessment Measures – each Regulated Project shall identify opportunities and constraints to
implement LID Stormwater Control Measures. Provide documentation of the following, as
appropriate to the development site:
a) Site topography including hydrologic features
b) Seasonal high groundwater
c) Geological analysis including geotechnical hazards
d) Soil type and hydrologic soil groups
e) Run-on characteristics and existing drainage infrastructure
2. Site Design Measures – Regulated Project shall optimize the use of LID site design measures, as
feasible and appropriate at the project site. Regulated Projects subject to Performance
Requirement No. 3 must augment design strategies required by Performance Requirement No. 1
with the following:
a) Define the development envelope and protected areas, identifying areas that are most
suitable for development and areas to be left undisturbed
b) Conserve natural areas, including existing trees, other vegetation, and soils
c) Limit the overall impervious footprint of the project
d) Construct streets, sidewalks, or parking lot aisles to the minimum widths necessary,
provided that public safety or mobility uses are not compromised
e) Set back development from creeks, wetlands, and riparian habitats
f) Conform the site layout along natural landforms
g) Avoid excessive grading and disturbance of vegetation and soils
3. Delineation of discrete Drainage Management Areas (DMAs) – Regulated Project shall delineate
DMAs to support a decentralized approach to stormwater management. A map or diagram
dividing the entire project site into discrete DMAs shall be provided.
Each Regulated Project shall account for the drainage from each DMA using measures identified
as follows:
a) Undisturbed and Natural Landscape Areas –each Regulated Project shall implement
appropriate Site Design and Runoff Reduction Measures in Performance Requirement
No. 1, to reduce the amount of runoff for which retention and treatment is required.
Runoff reduction measures that can be used to account for this reduction also include as
follows:
i. Undisturbed or areas planted with native, drought-tolerant, or LID appropriate
vegetation that do not receive runoff from other areas may be considered self-treating
and no additional stormwater management is required.
ii. Runoff from impervious surfaces, generated by the rainfall events identified in
Section B.4.c, may be directed to undisturbed or natural landscaped areas. When the
applicant can demonstrate that this runoff will be infiltrated and will not produce
runoff to the storm drain system, or a surface receiving waterbody, or create nuisance
ponding that may affect vegetation health or contribute to vector problems, then no
additional stormwater management is required for these impervious surfaces.
b) Structural Stormwater Control Measures – Where it has been demonstrated in the
Stormwater Control Plans, that the use of Site Design measures listed above, Runoff
Reduction measures listed in Performance Requirement No.1, and undisturbed and
natural landscape has been maximized to the extent feasible, Structural Stormwater
Control Measures designed for water quality treatment and/or flow control shall be used
to comply with Performance Requirement No. 3.
4. Ten Percent Adjustment for Sites with Technical Infeasibility – Where technical infeasibility, as
described in Section C.1.c., prevents full on-site compliance with the Runoff Retention
Performance Requirement, on-site retention of the full Retention Volume per Section B.4.d.vi. is
not required and the Regulated Project is required to dedicate no less than ten percent of the
Regulated Project’s Equivalent Impervious Surface Area5 to retention-based Stormwater Control
Measures.
5. Off-Site Mitigation – Off-site mitigation is required when Regulated Projects do not retain the
full Retention Volume, and 1) fail to demonstrate technical infeasibility of full retention; or 2)
demonstrate technical infeasibility of full retention AND fail to dedicate at least ten percent of the
Regulated Project’s Equivalent Impervious Surface Area to retention-based Stormwater Control
Measures.
6. Reporting Requirements – For each Regulated Project subject to the Runoff Retention
Performance Requirement, provide the following information in a Stormwater Control Plan. Final
project approval shall not be granted until the Stormwater Control Plan for the Regulated Project
sufficiently demonstrates the Regulated Project design meets the Water Quality Treatment and
Runoff Retention Performance Requirements.
Performance Requirement 4: Peak Management
All Regulated Projects that create and/or replace >22,500 square feet of impervious surface (collectively
over the entire project site) in Watershed Management Zones 1 and 2 shall manage peak stormwater
runoff to ensure that post-development peak flows, discharged from the site, do not exceed pre-project
peak flows for the 2- through 10- year storm events and shall meet Water Quality Treatment and Runoff
Retention Performance Requirements.
Reporting Requirements – For each Regulated Project subject to the Peak Management Performance
Requirement a Stormwater Control Plan shall be required. Final project approval shall not be granted until
the Stormwater Control Plan for the Regulated Project sufficiently demonstrates the Regulated Project
design meets the Water Quality Treatment, Runoff Retention, and Peak Management Requirements.
Performance Requirement 5: Special Circumstances
There are no Special Circumstance designations in the City of Arroyo Grande.
Alternative Compliance
The City will only consider alternative compliance for projects able to demonstration technical
infeasibility and who cannot satisfy the 10% adjustment criteria on site. Alternative Compliance may be
allowed under the following circumstances:
1. Technical Infeasibility - applicant must submit a site-specific hydrologic and/or design analysis
conducted and endorsed by a registered professional engineer, geologist, architect, and/or
landscape architect, demonstrating that compliance with the applicable numeric Post-
Construction Stormwater Management Requirements is technically infeasible.
2. The Regulated Project applicant must submit a description of the project(s) that will provide off-
site mitigation. The proposed off-site projects may be existing facilities and/or prospective
projects that are as effective in maintaining watershed processes as implementation of the
applicable Post-Construction Stormwater Requirements on-site.
The description shall include:
c) The location of the proposed off-site project(s) must be within the same watershed as the
Regulated Project. Alternative Compliance project sites located outside the watershed
may be approved by the Central Coast Water Board Executive Officer
d) A schedule for completion of offsite mitigation project(s), where the off -site mitigation
project(s) has not been constructed.
Technical infeasibility may be caused by site conditions, including:
a) Depth to seasonal high groundwater limits infiltration and/or prevents construction of sub-
grade stormwater control measures
b) Depth to an impervious layer such as bedrock limits infiltration
c) Sites where soil types significantly limit infiltration
d) Sites where pollutant mobilization in the soil or groundwater is a documented concern
e) Space constraints (e.g., infill projects, some redevelopment projects, high density
development)
f) Geotechnical hazards
g) Stormwater Control Measures located within 100 feet of a gro undwater well used for
drinking water
h) Incompatibility with surrounding drainage system (e.g., project drains to an existing
stormwater collection system whose elevation or location precludes connection to a properly
functioning treatment or flow control facility).
Operational and Maintenance Plans
The developer shall submit an Operation and Maintenance Plan and Maintenance Agreements (O&M
Plan) that clearly establish responsibility for all structural Water Quality Treatment, Runoff Retention,
and/or Peak Management controls for all Regulated Projects.
The Regulated Project applicant shall develop and implement a written O&M Plan. The O&M Plan shall
be submitted for approval prior to final approval/occupancy. The O&M Plan must include:
a. A site map identifying all structural Stormwater Control Measures (SCM's) requiring O&M
practices to function as designed
b. O&M procedures for each structural stormwater control measure including, but not limited to,
LID facilities, retention/detention basins, and proprietorship devices.
c. The O&M Plan will include short-and long-term maintenance requirements, recommended
frequency of maintenance, and estimated cost for maintenance.
Maintenance Agreement and Transfer of Responsibility for SCMs: Prior to issuing approval for final
occupancy the owner of the Regulated Project subject to these Post-Construction Requirements shall
provide verification of ongoing maintenance provisions for Structural Stormwater Control Measures,
including but not limited to legal agreements, covenants, CEQA mitigation requirements, and or
conditional use permits. Verification shall include, at a minimum:
a. The project owner’s signed statement accepting responsibility for the O&M of the installed onsite
and/or offsite structural treatment and flow control SCMs until such responsibility is legally
transferred to another entity; and either
i. A signed statement from the public entity assuming responsibility for structural treatment
and flow control SCM maintenance and stating that the SCM meets all local agency
design standards; or
ii. Written conditions in the sales or lease agreements or deed for the project that require the
buyer or lessee to assume responsibility for the O&M of the onsite and/or offsite
structural treatment and flow control SCM until such responsibility is legally transferred
to another entity; or
iii. Written text in project deeds, or conditions, covenants and restrictions for multi-unit
residential projects that require the homeowners association or, if there is no association,
each individual owner to assume responsibility for the O&M of the onsite and/or offsite
structural treatment and flow control SCM until such responsibility is legally transferred
to another entity; or
iv. Any other legally enforceable agreement or mechanism, such as recordation in the
property deed, that assigns responsibility for the O&M of the onsite and/or offsite
structural treatment and flow control SCM to the project owner(s).
Reporting
The Central Coast Regional Board requires two types of reporting.
Project applicant reporting to the City
Applicants of regulated projects subject to Runoff Retention Requirement (PR 3) are
required to demonstrate compliance with a decentralized approach to stormwater
management through a Stormwater Control Plan. Additional supporting information is
required if the project is subject to additional Performance Requirements.
Determine Watershed Management Zones
Projects creating or replacing 15,000 square feet or more of impervious surfaces, that are not a detached
single family residence, and do not qualify for a special circumstance designation, will need to know thei r
Watershed Management Zone (WMZ). Watershed management zones (WMZs) are based on common key
watershed processes and receiving water type. They are the basis for determining if the project is subject
to Runoff Retention Performance Standards, or Runoff Retention and Peak management Requirements
Performance Standards. There are two Watershed Management Zones located within the City of Arroyo
Grande. For a copy of the Watershed Management Zone map please visit the Engineer's office at City
Hall. If the project is located on one or the two WMZs, the applicant shall satisfy the requirements of each
zone proportionately. The City’s Engineering Division can assist if the maps does not clearly identify the
watershed management zone the project is located in.
Determine Applicable Rainfall Event
The performance criteria for post-construction retention standards is the percentile storm that best
represents the volume that is fully infiltrated in a natural condition, and thus should be managed onsite to
maintain the pre-development hydrology for duration, rate and volume of stormwater flows. The
percentile rain event is the 95th percentile rain event and shall be achieved by infiltration, rainwater
harvesting and/or evaporation for Watershed Management Zones 1 and 2. To obtain exhibits for the 85th
and 95th percentile, 24-hour storm exhibits go to
http://www.swrcb.ca.gov/rwqcb3/water_issues/programs/stormwater/docs/lid/lid_hydromod_c
harette_index.shtml
Stormwater Control Plan (SWCP) Coversheet
Owner/Engineer contact inform (as applicable)
Name: __________________________________________________________________________
Mailing Address: __________________________________________________________________
Email Address:____________________________ Phone number: ___________________________
Permit/Project Number: _______________ Property APN: _______________________
Impervious surface values:
Pre-Project: Post-Project:
Impervious Area (sf): _________ Impervious Surface Area (sf): ___________
New (%): ___________
Total Project Site Area (ac): _________ Replaced (%): ___________
Net Impervious Area (sf): ___________
Total Disturbed Area (ac): ___________
Applicable Performance Requirements
□ Performance Requirement 1: Site Design and Runoff Reduction
□ Performance Requirement 2: Water Quality Treatment
□ Performance Requirement 3: Runoff Retention
□ Performance Requirement 4: Peak Management
□ Performance Requirement 5:Special Circumstances
(select applicable special circumstance(s) from list below)
___ Highly Altered Channel
___ Intermediate Flow Control Facility
___ Historic Lake or Wetland
___ Technical Infeasibility: Equivalent Impervious Surface Area: _____________
Identify appropriate certification statement being pursued:
□ (Full compliance)
□ (Partial compliance on-site)
Location of off-site features: _____________________________________________
□ (Alternative Compliance)
Certification statement must be seal and signed by a State of California licensed civil engineer.
Stormwater Control Plan (SWCP) Checklist
Report
□ Cover Sheet.
□ Table of Contents.
□ Project type (commercial, industrial, multiunit residential, mixed use, public) and description.
□ Identify if project will be completed in phases, and if so, what phase is represented by the
SWCP.
□ List of permits requested and other permits required (401, 404, Caltrans Encroachment, etc).
□ Table of Performance Requirements applicable to project and if there are any adjustments
required.
□ Checklists from each applicable performance requirement or alternate compliance, as
appropriate.
□ Site assessment summary (Step 2 and 3). Reference Site Stormwater Assessment Exhibit
described below.
□ Summary of Site Design and Runoff Reduction, Water Quality Treatment, Runoff Reduction
and Structural Stormwater Control Measures, by Drainage Management Area, as well as the
entire site (as applicable to project). Reference Exhibits listed below. Identify types of retention
and non-retention based measures specified.
□ Supporting calculations to demonstrate compliance with applicable Performance Requirements.
□ Statements of Compliance.
Exhibits
□ Drainage Management Area (DMA) Exhibit.
o Uniquely identify each DMA and indicate if the DMA is self-retaining (zero discharge),
self-treating, or draining to a treatment/flow control facility.
o Include location of all infiltration, treatment, or flow-control facilities, their tributary
area and basis for sizing (rational C, NRCS CN value, Tc, etc)
o Potential pollutant source areas (if applicable), including loading docks, food service
areas, refuse areas, outdoor processes and storage, vehicle cleaning, repair or
maintenance, fuel dispensing, equipment washing, etc.
o Plan Set with Construction Details for drainage related items (as appropriate)
□ Site Stormwater Assessment Exhibit.
o Site map with (existing and proposed) topographic information
o Delineation of sensitive areas, native vegetation and soils types. (Can be provided on
multiple exhibits to supplement design strategy narrative)
Performance Requirement 1: Site Design and Runoff Reduction SWCP Checklist
DESIGN STRATEGY MEANS OF DEMONSTRATING
COMPLIANCE
1 Limit disturbance of creeks and natural drainage
features.
Pre and post drainage feature map.
Delineate natural drainage features on Site
Stormwater Assessment Exhibit and DMA
Exhibit, as applicable.
2 Minimize compaction of highly permeable soils. Site Stormwater Assessment Exhibit of soil
types, overlain with development footprint
3 Limit clearing and grading of native vegetation
at the site to the minimum area needed to build
the project, allow access, and provide fire
protection.
Site Stormwater Assessment Exhibit with
native vegetation, overlain with development
footprint.
4 Minimize impervious surfaces by concentrating
improvements on the least-sensitive portions of
the site, while leaving the remaining land in a
natural undisturbed state.
Site Stormwater Assessment Exhibit with
delineated sensitive areas overlain with
development footprint
MINIMIZE STORMWATER RUNOFF BY IMPLEMENTING ONE OR MORE OF THE FOLLOWING DESIGN MEASURES:
5
MANDATORY SITE DESIGN MEASURES
(SELECT AT LEAST ONE)
Selected Reason,
for not selecting
a. Roof runoff directed into cisterns or rain
barrels for reuse?
b.
Roof runoff directed into vegetated areas
(safely away from building foundations and
footings)?
c.
Runoff from sidewalks, walkways, and/or
patios directed onto vegetated areas
(safely away from the building foundations
and footings)?
d.
Runoff from driveways and/or uncovered
parking lots onto vegetated areas (safely
away from the building foundations and
footings)?
e.
Are bike lanes, driveways, uncovered
parking lots, sidewalks, walkways, and
patios constructed with permeable
surfaces?
NOTE: This checklist must be included with every project application (except for projects deemed
EXEMPT). See Figure xxx to determine if your project is considered exempt, or regulated.
Performance Requirement 2: Water Quality Treatment SWCP Checklist
□ Project Net Impervious Area
□ Certification that on-site water quality treatment measures have been met on site, or if not
achievable:
o Documentation of the volume of runoff for which compliance cannot be achieved on site and the
associated off-site compliance requirements;
o Statement of intent to comply with Water Quality Treatment Performance Requirement through
Alternative Compliance.
For each Drainage Management Area, provide:
□ Unique DMA Number, area, and likely pollutant(s) of concern
□ Water Quality Treatment Approach
N/A if self-treating, or,
Through the use of LID, Biofiltration or Non-retention Based Treatment System);
□ Supporting calculations demonstrating compliance with Treatment Performance Requirement;
□ Plan sheet page and detail number (if appropriate) of Drainage Management Areas (DMA) Exhibit
where construction details are provided for each DMA.
For DMAs using Low Impact Development Treatment Systems, provide:
□ 85th percentile 24-hour storm event value, and basis of determination.
For DMAs using Biofiltration Systems, provide:
□ Statement indicating why an LID treatment system was not appropriate;
□ Surface loading rate approach, and basis of determination;-
(0.2 x per hour intensity, or 2 x 85th percentile hourly rainfall intensity);
□ Calculations to demonstrate that the minimum surface reservoir volume is equal to the biofiltration
treatment system surface area time a depth of 6-inches;
□ Construction detail (or reference to page on plans) which provides:
o Minimum planting depth
o Planting medium specifications. Either:
Specify 60 to 70% ASTM C33 sand, with 30-40% compost , or
Provide testing documentation demonstrating planting medium specified can minimally
infiltrate at a rate of 5 inches per hour)
o Plant selection consistent with Appendix L;
o Subsurface drainage/storage (gravel) layer with an area equal to the biofiltration treatment
system surface area and having a minimum depth of 12 inches;
o Underdrain with discharge elevation at top of gravel layer;
o No compaction of soils beneath the biofiltration facility (ripping/loosening of soils required if
compacted);
o No liners or other barriers interfering with infiltration, except for situations where lateral
infiltration is not technically feasible.
For DMAs using Non-Retention Based Treatment Systems, provide:
□ Statement indicating why an LID, or Biofiltration treatment system was not appropriate
□ Hydraulic Sizing Criteria used, and basis of determination (Volume = to 85th percentile, 24-hour storm, or flow
b basis (2 x 85th percentile hourly rainfall intensity or 0.2 x inches per hour intensity)
Performance Requirement 3: Runoff Retention SWCP Checklist
SITE ASSESSMENT MEASURES: (see table 3.5)
Include an exhibit and narrative of the opportunities and constraints to implementing LID Stormwater
Control measures based on the following items (as applicable):
□ Site topography
□ Hydrologic features including contiguous
n natural areas, wetlands, watercourses, seeps,
o or springs
□ Depth to seasonal high groundwater
□ Locations of groundwater wells used for
drinking water
□ Depth to an impervious layer such as
Bedrock
□ Presence of unique geology (e.g., karst)
□ Geotechnical hazards
□ Documented soil and/or groundwater
Contamination
□ Soil types and hydrologic soil groups
□ Vegetative cover/trees
□ Run-on characteristics (source and estimated runoff from
o offsite which discharges to the project area)
□ Existing drainage infrastructure for the site and nearby
a areas, including the location of municipal storm drains
□ Structures, including retaining walls
□ Utilities
□ Easements
□ Covenants
□ Zoning/Land Use
□ Setbacks
□ Open space requirements
□ Other pertinent overlay(s)
SITE DESIGN MEASURES
Include in narrative, and provide supporting exhibits as necessary, to demonstrate that the project
design has implemented the following design strategies (as applicable)
DESIGN STRATEGY MEANS OF DEMONSTRATING COMPLIANCE
1.
Define the development envelope and protected
areas, identifying areas that are most suitable for
development and areas to be left undisturbed.
Site Stormwater Assessment Exhibit.
2.
Conserve natural areas, including existing trees,
other vegetation, and soils
Site Stormwater Assessment Exhibit with native vegetation,
overlain with development footprint
3.
Limit the overall impervious footprint of the
project
Discussion regarding other building configurations
considered (and ultimately rejected)
4.
Construct streets, sidewalks, or parking lot aisles
to the minimum widths necessary, provided that
public safety or mobility uses are not
compromised
Discussion on minimum allowable widths, and rationale for
using larger values (if applicable) or confirmation that
minimum values were used (where applicable).
Set back development from creeks, wetlands, Discussion on set-back dimensions chosen.
5. and riparian habitats
6.
Conform the site layout along natural landforms
Within the Drainage Management Area (DMA) Exhibit, show
Topo survey with existing and planned contours cut and fill
lines. Discussion of grading approach.
7.
Avoid excessive grading and disturbance of
vegetation and soils
Exhibit with native vegetation, overlain with planned
disturbed area limits.
Performance Requirement 3: Runoff Retention SWCP Checklist Continued
STORMWATER STRUCTURAL CONTROL MEASURE SIZING
For Overall project,
□ Certification statement indicating that the selection, sizing, and design of Stormwater Control
measures meets the applicable Water Quality Treatment and Runoff Retention Performance
Requirements, or, if not achievable
o Provide documentation of the volume of runoff for which compliance cannot be achieved
on-site and the associated off-site compliance volume
o Statement of intent to comply with Water Quality Treatment and Runoff Retention
Performance Requirements through an Alternative Compliance Agreement
□ Documentation demonstrating percentage of the project’s Equivalent Impervious Surface Area
dedicated to retention-based Stormwater Control Measures
For each DMA,
□ Indicate sizing strategy used
o Hydrologic analysis and sizing methods
o Locally/regionally calibrated continuous simulation model that results in equivalent
optimization of on-site runoff retention volumes
o Hydrologic analysis and sizing methods, equally effective in optimizing on -site retention
volumes of the runoff generated by the rainfall events specified
□ Provide supporting calculations demonstrating compliance with Runoff Retention Performance
Requirement
□ Indicate if a ten percent adjustment (based on technical infeasibility) is included in design
approach
□ Indicate if off-site mitigation is included in design approach
NOTE: This checklist must be included with every project application (except for projects
deemed EXEMPT). See Figure xxx to determine if your project is considered exempt, or
regulated.
Performance Requirement 4: Peak Management SWCP Checklist
Project Level Documentation, identify
□ Point source discharge locations
□ Hydraulic Report demonstrating that post development storm water runoff peak flows discharged
from the site do not exceed pre-project peak flows for the 2- through 10-yer storm events)
□ Certification that on-site water quality treatment measures have been met on site, or if not
achievable:
o Documentation of the volume of runoff for which compliance cannot be achieved on site and the
associated off-site compliance requirements
o Statement of intent to comply with Water Quality Treatment Performance Requirement through
Alternative Compliance
NOTE: This checklist must be included with every project application (except for projects
deemed EXEMPT). See Figure xxx to determine if your project is considered exempt, or
regulated.
Performance Requirement 5: Special Circumstances SWCP Checklist
Project Level Documentation, identify
□ Which types of Special Circumstances apply
□ Which Watershed Management Zones (WMZ) the project is located in
□ Identification if the project is located atop of a designated Groundwater Basin
□ Proposed Performance Requirement modifications based on special circumstances
o Peak Management
o Runoff Retention
For highly altered channels,
□ Vicinity map indicating channel location relative to project, and downstream receiving waters
□ Narrative, and supporting calculations (as applicable) regarding anticipated impacts to downstream
waters
For intermediate flow control facilities,
□ Vicinity map indicating location of intermediate flow control facilities relative to project, and
downstream receiving waters
□ Quantification of pre-project tributary area to intermediate flow control facility performance
□ Quantification of proposed post-project tributary area to intermediate flow control facility
Performance
□ Summarize flow control performance data (pre and post) and include supporting performance
information based on numeric, hydraulic modeling, including flow volumes, durations and velocities
□ Narrative, and supporting calculations (as applicable) regarding anticipated impacts to downstream
Waters
□ Vicinity map delineating location of historic lake and/or wetlands relative to project
□ Supporting technical information to substantiate the request
□ Narrative, and supporting calculations (as applicable) regarding anticipated impacts to downstream
Waters
□ Stamped submittal (by registered professional engineer, geologist, architect, and/or landscape
architect)
NOTE: This checklist must be included with every project application (except for projects
deemed EXEMPT). See Figure 1 to determine if your project is considered exempt, or
regulated.
Alternate Compliance (Off-Site Compliance) SWCP Checklist
The City will only consider alternative compliance for project s able to demonstrate technical infeasibility. Projects
approved for alternative compliance must identify and secure rights to use an alternative site. Potential off-site
compliance alternative projects might i nclude green streets retrofits, off-site drainage features, riparian habitat
restoration projects, etc. The off-site compliance alternative project must be located within the same watershed as the
project and retain the storm water retention volume required by the original project. It is recommended that
discussions with City staff begin early in the development process regarding the acceptability of an off-site
compliance alternative project.
Project Level Documentation, identify
□ Indication of site conditions which are resulting in LID technical infeasibility
o Depth to seasonable high groundwater limits infiltration and/or prevents construction of subgrade stormwater
c control measures
o Depth to an impervious layer such as bedrock limits infiltration
o Sites where soil types significantly limit infiltration
o Sites where pollutant mobilization in the soil or groundwater is a documented concern
o Space constraints (e.g., infill projects, some redevelopment projects, high density development)
o Geotechnical hazards
o Stormwater Control Measures located within 100 feet of a groundwater well used for drinking water
o Incompatibility with surrounding drainage system (e.g., project drains to an existing stormwater collection
s system whose elevation or location precludes connection to a properly functioning treatment or flow control
f facility)
□ Indication of site conditions which are resulting in Bioretention technical infeasibility
o Biofiltration is not compatible with surrounding drainage system
o Location available for biofiltration facility is in an area with identified erosion or landslide hazards
o Location available for biofiltration facility is on a slope equal to or in excess of 8 percent
o Location available for biofiltration facility is within 50-feet from the projected top of the slope (using projected
a angle of repose) that is great than 20%
o Areas where runoff potentially contains industrial wastes
o Areas where there is a higher risk of concentrated spills (such as gas stations, truck stops)
□ Site-specific hydrologic and/or design analysis conducted and endorsed by a registered professional engineer,
g geologist, architect, and/or landscape architect, demonstrating that compliance with the applicable numeric Post-
C Construction Stormwater Management requirements is technically infeasible.
□ Schedule for completion of offsite project with milestone dates to identify funding, design, and construction of the
o off-site project(s) Note, Approved Watershed or Regional Plans, and projects situated within designated Urban
S Sustainability Areas are currently not acceptable reasons for pursuing off-site compliance
NOTE: This checklist must be included with every project application (except for projects
deemed EXEMPT). See Figure xxx to determine if your project is considered exempt, or
regulated.
CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD
CENTRAL COAST REGION
895 Aerovista Place, Suite 101
San Luis Obispo, California
RESOLUTION NO. R3-2013-0032
APPROVING POST-CONSTRUCTION STORMWATER MANAGEMENT REQUIREMENTS
FOR DEVELOPMENT PROJECTS IN THE CENTRAL COAST REGION
The Central Coast Regional Water Quality Control Board (Central Coast Water Board) finds
that:
Background
1.On December 8, 1999, USEPA promulgated regulations, known as Phase II, requiring
permits for stormwater discharges from small Municipal Separate Storm Sewer Systems
(MS4s) and from construction sites disturbing one and five acres of land. On February 5,
2013, the State Water Resources Control Board (State Water Board) adopted the National
Pollutant Discharge Elimination System (NPDES) General Permit for the Discharge of Storm
Water from Small Municipal Separate Storm Sewer Systems, Order No. 2013-0001-DWQ
(Phase II Municipal General Permit). Regulated small MS4s are required to apply to obtain
coverage under the Phase II Municipal General Permit and complete a Guidance Document.
Under the previous Municipal General Permit (Order No. 2003-0005-DWQ), the MS4s were
required to complete a Storm Water Management Plan (SWMP). The Central Coast Water
Board implements the Phase II Municipal General Permit to be consistent with its Water
Quality Control Plan, Central Coast Region (Basin Plan) to ensure protection of water
quality, beneficial uses, and the biological and physical integrity of watersheds in the Central
Coast region. The Central Coast Water Board Executive Officer requires specific conditions
for MS4s pursuant to the federal Clean Water Act, the Basin Plan, and the Phase II
Municipal General Permit.
2.The Phase II Municipal General Permit requires regulated small MS4s to develop and
implement Best Management Practices (BMPs), measurable goals, and timetables for
implementation, designed to reduce the discharge of pollutants to the maximum extent
practicable (MEP) and to protect water quality. The Phase II Municipal General Permit
requires regulated small MS4s to address stormwater runoff from development and
redevelopment projects through post-construction stormwater management requirements.
Phase II Municipal General Permit section E.12.k requires the Permittee to comply with
alternative post-construction storm water management requirements based on a watershed-
process approach after development and approval by the Central Coast Water Board.
3.The Central Coast Water Board approved Post-Construction Storm Water Management
Requirements for Development Projects in the Central Coast (Post-Construction
Requirements) on September 6, 2012 through adoption of Resolution R3-2012-0025.
Resolution R3-2012-0025 made findings that Central Coast municipalities must implement
the Post-Construction Requirements to comply with the statewide Phase II Municipal
General Permit, Order No. 2003-0005-DWQ in effect at that time.
ATTACHMENT 1
Resolution No. R3-2013-0032 -2- July 12, 2013
4. At the time of adoption of Resolution R3-2012-0025 by the Central Coast Water Board,
State Water Board staff was preparing to reissue the Phase II Municipal General Permit.
The State Water Board reissued the permit on February 5, 2013.
5. The reissued Phase II Municipal General Permit included several new provisions affecting
the implementation of post-construction requirements on the Central Coast. First, the
reissued Phase II Municipal General Permit allows for implementation of the Central Coast
Post-Construction Requirements in the Central Coast (Section E.12.k, Order No. 2013-
0001-DWQ). Second, it identifies the Cites of Greenfield, Gonzales, and Guadalupe, as
new Traditional MS4s (Attachment A, Order No. 2013-0001-DWQ). Third, it requires the
Guidance Document for Renewal Permittees to (1) identify and describe each BMP and
associated measurable goal, included in the Permittee’s most current SWMP, that
constitutes a more specific local or tailored level of implementation that may be more
protective of water quality than the minimum requirements of the Phase II Municipal General
Permit; and (2) for any more protective, locally-tailored BMP and associated measurable
goal for which the Renewal Permittee will reduce or cease implementation, provide a
demonstration to the Executive Officer of the relevant Regional Water Board that the
reduction or cessation is in compliance with Phase II Municipal General Permit and the
maximum extent practicable standard, and will not result in increased pollutant discharges
(Section A.1.b.4., Order No. 2013-0001-DWQ). All of the municipalities participating in the
Central Coast Joint Effort for Low Impact Development and Hydromodification Control (Joint
Effort MS4s) are Renewal Permittees under the reissued permit.
6. The Central Coast Water Board’s September 6, 2012 Resolution R3-2012-0025, which
approved the Post-Construction Requirements, must be re-adopted by the Central Coast
Water Board after a public process for consistency with the reissued Phase II Municipal
General Permit. The language of the Central Coast Water Board’s September 6, 2012
Resolution R3-2012-0025: refers to the former Phase II Municipal General Permit, Order No.
2003-0005-DWQ instead of the current Phase II Municipal General Permit, Order No. 2013-
0001-DWQ; cites the section numbers for post construction requirements as per Order No.
2003-0005-DWQ instead of the reissued Phase II Municipal General Permit section
numbers; and describes implementation via SWMPs as in Order No. 2003-0005-DWQ
instead of directly through permit requirements as in the reissued Phase II Municipal
General Permit.
7. On February 15, 2008, the Central Coast Water Board Executive Officer notified un-enrolled
traditional, small MS4 stormwater dischargers and two un-enrolled non-traditional, small
MS4 stormwater dischargers (University of California at Santa Barbara and Santa Cruz) of
the process the Central Coast Water Board would follow for enrolling the MS4s under the
Phase II Municipal General Permit. In the February 15, 2008 correspondence, the Central
Coast Water Board Executive Officer stated his intent to require MS4s to include in their
SWMPs a schedule for development and adoption of hydromodification control standards.
Subsequently, the Executive Officer required the MS4s’ SWMPs to include provisions for
development and implementation of hydromodification control criteria. For MS4s previously
enrolled, the Central Coast Water Board Executive Officer generally required those MS4s’
SWMPs to be updated with hydromodification control provisions.
8. On August 4, 2009 and October 20, 2009, the Central Coast Water Board Executive Officer
notified the MS4s of the option to participate in the Central Coast Joint Effort for developing
hydromodification control criteria (Joint Effort) as a means to meet the hydromodification
control criteria development, adoption, and implementation commitments in the MS4s’
Resolution No. R3-2013-0032 -3- July 12, 2013
SWMPs. MS4s agreeing to participate in the Joint Effort (Joint Effort MS4s) submitted a
written declaration of their intent to meet the terms of participation.
9. Between January and August 2010, Central Coast Joint Effort MS4s amended their SWMPs
to include BMPs to codify steps the Central Coast Water Board Executive Officer required of
them to participate in the Joint Effort. These BMPs included development and
implementation of hydromodification control criteria and selection of applicability thresholds
pursuant to the Joint Effort.
10. On September 28, 2010, the Central Coast Water Board Executive Officer notified the Joint
Effort MS4s of the commencement of the Joint Effort.
11. On December 2, 2009, the City of Salinas requested to participate in the Joint Effort. On
May 17, 2011, Central Coast Water Board Executive Officer outlined to the City of Salinas
the steps they needed to take to formalize participation in the Joint Effort. On August 16,
2011, the City of Salinas modified its SWMP to include these steps. On May 3, 2012, the
Central Coast Water Board approved Order No. R3-2012-0005, NPDES Permit No.
CA0049981, Waste Discharge Requirements for City of Salinas Municipal Stormwater
Discharges. Order No. R3-2012-0005, Provision J requires the City of Salinas to revise its
Stormwater Development Standards to incorporate the Post-Construction Requirements,
developed by the Joint Effort.
Stormwater Management to Protect Beneficial Uses
12. Prior to the Joint Effort, information on the local characteristics of Central Coast watersheds
was inadequate for MS4s to develop Post-Construction Requirements that protect
watershed processes so that beneficial uses of receiving waters are maintained and, where
applicable, restored. The Central Coast Water Board secured funds from the State Water
Quality Control Board’s Cleanup and Abatement Account to support acquisition and
assessment of information to inform the development of hydromodification control criteria
and related Post-Construction Requirements. These funds were used to establish an expert
team of scientists that would characterize the Central Coast region’s watersheds and help
create a methodology for developing Post-Construction Requirements based on that
characterization. The Post-Construction Requirements included in this Resolution
(Attachment 1) are based on the methodology, which has been summarized in the Technical
Support Document for Post-Construction Stormwater Management Requirements for
Development Projects in the Central Coast Region (Technical Support Document)
(Attachment 2).
13. The Technical Support Document (Attachment 2) contains rationale, justification, and
explanation for the Post-Construction Requirements. This information is hereby
incorporated by reference.
14. Urban runoff is a leading cause of pollution throughout the Central Coast region.
Development and urbanization increase pollutant loading and volume, velocity, frequency,
and discharge duration of stormwater runoff. First, natural vegetated pervious ground cover
is converted to impervious surfaces such as highways, streets, rooftops and parking lots.
While natural vegetated soil can both absorb rainwater and remove pollutants, providing an
effective natural purification process, impervious surfaces, in contrast, can neither absorb
water nor remove pollutants, and thus the natural purification characteristics are lost.
Second, urban development creates new pollution sources as the increased density of
Resolution No. R3-2013-0032 -4- July 12, 2013
human population brings proportionately higher levels of vehicle emissions, vehicle
maintenance wastes, pesticides, household hazardous wastes, pet wastes, trash, and other
anthropogenic pollutants, which can either be washed or directly dumped into the MS4. As
a result, the runoff leaving the developed urban area is significantly greater in pollutant load
than the pre-development runoff from the same area. These increased pollutant loads must
be controlled to protect downstream receiving water quality. Additionally, the increased
volume, increased velocity, and discharge duration of stormwater runoff from developed
areas, has the potential to accelerate downstream erosion, reduce groundwater recharge,
and impair stream habitat in natural drainages.
15. A higher percentage of impervious area correlates to a greater pollutant loading, resulting in
turbid water, nutrient enrichment, bacterial contamination, organic matter loads, toxic
compounds, temperature increases, and increases of trash or debris.
16. The discharge of pollutants and/or increased flows from MS4s can cause or threaten to
cause exceedances of applicable receiving water quality objectives, impair or threaten to
impair designated beneficial uses, and result in a condition of pollution (i.e., unreasonable
impairment of water quality for designated beneficial uses), contamination, hazard, or
nuisance.
17. Maintenance and restoration of watershed processes impacted by stormwater management
is necessary to protect water quality and beneficial uses. Watershed processes affected by
stormwater, by actions to manage stormwater, and/or by land uses that alter stormwater
runoff patterns include the following: 1) overland flow, 2) groundwater recharge, 3) interflow,
4) evapotranspiration, 5) delivery of sediment and organic matter to receiving waters, and 6)
chemical and biological transformations. These watershed processes must be maintained
and protected in order to support beneficial uses throughout the Central Coast region’s
watersheds. Restoration of degraded watershed processes, impacted by stormwater
management, is necessary to protect water quality and re-establish impacted beneficial
uses. New development, redevelopment, and existing land use activities create alterations
to stormwater runoff conditions which in turn result in changes to watershed processes that
can cause or contribute to impairment of beneficial uses and violations of water quality
standards. Future growth planned within the Central Coast region will degrade watershed
processes if not managed properly.
18. Low Impact Development (LID) is an effective approach to managing stormwater to minimize
the adverse effects of urbanization and development on watershed processes and beneficial
uses resulting from changes in stormwater runoff conditions. LID strategies can achieve
significant reductions in pollutant loading and runoff volumes as well as greatly enhanced
groundwater recharge rates. The proper implementation of LID techniques results in greater
benefits than single purpose stormwater and flood control infrastructure.
19. Controlling urban runoff pollution by using a combination of onsite source control and LID
BMPs augmented with treatment control BMPs before the runoff enters the MS4 is important
for the following reasons: 1) many end-of-pipe BMPs (such as diversion to the sanitary
sewer) are typically ineffective during significant storm events, but onsite source control
BMPs can be applied during all runoff conditions; 2) end-of-pipe BMPs are often incapable
of capturing and treating the wide range of pollutants which can be generated on a sub-
watershed scale; 3) end-of-pipe BMPs are more effective when used as polishing BMPs,
rather than the sole BMP to be implemented; 4) end-of-pipe BMPs do not protect the quality
or beneficial uses of receiving waters between the source and the BMP; and 5) offsite end-
Resolution No. R3-2013-0032 -5- July 12, 2013
of-pipe BMPs do not aid in the effort to educate the public regarding sources of pollution and
their prevention.
20. The risks associated with infiltration can be properly managed by many techniques,
including: 1) designing landscape drainage features that promote infiltration of runoff, but do
not “inject” runoff (injection bypasses the natural processes of filtering and transformation
that occur in the soil), 2) taking reasonable steps to prevent the illegal disposal of wastes, 3)
protecting footings and foundations, and 4) ensuring that each drainage feature is
adequately maintained in perpetuity. However, in some circumstances, site conditions (e.g.,
historical soil contamination) and the type of development (i.e., urban infill) can limit the
feasibility of retaining, infiltrating, and reusing stormwater at sites.
21. Redevelopment projects involve work on sites with existing impervious surfaces and other
disturbances that contribute pollutants to receiving waters and potentially impact watershed
processes such as infiltration. Though implementation of infiltration based LID measures
may be constrained by these conditions, post-construction stormwater management applied
to redevelopment projects still holds the potential to partially mitigate these existing impacts
as well as the impacts associated with the new or expanded portions of the project.
22. Providing long-term operation and maintenance of structural flow/volume control and
treatment BMPs is necessary so that the BMPs maintain their intended effectiveness at
managing runoff flow/volume and removing pollutants. If BMPs are not properly maintained,
new development and redevelopment will cause degradation of watershed processes.
23. When water quality impacts are considered during the planning stages of a project, new
development and many redevelopment projects can more efficiently incorporate measures
to protect water quality and beneficial uses. Planning decisions should account for potential
stormwater impacts to reduce pollutant loading and manage flows in order to maintain and
restore watershed processes as necessary to protect water quality and beneficial uses.
24. Infiltration and subsurface flow are the dominant hydrologic processes across all intact
watersheds of the Central Coast region. Different physical landscapes, defined by their
surface geology and slope, respond differently to the changes in watershed processes
imposed by urbanization, but the shift from infiltration to surface flow is ubiquitous.
25. The Post-Construction Requirements’ emphasis on protecting and, where degraded,
restoring key watershed processes is necessary to create and sustain linkages between
hydrology, channel geomorphology, and biological health necessary for healthy watersheds.
These linkages cannot be created by fine-tuning any particular flow attribute (e.g., peak,
duration) or reconstructing a desired geomorphic feature alone. Instead, these critical
linkages only occur where key watershed processes are intact.
26. Section 402 (p) of the Clean Water Act requires the Administrator of the United States
Environmental Protection Agency (USEPA) or her designated agent, in this instance, the
Central Coast Water Board, to require as part of the stormwater program “controls to reduce
the discharge of pollutants to the maximum extent practicable, including management
practices, control techniques and system, design and engineering methods, and such other
provisions as the Administrator or the State determines appropriate for the control of such
pollutants.” [USC Section 1342 (p)(3)(B)]. The maximum extent practicable (MEP) standard
is an ever-evolving, flexible, and advancing concept, which considers technical and
Resolution No. R3-2013-0032 -6- July 12, 2013
economic feasibility. As knowledge about controlling urban runoff continues to evolve, so
does that which constitutes MEP. Reducing the discharge of stormwater pollutants to the
MEP in order to protect beneficial uses requires review and improvement, which includes
seeking new opportunities, such as establishing these Post-Construction Requirements.
27. In cases of stormwater retention technical infeasibility, the dedication of an area equal to ten
percent of a site’s Effective Impervious Surface Area is practicable, because ten percent of
a site is a typical municipal landscape requirement.
Establishing Post-Construction Requirements
28. This Resolution enacts Post-Construction Requirements which include the components for
post-construction requirements based on a watershed-process approach that are identified
in section E.12.k of the Phase II Municipal General Permit, Order No. 2013-0001 DWQ.
29. The Post-Construction Requirements enacted by this Resolution protect the beneficial uses
of Waters of the United States. The intent of the Post-Construction Requirements enacted
by this Resolution is to focus on those discharges that threaten beneficial uses, and to
require implementation of BMPs to reduce stormwater pollutant discharges to the MEP and
protect water quality and beneficial uses. The Post-Construction Requirements enacted by
this Resolution are consistent with the evolving MEP standard.
30. The Post-Construction Requirements constitute a more specific local or tailored level of
implementation that may be more protective of water quality than the minimum requirements
of the Phase II Municipal General Permit.
31. This action to adopt this Resolution is exempt from the provisions of the California
Environmental Quality Act (Public Resources Code §21100, et seq.) in accordance with
section 13389 of the Porter-Cologne Water Quality Control Act (Porter-Cologne, Division 7
of the California Water Code).
32. The Post-Construction Requirements, developed by the Joint Effort, will become effective
upon approval of this Resolution by the Central Coast Water Board.
Stakeholder Involvement
33. On August 27, 2009, September 3, 2009, and September 8, 2009, Central Coast Water
Board staff held stakeholder workshops around the Central Coast region to provide an
opportunity for stakeholders to help select project milestones for the two-year Joint Effort
process. At the October 23, 2009, December 9, 2010, December 11, 2011, and March 15,
2012 Central Coast Water Board Meetings, staff provided updates on the Joint Effort to the
public and Board Members. Central Coast Water Board staff established the Joint Effort
Review Team (JERT), consisting of stakeholders representing the regulated governmental
agencies, environmental management agencies, developers, and technical consultants, to
provide review of Joint Effort project deliverables. The JERT met for the first time December
15, 2010, and held its seventh meeting March 28, 2012. On February 9 and October 31,
2011, Central Coast Water Board staff distributed to stakeholders Joint Effort updates and
status reports. In December 2011 and January 2012, Central Coast Water Board staff
conducted outreach to Joint Effort MS4s on the status of the Joint Effort. On February 15
and 16, 2012, Central Coast Water Board staff conducted workshops to provide updates on
the Joint Effort.
Resolution No. R3-2013-0032 -7- July 12, 2013
34. Central Coast Water Board staff implemented a process to inform interested persons and
the public and solicit comment on the Post-Construction Requirements developed through
the Joint Effort. On June 5th and 6th, 2012, Central Coast Water Board staff conducted
workshops on the Post-Construction Requirements. On May 14, 2012, staff issued a public
notice indicating that the Central Coast Water Board would consider adoption of the Post-
Construction Requirements. The public notice provided the public a 53-day public comment
period preceding the Central Coast Water Board hearing. Central Coast Water Board staff
responded to oral and written comments received from the public. All public comments were
considered. Public notice of the public hearing was given by electronic mail on May 14,
2012. Relevant documents and notices were also made available on the Central Coast
Water Board website.
35. On September 6, 2012, in San Luis Obispo California, the Central Coast Water Board held a
public hearing and heard and considered all public comments and evidence in the record.
The Central Coast Water Board adopted Resolution R3-2012-0025, approving the Post-
Construction Requirements for the first time on that date.
36. Upon adoption of Resolution R3-2012-0025 on September 6, 2012, the Central Coast Water
Board directed Central Coast Water Board staff to continue working with stakeholders to
identify potential obstacles over the one-year period leading up to implementation. This
Resolution R3-2013-0032 removes an obstacle identified during Central Coast Water Board
staff implementation of that process: overly conservative stormwater retention facility sizing.
37. On February 1 and March 14, 2013, Central Coast Water Board staff provided updates to
the Central Coast Water Board on the status of implementation of the Post-Construction
Requirements and how the Post-Construction Requirements interact with the Phase II
Municipal General Permit, Order No. 2013-0001 DWQ. On April 8, 2013, staff issued a
public notice indicating that the Central Coast Water Board would consider re-adopting the
Post-Construction Requirements. The public notice provided the public a 32-day public
comment period preceding the Central Coast Water Board hearing. Central Coast Water
Board staff responded to oral and written comments received from the public. All public
comments were considered. Public notice of the public hearing was given by electronic mail
to all stakeholders on April 8, 2013. The public notice and relevant documents were also
made available on the Central Coast Water Board website.
38. On July 12, 2013, in Watsonville California, the Central Coast Water Board held a public
hearing and heard and considered all public comments and evidence in the record.
THEREFORE, be it resolved that:
1. The Post-Construction Requirements, as defined in Attachment 1 are appropriate and
effective requirements for small MS4s subject to the post-construction requirements of the
current and subsequent Phase II Municipal General Permits to apply to development
projects, in order to protect watershed processes so that beneficial uses of receiving waters
affected by stormwater management are maintained and, where applicable, restored.
2. The Central Coast Water Board adopts the Post-Construction Requirements, as defined in
Attachment 1, as the minimum post-construction criteria that Central Coast Traditional
MS4s, the University of California at Santa Barbara and Santa Cruz, and any other
Resolution No. R3-2013-0032 -8- July 12, 2013
municipal discharger who chooses to implement these requirements, must apply to
applicable new development and redevelopment projects in order to protect water quality
and comply with the MEP standard and Phase II Municipal General Permit section E.12.k.
Section E.12.k requires Traditional MS4s to comply with post-construction storm water
management requirements based on a watershed-process approach developed by Regional
Water Boards. For the Non-Traditional MS4s already undertaking implementation of the
Post-Construction Requirements through implementation of BMPs in their SWMPs – the
University of California at Santa Barbara and Santa Cruz – the Post-Construction
Requirements constitute a more specific local or tailored level of implementation that may be
more protective of water quality than the minimum requirements of the Phase II Municipal
General Permit.
3. As minimum criteria, MS4s may establish criteria more stringent than the Post-Construction
Requirements as defined in Attachment 1. The MS4 may determine the need for greater
stringency based on specific factors and conditions affecting implementation of the Post-
Construction Requirements. Greater stringency may be achieved by lower applicability
thresholds where practical; additional site design and runoff reduction requirements; and
more rigorous flow control (peak management) criteria than indicated in the Post-
Construction Requirements as defined in Attachment 1.
4. The Central Coast Water Board Executive Officer may approve non-substantive changes to
the Post-Construction Requirements that improve clarity without altering the intent of the
requirements.
5. By March 6, 2014, the Central Coast Renewal Traditional MS4s, and applicable Non-
Traditional MS4s, shall apply the Post-Construction Requirements to all regulated projects
as defined in Attachment 1. Central Coast Traditional MS4s, and applicable Non-Traditional
MS4s, shall continue to apply the Post-Construction Requirements to all regulated projects
as defined in Attachment 1, pursuant to subsequent Phase II Municipal General Permits,
unless the Central Coast Water Board Executive Officer requires otherwise.
6. By July 1, 2014, the Central Coast New Traditional MS4s (Cities of Greenfield, Gonzales,
and Guadalupe) shall apply the Post-Construction Requirements to all regulated projects as
defined in Attachment 1.
7. The Central Coast Water Board adopts the Post-Construction Requirements, as defined in
Attachment 1, as the minimum post-construction criteria that the City of Salinas must apply
to applicable new development and redevelopment projects in order to protect water quality
and comply with the MEP standard and Order No. R3-2012-0005, NPDES Permit No.
CA0049981, Waste Discharge Requirements for City of Salinas Municipal Stormwater
Discharges.
I, Kenneth A. Harris Jr., Interim Executive Officer, do hereby certify the foregoing is a full, true,
and correct copy of the resolution adopted by the California Regional Water Quality Control
Board, Central Coastal Region on July 12, 2013.
______________________________
Kenneth A. Harris Jr.
Interim Executive Officer
Resolution No. R3-2013-0032 -9- July 12, 2013
ATTACHMENT 1: Post-Construction Stormwater Management Requirements for Development
Projects in the Central Coast Region
ATTACHMENT 2: Technical Support Document for Post-Construction Stormwater Management
Requirements for Development Projects in the Central Coast Region
Resolution No. R3-2013-0032 ATTACHMENT 1
POST-CONSTRUCTION STORMWATER MANAGEMENT REQUIREMENTS FOR
DEVELOPMENT PROJECTS IN THE
CENTRAL COAST REGION
July 12, 2013
CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD
CENTRAL COAST REGION
895 Aerovista Place, Suite 101, San Luis Obispo, California 93401
Phone y (805) 549-3147
http://www.waterboards.ca.gov/centralcoast/
To request copies of this report please contact
Dominic Roques at (805) 542-4780, or by email at:
droques@waterboards.ca.gov
Documents also are available at:
http://www.waterboards.ca.gov/centralcoast/water_issues/programs/stormwater/docs/lid/lid_hyd
romod_charette_index.shtml
Resolution No. R3-2013-0032 ATTACHMENT 1
POST-CONSTRUCTION STORMWATER MANAGEMENT REQUIREMENTS FOR
DEVELOPMENT PROJECTS IN THE CENTRAL COAST REGION
TABLE of CONTENTS
A. Watershed Management Zones (WMZs) ............................................................................ 1
B. Post-Construction Requirements ........................................................................................ 1
1) Regulated Projects .......................................................................................................... 1
2) Performance Requirement No. 1: Site Design and Runoff Reduction .............................. 3
3) Performance Requirement No. 2: Water Quality Treatment ............................................. 3
4) Performance Requirement No. 3: Runoff Retention ......................................................... 5
5) Performance Requirement No. 4: Peak Management .....................................................10
6) Performance Requirement No. 5: Special Circumstances ..............................................11
C. Alternative Compliance (Off-Site Compliance) ...................................................................12
1) Technical Infeasibility .....................................................................................................12
2) Approved Watershed or Regional Plan ...........................................................................13
3) Approved Urban Sustainability Area ...............................................................................14
D. Field Verifications of Post-Construction Stormwater Control Measures ..............................15
E. Operation and Maintenance for Structural SCMs ...............................................................16
1) O&M Plan .......................................................................................................................16
2) Maintenance Agreement and Transfer of Responsibility for SCMs .................................16
3) Structural Stormwater Control Measure O&M Database .................................................16
F. Permittee Reporting Requirements ....................................................................................17
G. Pre-existing Programs ........................................................................................................18
ATTACHMENT A: Watershed Management Zones ..................................................................19
ATTACHMENT B: Designated Groundwater Basins ................................................................20
ATTACHMENT C: Definitions Related to Post-Construction Requirements .............................23
ATTACHMENT D: Hydrologic Analysis and Stormwater Control Measure Sizing Guidance…..27
ATTACHMENT E: Ten Percent Adjustment to Retention Requirement – Calculation
Instructions……………………………………………………………………………………………….31
ATTACHMENT F: Calculating Off-Site Retention Requirements…………………………………33
Resolution No. R3-2013-0032 ATTACHMENT 1
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A. Watershed Management Zones (WMZs)
The urbanized portions of the Central Coast Region are categorized into 10 Watershed
Management Zones (WMZs), based on common key watershed processes and receiving water
type (creek, marine nearshore waters, lake, etc). Maps in Attachment A illustrate the WMZs for
the Central Coast Region’s urbanized areas. Designated Groundwater Basins of the Central
Coast Region (Attachment B) underlie some but not all WMZs in urbanized portions of the
Central Coast Region. The map and table in Attachment B illustrates the Groundwater Basins
of the Central Coast Region. Each WMZ and, where present, Groundwater Basin, is aligned
with specific Post-Construction Stormwater Management Requirements to address the impacts
of development on those watershed processes and beneficial uses.
1) The Permittee shall maintain the ability to identify the WMZs and their boundaries, and to
determine the WMZ in which development projects are proposed, throughout the urbanized
portions of their jurisdiction corresponding with the Phase I or Phase II Municipal Stormwater
Permit boundary.
2) The Permittee shall maintain the ability to determine whether development projects are
proposed in areas overlying designated Groundwater Basins, throughout the urbanized
portions of their jurisdiction subject to either a Phase I or Phase II Municipal Stormwater
Permit.
B. Post-Construction Requirements
The primary objective of these Post-Construction Stormwater Management Requirements
(hereinafter, Post-Construction Requirements) is to ensure that the Permittee is reducing
pollutant discharges to the Maximum Extent Practicable and preventing stormwater discharges
from causing or contributing to a violation of receiving water quality standards in all applicable
development projects that require approvals and/or permits issued under the Permittee’s
planning, building, or other comparable authority. The Post-Construction Requirements
emphasize protecting and, where degraded, restoring key watershed processes to create and
sustain linkages between hydrology, channel geomorphology, and biological health necessary
for healthy watersheds. Maintenance and restoration of watershed processes impacted by
stormwater management is necessary to protect water quality and beneficial uses.
1) Regulated Projects
Regulated Projects include all New Development or Redevelopment projects that create
and/or replace >2,500 square feet of impervious surface (collectively over the entire project
site)
a) Regulated Projects include, but are not limited to the following road projects/practices:
i) Removing and replacing a paved surface resulting in alteration of the original line
and grade, hydraulic capacity or overall footprint of the road
ii) Extending the pavement edge, or paving graveled shoulders
iii) Resurfacing by upgrading from dirt to asphalt, or concrete; upgrading from gravel to
asphalt, or concrete; or upgrading from a bituminous surface treatment (“chip seal”)
to asphalt or concrete
b) Regulated Projects do not include:
i) Road and Parking Lot maintenance:
(1) Road surface repair including slurry sealing, fog sealing, and pothole and square
cut patching
(2) Overlaying existing asphalt or concrete pavement with asphalt or concrete
without expanding the area of coverage
(3) Shoulder grading
(4) Cleaning, repairing, maintaining, reshaping, or regrading drainage systems
Resolution No. R3-2013-0032 ATTACHMENT 1
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(5) Crack sealing
(6) Resurfacing with in-kind material without expanding the road or parking lot
(7) Practices to maintain original line and grade, hydraulic capacity, and overall
footprint of the road or parking lot
(8) Repair or reconstruction of the road because of slope failures, natural disasters,
acts of God or other man-made disaster
ii) Sidewalk and bicycle path or lane projects, where no other impervious surfaces are
created or replaced, built to direct stormwater runoff to adjacent vegetated areas
iii) Trails and pathways, where no other impervious surfaces are replaced or created,
and built to direct stormwater runoff to adjacent vegetated areas
iv) Underground utility projects that replace the ground surface with in-kind material or
materials with similar runoff characteristics
v) Curb and gutter improvement or replacement projects that are not part of any
additional creation or replacement of impervious surface area (e.g., sidewalks,
roadway)
vi) Second-story additions that do not increase the building footprint
vii) Raised (not built directly on the ground) decks, stairs, or walkways designed with
spaces to allow for water drainage
viii) Photovoltaic systems installed on/over existing roof or other impervious surfaces,
and panels located over pervious surfaces with well-maintained grass or vegetated
groundcover, or panel arrays with a buffer strip at the most down gradient row of
panels
ix) Temporary structures (in place for less than six months)
x) Electrical and utility vaults, sewer and water lift stations, backflows and other utility
devices
xi) Above-ground fuel storage tanks and fuel farms with spill containment system
c) The Permittee shall apply the Post-Construction Requirements by March 6, 20141, to all
applicable Regulated Projects that require approvals and/or permits issued under the
Permittee’s planning, building, or other comparable authority. Applicable Regulated
Projects include both private development requiring permits, and public projects:
i) Private Development Projects
(1) Discretionary Projects – The Permittee shall apply the Post-Construction
Requirements to those projects that have not received the first discretionary
approval of project design.
(2) Ministerial Projects – If the project is only subject to ministerial approval, the
Permittee shall apply the Post-Construction Requirements to those projects that
have not received any ministerial approvals. If the ministerial project receives
multiple ministerial approvals, the Permittee shall apply the Post-Construction
Requirements to the first ministerial approval. Ministerial approvals include, but
are not limited to, building permits, site engineering improvements, and grading
permits.
ii) Public Development Projects
(1) The Permittee shall develop and implement an equivalent approach, to the above
approach used for private development projects, to apply the Post-Construction
Requirements to applicable public development projects, including applicable
university development projects
iii) Exemptions – The Permittee may propose, to the Central Coast Water Board
Executive Officer, a lesser application of the Post-Construction Requirements for
1 Newly enrolled Permittees Gonzales, Greenfield, and Guadalupe shall apply the Post-Construction
Requirements by July 1, 2014.
Resolution No. R3-2013-0032 ATTACHMENT 1
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projects with completed project applications dated prior to September 6, 2012. The
Permittee must demonstrate that the application of the Post-Construction
Requirements would pose financial infeasibility for the project. The Permittee shall
not grant any exemptions without prior approval from the Central Coast Water Board
Executive Officer.
2) Performance Requirement No. 1: Site Design and Runoff Reduction
a) The Permittee shall require all Regulated Projects that create and/or replace > 2,500
square feet of impervious surface (collectively over the entire project site), including
detached single-family home projects, to implement at least the following design
strategies throughout the Regulated Project site:
i) Limit disturbance of creeks and natural drainage features
ii) Minimize compaction of highly permeable soils
iii) Limit clearing and grading of native vegetation at the site to the minimum area
needed to build the project, allow access, and provide fire protection
iv) Minimize impervious surfaces by concentrating improvements on the least-sensitive
portions of the site, while leaving the remaining land in a natural undisturbed state
v) Minimize stormwater runoff by implementing one or more of the following site design
measures:
(1) Direct roof runoff into cisterns or rain barrels for reuse
(2) Direct roof runoff onto vegetated areas safely away from building foundations
and footings, consistent with California building code
(3) Direct runoff from sidewalks, walkways, and/or patios onto vegetated areas
safely away from building foundations and footings, consistent with California
building code
(4) Direct runoff from driveways and/or uncovered parking lots onto vegetated areas
safely away from building foundations and footings, consistent with California
building code
(5) Construct bike lanes, driveways, uncovered parking lots, sidewalks, walkways,
and patios with permeable surfaces
b) The Permittee shall confirm that projects comply with Site Design and Runoff Reduction
Performance Requirements by means of appropriate documentation (e.g., check lists)
accompanying applications for project approval.
3) Performance Requirement No. 2: Water Quality Treatment
a) The Permittee shall require Regulated Projects, except detached single-family homes, >
5,000 square feet of Net Impervious Area, and detached single-family homes > 15,000
square feet of Net Impervious Area, to treat stormwater runoff as required in the Water
Quality Treatment Performance Requirements in Section B.3.b. to reduce pollutant loads
and concentrations using physical, biological, and chemical removal.
i) Net Impervious Area is the total (including new and replaced) post-project impervious
areas, minus any reduction in total imperviousness from the pre-project to post-
project condition: Net Impervious Area = (New and Replaced Impervious Area) -
(Reduced Impervious Area Credit), where Reduced Impervious Area Credit is the
total pre-project to post-project reduction in impervious area, if any.
b) The Permittee shall require each Regulated Project subject to Water Quality Treatment
Performance Requirements to treat runoff generated by the Regulated Project site using
the onsite measures below, listed in the order of preference (highest to lowest). Water
Quality Treatment Performance Requirements shall apply to the runoff from existing,
new, and replaced impervious surfaces on sites where runoff from existing impervious
surfaces cannot be separated from runoff from new and replaced impervious surfaces.
Resolution No. R3-2013-0032 ATTACHMENT 1
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i) Low Impact Development (LID) Treatment Systems – Implement harvesting and use,
infiltration, and evapotranspiration Stormwater Control Measures that collectively
achieve the following hydraulic sizing criteria for LID systems:
(1) Hydraulic Sizing Criteria for LID Treatment Systems – LID systems shall be
designed to retain stormwater runoff equal to the volume of runoff generated by
the 85th percentile 24-hour storm event, based on local rainfall data.
ii) Biofiltration Treatment Systems – Implement biofiltration treatment systems using
facilities that must be demonstrated to be at least as effective as2 a biofiltration
treatment system with the following design parameters:
(1) Maximum surface loading rate appropriate to prevent erosion, scour and
channeling within the biofiltration treatment system itself and equal to 5 inches
per hour, based on the flow of runoff produced from a rain event equal to or at
least:
(a) 0.2 inches per hour intensity; or
(b) Two times the 85th percentile hourly rainfall intensity for the applicable area,
based on historical records of hourly rainfall depth
(2) Minimum surface reservoir volume equal to the biofiltration treatment system
surface area times a depth of 6 inches
(3) Minimum planting medium depth of 24 inches. The planting medium must sustain
a minimum infiltration rate of 5 inches per hour throughout the life of the project
and must maximize runoff retention and pollutant removal. A mixture of sand
(60%-70%) meeting the specifications of American Society for Testing and
Materials (ASTM) C33 and compost (30%-40%) may be used. A Regulated
Project may utilize an alternative planting medium if it demonstrates its planting
medium is equal to or more effective at attenuating pollutants than the specified
planting medium mixture.
(4) Proper plant selection3
(5) Subsurface drainage/storage (gravel) layer with an area equal to the biofiltration
treatment system surface area and having a minimum depth of 12 inches
(6) Underdrain with discharge elevation at top of gravel layer
(7) No compaction of soils beneath the biofiltration facility (ripping/loosening of soils
required if compacted)
(8) No liners or other barriers interfering with infiltration, except for situations where
lateral infiltration is not technically feasible.
iii) Non-Retention Based Treatment Systems – Implement Stormwater Control
Measures that collectively achieve at least one of the following hydraulic sizing
criteria for non-retention based treatment systems:
(1) Hydraulic Sizing Criteria for Non-Retention Based Treatment Systems:
(a) Volume Hydraulic Design Basis – Treatment systems whose primary mode of
action depends on volume capacity shall be designed to treat stormwater
runoff equal to the volume of runoff generated by the 85th percentile 24-hour
storm event, based on local rainfall data.
2 Facilities or a combination of facilities, of a different design than in Section B.3.b.ii. may be permitted if
all of the following measures of equivalent effectiveness are demonstrated: 1) equal or greater amount of
runoff infiltrated or evapotranspired; 2) equal or lower pollutant concentrations in runoff that is discharged
after biofiltration; 3) equal or greater protection against shock loadings and spills; and 4) equal or greater
accessibility and ease of inspection and maintenance.
3 Technical guidance for designing bioretention facilities is available from the Central Coast LID Initiative.
The guidance includes design specifications and plant lists appropriate for the Central Coast climate.
(http://www.centralcoastlidi.org/Central_Coast_LIDI/LID_Structural_BMPs.html)
Resolution No. R3-2013-0032 ATTACHMENT 1
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(b) Flow Hydraulic Design Basis – Treatment systems whose primary mode of
action depends on flow capacity shall be sized to treat:
(i) The flow of runoff produced by a rain event equal to at least two times the
85th percentile hourly rainfall intensity for the applicable area, based on
historical records of hourly rainfall depths; or
(ii) The flow of runoff resulting from a rain event equal to at least 0.2 inches
per hour intensity.
c) Stormwater Control Plan Requirements – For each Regulated Project subject to the
Water Quality Treatment Performance Requirement, the Permittee shall require the
Project Applicant to provide the below information in a Stormwater Control Plan. The
Permittee shall not grant final project approval, until the Stormwater Control Plan for the
Regulated Project sufficiently demonstrates the Regulated Project design meets the
Water Quality Treatment Performance Requirements.
i) Project name, application number, location including address and assessor’s
parcel number
ii) Name of Applicant
iii) Project Phase number (if project is being constructed in phases)
iv) Project Type (e.g., commercial, industrial, multi-unit residential, mixed-use, public),
and description
v) Total project site area
vi) Total new impervious surface area, total replaced impervious surface area, total
new pervious area, and calculation of Net Impervious Area
vii) Statement of Water Quality Treatment Performance Requirements that apply to the
Project
viii) Summary of Site Design and Runoff Reduction (Performance Requirement No. 1)
measures selected for the project
ix) Description of all post-construction structural Stormwater Control Measures
x) Supporting calculations used to comply with the applicable Water Quality
Treatment Performance Requirements
xi) Documentation certifying that the selection, sizing, and design of the Stormwater
Control Measures meet the full or partial Water Quality Treatment Performance
Requirement
xii) Water quality treatment calculations used to comply with Water Quality Treatment
Performance Requirement and any analysis to support infeasibility determination
xiii) Statement of Compliance:
(1) Statement that Water Quality Treatment Performance Requirement has been
met on-site, or, if not achievable:
(a) Documentation of the volume of runoff for which compliance cannot be
achieved on-site and the associated off-site compliance requirements.
(b) Statement of intent to comply with Water Quality Treatment Performance
Requirement through Alternative Compliance
4) Performance Requirement No. 3: Runoff Retention
a) The Permittee shall require Regulated Projects, except detached single-family homes,
that create and/or replace >15,000 square feet of impervious surface (collectively over
the entire project site), and detached single-family homes > 15,000 square feet of Net
Impervious Area, in WMZs 1, 2, 5, 6, 8 and 9, and those portions of WMZs 4, 7, and 10
that overlie designated Groundwater Basins (Attachment B) to meet the Runoff
Retention Performance Requirements in Sections B.4.b. and B.4.c. using the LID
Development Standards in Section B.4.d. for optimal management of watershed
processes.
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b) Adjustments to the Runoff Retention Performance Requirements for Redevelopment –
Where the Regulated Project includes replaced impervious surface, the below
adjustments apply. These adjustments are accounted for in the Retention Tributary
Area calculation in Attachment D.
i) Redevelopment Projects outside an approved Urban Sustainability Area, as
described in Section C.3. – The total amount of replaced impervious surface shall be
multiplied by 0.5 when calculating the volume of runoff subject to Runoff Retention
Performance Requirements.
ii) Redevelopment Projects located within an approved Urban Sustainability Area
(Section C.3.) – The total amount of runoff volume to be retained from replaced
impervious surfaces shall be equivalent to the pre-project runoff volume retained.
c) The Permittee shall require Regulated Projects, subject to the Runoff Retention
Performance Requirements, to meet the following Performance Requirements:
i) Watershed Management Zone 1 and portions of Watershed Management Zones 4,
7 and 10 which overlie designated Groundwater Basins:
(1) Retain 95th Percentile Rainfall Event – Prevent offsite discharge from events up
to the 95th percentile 24-hour rainfall event as determined from local rainfall
data.4
(2) Compliance must be achieved by optimizing infiltration. Compliance for retention
of the remaining volume must be achieved via storage, rainwater harvesting
and/or evapotranspiration.
ii) Watershed Management Zone 2:
(1) Retain 95th Percentile Rainfall Event – Prevent offsite discharge from events up
to the 95th percentile 24-hour rainfall event as determined from local rainfall data.
(2) Compliance must be achieved via storage, rainwater harvesting, infiltration,
and/or evapotranspiration.
iii) Watershed Management Zones 5 and 8:
(1) Retain 85th Percentile Rainfall Event – Prevent offsite discharge from events up
to the 85th percentile 24-hour rainfall event as determined from local rainfall data.
(2) Compliance must be achieved by optimizing infiltration. Compliance for retention
of the remaining volume must be achieved via storage, rainwater harvesting
and/or evapotranspiration.
iv) Watershed Management Zones 6 and 9:
(1) Retain 85th Percentile Rainfall Event – Prevent offsite discharge from events up
to the 85th percentile 24-hour rainfall event as determined from local rainfall data.
(2) Compliance must be achieved via storage, rainwater harvesting, infiltration,
and/or evapotranspiration.
d) LID Development Standards – The Permittee shall require Regulated Projects, subject to
Runoff Retention Performance Requirements, to meet Runoff Retention Performance
Requirements (Sections B.4.b. and B.4.c.) using the following LID Development
Standards:
i) Site Assessment Measures – Permittees shall require the applicant for each
Regulated Project to identify opportunities and constraints to implement LID
Stormwater Control Measures. Permittees shall require the applicant to document
the following, as appropriate to the development site:
4 Use either the methodology provided in Part I.D of the December 2009 Technical Guidance on
Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy
Independence and Security Act, or, rainfall statistics provided by the Central Coast Water Board,
whichever produces a more accurate value for rainfall depth.
Resolution No. R3-2013-0032 ATTACHMENT 1
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• Site topography
• Hydrologic features including contiguous natural areas, wetlands, watercourses,
seeps, or springs
• Depth to seasonal high groundwater
• Locations of groundwater wells used for drinking water
• Depth to an impervious layer such as bedrock
• Presence of unique geology (e.g., karst)
• Geotechnical hazards
• Documented soil and/or groundwater contamination
• Soil types and hydrologic soil groups
• Vegetative cover/trees
• Run-on characteristics (source and estimated runoff from offsite which discharges
to the project area)
• Existing drainage infrastructure for the site and nearby areas including the
location of municipal storm drains
• Structures including retaining walls
• Utilities
• Easements
• Covenants
• Zoning/Land Use
• Setbacks
• Open space requirements
• Other pertinent overlay(s)
ii) Site Design Measures – Permittees shall require the applicant for each Regulated
Project to optimize the use of LID site design measures, as feasible and appropriate
at the project site. Regulated Projects subject to Performance Requirement No. 3
must augment design strategies required by Performance Requirement No. 1
(Section B.2.a.i-v) with the following:
• Define the development envelope and protected areas, identifying areas that are
most suitable for development and areas to be left undisturbed
• Conserve natural areas, including existing trees, other vegetation, and soils
• Limit the overall impervious footprint of the project
• Construct streets, sidewalks, or parking lot aisles to the minimum widths
necessary, provided that public safety or mobility uses are not compromised
• Set back development from creeks, wetlands, and riparian habitats
• Conform the site layout along natural landforms
• Avoid excessive grading and disturbance of vegetation and soils
iii) Delineation of discrete Drainage Management Areas (DMAs) – The Permittee shall
require each Regulated Project to delineate DMAs to support a decentralized
approach to stormwater management.
(1) The Permittee shall require the applicant for each Regulated Project to provide a
map or diagram dividing the entire project site into discrete DMAs
(2) The Permittee shall require the applicant for each Regulated Project to account
for the drainage from each DMA using measures identified in Sections B.4.d.iv.
and B.4.d.v., below.
iv) Undisturbed and Natural Landscape Areas – Permittees shall require each
Regulated Project to implement appropriate Site Design (Section B.4.d.ii.), and
Runoff Reduction Measures in Performance Requirement No. 1, to reduce the
amount of runoff for which retention and treatment is required. Runoff reduction
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measures that can be used to account for this reduction also include the below
measures. The Retention Tributary Area calculation in Attachment D accounts for
these reductions.
(1) Undisturbed or areas planted with native, drought-tolerant, or LID appropriate
vegetation that do not receive runoff from other areas may be considered self-
treating and no additional stormwater management is required.
(2) Runoff from impervious surfaces, generated by the rainfall events identified in
Section B.4.c, may be directed to undisturbed or natural landscaped areas.
When the applicant can demonstrate that this runoff will be infiltrated and will not
produce runoff to the storm drain system, or a surface receiving waterbody, or
create nuisance ponding that may affect vegetation health or contribute to vector
problems, then no additional stormwater management is required for these
impervious surfaces.
v) Structural Stormwater Control Measures – Where Regulated Project Applicants have
demonstrated in their Stormwater Control Plans, and the Permittee has confirmed,
that use of Site Design measures listed in Section B.4.d.ii., Runoff Reduction
measures listed in Performance Requirement No.1, and undisturbed and natural
landscape areas discussed in Section B.4.d.iv., has been maximized to the extent
feasible, Structural Stormwater Control Measures designed for water quality
treatment and/or flow control shall be used to comply with Performance Requirement
No. 3.
(1) The Permittee shall require the Regulated Project applicant to use structural
Stormwater Control Measures that optimize retention and result in optimal
protection and restoration of watershed processes, such as Structural Control
Measures associated with small-scale, decentralized facilities designed to
infiltrate, evapotranspirate, filter, or capture and use stormwater.
vi) Hydrologic Analysis and Structural Stormwater Control Measure Sizing – To
determine Stormwater Control Measure sizing and design, Permittees shall require
Regulated Project applicants to use one of the following: 1) hydrologic analysis and
sizing methods as outlined in Attachment D; 2) locally/regionally calibrated
continuous simulation model that results in equivalent optimization of on-site runoff
volume retention; or 3) hydrologic analysis and sizing methods, equally effective in
optimizing on-site retention of the runoff generated by the rainfall event specified in
Section B.4.c, that have been approved by the Central Coast Water Board Executive
Officer.
e) Ten Percent Adjustment for Sites with Technical Infeasibility – Where technical
infeasibility, as described in Section C.1.c., prevents full on-site compliance with the
Runoff Retention Performance Requirement, on-site retention of the full Retention
Volume per Section B.4.d.vi. is not required and the Regulated Project is required to
dedicate no less than ten percent of the Regulated Project’s Equivalent Impervious
Surface Area5 to retention-based Stormwater Control Measures.
i) Use the Attachment E instructions to calculate the ten percent adjustment for
applying the Runoff Retention Performance Requirement.
ii) The Water Quality Treatment Performance Requirement is not subject to this
adjustment, i.e., mitigation to achieve full compliance with the Water Quality
Treatment Performance Requirement is required on- or off-site.
f) Off-Site Mitigation – Off-site mitigation is required when Regulated Projects do not retain
the full Retention Volume per Section B.4.b and B.4.c, and 1) fail to demonstrate
technical infeasibility of full retention; or 2) demonstrate technical infeasibility of full
5 Calculate Equivalent Impervious Surface Area using guidance in Attachment E
Resolution No. R3-2013-0032 ATTACHMENT 1
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retention AND fail to dedicate at least ten percent of the Regulated Project’s Equivalent
Impervious Surface Area to retention-based Stormwater Control Measures.
i) Use the Attachment F instructions to calculate the Off-Site retention requirements
when a Regulated Project subject to the Runoff Retention Performance Requirement
does not allocate the full ten percent of the project site’s Equivalent Impervious
Surface Area to retention-based Stormwater Control Measures.
g) Reporting Requirements – For each Regulated Project subject to the Runoff Retention
Performance Requirement, the Permittee shall require the Project Applicant to provide
the below information in a Stormwater Control Plan. The Permittee shall not grant final
project approval, until the Stormwater Control Plan for the Regulated Project sufficiently
demonstrates the Regulated Project design meets the Water Quality Treatment and
Runoff Retention Performance Requirements.
i) Project Name, application number, and location including address and assessor’s
parcel number
ii) Name of Applicant
iii) Project Phase number (if project is being constructed in phases)
iv) Project Type (e.g., commercial, industrial, multiunit residential, mixed-use, public),
and description
v) Total project site area
vi) Total new and/or replaced impervious surface area
vii) Statement of Water Quality Treatment and Runoff Retention Performance
Requirements that apply to the Project
viii) Adjusted Requirements based on the local jurisdiction’s approval, that the Project
is allowed a Special Circumstance, Watershed or Regional Plan, or Urban
Sustainability Area designation
ix) Site assessment summary
x) LID Measures used:
(1) Site design measures
(2) Runoff Reduction Measures
(3) Post-construction structural Stormwater Control Measures
xi) Summary of Runoff Reduction Measures and Structural Stormwater Control
Measures, by Drainage Management Area, as well as for the entire site
xii) Supporting calculations used to comply with the applicable Water Quality
Treatment and Runoff Retention Performance Requirements
xiii) Documentation demonstrating infeasibility where Site Design and Runoff
Reduction measures cannot retain required runoff volume
xiv) Documentation demonstrating infeasibility where retention-based Stormwater
Control Measures cannot retain and/or treat the required runoff volume
xv) Documentation demonstrating infeasibility where on-site compliance cannot be
achieved
xvi) Documentation demonstrating percentage of the project’s Equivalent Impervious
Surface Area dedicated to retention-based Stormwater Control Measures
xvii) Documentation of certification that the selection, sizing, and design of the
Stormwater Control Measures meets the applicable Water Quality Treatment and
Runoff Retention Performance Requirement
xviii) O&M Plan for all structural Stormwater Control Measures to ensure long-term
performance
xix) Owner of facilities
xx) Statement of Compliance:
(1) Statement that the Water Quality Treatment and Runoff Retention Performance
Requirements have been met on-site, or, if not achievable:
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(a) Documentation of the volume of runoff for which compliance cannot be
achieved on-site and the associated off-site compliance volume.
(b) Statement of intent to comply with Water Quality Treatment and Runoff
Retention Performance Requirements through an Alternative Compliance
agreement.
5) Performance Requirement No. 4: Peak Management
The Permittee shall require all Regulated Projects that create and/or replace >22,500
square feet of impervious surface (collectively over the entire project site) in Watershed
Management Zones 1, 2, 3, 6, and 9 to manage peak stormwater runoff as required below
(Section B.5.a.i.), and to meet Water Quality Treatment and Runoff Retention Performance
Requirements.
a) The Permittee shall apply the following Peak Management Performance Requirements:
i) Post-development peak flows, discharged from the site, shall not exceed pre-project
peak flows for the 2- through 10-year storm events.
b) Reporting Requirements – For each Regulated Project subject to the Peak Management
Performance Requirement, the Permittee shall require the Project Applicant to provide
the below information in a Stormwater Control Plan. The Permittee shall not grant final
project approval, until the Stormwater Control Plan for the Regulated Project sufficiently
demonstrates the Regulated Project design meets the Water Quality Treatment, Runoff
Retention, and Peak Management Requirements.
i) Project Name, application number, and location including address and assessor’s
parcel number
ii) Name of Applicant
iii) Project Phase number (if project is being constructed in phases)
iv) Project Type (e.g., commercial, industrial, multiunit residential, mixed-use, public),
and description
v) Total project site area
vi) Total new and/or replaced impervious surface area
vii) Statement of Water Quality Treatment, Runoff Retention, and Peak Management
Performance Requirements that apply to the Project
viii) Adjusted Requirements based on the local jurisdiction’s approval, that the Project
is allowed a Special Circumstance, Watershed or Regional Plan, or Urban
Sustainability Area designation
ix) Site assessment summary
x) LID Measures used:
(1) Site design measures
(2) Runoff Reduction Measures
(3) Post-construction structural Stormwater Control Measures
xi) Summary of Runoff Reduction Measures and Structural Stormwater Control
Measures, by Drainage Management Area, as well as for the entire site
xii) Supporting calculations used to comply with the applicable Water Quality
Treatment, Runoff Retention, and Peak Management Performance Requirements
xiii) Documentation demonstrating infeasibility where on-site compliance cannot be
achieved
xiv) Documentation of certification that the selection, sizing, and design of the
Stormwater Control Measures meets the applicable Water Quality Treatment,
Runoff Retention, and Peak Management Performance Requirements
xv) O&M Plan for all structural SCMs to ensure long-term performance
xvi) Owner of facilities
xvii) Statement of Compliance:
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(1) Statement that the Water Quality Treatment, Runoff Retention, and Peak
Management Performance Requirements have been met on-site, or, if not
achievable:
(a) Documentation of the volume of runoff for which compliance cannot be
achieved on-site and the associated off-site compliance requirements.
(b) Statement of intent to comply with Water Quality Treatment, Runoff
Retention, and Peak Management Performance Requirements through an
Alternative Compliance agreement.
6) Performance Requirement No. 5: Special Circumstances
The Permittee may designate Regulated Projects as subject to Special Circumstances
based on certain site and/or receiving water conditions. The Special Circumstances
designation exempts a Regulated Project from Runoff Retention and/or Peak Management
Performance Requirements where those Performance Requirements would be ineffective to
maintain or restore beneficial uses of receiving waters. The Regulated Project subject to
Special Circumstances must still comply with the Water Quality Treatment Performance
Requirements.
a) Special Circumstances include:
i) Highly Altered Channel Special Circumstance:
The Permittee may designate Regulated Projects as subject to Special Circumstances
for Highly Altered Channels for the following conditions:
(1) Project runoff discharges into stream channels that are concrete-lined or
otherwise continuously armored from the discharge point to the channel’s
confluence with a lake, large river (>200-square mile drainage area).
(2) Project runoff discharges to a continuous underground storm drain system that
discharges directly to a lake, large river (>200-square mile drainage area), the
San Lorenzo River in the City of Santa Cruz, or marine nearshore waters
(3) Project runoff discharges to other areas identified by the Central Coast Water
Board
(4) Under no circumstance described in 6.a.i. can runoff from the Regulated Project
result in adverse impacts to downstream receiving waters
ii) Intermediate Flow Control Facility Special Circumstance:
(1) The Permittee may designate Regulated Projects as subject to Special
Circumstances for Intermediate Flow Control Facilities if the project runoff
discharges to an existing (as of the date when the Central Coast Water Board
approved Resolution R3-2012-0025) flow control facility that regulates flow
volumes and durations to levels that have been demonstrated to be protective of
beneficial uses of the receiving water downstream of the facility.
(2) The flow control facility must have the capacity to accept the Regulated Project’s
runoff.
(3) Demonstration of facility capacity to accept runoff and to regulate flow volumes
and durations must include quantitative analysis based on numeric, hydraulic
modeling of facility performance.
(4) Under no circumstance described in Section B.6.a.ii. can runoff from the
Regulated Project result in adverse impacts to downstream receiving waters.
iii) Historic Lake and Wetland Special Circumstance:
(1) The Permittee may designate Regulated Projects as subject to Special
Circumstances for Historic Lakes and Wetlands for the following conditions:
(a) Project is located where there was once a historic lake or wetland where pre-
development hydrologic processes included filtration and storage but no
significant infiltration to support downstream receiving water.
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(b) The Special Circumstance has been established based on a delineation of
the historic lake or wetland approved by the Central Coast Water Board
Executive Officer
b) Performance Requirements for Highly Altered Channel and/or Intermediate Flow Control
Facility Special Circumstances:
i) For Regulated Projects that: 1) create and/or replace >22,500 square feet of
impervious surface; 2) are located in WMZs 1, 2, 5, and 8, and those portions of
WMZs 4, 7, and 10 that overlie a designated Groundwater Basin:
(1) Water Quality Treatment (Performance Requirement No. 2)
(2) Runoff Retention (Performance Requirement No. 3)
ii) For Regulated Projects that: 1) create and/or replace >22,500 square feet of
impervious surface; and 2) are located in WMZs 3, 6, and 9, and those portions of
WMZs 4, 7, and 10 that do not overlie a designated Groundwater Basin:
(1) Water Quality Treatment (Performance Requirement No. 2)
c) Performance Requirements for Historic Lake and Wetland Special Circumstances
i) For Regulated Projects that create and/or replace >15,000 and < 22,500 square feet
of impervious surface and meet the Historic Lake and Wetland Special
Circumstance:
(1) Water Quality Treatment (Performance Requirement No. 2)
(2) Detention: Detain runoff such that the post-project peak discharge rate does not
exceed the pre-project rate for all runoff up to the 95th percentile 24-hr rainfall
event, or a more protective rate consistent with the Permittee’s own development
requirements
ii) For Regulated Projects that create and/or replace >22,500 square feet of impervious
surface and meet the Historic Lake and Wetland Special Circumstance:
(1) Water Quality Treatment (Performance Requirement No. 2)
(2) Peak Management: Detain runoff such that the post-project peak discharge rate
does not exceed the pre-project rate for the 95th percentile 24-hr rainfall event
and the 2- through 10-yr storm events or a more protective rate consistent with
the Permittee’s own development requirements.
d) Documentation and Approval of Special Circumstances – The Permittee shall provide
reasonable documentation to justify that a Regulated Project is more appropriately
categorized under the Special Circumstances category.
i) Historic Lake and Wetland Special Circumstance – Prior to granting a Regulated
Project Special Circumstances, the Permittee shall submit a proposal to the Central
Coast Water Board Executive Officer for review and approval. The proposal shall
include, at a minimum:
(1) Delineation of historic lakes and wetlands and any supporting technical
information to substantiate the requested Special Circumstances designation;
and
(2) Documentation that the proposal was completed by a registered professional
engineer, geologist, architect, and/or landscape architect.
C. Alternative Compliance (Off-Site Compliance)
Alternative Compliance refers to Water Quality Treatment, Runoff Retention and Peak
Management Performance Requirements that are achieved off-site through mechanisms such
as developer fee-in-lieu arrangements and/or use of regional facilities. Alternative Compliance
may be allowed under the following circumstances:
1) Technical Infeasibility
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Off-site compliance with Water Quality Treatment, Runoff Retention, or Peak Management
Performance Requirements may be allowed when technical infeasibility limits or prevents
use of structural Stormwater Control Measures.
a) To pursue Alternative Compliance based on technical infeasibility, the Regulated Project
applicant, for Regulated Projects outside of Urban Sustainability Areas, must submit a
site-specific hydrologic and/or design analysis conducted and endorsed by a registered
professional engineer, geologist, architect, and/or landscape architect, demonstrating
that compliance with the applicable numeric Post-Construction Stormwater Management
Requirements is technically infeasible
b) The Regulated Project applicant must submit a description of the project(s) that will
provide off-site mitigation. The proposed off-site projects may be existing facilities and/or
prospective projects that are as effective in maintaining watershed processes as
implementation of the applicable Post-Construction Stormwater Requirements on-site.
The description shall include:
i) The location of the proposed off-site project(s) must be within the same watershed
as the Regulated Project. Alternative Compliance project sites located outside the
watershed may be approved by the Central Coast Water Board Executive Officer
ii) A schedule for completion of offsite mitigation project(s), where the off-site mitigation
project(s) has not been constructed.
c) Technical infeasibility may be caused by site conditions, including:
i) Depth to seasonal high groundwater limits infiltration and/or prevents construction of
subgrade stormwater control measures6
ii) Depth to an impervious layer such as bedrock limits infiltration
iii) Sites where soil types significantly limit infiltration
iv) Sites where pollutant mobilization in the soil or groundwater is a documented
concern
v) Space constraints (e.g., infill projects, some redevelopment projects, high density
development)
vi) Geotechnical hazards
vii) Stormwater Control Measures located within 100 feet of a groundwater well used for
drinking water
viii) Incompatibility with surrounding drainage system (e.g., project drains to an existing
stormwater collection system whose elevation or location precludes connection to a
properly functioning treatment or flow control facility)
2) Approved Watershed or Regional Plan
An approved Watershed or Regional Plan as described below (Section C.2.a.), may be used
to justify Alternative Compliance for a Regulated Project’s numeric Runoff Retention and
Peak Management Performance Requirements without demonstrating technical infeasibility.
a) The Permittee must submit the proposed Watershed or Regional Plan to the Central
Coast Water Board Executive Officer for approval. Watershed and Regional Plans must
take into consideration the long-term cumulative impacts of urbanization including
existing and future development and include, at minimum:
6 According to the CASQA Frequently Asked Questions about LID, “some MS4 permits and BMP
guidance manuals require anywhere from 3-10 feet of separation from the groundwater level for
infiltration practices. This distance depends on the soil type, pollutants of concern, and groundwater
use. In some cases, however, where there may be groundwater or soil contamination, LID infiltrative
practices may be restricted completely. (p. 7 in https://www.casqa.org/Portals/0/LID/CA_LID_FAQ_06-
28-2011.pdf)
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i) A description of the project(s) that will provide off-site mitigation. The proposed off-
site projects may be existing facilities and/or prospective projects.
ii) The location of the proposed off-site project(s), which must be within the same
watershed as the Regulated Project. Alternative Compliance project sites located
outside the watershed may be approved by the Central Coast Water Board
Executive Officer.
iii) Demonstration that implementation of projects per the Watershed or Regional Plan
will be as effective in maintaining watershed processes as implementation of the
applicable Post-Construction Stormwater Requirements on-site. The proposal must
include quantitative analysis (e.g., calculations and modeling) used to evaluate off-
site compliance.
iv) A schedule for completion of offsite mitigation project(s), where the off-site mitigation
project(s) has not been constructed.
b) The Permittee may use projects identified per the Watershed or Regional Plan to meet
Water Quality Treatment Performance Requirements off-site only when:
i) The Regulated Project applicant has demonstrated that on-site water quality
treatment is infeasible as described in Sections C.1.a and C.1.c., and
ii) The proposed off-site project(s) has been demonstrated to comply with the Water
Quality Treatment Performance Requirements for the Regulated Project.
c) The Central Coast Water Board Executive Officer will deem complete a Permittee’s
Watershed or Regional Plan proposal within 60 days of receiving a complete proposal.
The Central Coast Water Board Executive Officer will approve or deny the proposal
within 120 days of a proposal being deemed complete.
3) Approved Urban Sustainability Area
The Permittee may allow Regulated Projects located within an approved Urban
Sustainability Area to pursue Alternative Compliance for numeric Runoff Retention and Peak
Management Performance Requirements without demonstrating technical infeasibility.
a) The Urban Sustainability Area shall encompass high density urban centers (but not
limited to incorporated jurisdictional areas) where the Permittee’s documented objective
is to preserve or enhance an existing pedestrian-oriented and/or public transit-oriented
type of urban design through the promotion of high density redevelopment and infill. The
Permittee must submit a proposal to the Central Coast Water Board Executive Officer for
approval of an Urban Sustainability Area. The USA proposal must include, at minimum:
i) A definition and delineation of the USA for high-density infill and redevelopment for
which area-wide approval for Alternative Compliance is sought.
ii) Information and analysis that supports the Permittee’s intention to balance water
quality protection with the needs for adequate housing, population growth, public
transportation, land recycling, and urban revitalization.
iii) Demonstration that implementation of Alternative Compliance for Regulated Projects
in the USA will meet or exceed the on-site requirements for Runoff Retention and
Peak Management. The proposal must include quantitative analysis (e.g.,
calculations and modeling) used to evaluate off-site compliance. Identification of
specific off-site projects is not necessary for approval of the USA designation.
b) The Permittee may allow Regulated Projects in a USA to meet Water Quality Treatment
Performance Requirements off-site only when:
i) The Regulated Project applicant has demonstrated that on-site water quality
treatment is infeasible as described in Sections C.1.a. and C.1.c., and
ii) The proposed off-site project(s) have been demonstrated to comply with the Water
Quality Treatment Performance Requirements.
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c) The Central Coast Water Board Executive Officer will deem complete a Permittee’s USA
proposal within 60 days of receiving a complete proposal. The Central Coast Water
Board Executive Officer will approve or deny the proposal within 120 days of a proposal
being deemed complete.
4) Other situations as approved by the Central Coast Water Board Executive Officer
5) Location of Alternative Compliance Project(s) – The location of the proposed off-site
project(s) must be within the same watershed as the Regulated Project. Alternative
Compliance project sites located outside the watershed may be approved by the Central
Coast Water Board Executive Officer.
6) Timing and Funding Requirements for Alternative Compliance Projects – The Permittee
shall develop a schedule for the completion of off-site mitigation projects, including
milestone dates to identify funding, design, and construction of the off-site projects.
a) Complete the project(s) as soon as practicable and no longer than four years from the
date of the certificate of occupancy for the project for which off-site mitigation is required,
unless a longer period is otherwise authorized by the Central Coast Water Board
Executive Officer.
b) The timeline for completion of the off-site mitigation project may be extended, up to five
years with prior Central Coast Water Board Executive Officer approval. Central Coast
Water Board Executive Officer approval will be granted contingent upon a demonstration
of good faith efforts to implement an Alternative Compliance project, such as having
funds encumbered and applying for the appropriate regulatory permits.
c) Require sufficient funding be transferred to the Permittee for public off-site mitigation
projects. Require private off-site mitigation projects to transfer sufficient funding to a
Permittee controlled escrow account, or provide the Permittee with appropriate project
bonding within one year of the initiation of construction of the Regulated Project.
d) The Permittee may establish different timelines and requirements that are more
restrictive than those outlined above.
D. Field Verifications of Post-Construction Stormwater Control Measures
1) The Permittee shall establish and implement a mechanism (a checklist or other tools) to
verify7 that structural Water Quality Treatment, Runoff Retention, and/or Peak Management
controls are designed and constructed in accordance with these Post-Construction
Stormwater Management Requirements
2) Prior to occupancy of each Regulated Project, the Permittee shall field verify that the Site
Design, Water Quality Treatment, Runoff Retention, and/or Peak Management controls
have been implemented in accordance with these Post-Construction Requirements
a) The Permittee may accept third-party verification of SCMs conducted and endorsed by a
registered professional engineer, geologist, architect, and/or landscape architect
b) The Permittee shall ensure, through conditions of approval or other legally enforceable
agreements or mechanisms, that site access is granted to all representatives of the
Permittee for the sole purpose of performing operation and maintenance (O&M)
inspections of the installed Stormwater Control Measures
7 A series of checklists that can be used by both inspectors and maintenance personnel is available in
the City of Santa Barbara Storm Water BMP Guidance Manual, Appendix H: Facility Inspection and
Maintenance Checklists. GeoSyntec Consultants, July 2008.
http://www.santabarbaraca.gov/Resident/Community/Creeks/Low_Impact_Development.htm
Resolution No. R3-2013-0032 ATTACHMENT 1
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E. Operation and Maintenance for Structural SCMs
The Permittee shall require O&M Plans and Maintenance Agreements that clearly establish
responsibility for all structural Water Quality Treatment, Runoff Retention, and/or Peak
Management controls on private and public Regulated Projects. The Permittee shall also
maintain a structural SCM tracking database to support long-term performance of structural
SCMs.
1) O&M Plan
The Regulated Project applicant shall develop and implement a written O&M Plan that, at a
minimum, includes each component listed below. The Permittee may allow the Regulated
Project applicant to include the O&M Plan components in the Stormwater Control Plan in
place of developing a separate document. The Permittee shall approve the O&M Plan prior
to final approval/occupancy. The O&M Plan must include, at minimum:
a) A site map identifying all structural Stormwater Control Measures requiring O&M
practices to function as designed
b) O&M procedures for each structural stormwater control measure including, but not
limited to, LID facilities, retention/detention basins, and proprietorship devices.
c) The O&M Plan will include short-and long-term maintenance requirements,
recommended frequency of maintenance, and estimated cost for maintenance.
2) Maintenance Agreement and Transfer of Responsibility for SCMs
Prior to issuing approval for final occupancy each Permittee shall require that Regulated
Projects subject to these Post-Construction Requirements provide verification of ongoing
maintenance provisions for Structural Stormwater Control Measures, including but not
limited to legal agreements, covenants, CEQA mitigation requirements, and or conditional
use permits. Verification shall include, at a minimum:
a) The project owner’s signed statement accepting responsibility for the O&M of the
installed onsite and/or offsite structural treatment and flow control SCMs until such
responsibility is legally transferred to another entity; and either
i) A signed statement from the public entity assuming responsibility for structural
treatment and flow control SCM maintenance and stating that the SCM meets all
local agency design standards; or
ii) Written conditions in the sales or lease agreements or deed for the project that
require the buyer or lessee to assume responsibility for the O&M of the onsite and/or
offsite structural treatment and flow control SCM until such responsibility is legally
transferred to another entity; or
iii) Written text in project deeds, or conditions, covenants and restrictions for multi-unit
residential projects that require the homeowners association or, if there is no
association, each individual owner to assume responsibility for the O&M of the onsite
and/or offsite structural treatment and flow control SCM until such responsibility is
legally transferred to another entity; or
iv) Any other legally enforceable agreement or mechanism, such as recordation in the
property deed, that assigns responsibility for the O&M of the onsite and/or offsite
structural treatment and flow control SCM to the project owner(s) or the Permittee
3) Structural Stormwater Control Measure O&M Database
The Permittee shall develop a database with information regarding each structural
Stormwater Control Measure installed per these Post-Construction Stormwater Management
Requirements. The Database shall contain, at a minimum, fields for:
a) SCM identification number and location/address
b) Type of SCM
c) Completion date of the following project stages, where applicable:
i) Construction
ii) Field verification of SCM
Resolution No. R3-2013-0032 ATTACHMENT 1
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iii) Final Project approval/occupancy
iv) O&M plan approval by Permittee
d) Location (physical and/or electronic) where the O&M Plan is available to view
e) Party responsible for O&M
f) Source of funding for O&M
g) Verification that responsible party has maintained the SCM as outlined in the O&M Plan,
or, indication that a self-inspection program is in place to verify that the SCM continues
to function as designed and to repair and/or replace the SCM if it is not functioning as
designed
h) Any problems identified during inspections including any vector or nuisance problems.
F. Permittee Reporting Requirements
1) The Permittee shall submit a sample checklist and the number of permits regulated under
the Site Design and Runoff Reduction Requirement (No. 1) as part of Stormwater Program
Annual Reporting. This information must demonstrate the Site Design and Runoff
Reduction Performance Requirement (No. 1) is applied to all applicable projects.
2) The Permittee shall report the following for all Regulated Projects subject to numeric
Performance Requirements (Nos. 2, 3, 4, and 5) in Stormwater Program Annual Reporting:
a) The total number of completed Regulated Projects
b) The total number of Regulated Projects within each of the following categories of new
and/or replaced impervious surface:
i) > 5,000 and <15,000 (based on Net Impervious Area)
ii) > 15,000 and < 22,500
iii) > 22,500
c) A list of which projects were granted each of the following :
i) Special Circumstances – Highly Altered Channel
ii) Special Circumstances – Intermediate Flow Control Facility
iii) Special Circumstances – Historic Lake or Wetland
iv) Alternative Compliance – Technical Infeasibility
(1) Performance Requirement No. 2: Water Quality Treatment
(2) Performance Requirement No. 3: Runoff Retention
(3) Performance Requirement No. 4: Peak Management
v) Alternative Compliance – Watershed or Regional Plan
vi) Alternative Compliance – Urban Sustainability Area
vii) Other Technical Infeasibility
(1) Technical infeasibility to retain the required runoff volume (per Performance
Requirement No. 3: Runoff Retention) using Site Design and Runoff Reduction
measures
(2) Technical infeasibility to retain and/or treat the required runoff volume (per
Performance Requirement No. 3: Runoff Retention) using retention-based
Stormwater Control Measures
d) Confirmation by the Permittee that for all Permittee-approved technical infeasibility
determinations, the Regulated Project’s Stormwater Control Plan adequately
demonstrated the basis for the technical infeasibility
e) A list of mitigation projects constructed for Alternative Compliance and the following
project information:
i) A summary description of mitigation projects constructed during the reporting period
comparing the expected aggregate results of Alternative Compliance projects to the
results that would otherwise have been achieved by meeting the numeric
Performance Requirements on-site
Resolution No. R3-2013-0032 ATTACHMENT 1
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ii) For public offsite mitigation projects, a summation of total offsite mitigation funds
raised to date and a description (including location, general design concept, volume
of water expected to be retained, and total estimated budget) of all pending public
offsite mitigation projects
f) Number of Regulated Projects where Field Verification of Post-Construction Stormwater
Management Measures was required and was NOT completed
g) Number of Regulated Projects where the required O&M Plan was NOT
submitted/completed
h) Number of Regulated Projects where Ownership and Responsibility of structural
Stormwater Control Measures was not completed
i) Structural Stormwater Control Measure O&M Database, including elements identified in
Section E.3. Tabular spreadsheet data are acceptable.
i) The Permittee shall provide Central Coast Water Board staff electronic access to the
database.
G. Pre-existing Programs
a) A Permittee may propose, for Central Coast Water Board Executive Officer approval,
implementation of pre-existing post-construction stormwater management requirements
for development projects in the Permittee’s jurisdictional coverage area, in place of
implementing the requirements set forth in the Post-Construction Requirements. To be
eligible for consideration and approval, the proposal must demonstrate the following:
i) The Permittee’s pre-existing post-construction stormwater management
requirements are as effective as the Post-Construction Requirements in maintaining
watershed processes, impacted by stormwater management, that are necessary to
protect water quality and beneficial uses;
ii) The Permittee was implementing its pre-existing post-construction stormwater
management requirements prior to Central Coast Water Board approval of the Post-
Construction Requirements; and
iii) The Permittee’s pre-existing post-construction stormwater management
requirements include LID site design and runoff reduction measures, numeric runoff
treatment controls, numeric runoff retention controls, numeric runoff peak
management controls, and project applicability thresholds as effective as those
included in the Post-Construction Requirements.
b) A Permittee must submit its proposal within 30 days of adoption of the Post-Construction
Requirements by the Central Coast Water Board. The Central Coast Water Board
Executive Officer will approve or deny the proposal within 90 days of receipt of a
proposal.
c) If the Central Coast Water Board Executive Officer denies a Permittee’s proposal, the
Permittee shall adhere to the Post-Construction Requirements provisions and deadlines.
Resolution No. R3-2013-0032 ATTACHMENT 1
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ATTACHMENT A: Watershed Management Zones
Available electronically at:
http://www.waterboards.ca.gov/centralcoast/water_issues/programs/stormwater/docs/lid/
lid_hydromod_charette_index.shtml
Resolution No. R3-2013-0032 ATTACHMENT 1
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ATTACHMENT B: Designated Groundwater Basins
Groundwater basin areas are defined by the California Department of Water Resources
(CDWR)8 and used in the Central Coast Water Board Joint Effort for Hydromodification Control
to identify groundwater receiving-water issues and areas where recharge is a key watershed
process. CDWR based identification of the groundwater basins on the presence and areal
extent of unconsolidated alluvial soils identified on a 1:250,000 scale from geologic maps
provided by the California Department of Conservation, Division of Mines and Geology. CDWR
then further evaluated identified groundwater basin areas through review of relevant geologic
and hydrogeologic reports, well completion reports, court-determined adjudicated basin
boundaries, and contact with local agencies to refine the basin boundaries.
Designated Groundwater Basins include those identified in the CDWR Groundwater Basins
Map. Numbers correspond to Groundwater Basins in Table 1.
8 California Department of Water Resources. 2004. Groundwater basin map.
<http://www.water.ca.gov/groundwater/bulletin118/gwbasin_maps_descriptions.cfm>. Accessed
September 15, 2006.
Resolution No. R3-2013-0032 ATTACHMENT 1
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Resolution No. R3-2013-0032 ATTACHMENT 1
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Table 1: Groundwater Basins in the Central Coast Region by GIS Basin Number (See
Map)
GIS BASIN
NUMBER
GROUNDWATER BASIN NAME GIS BASIN
NUMBER
GROUNDWATER BASIN
NAME
1 Carpinteria 35 Peach Tree valley
2 Santa Barbara 36 Hernandez valley
3 Montecito 37 Salinas valley
4 Foothill 38 Bitter Water valley
5 Goleta 39 Dry Lake valley
6 Santa Ynez River valley 40 Carmel valley
7 Santa Ynez River valley 41 Salinas valley
8 Lockwood valley 42 San Benito river valley
9 Mil Potrero area 43 Salinas valley
10 San Antonio Creek valley 44 Tres Pinos valley
11 Huasna valley 45 Salinas valley
12 Santa Maria 46 Upper Santa Ana valley
13 Cuyama valley 47 Salinas valley
14 Big Spring area 48 Salinas valley
15 Rafael valley 49 Santa Ana valley
16 San Luis Obispo valley 50 Quien Sabe valley
17 Los Osos valley 51 Gilroy-Hollister valley
18 Rinconada valley 52 Needle Rock point
19 Pozo valley 53 Gilroy-Hollister valley
20 Chorro valley 54 West Santa Cruz terrace
21 Morro valley 55 West Santa Cruz terrace
22 Toro valley 56 Majors creek
23 Carrizo Plain 57 Soquel valley
24 Cayucos valley 58 West Santa Cruz terrace
25 Old valley 59 West Santa Cruz terrace
26 Villa valley 60 Gilroy-Hollister valley
27 Santa Rosa valley 61 Pajaro valley
28 San Simeon valley 62 Scotts valley
29 Arroyo de la Cruz valley 63 Felton area
30 San Carpoforo valley 64 Santa Cruz Purisima formation
31 Cholame valley 65 Ano Nuevo area
32 Salinas valley 66 Gilroy-Hollister valley
33 Lockwood valley 67 Pescadero valley
34 Salinas valley 68 Santa Clara valley
Resolution No. R3-2013-0032 ATTACHMENT 1
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ATTACHMENT C: Definitions Related to Post-Construction Requirements
Bioretention – A Stormwater Control Measure designed to retain stormwater runoff using
vegetated depressions and soils engineered to collect, store, treat, and infiltrate runoff.
Bioretention designs do not include underdrains.
Biotreatment or Biofiltration Treatment –A Stormwater Control Measure designed to detain
stormwater runoff, filter stormwater through soil media and plant roots, and release the treated
stormwater runoff to the storm drain system. Biotreatment systems include an underdrain.
Discretionary Approval – A project approval which requires the exercise of judgment or
deliberation when the MS4 decides to approve or disapprove a particular activity, as
distinguished from situations where the MS4 merely has to determine whether there has been
conformity with applicable statutes, ordinances, or regulations.
Dispersion – The practice of routing stormwater runoff from impervious areas, such as
rooftops, walkways, and patios, onto the surface of adjacent pervious areas. Stormwater runoff
is dispersed via splash block, dispersion trench, or sheet flow and soaks into the ground as it
moves slowly across the surface of the pervious area.
Drainage Management Area (DMAs) – Following the low impact development principle of
managing stormwater through small-scale, decentralized measures, DMAs are designated
individual drainage areas within a Regulated Project that typically follow grade breaks and roof
ridge lines and account for each surface type (e.g., landscaping, pervious paving, or roofs).
Stormwater Control Measures for runoff reduction and structural facilities are designed for each
DMA.
Equivalent Impervious Surface Area – is equal to Impervious Tributary Surface Area (ft2) +
Pervious Tributary Surface Area (ft2), where Impervious Tributary Surface Area is defined as the
sum of all of the site’s conventional impervious surfaces, and Pervious Tributary Surface Area is
defined as the sum of all of the site’s pervious surfaces, corrected by a factor equal to the
surface’s runoff coefficient (see Attachment E for how to calculate).
Evapotranspiration (ET) – The loss of water to the atmosphere by the combined processes of
evaporation (from soil and plant surfaces) and transpiration (from plant tissues).
Flow-Through Water Quality Treatment Systems – Stormwater Control Measures that are
designed to treat stormwater through filtration and/or settling. Flow-through systems do not
provide significant retention or detention benefits for stormwater volume control.
Groundwater Basins – Groundwater basin areas defined by the California Department of
Water Resources (DWR) and used in the Central Coast Water Board Joint Effort for
Hydromodification Control to identify groundwater receiving-water issues and areas where
recharge is a key watershed process. DWR based identification of the groundwater basins on
the presence and areal extent of unconsolidated alluvial soils identified on a 1:250,000 scale
from geologic maps provided by the California Department of Conservation, Division of Mines
and Geology. DWR then further evaluated identified groundwater basin areas through review of
relevant geologic and hydrogeologic reports, well completion reports, court-determined
adjudicated basin boundaries, and contact with local agencies to refine the basin boundaries.
Resolution No. R3-2013-0032 ATTACHMENT 1
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Impervious Surface – A hard, non-vegetated surface area that prevents or significantly limits
the entry of water into the soil mantle, as would occur under natural conditions prior to
development. Common impervious surfaces include, but are not limited to, roof tops, walkways,
patios, driveways, parking lots or storage areas, concrete or asphalt paving, oiled, macadam or
other surfaces which similarly impede the natural infiltration of stormwater. Open, uncovered
retention/detention facilities shall not be considered as impervious surfaces for purposes of
determining whether the thresholds for application of Performance Requirements are exceeded.
However, for modeling purposes, open, uncovered facilities that retain/detain water (e.g.,
retention ponds, pools) shall be considered impervious surfaces.
Land recycling – The reuse of abandoned, vacant, or underused properties for redevelopment
or repurposing
Landscaped Areas – Areas of soil and vegetation not including any impervious surfaces of
ancillary features such as impervious patios, BBQ areas, and pools.
Large River – A river draining 200 square miles or more.
Low Impact Development (LID) – A stormwater and land use management strategy that
strives to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage,
evaporation, and transpiration by emphasizing conservation, use of on-site natural features, site
planning, and distributed stormwater management practices that are integrated into a project
design.
Ministerial Approval – A project approval which involves little or no personal judgment by
the MS4 as to the wisdom or manner of carrying out the project and only involves the use of
fixed standards or objective measurements.
Native Vegetation – Vegetation comprised of plant species indigenous to the Central Coast
Region and which reasonably could have been expected to naturally occur on the site.
Net Impervious Area – The sum of new and replaced post-project impervious areas, minus any
reduction in total imperviousness from the pre-project to post-project condition: Net Impervious
Area = (New and Replaced Impervious Area) – (Reduced Impervious Area Credit), where
Reduced Impervious Area Credit is the total pre-project to post-project reduction in impervious
area, if any.
New Development – Land disturbing activities that include the construction or installation of
buildings, roads, driveways and other impervious surfaces. Development projects with pre-
existing impervious surfaces are not considered New Development.
Percentile Rainfall Event (e.g., 85th and 95th) – A percentile rainfall event represents a rainfall
amount which a certain percent of all rainfall events for the period of record do not exceed. For
example, the 95th percentile rainfall event is defined as the measured rainfall depth
accumulated over a 24-hour period, for the period of record, which ranks as the 95th percentile
rainfall depth based on the range of all daily event occurrences during this period.
Permeable or Pervious Surface – A surface that allows varying amounts of stormwater to
infiltrate into the ground. Examples include pasture, native vegetation areas, landscape areas,
and permeable pavements designed to infiltrate.
Resolution No. R3-2013-0032 ATTACHMENT 1
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Pre-Project – Stormwater runoff conditions that exist onsite immediately before development
activities occur. This definition is not intended to be interpreted as that period before any
human-induced land activities occurred. This definition pertains to redevelopment as well as
initial development.
Project Site – The area defined by the legal boundaries of a parcel or parcels of land within
which the new development or redevelopment takes place and is subject to these Post-
Construction Stormwater Management Requirements.
Rainwater Harvest – Capture and storage of rainwater or stormwater runoff for later use, such
as irrigation (without runoff), domestic use (e.g. toilets), or storage for fire suppression.
Receiving Waters – Bodies of water, surface water systems or groundwater that receive
surface water runoff through a point source, sheet flow or infiltration.
Redevelopment – On a site that has already been developed, construction or installation of a
building or other structure subject to the Permittee’s planning and building authority including: 1)
the creation or addition of impervious surfaces; 2) the expansion of a building footprint or
addition or replacement of a structure; or 3) structural development including construction,
installation or expansion of a building or other structure. It does not include routine road
maintenance, nor does it include emergency construction activities required to immediately
protect public health and safety.
Replaced Impervious Surface – The removal of existing impervious surfaces down to bare soil
or base course, and replacement with new impervious surface. Replacement of impervious
surfaces that are part of routine road maintenance activities are not considered replaced
impervious surfaces.
Retention Tributary Area – The entire project area except for undisturbed areas, planted areas
with native, drought-tolerant, or LID appropriate vegetation that do not receive runoff from other
areas, and impervious surface areas that discharge to infiltrating areas that will not produce
runoff or create nuisance ponding. The Drainage Management Areas are smaller Retention
Tributary Areas that cumulatively make up the Retention Tributary Area for the entire site.
Routine Road Maintenance – includes pothole and square cut patching; overlaying existing
asphalt or concrete pavement with asphalt or concrete without expanding the area of coverage;
shoulder grading; reshaping/regrading drainage systems; crack sealing; resurfacing with in-kind
material without expanding the road prism or altering the original line and grade and/or hydraulic
capacity of the road.
Self-Retaining Areas – (also called “zero discharge” areas), are designed to retain some
amount of rainfall (by ponding and infiltration and/or evapotranspiration) without producing
stormwater runoff. Self-Retaining Areas may include graded depressions with landscaping or
pervious pavement.
Self-Treating Areas – are a portion of a Regulated Project in which infiltration,
evapotranspiration and other natural processes remove pollutants from stormwater. The self-
treating areas may include conserved natural open areas and areas planted with native,
drought-tolerant, or LID appropriate vegetation. The self-treating area only treats the rain falling
on itself and does not receive stormwater runoff from other areas.
Resolution No. R3-2013-0032 ATTACHMENT 1
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Single-Family Residence – The building of one single new house or the addition and/or
replacement of impervious surface associated with one single existing house, which is not part
of a larger plan of development.
Stormwater Control Measures – Stormwater management measures integrated into project
designs that emphasize protection of watershed processes through replication of pre-
development runoff patterns (rate, volume, duration). Physical control measures include, but
are not limited to, bioretention/rain gardens, permeable pavements, roof downspout controls,
dispersion, soil quality and depth, minimal excavation foundations, vegetated roofs, and water
use. Design control measures include but are not limited to conserving and protecting the
function of existing natural areas, maintaining or creating riparian buffers, using onsite natural
drainage features, directing runoff from impervious surfaces toward pervious areas, and
distributing physical control measures to maximize infiltration, filtration, storage, evaporation,
and transpiration of stormwater before it becomes runoff.
Stormwater Control Plan – A plan, developed by the Regulated Project applicant, detailing
how the project will achieve the applicable Post-Construction Stormwater Management
Requirements (for both onsite and offsite systems).
Resolution No. R3-2013-0032 ATTACHMENT 1
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ATTACHMENT D: Hydrologic Analysis and Stormwater Control Measure Sizing Guidance
Project site conditions will influence the ability to comply with the Water Quality Treatment and
Runoff Retention Performance Requirements. This Appendix provides the acceptable
Stormwater Control Measure (SCM) sizing methodology to evaluate runoff characteristics. This
guidance provides a simple event-based approach and a runoff routing approach. Both of these
approaches are based on sizing for a single-event and avoid the necessity of using calibrated,
continuous simulation modeling. The Permittee can allow project applicants to use a
locally/regionally calibrated continuous simulation-based model to improve hydrologic analysis
and SCM sizing.
1) Determination of Retention Tributary Area
Determining the Retention Tributary Area is the basis for calculating the runoff volumes
subject to Performance Requirement Number 3. Retention Tributary Area should be
calculated for each individual Drainage Management Area to facilitate the design of SCMs
for each Drainage Management Area. The generic equation below illustrates how various
portions of the site are addressed when determining the Retention Tributary Area. The
Retention Tributary Area calculation must also account for the adjustments for
Redevelopment Projects subject to Performance Requirement No. 3.
a) Compute the Retention Tributary Area, using the equation:
Retention Tributary Area = (Entire Project Area) – (Undisturbed or Planted Areas)* –
(Impervious Surface Areas that Discharge to Infiltrating Areas)**
*As defined in Section B.4.d.iv.1.
** As defined in Section B.4.d.iv.2.
b) Adjustments for Redevelopment Project Retention Tributary Area – Where the
Regulated Project includes replaced impervious surface, the following Retention
Tributary Area adjustments apply:
i) Redevelopment Projects outside an approved Urban Sustainability Area, as
described in Section C.3. – The total amount of replaced impervious surface area
shall be multiplied by 0.5 when calculating the Retention Tributary Area.
ii) Redevelopment Projects located within an approved Urban Sustainability Area
(Section C.3) – The replaced impervious surface areas may be subtracted from the
Retention Tributary Area. The total amount of runoff volume to be retained from
replaced impervious surfaces shall be equivalent to the pre-project runoff volume
retained.
2) Determination of Retention Volume
a) Based on the Regulated Project’s Watershed Management Zone, determine the
Regulated Project’s Runoff Retention Requirement (e.g., Retain 95th Percentile 24-hour
Rainfall Event, or, Retain 85th Percentile 24-hour Rainfall Event).
b) Determine the 85th or 95th percentile 24-hour rainfall event:
Use either the methodology provided in Part I.D of the December 2009 Technical
Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects
Resolution No. R3-2013-0032 ATTACHMENT 1
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under Section 438 of the Energy Independence and Security Act,9 or, rainfall statistics
provided by the Central Coast Water Board, whichever produces a more accurate value
for rainfall depth.
c) Compute the Runoff Coefficient10 “C” for the area tributary to the SCMs, using the
equation:
C = 0.858i 3- 0.78i2 + 0.774i + 0.04
Where “i” is the fraction of the tributary area that is impervious11
d) Compute Retention Volume:
Retention Volume for 95th Percentile 24-hr Rainfall Depth = C x Rainfall Depth95th x Retention
Tributary Area
or,
Retention Volume for 85th Percentile 24-hr Rainfall Depth = C x Rainfall Depth85th x Retention
Tributary Area
All rainfall directly incident to each SCM must be considered in determining runoff, including:
tributary landscaping, impervious areas, pervious pavements, and bioretention features.
Note: For redevelopment projects located within an approved Urban Sustainability Area
(Section C.3.), the total amount of runoff volume to be retained from replaced impervious
surfaces shall be equivalent to the pre-project runoff volume retained.
3) Structural Stormwater Control Measure Sizing
The Permittee shall require the Regulated Project applicant to use structural SCMs that optimize
retention and result in optimal protection and restoration of watershed processes, such as
Structural Control Measures associated with small-scale, decentralized facilities designed to
infiltrate, evapotranspirate, filter, or capture and use stormwater, to address the volumes
calculated in 2 (above). Where the Regulated Project is within a Watershed Management Zone
where infiltration is required, Permittees must use SCM designs that optimize infiltration of the
entire Retention Volume to minimize the potential need for off-site mitigation. Various resources
provide design guidance for fully infiltrative SCMs including:
- The Contra Costa C.3 Manual
- The City of Santa Barbara LID BMP Manual
- The City of San Diego LID Design Manual, July 2011
- Central Coast LID Initiative Bioretention Design Guidance
a) Calculate SCM Capture Volume – Calculate the required SCM Capture Volume,
associated with the Regulated Project’s Runoff Retention Requirement, by one of the
following methods:
Method 1: Simple Method
9 USEPA, 841-B-09-00. http://www.epa.gov/owow/NPS/lid/section438/pdf/final_sec438_eisa.pdf
10 As set forth in WEF Manual of Practice No. 23/ASCE Manual of Practice No. 87, (1998), pages 175-
178 and based on the translation of rainfall to runoff using a runoff regression equation developed using
two years of data from more than 60 urban watersheds nationwide.
11 As defined in Post-Construction Requirements Attachment C.
Resolution No. R3-2013-0032 ATTACHMENT 1
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SCM Capture Volume = Retention Volume for 95th Percentile 24-hr Rainfall Depth
or,
SCM Capture Volume = Retention Volume for 85th Percentile 24-hr Rainfall Depth
Method 2: Routing Method
Use a hydrograph analysis12 to determine the SCM Capture Volume needed to retain the
Retention Volume for 95th or 85th Percentile 24-hr Rainfall Depth calculated in 2 (above). The
SCM Capture Volume shall be based on both the rate of flow from tributary areas into the SCM,
and the rate of flow out of the SCM through infiltration into the underlying soil during the rain
event. When conducting the hydrograph analysis, adhere to the criteria included in Table 1.
The SCM shall be designed such that a single 95th or 85th Percentile 24-hr Rainfall Event will not
overflow the SCM.
If the Retention Volume cannot infiltrate within 48-hours, a multiplier of 1.20 shall be applied to
the SCM Capture Volume calculated through the routing method.
TABLE 1: Routing Method Criteria
Parameter Criteria
Hydrograph Analysis Method National Resources Conservation Service or Santa
Barbara Urban Hydrograph
Pond Routing Method Storage-indication, unless otherwise justified to be
more correct based on site and storage conditions.
Infiltration Rate
Underlying soil saturated infiltration rate, as indicated
by locally accepted data approved by the Permittee
and/or by on-site testing, whichever is more accurate.
Rainfall Distribution National Resources Conservation Service Type I13 or
based on local rainfall data
Time of Concentration Permittee’s current drainage and flood control standard
Time Increment 0.10 hour, unless otherwise justified to be more correct
based on rainfall distribution
b) Demonstration of Compliance – Permittees shall require Regulated Projects to
demonstrate that site SCMs: a) will infiltrate and/or evapotranspirate the Retention
Volume or, b) will provide sufficient Capture Volume to retain the Retention Volume.
Any outlet (i.e., underdrain) installed in a structural SCM shall be installed above the
elevation of any portion of the structural SCM dedicated to Retention Volume storage.
12 HydroCAD is an example of a commonly used and widely accepted program for performing
hydrograph analyses and design of stormwater infrastructure. HydroCAD is based on U.S. Department of
Agriculture Soil Conservation Service’s (now Natural Resources Conservation Service) TR-55: Urban
Hydrology for Small Watersheds.
13 The National Resources Conservation Service developed standard 24-hour rainfall distributions for
hydrograph analyses. These rainfall distributions were intended to represent intensities associated with
shorter duration storms, ranging from durations of 30 minutes to 12 hours. The National Resources
Conservation Service Type 1 storm applies to the California West Coast, including the Central Coast
Region. The Type 1 rainfall distribution was derived using National Oceanic Atmospheric Administration
Atlas 2 rainfall statistics for the 1-year through 100-year storm.
Resolution No. R3-2013-0032 ATTACHMENT 1
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c) Compliance with Water Quality Treatment Performance Requirement – Permittees shall
require Regulated Projects that propose to use the retention-based structural
Stormwater Control Measures to also meet the Water Quality Treatment Performance
Requirement, to demonstrate, in the Stormwater Control Plan, that the Water Quality
Treatment Performance Requirement is being fully met.
Resolution No. R3-2013-0032 ATTACHMENT 1
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ATTACHMENT E: Ten Percent Adjustment to Retention Requirement – Calculation
Instructions
Where technical infeasibility, as described in Section C.1.c., prevents full on-site compliance
with the Runoff Retention Performance Requirement, on-site retention of the full Retention
Volume per Section B.4.d.vi. is not required and the Regulated Project is required to dedicate
no less than ten percent of the Regulated Project’s Equivalent Impervious Surface Area to
retention-based Stormwater Control Measures. The Water Quality Treatment Performance
Requirement is not subject to this adjustment, i.e., mitigation to achieve full compliance is
required on- or off-site.
Calculating Ten Percent of a Project’s Equivalent Impervious Surface Area
The area of the project that must be dedicated to structural SCMs to waive off-site compliance
with the Runoff Retention Requirement is equal to ten percent of the project’s Equivalent
Impervious Surface Area, defined as:
Equivalent Impervious Surface Area (ft2) = (Impervious Tributary Surface Area (ft2) + (Pervious
Tributary Surface Area (ft2))
Impervious Tributary Surface Area is defined as the sum of all of the site’s conventional
impervious surfaces. When calculating Impervious Tributary Area:
• Do include: concrete, asphalt, conventional roofs, metal structures and similar surfaces
• Do not include: green roofs
Pervious Tributary Surface Area is defined as the sum of all of the site’s pervious surfaces,
corrected by a factor equal to the surface’s runoff coefficient. When calculating Pervious
Tributary Surface Area:
• Do include surfaces such as: unit pavers on sand; managed turf14; disturbed soils; and
conventional landscaped areas (see Table 1 for correction factors).
Example:
Project Site includes 500 ft2 of unit pavers on sand.
Pervious Tributary Surface Area = 500 ft2 x C = 50 ft2
Where C = Correction Factor for unit pavers, 0.1, from Table 1.
• Do not include: Infiltration SCM surfaces (e.g., SCMs designed to specific performance
objectives for retention/infiltration) including bioretention cells, bioswales; natural and
undisturbed landscape areas, or landscape areas compliant with the Model Water
Efficient Landscape Ordinance (California Code of Regulations, Title 23. Waters,
Division 2. Department of Water Resources, Chapter 2.7.), or a local ordinance at least
as effective as the Model Water Efficient Landscape Ordinance.
14 Managed Turf includes turf areas intended to be mowed and maintained as turf within residential,
commercial, industrial, and institutional settings.
Resolution No. R3-2013-0032 ATTACHMENT 1
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TABLE 1: Correction Factors15 for Use in Calculating
Equivalent Impervious Surface Area
Pervious Surface Correction
Factor
Disturbed Soils/Managed Turf
(dependent on original Hydrologic Soil
Group)
A: 0.15
B: 0.20
C: 0.22
D: 0.25
Pervious Concrete 0.60
Cobbles 0.60
Pervious Asphalt 0.55
Natural Stone (without grout) 0.25
Turf Block 0.15
Brick (without grout) 0.13
Unit Pavers on Sand 0.10
Crushed Aggregate 0.10
Grass 0.10
15 Factors are based on runoff coefficients selected from different sources: Turf and Disturbed Soils from
Technical Memorandum: The Runoff Reduction Method. Center for Watershed Protection & Chesapeake
Stormwater Network. p.13, April 18, 2008.
http://town.plympton.ma.us/pdf/land/scheuler_runoff_reduction_method_techMemo.pdf. All other
correction factors from C.3 Stormwater Handbook, Santa Clara Valley Urban Runoff Pollution Prevention
Program, Appendix F, p. F-9., May 2004.
http://www.sanjoseca.gov/planning/stormwater/pdfs/appendices_files/Appendix_F_Final.pdf
Resolution No. R3-2013-0032 ATTACHMENT 1
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ATTACHMENT F: Calculating Off-Site Retention Requirements When Less Than 10
Percent of the Project Site Equivalent Impervious Surface Area is Allocated to Retention-
Based Structural Stormwater Control Measures
The following instructions demonstrate how to determine the Off-Site Retention Requirements
when a Regulated Project subject to the Runoff Retention Performance Requirement, cannot
allocate the full 10% of the project site’s Equivalent Impervious Surface Area16 to retention-
based Stormwater Control Measures (SCMs).
STEP A. Potential Off-Site Mitigation Retention Volume
First calculate the Potential Off-Site Mitigation Retention Volume, which represents the
additional volume of runoff that would have been retained on-site, had the full 10% of Equivalent
Impervious Surface Area been dedicated to retention-based SCMs.
Equation A:
Potential Off-Site Mitigation Retention Volume = (the portion of the 10% Equivalent Impervious
Area not allocated on-site) X (the On-Site Retention Feasibility Factor)
Where:
The portion of the 10% Equivalent Impervious Surface Area not allocated on-site is that
portion not allocated to on-site structural retention-based SCMs. For example, if 10% of
Equivalent Impervious Surface Area is 1,000 ft2 and only 8% (800 ft2) is allocated to
retention-based SCMs, the remaining 2% (200 ft2) is the value inserted in the equation.
The On-Site Retention Feasibility Factor is the ratio of Design Retention Volume17
managed on-site (ft3), to actual area (ft2) allocated to structural SCMs. This establishes
the site’s retained volume:area ratio, expressed as cubic feet of retained runoff volume per
square foot of area. For example, if a project is able to infiltrate 3,500 ft3 of runoff over an
800-ft2 area, this ratio of 3,500:800, or 4.38, is the On-Site Retention Feasibility Factor.
STEP B. Actual Off-Site Mitigation Retention Volume
Next, determine the Actual Off-Site Mitigation Retention Volume, which may be less than the
Potential Off-Site Mitigation Retention Volume. The Actual Off-Site Mitigation Retention
Volume is the lesser of the volume calculated in Equation A, and the remaining portion of the
Design Retention Volume, calculated per Attachment D, not controlled on-site. There are two
possible outcomes when the Runoff Retention Performance Requirement is not met on-site
and less than 10% of the site’s Equivalent Impervious Surface Area is allocated to retention-
based SCMs:
Potential Off-Site Mitigation Retention Volume is the Actual Off-Site Mitigation Retention
Volume
Remaining Design Retention Volume represents Actual Off-Site Design Retention
Mitigation Volume
16 Calculate Equivalent Impervious Surface Area using guidance in Post-Construction Requirements
Attachment E
17 Calculate Design Retention Volume using guidance in Post-Construction Requirements Attachment D,
or equivalent method. Final Design Retention Volumes should reflect the applicant’s demonstrated
effort to use non-structural design measures to reduce the amount of runoff (e.g., reduction of
impervious surfaces) as required by the Post-Construction Requirements’ LID Development Standards
(Section B.4.d).