CC 2016-04-12_09g Engineering Standards Attachment 2
CITY OF ARROYO GRANDE – ENGINEERING STANDARD PLAN
1010
Issue Date: March 2016 UNIFORM DESIGN CRITERIA Sheet 51 of 66
3. Dissipater trench dimensions
4. Rock placement method
5. RSP fabric type
Culvert energy dissipaters must be designed for the flow from the Design Storm. Rock slope
protection gradation shall conform to Section 72 of the State Standard Specifications.
5.2 Post Construction Stormwater Compliance
5.2.1 Performance Requirements
A. Performance Requirement No. 1: Site Design and Runoff Reduction
Projects that create and/or replace 2,500 square feet or more of impervious surface must:
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:
a. Direct roof runoff into cisterns or rain barrels for reuse
b. Direct roof runoff onto vegetated areas safely away from building foundations and
footings, consistent with California building code
c. Direct runoff from sidewalks, walkways, and/or patios onto vegetated areas safely away
from building foundations and footings, consistent with California building code
d. Direct runoff from driveways and/or uncovered parking lots onto vegetated areas safely
away from building foundations and footings, consistent with California building code
e. Construct bike lanes, driveways, uncovered parking lots, sidewalks, walkways, and patios
with permeable surfaces
6. Define the development envelope and protected areas, identifying areas that are most
suitable for development and areas to be left undisturbed
7. Conserve natural areas, including existing trees, other vegetation, and soils
8. Limit the overall impervious footprint of the project
9. Set back development from creeks, wetlands, and riparian habitats
10. Conform the site layout along natural landforms
11. Avoid excessive grading and disturbance of vegetation and soils
B. Performance Requirement No. 2: Water Quality Treatment
Projects that create and/or replace 5,000 square feet or more of impervious surface must treat
stormwater 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. Water Quality Treatment must be treated onsite and designed to treat stormwater runoff
equal to the volume of runoff generated by the 85th percentile 24-hour storm event, of 1 inch.
Water Quality Treatment may implement a treatment system that use multiple systems to complete
Water Quality Treatment. The Water Quality Treatment system must first implement Low Impact
Development Treatment Systems, then may implement Biofiltration Systems, and then finally may
CITY OF ARROYO GRANDE – ENGINEERING STANDARD PLAN
1010
Issue Date: March 2016 UNIFORM DESIGN CRITERIA Sheet 52 of 66
implement Non-Retention Based Treatment Systems. Projects subject to Performance
Requirement No. 2 must also include design strategies required by Performance Requirement No.
1.
1. Low Impact Development Treatment System – Design details may be found in Appendix K of
the Engineering Standards.
2. Biofiltration treatment systems with the following design parameters:
a. Prevent erosion, scour and channeling within the biofiltration treatment system based on
the flow of runoff produced from a rain event equal to 0.2 inches per hour intensity.
b. Minimum surface reservoir volume equal to the biofiltration treatment system surface
area times a depth of 6 inches
c. 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:
i. Sand - 60% to 70% meeting ASTM C33
ii. Compost - 30% to 40% may be used
d. Proper plant selection
e. Subsurface drainage/storage (gravel) layer with an area equal to the biofiltration
treatment system surface area and having a minimum depth of 12 inches
f. Underdrain with discharge elevation at top of gravel layer
g. No compaction of soils beneath the biofiltration facility (ripping/loosening of soils required
if compacted)
h. No liners or other barriers interfering with infiltration, except for situations where lateral
infiltration is not technically feasible.
3. Non-Retention Based Treatment Systems must collectively achieve at least one of the following
hydraulic sizing criteria:
a. Hydraulic Sizing Criteria for Non-Retention Based Treatment Systems:
i. Volume Hydraulic Design Basis must be based on the 85th percentile 24-hour
storm event of 1 inch.
ii. Flow Hydraulic Design Basis must be based on the flow of runoff resulting from a
rain event equal to at least 0.2 inches per hour intensity.
iii. See Performance Requirement No. 3: Runoff Retention for information to calculate
the required water quality volume.
C. Performance Requirement No. 3: Runoff Retention
Projects that create and/or replace 1 5,000 square feet or more of impervious surface must retain
runoff for optimal management of watershed processes. Projects subject to Performance
Requirement No. 3 must also include design strategies required by Performance Requirement No.
2 and 1.
1. Adjustments for Redevelopment – Replaced impervious surface, shall be multiplied by 0.5
when calculating the volume of runoff subject to Runoff Retention Performance
Requirements.
2. Retention must meet the following performance requirements:
a. Prevent offsite discharge from events up to the 95th percentile 24-hour rainfall event, 1.8
inches.
b. Achieve retention by:
i. optimizing soil infiltration
See Sheet 2 of 2 for paving llmlts
6" ·.
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dimension at all pipe
joints.
NOTE:S:
1. See Standard Specification Section 26-1 and 77-1 for backfill material and bedding requirements.
2. Prior to any beginning any trench cut, Contractors shall mNt with the Pub/le Works Department
to determine If pavement cuts are restricted by the City of AffO)O Grande Pavement Cut Polley
(Resolution No. 3819) •
.J. It is the responsib17ity of the Contractor or Premittee to contact ·underground Service Alert•
{USA) to determine locations for existing Power, Telephone, 017, Natural Gos, Coble rv. Sewer,
Storm Drainage, and any other underground facHltles.
4. Initial trench width may be only as wide as necessary to complete utility installation.
5. Pipe bedding shall be clean sand compacted to a 95X Minimum.
6. Number 12 insulated THW copper wire shall be placed continuously Ollflf' all pi;r: pipe and shall be
brought to the surface In valve wells, meter boxes, ct similar structures.
7. Trench backfill shall be 1-Sock (sand/cement -94 lbs cement per cubic yard) sluffy.
8. Temporary trench paving shall be 3• Min. of Cold Mix Asphalt. (Steel Traffic Plates may be used
in lieu of temporary paving upon approval of the City Engineer).
9. Provide vertical surfaces and square corners. Overlapping cuts shall not be permitted
10. ~en edge of trench cut Is with 3' of on existing trench or gutter, pavement cut shall extend to
existing trench or curb.
11. All pavements markings and markers which ore disturbed during trenching and asphalt
replocemen t shall be replaced
12. ~en the existing roadway asphalt concrete section depth is s• thick or greater, the Contractor
may grind and replace to a depth of 4• within paving limits of work that Is not directly over
trench area. Alternatively, the Contractor may remove and replace the entire asphalt concrete
section depth plus 1".
CITY OF ARROYO GRANDE -ENGINEERING STANDARD PLAN 6020
ISSUE DATE: MARCH 2016 TRENCH DETAIL SHEET10F2