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PC R 18-2293 RESOLUTION NO. 18-2293 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 RESOLUTION NO. 18-2293 PAGE2 the California Porter-Cologne Water Quality Control Act and 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 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 George, seconded by Commissioner Mack and by the following roll call vote to wit: RESOLUTION NO. 18-2293 PAGE 3 AYES: George, Mack, Fowler-Payne, Schiro, Martin NOES: None ABSENT: None the foregoing Resolution was adopted this 16th day of January 2018. GLENN MARTIN, CHAI ATTEST: DEBBIE WEICHINGER SECRETARY TO THE COMMISSION AS TO CONTENT: 022 TER A LISH 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. 40% evapotranspiration 38% evapotranspiration RP° h j y LL p j ,t 1- i Itt '''':: '''''Ct'''''''‘'V 944:1.1' '‘..ti;, 'II, ' .,, ,A.,,a: .*Isrios,, 1.1a,. ,ret..,,,,,,, . 1 Ali -°?,4§,1,44,.4,- 4„,. t .� ,• t,, , runoff t,.' runoff Sk..., 25% shallow 21% shallow ` nfiltration. infiltration _` 25%deep ; ' 21%° deep` infiltration infiltration Natural Ground Cover , 10%-2Q% Impervious Surface 35% evapotranspiration 30% evapotranspiration I ,E3Q #;,E a ,®®®® f yl © toa`Q 30%I n U, io©e�rt '.� � 55o'/0 s-,- ' ruri i10ff ` /fs4fa® 43 r3 01runoff ' �(©el c7 t .o it iri :©E:1S t d` ,.... to ri,,, r — . ,--- I [.,,,s . C3 LID G3+ 2'0% shallow 4, , ' 10%shallow .,, — 'i infiltration `' infiltration 1: Iui 15% deep ; "5% deep ' -- . .. - infiltration , . infiltration - 35%-50% Impervious Surface 75%-100/0 Impervious'.Surf ace 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 waters 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). :,A re 3-e-:.' hVY,1 7- 7'' t::'.,'ements,Ptppty�:,* ti., .:' ,.. �`a t '' a '- 'Iy, -` . �7+ $° ^`-,,4 r-., ;;7- �,a a " .,- ,. . € ,.7 TM ,, +n Do sp I eta ti fy Dae P 0.00! • '�� . $ _✓'ted. 1 pro t �" 711.'1:4•7".' flatnrmi e t ;'+e'•x T eondibonin table �.�, 'teroato orroplaee t ���a� dingy - '. +'r�0. , .p -_ dotae -d angle 4, AC f in't ddnca a .,ort •smgls item > it 1� ... prajuctT.. 'from bis 3 2? aQ-2g rt n7 .x 1110��1" AYES YES Ur YES a n,• r I �' Do s pro)et r t �' D je t o to NR Nu Table 3 1 -. rr0o '''y'' . . 0 c 5 000 s1 01:n t, c 16,D0D I of n t ®Corn aroly drams to en n s to undrain d s req mpervious area? ,�'"YES • Y�sS „-, �1 impervious area? Pi i gtr.locate d outside of County„ • ;• ' '=,,`' =t tviS4 Permit Ama Isco App Al -s--- -'T'able � ., y•�Roadadd pa krglotmei toeiv) i ®b y eel°end b cpcta I e p Iso t3 twl ern no othe P t r to •P, Noe PR Na O pyre t oto mupervueus s rfaoas are created or rsplacadt;end ,<1.0o. ,I f net c 22 500 f f-new u-Ga todt ctsmmrvvat r to ndjacaRty geared ` n�orvi a7 and ump rvro s ,. R�6smaa... ES • e�YE surfena7 a.t Trail:and pathways(whore no•a1lydr anpetulane '—""""" '"""""`"""' -' Eil nutlatus ore created p"teoodl;:end bo ltto donut. '` ' --' tormwatar ru off to nd7aw t. agetatod ar . :,•;' ` S.� Q� 4 Dnd ground{aril tY proPcts that pt co the g o d� _ E H f do til k d m to al r reatan is til similar runoff he eter slues * 3 Curb and gutter umproyemont er rap!acament , • r ,` p aiecfs tt at nue at part f‘‘<,,'.'!'"f`''' h ony a lr ttonet. D Pro) t I creation or replecament at rmpery as'stnfam.area G,. m dor ny sp i 0 udowatks,co dw yl ® i crf to QR Man ac ®So'..dere y ddeon til t do not incrsese[Im do ug e n? �y - YES°-' :7f b lddg faatp mt ._.,. ,„ •. _ RoePd{notb It du'oody on thegoundl dabs vii metre or Walkways dm grad w th spans to allow lot wet'rdra:ag _ ^,, Phdtovcl[aidsysteios instaued anlaver ez nog,too .i p l i pe otanos c s node al locate. .r Pt 2 vti} P r pain u su f lees until NroRma„;„de loc.- s or D ori.. .,. :..� - • t *° n utaiodco al 'rpa le ysLall b rstnp H2:Dotermmq III x n t 3 "4'7'':71-"'"' til m t d gradin t ow t p Applreablo Naw Foil , Doris pr out create T pa[ary svoctarus I n planetar ro tha s �.3 �sEvant 1 J >22 500 !n _ , ,$Ic m nths} °.. reel=,,. , E1 tr eel nd uhi ty yalyos, t a d wet r 1 ft i ✓ YES .' is st bon backftovvs end of hkty d Huta S � and d irnjuary ous C< IllAbo4a Around Itaj store ,tanks`and faai far !with ?.," c towneri SySiDm _._.-�.-'!»..,,,,f,_„„,,,,,, ,,„___,......-,t 1, ',{ 4r • e t PH P1 . . '11:41 ,-(..1.:' ..• t s ,1 '', 4 ', ,a Perform n , eloui t 1 t '}f5�f a d ..&,7.--,?r,,,,,t 1.IJ "int; o nt '�� Roqu rvmontsx teokup .SRH St,2V� � ` s f- s APPI a file to 8 .W N. r �nESi P joot tP .,.0A1$.,•. 541:3 c ':::::'." 4;,',. :.:,,,,`.;;:,::,7:.;,:,:!: ,.i L * '.i axe £ t, • See Appendtz fT it Situ ofubte to ret n d m -7-•';'''• p a 9 t- �- r,°,9,'"•1; PR3 Vptumem r q a , • b 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 - • -.. . D ' - I £! '- :• ' -: .' - . _• :. Typically, maintenance and utility projects aro also considered "exempt" • ..' . . • ' _ :-. A list of projects not included as Regulated-Projects can be4-found in Resolution,No R3 2013 0032 Section B Post Construction Requirenients 1b Examples of exempt ®_ non-regulated projects include routine overlays and slurry seals:, - . . - . . ' . .• -- . - "= -• . " •• •.= the installation of an ADA ramps, or solar panels on rooftops, or repair of roads or aerial utilities. -• ---- -- ' ' "- Non=Re elated Pro ects are still encouraged to implement practices that will reduce stormwater impacts associated with their proposed development. 111 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 New and replaced Pre-project to post-project impervious = impervious area - reduction in impervious area, if area 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 biofiltration 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 groundwater 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 111 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 their 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://wwwswrcb.ca.gov/rwgcb3/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). o 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. o 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. o Supporting calculations to demonstrate compliance with applicable Performance Requirements. o Statements of Compliance. Exhibits o 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 Pre and post drainage feature map. features. 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 Site Stormwater Assessment Exhibit with at the site to the minimum area needed to build native vegetation, overlain with development the project, allow access, and provide fire footprint. protection. 4 Minimize impervious surfaces by concentrating Site Stormwater Assessment Exhibit with improvements on the least-sensitive portions of delineated sensitive areas overlain with the site,while leaving the remaining land in a development footprint natural undisturbed state. MINIMIZE STORMWATER RUNOFF BY IMPLEMENTING ONE OR MORE OF THE FOLLOWING DESIGN MEASURES: MANDATORY SITE DESIGN MEASURES Selected Reason, (SELECT AT LEAST ONE) for not selecting a. Roof runoff directed into cisterns or rain barrels for reuse? Roof runoff directed into vegetated areas b. (safely away from building foundations and footings)? Runoff from sidewalks,walkways, and/or 5 c. patios directed onto vegetated areas (safely away from the building foundations and footings)? Runoff from driveways and/or uncovered d. parking lots onto vegetated areas(safely away from the building foundations and footings)? Are bike lanes, driveways, uncovered e. 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 o Project Net Impervious Area o 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: o Unique DMA Number, area, and likely pollutant(s)of concern o Water Quality Treatment Approach N/A if self-treating, or, Through the use of LID, Biofiltration or Non-retention Based Treatment System); o 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: o Statement indicating why an LID treatment system was not appropriate; o Surface loading rate approach, and basis of determination;- (0.2 x per hour intensity, or 2 x 85th percentile hourly rainfall intensity); o Calculations to demonstrate that the minimum surface reservoir volume is equal to the biofiltration treatment system surface area time a depth of 6-inches; o 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: o Statement indicating why an LID, or Biofiltration treatment system was not appropriate o Hydraulic Sizing Criteria used, and basis of determination (Volume = to 85th percentile, 24-hour storm, or flow 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): o Site topography o Run-on characteristics(source and estimated runoff from offsite which discharges to the project area) o Hydrologic features including contiguous natural areas,wetlands,watercourses, seeps, o Existing drainage infrastructure for the site and nearby or springs areas, including the location of municipal storm drains o Depth to seasonal high groundwater o Structures, including retaining walls o Locations of groundwater wells used for o Utilities drinking water o Easements o Depth to an impervious layer such as Bedrock ❑ Covenants o Presence of unique geology(e.g., karst) o Zoning/Land Use o Geotechnical hazards o Setbacks o Documented soil and/or groundwater o Open space requirements Contamination o Other pertinent overlay(s) o Soil types and hydrologic soil groups o Vegetative cover/trees 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 Define the development envelope and protected Site Stormwater Assessment Exhibit. areas, identifying areas that are most suitable for 1. development and areas to be left undisturbed. Conserve natural areas, including existing trees, Site Stormwater Assessment Exhibit with native vegetation, 2. other vegetation, and soils overlain with development footprint Limit the overall impervious footprint of the Discussion regarding other building configurations 3. project considered (and ultimately rejected) Construct streets,sidewalks, or parking lot aisles Discussion on minimum allowable widths, and rationale for to the minimum widths necessary, provided that using larger values (if applicable)or confirmation that 4. public safety or mobility uses are not minimum values were used (where applicable). compromised Set back development from creeks,wetlands, Discussion on set-back dimensions chosen. 5. and riparian habitats 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 6. lines. Discussion of grading approach. Avoid excessive grading and disturbance of Exhibit with native vegetation, overlain with planned 7. vegetation and soils disturbed area limits. Performance Requirement 3: Runoff Retention SWCP Checklist Continued STORMWATER STRUCTURAL CONTROLMEASURE 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 111 ❑ 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 O 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, O 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 projects 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 include 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 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 system whose elevation or location precludes connection to a properly functioning treatment or flow control facility) o 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 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) o 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. o Schedule for completion of offsite project with milestone dates to identify funding, design, and construction of the off-site project(s) Note, Approved Watershed or Regional Plans, and projects situated within designated Urban 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.