012-08 - Ordinance - Amending Stormwater Design ManualORDINANCE NO. 012-08
AN ORDINANCE OF THE CITY OF PORT ORCHARD, WASHINGTON
AMENDING THE CITY OF PORT ORCHARD'S STORMWATER DESIGN
MANUAL BY INCORPORATING LOW IMPACT DEVELOPMENT
GUIDELINES AND PRACTICES
WHEREAS, on December 19, 2007, the City Council adopted Ordinance No. 046-07
which established Port Orchard Municipal Code Title 16 "Land Use Regulatory Code;" and
WHEREAS, Land Use Regulatory Code Chapter 16.80 provides for voluntary
compliance with the Planned Low Impact Development (PLID) — Alternative Development
Regulations set forth therein; and
WHEREAS, Land Use Regulatory Code Chapter 16.80 contemplates that at some
future date, the City will adopt comprehensive low impact development standards for residential and
commercial design and construction; and
WHEREAS, pending the future adoption of such standards, City staff has proposed
that the Port Orchard Stormwater Design Manual be amended to incorporate certain low impact
development guidelines and practices to provide guidance to those property owners and developers
that wish to voluntarily comply with the requirements of Chapter 16.80; now, therefore,
THE CITY COUNCIL OF THE CITY OF PORT ORCHARD, WASHINGTON, DO ORDAIN
AS FOLLOWS:
SECTION 1. The City of Port Orchard Stormwater Design Manual is hereby
amended by adding a new Appendix 5A relating to Low Impact Development Guidelines to read as
follows:
Appendix 5A; Low Impact Development Guidelines
Low Impact Development Guidelines
These guidelines outline methods to reduce the amount of stormwater runoff generated on
developed sites. Throughout this Appendix 5A, unless otherwise specified, all references to
this Manual shall mean the City of Port Orchard Stormwater Design Manual. The following
references may also be of assistance in designing low impact development sites. In cases
where these references differ with this Manual, this Manual will prevail.
Ordinance No. 012-08
Page 2 of 13
1. Low Im act Develo ment Technical Guidance Manual for Puget Sound, Puget
Sound Action Team and Washington State University, Pierce County
Extension, January 2005.
2. Stormwater Management Manual for Western Washington, Washington
Department of Ecology, February 2005.
3. Low Impact Development Design Strategies, An Integrated Design Approach,
Prince George's County, MD, June 1999.
4. Low Impact Development Hydrologic Analysis, Prince George's County, MD,
July 1999.
5. Bioretention Manual, Prince George's County, MD, December 2002.
5A.1 Site Design
Designers are referred to the Low Impact Development Technical Guidance Manual for Puget
Sound for detailed guidance on how to plan a low impact development project.
5A,2 Reduced Impervious Surface
5A.2.1 Permeable Pavement
A. The initial 5,000 square feet for each project of permeable pavement shall not be
considered impervious when determining whether or not a project meets the
definition of a major development. All permeable pavement and shall be modeled as
a dirt road or parking lot per table 5-2 for sizing of water quality and quality facilities
if:
1. The paving system does not have an under drain system connected to a
conveyance system
2. The site has a recorded covenant requirement paving systems maintenance
per section 5A.2.1.E.
3. The permeable pavement does not receive stormwater runoff from a
separate area larger than 10% of the permeable pavement area.
4. The permeable pavement product has an installed infiltration rate of at least
10 inches per hour.
B. Permeable pavement may not be used in areas with heavy pollutant loading or high
chemical spill risk such as but not limited to gas stations, heavy industrial areas, auto
body/repair shops, auto wash areas, commercial truck parking areas, areas with
heavy industrial activity (as defined by USEPA regulations), or areas with high
pesticide use.
Ordinance No. 012-08
Page 3 of 13
C. Permeable pavement overlying an underground infiltration system shall be modeled
as an impervious surface draining to an infiltration system designed per section 5.3.5
of this Manual. This system does not require a separate water quality device.
D. Permeable pavement post construction infiltration testing.
1. Bucket Test— Surfaces can be tested by simply throwing a 5-gallon bucket of
water on the surface. If the water puddles, remains on the surface greater
than 2 minutes, or runs off the surface, 6-inch ring testing is required prior to
accepting the construction.
2. A 6-inch ring infiltration test: Seal a 6-inch ring to the base of the road
surface. Wet the surface continuously for 10 minutes. The surface must
infiltrate at least 10 inches per hour to be considered permeable. (See the
Stormwater Management Manual for Western Washington, Volume III,
Appendix III C or Appendix 8A of this Manual).
3. Surfaces shall be tested at least once per 10,000 sq. ft. Clogged surfaces
(infiltration rate less than 10 inches per hour) totaling greater than 10% of
the total surface area shall be removed/replaced and retested.
E. Permeable Pavement Post Construction Maintenance Requirements
1. Surfaces shall be swept with a high -efficiency or vacuum sweeper at least
once per year. The optimum time is in the autumn after leaf fall.
2. Porous asphalt and concrete shall be cleaned with a high-pressure hose
following one of the annual sweepings at least once every other year.
3. A Bucket Test, and if necessary 6-inch ring infiltration test, shall be
performed after the first year of use and each 5 years thereafter. Surfaces
shall be tested at least once per 10,000 sq. ft. If this test indicates the
infiltration test is less than 10 inches per hour then the sweeping frequency
shall be increased to semi-annually and the high pressure washing annually.
(See the Stormwater Management Manual for Western Washington, Volume
III, Appendix III C or Appendix 8A of this Manual for more details on the 6-
inch ring test).
4. Owner/Operator must keep at least 5 years of written documentation of all
cleaning and infiltration tests and make them available to City inspectors
upon request.
5A.3 Dispersion
5A.3.1 Full Dispersion:
Impervious surfaces dispersed over native vegetation per section 5.A.2 of this Manual shall
be considered mitigated and require no further water quantity controls.
Ordinance No. 012-08
Page 4 of 13
5A.3.2 Partial Dispersion:
A. Impervious surfaces dispersed as sheet flow over a flow length of at least 50 ft. of
native vegetation or lawn/landscaped areas shall be considered partially dispersed.
B. Lawn or landscaped areas shall meet the soil amendment requirements of this
Manual.
C. Partially dispersed impervious surfaces shall meet all design criteria per section
5.A.2. of this Manual except the minimum flow length shall be 50ft.
D. Partially dispersed impervious surfaces shall be considered impervious when
determining whether or not a project meets the definition of a major development.
F. Partially dispersed impervious surfaces shall be modeled as open space in good
conditions per table 5-2 "Modified Curve Numbers" of this Manual.
5A.3.3 Vegetated Roofs:
A. Design details for vegetated roofs can be found in section 6.4 of the Low Impact
Development Technical Guidance Manual for Puget Sound,
B. Vegetated portions of roofs shall be modeled using the Curve Numbers from table 5-
2 of this Manual as:
1. Open Space Good Condition on till (C soil) for the areas with 3-8 inches of
growing media.
2. Meadow or Pasture on till (C soil) for areas with greater than 8 inches of
growing media.
5AA Minimum Excavation Foundations:
See section 7.6 of the Low Impact Development Technical Guidance Manual for Puget Sound
for modeling credits given for this technology.
5A.5 Bioretention Facilities
A. Bioretention facilities shall be designed as either filtration systems or retention
systems.
1. Facilities designed as retention systems do not require pretreatment per section
5.3.5 of this Manual.
Ordinance No. 012-08
Page 5 of 13
2. For facilities that receive runoff from less than or equal to 5000 sq. ft. of
impervious surface subject to vehicular traffic, there shall be 1 ft. of clearance
from the bottom of the facility to the seasonal high water table.
3. For facilities that receive runoff from greater than 5000 sq. ft. of impervious
surface subject to vehicular traffic, there shall be 3 ft. of clearance from the
bottom of facility to the seasonal high water table.
4. Facilities designed only as filtration systems (water quality devices) shall have an
under drain system to ensure that the Stormwater is filtrated through the media
prior to discharge.
5. Maximum ponding for bioretention systems shall be 12 inches. However, a
maximum ponding depth of 6-8 inches is preferred.
6. Maximum draw down time shall be 24 hours. Draw down volume includes the
engineered soil and gravel void spaces within the facility.
7. Filtration systems shall be modeled as having a flow rate equivalent to a filtration
rate of 1 inch per hour (no safety factor applied to the 1 inch/hr).
8. Retention systems shall be modeled per section 5.35 of this Manual using the
native soil infiltration rates. Soil void space shall be assumed to be 40%. Only
void space below any under drains may be used for storage calculations.
9. Facilities will have an overflow system that connects the facility to either a
designated dispersion area or downstream conveyance system.
10. Facilities receiving runoff from up to 5000 sq, ft. of impervious area shall have a
minimum engineered soil depth of 18 inches for systems and an optional mulch
layer of 2-3 inches. Facilities receiving runoff from greater than 5000 sq. ft. of
impervious surface shall have a minimum engineering soil depth of 24 inches and
a mulch layer of 2-3 inches.
11. Figures 5-31a, b, c show possible bioretention facility configurations. Designers
may find other configurations in the references listed at the beginning of this
Appendix.
12. Engineering soils shall be compacted.
B. the engineered soil component shall meet one of the following soil specifications:
1. On -Site Soil Mix
a) Reuses native soils;
b) 65% on -site soil, 35% compost (See 5A.1.0);
c) Mix shall be free of debris, rocks, garbage and organic material
greater than 2 inches in any dimension;
d) On -site soils shall be loam, sandy loam or loamy sand per figure 5-
14. Soils may be amended with sand to meet this specification;
and
e) Mix shall be well blended and covered to prevent wetting and
saturation.
Ordinance No. 012-08
Page 6 of 13
2. Off -Site Soil Mix
a) Uses imported soils;
b) 65 to 70% gravelly sand per ASTM D 422;
Sieve Size %Passing
US No 4
100
US No 6
88-100
US No 8
79-97
US No 50
11-35
US No 200
5-15
c) 30 to 35% compost (See 5A.1.0);
d) Clay content shall be less than 5%;
e) Mix shall be free of debris, rocks, garbage and organic material
greater than 2" in any dimension;
f) Mix shall be well blended and covered to prevent wetting and
saturation; and
g) This blend is available commercially as "Vegetable Garden Mix" by
Cedar Grove Composting.
C. Maintenance: Facilities shall be maintained per Appendix 8A of this Manual.
Ordinance No. 012-08
Page 7 of 13
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Ordinance No. 012-08
Page 8 of 13
Ordinance No. 012-08
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Ordinance No. 012-08
Page 10 of 13
SECTION 2. The City of Port Orchard Stormwater Design Manual is hereby
amended by adding a new Appendix 8A to read as follows:
Appendix 8A
6-inch ring infiltration test
Equipment:
1. An open-ended cylinder of approximately 6-inch diameter, ;>-12 inches in length
2. Bucket or other device to contain 2.5 to 5 gallons of water
3. Plumbers putty or other temporary waterproof sealant
4. Stop watch or other time keeping device accurate to a second
5. Infiltration test data sheet (page 8A-11)
Procedure:
1. Measure cylinder interior diameter in inches and note on test data sheet
2. Seal one end of the cylinder interior and exterior to the test surface.
3, Place a mark on the interior of the cylinder approximately 6 inches from the bottom.
4. Wet the test surface inside the cylinder continuously for 10 minutes.
5. Allow water to completely infiltrate.
6. Measure a known quantity of water between 1 and 5 gallons.
7. Start stop watch
8. Pour measured water into cylinder as needed until all the water has infiltrated. Do not fill the
cylinder above the 6-inch mark.
9. Record time required to infiltrate measured water.
10. Determine infiltration rate
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Appendix HA
6-inch ring infiltration test
Equipment:
Ordinance No. 012-08
Page 11 of 13
1. An open-ended cylinder of approximately 6-inch diameter, more than or equal to 12
inches in length.
2. Bucket or other device to contain 2.5 to 5 gallons of water.
3. Plumbers putty or other temporary waterproof sealant.
4. Stop watch or other time keeping device accurate to a second.
5. Infiltration rate data sheet (page 8A-11).
Procedures:
1. Measure cylinder interior in inches and note on rate data sheet.
2. Seal one end of the cylinder interior and exterior to the test surface.
3. Place a mark on the interior of the cylinder approximately 6 inches from the bottom.
4. Wet the test surface inside the cylinder continuously for 10 minutes.
5. Allow water to completely infiltrate.
6. Measure a known quantity of water between 1 and 5 gallons.
7. Start stop watch.
8. Pour measured water into cylinder as needed until all the water has infiltrated. Do not
fill the cylinder above 6-inch mark.
9. Record time required to infiltrate measured water.
10. Determine infiltration rate.
Ordinance No. 012-08
Page 12 of 13
Appendix 8A
Infiltration Rate Data Sheet
Date: Operator:
Test Site:
Quantity of Water Infiltrated: Gallons (W in equations below)
Time to Infiltrate Water: Seconds (T in equations below)
Cylinder Interior Diameter: Inches (D in equations below)
Determine Test Surface Area as follows: (A in equations below)
A = [D/2]2 x P1/144
Divide Diameter by 2. Square resultant number. Multiply by 3,14. Divide by 144
(If interior diameter is 6 inches, A -- U ft2) .
Determine Infiltration rate as follows: Inches per hour
Divide W by 7.48
[ W / 7.48 ]
Divide resultant number by T
[ W/7A8] 1 [T]
Divide resultant number by A
[ W/7.48]/M/[A]
Multiply resultant number by 3600
[ W/7,48]1[11I[A] x 3600
Multiply resultant number by 12
[ W/7,48]/M/[A] x 3600 x 12
(if 2.5 gallons infiltrates in a 6 inch cylinder in 600 seconds, rate — 120 inches per hour)
SECTION 3. This ordinance shall be in full force and effect five (5) days after posting and
publication as required by law. A summary of this Ordinance may be published in lieu of the entire
ordinance, as authorized by State Law.
Ordinance No. 012-08
Page 13of13
PASSED by the City Council of the City of Port Orchard, APPROVED by the Mayor and
attested by the Clerk in authentication of such passage this 25th day gf'lch 2008.
ATTEST:
Michelle Merlino, City Clerk
APPROVED AS TO FORM:
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City Attorne
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Sponsored by:
Robert Putaansuu, Councilman