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40.380.040 Stormwater Control
A. Design Standards.
1. Stormwater facilities shall be designed and constructed in accordance with 1998 Standard Specifications for Road, Bridge, and Municipal Construction, and updates as prepared by Washington Department of Transportation (WSDOT); and the BMP manual.
B. Water Quality Treatment.
1. General Standards.
a. All development activities and redevelopment, unless exempted in Sections 40.380.020 and 40.380.030, shall provide treatment of stormwater runoff through the use of BMPs specified in this section and in accordance with the BMP manual.
b. Treatment BMPs shall be sized to capture, hold and treat the water quality design storm, defined as seventy percent (70%) of the two (2) year recurrence interval twenty-four (24) hour storm runoff event.
c. If site conditions are appropriate and groundwater quality will not be impaired, infiltration is the preferred BMP. Direct discharge of untreated stormwater to groundwater is prohibited. All discharges to groundwater shall comply with the following state laws: the Water Pollution Control Act (Chapter 90.48 RCW), the Water Resources Act (Chapter 90.54 RCW), and Water Quality Standards for Ground Waters of the State of Washington (Chapter 173-200 WAC). Infiltration may be limited near public water supply wells.
d. The BMPs cited in this section shall be sited, designed and constructed in accordance with the requirements detailed in the BMP manual for each BMP, with the following exceptions:
(1) For biofiltration swales (RB.05) and vegetative filter strips (RB.10) alternative design criteria from the publication Biofiltration Swale Performance, Recommendations, and Design Considerations – Appendix G by the Municipality of Metropolitan Seattle, water pollution control department, dated October 5, 1992, shall be used;
(2) Where provisions of this section conflict with the BMP manual or other cited design guidance, this chapter shall take precedence.
2. Off-Site Analysis.
a. All development activities and redevelopment required to prepare a final stormwater control plan shall conduct an analysis of off-site water quality impacts resulting from the development activity or redevelopment and shall mitigate these impacts. The analysis shall extend a minimum of one-fourth (1/4) of a mile downstream from the development site. The applicant shall use best efforts to obtain this data while respecting private property. The existing conditions and potential impacts to be evaluated shall include, at a minimum, but not be limited to:
(1) Excessive sedimentation;
(2) Streambank erosion;
(3) Polluted discharges to groundwater contributing to recharge zones;
(4) Violations of water quality standards;
(5) Spills and discharges of priority pollutants, as defined by the Federal Clean Water Act.
b. Existing off-site impacts that are not affected by the development activity or redevelopment do not require mitigation. However, in cases where the subject property was the cause of the existing impact, it is the responsibility of the applicant to either mitigate or provide technical information and analysis demonstrating that no increased impact will result.
c. The above mitigation, where required for streambank erosion, is in addition to the basic requirement for water quantity control. The additional mitigation will take the form of acceptable BMPs for downstream erosion control (Section 40.380.040(B)(4)(c)). The publication entitled Integrated Streambank Protection Guidelines published by the Washington Department of Fish and Wildlife shall be utilized to guide design and installation of streambank erosion BMPs within and adjacent to streams. Other types of impacts will require mitigation of a type to be determined by the responsible official.
d. All discharges to surface waters shall comply with the following state laws: the Water Pollution Control Act (Chapter 90.48 RCW) and Water Quality Standards for Surface Waters of the State of Washington (Chapter 173-201A WAC).
e. Except within the Lacamas Basin where additional nutrient removal treatment is required pursuant to Section 40.380.040(B)(5), treatment of runoff from sidewalks, pedestrian pathways and bike paths is not required if the stormwater drains away from roadways. Runoff from sidewalks, pedestrian pathways and bike paths that mix with roadways will require treatment.
3. Redevelopment.
a. Where redevelopment of five thousand (5,000) square feet or more occurs within urban growth boundaries, rural centers, or urban reserve zoning districts, the requirements of Sections 40.380.020, 40.380.030, 40.380.040, and 40.380.050 shall apply to that portion of the site that is being redeveloped; provided, that non-pollution-generating land-disturbing activities are exempted from the water quality provisions of Section 40.380.040(B)(4). Source control BMPs of Article III (Chapter 1-4 of the BMP manual) shall be applied to the entire parcel(s) upon which the redevelopment project is occurring. A stormwater plan shall be prepared and shall include a maximum five (5) year schedule for implementing source control BMPs.
b. In addition to the above requirements, where one (1) or more of the following conditions apply, a stormwater plan shall include a maximum five (5) year schedule for: implementing all of the water quality requirements of Section 40.380.040 for those areas of the parcel draining into or from the redeveloped area; and implementing all quantity control requirements to the maximum extent practicable for the entire parcel(s) upon which the redevelopment project is occurring. An adopted and implemented basin plan may be used to establish redevelopment requirements that are tailored to a specific basin.
(1) Existing parcel(s) greater than one (1) acre in size with fifty percent (50%) or more impervious surface;
(2) Parcel(s) that discharge to receiving water that has a documented water quality problem shall implement all of the requirements appropriate to the identified problem. The responsible official shall maintain and make available to applicants a listing of water bodies with documented water quality problems.
c. An applicant may apply for a modification to the requirements of Section 40.380.040(B)(3)(b). The request for modification shall be proposed in conjunction with the application for the underlying redevelopment proposal in accordance with Chapter 40.500; provided, that where the modification request is filed subsequent to the decision on the development proposal, such request shall be processed in accordance with the post-decision review procedures of Section 40.520.060 and subject to the fees listed in Section 6.110A.020. The modification application, to be filed with the Community Development department shall include a written request including the following information:
(1) A map showing applicable existing conditions such as contours, wetlands, significant trees, lakes and rivers, utilities, property lines, existing and proposed roads and roadways, existing structures in impervious areas, existing drainage conditions and flow patterns, and existing stormwater facilities;
(2) An analysis of existing stormwater runoff quality and quantity being discharged from the entire parcel(s);
(3) An analysis of the stormwater quality and quantity impacts of the redevelopment proposal;
(4) The requirements of Section 40.380.040(B)(3)(b) which the applicant is requesting to be modified or waived;
(5) A comparison of the cost of compliance with the water quality and quantity control requirements of Section 40.380.040(B)(3)(b) contrasted to the costs of compliance with the water quality and quantity control requirements of Section 40.380.040(B)(3)(a);
(6) An analysis of the differences in stormwater quality and quantity treatment that would result from granting the request for modification as opposed to complying with the requirements of Section 40.380.040(B)(3)(b); and
(7) A discussion of the reasons why compliance with the requirements of Section 40.380.040(B)(3)(b) is unduly burdensome.
d. A stormwater variance to modify the requirements of Section 40.380.040(B)(3)(b) shall be granted if it is determined that complying with those requirements would be unduly burdensome. In making this determination, consideration shall be given to:
(1) The seriousness of existing stormwater quantity and quality problems generated from the site;
(2) The extent to which the stormwater problems would be aggravated by the redevelopment proposal;
(3) The degree to which the imposition of the requirements of Section 40.380.040(B)(3)(b) would alleviate the stormwater problems;
(4) The extent to which the request for modification would lessen the treatment of the stormwater problems;
(5) The cost of complying with Section 40.380.040(B)(3)(b) as compared to the cost of complying with the requirements of Section 40.380.040(B)(3)(a). If the cost of complying with Section 40.380.040(B)(3)(b) is more than the cost of Article III requirements imposed by Section 40.380.040(B)(3)(a), then it shall be presumed that Section 40.380.040(B)(3)(b) requirements are unduly burdensome.
e. If it is determined that full compliance with the requirements of Section 40.380.040(B)(3)(b) would be unduly burdensome, the review authority shall determine which requirements of Section 40.380.040(B)(3)(b) shall be imposed to provide the most effective treatment of stormwater impacts generated by the entire site without imposing an undue burden on the applicant.
f. In the case of public works projects to widen or otherwise redevelop an existing road, the site for which water quality treatment is required shall include only the portion of road which fronts on, and drains to or from, the redevelopment.
4. Standard BMPs.
a. Standard stormwater treatment BMPs shall be used to treat stormwater throughout Clark County, except for certain development activities in the Lacamas watershed as noted in Section 40.380.040(C)(5)(a).
b. Acceptable standard treatment BMPs may, depending upon circumstances and site characteristics, include the following from the BMP manual (Chapters III-3, III-4, and III-6):
(1) R1.05 – WQ Infiltration basin;
(2) R1.10 – WQ Infiltration trench;
(3) R1.15 – Roof downspout system;
(4) RD.09 – Constructed wetland;
(5) RD.06 – Wet pond with marsh;
(6) RD.05 – Wet pond without marsh;
(7) RB.05 – Biofiltration swale;
(8) RB.10 – Vegetative filter strip;
(9) RF.05 – Sand filtration basin;
(10) RF.10 – Sand filtration trench;
(11) Cartridge filters using compost, perlite, and geolite.
c. Acceptable BMPs for downstream erosion control may, depending upon circumstances and site characteristics, include the following from the BMP manual:
(1) E1.25 – Preserving natural vegetation;
(2) E1.30 – Buffer zones;
(3) E1.35 – Permanent seeding and planting;
(4) E1.40 – Sodding;
(5) E1.45 – Topsoiling;
(6) E2.70 – Outlet protection;
(7) E2.75 – Riprap;
(8) E2.80 – Vegetative streambank stabilization;
(9) E2.85 – Bioengineering methods of streambank stabilization;
(10) E2.90 – Structural streambank stabilization;
(11) The applicant may elect to provide additional BMPs in accordance with the following chapters and sections of the BMP manual as a method of mitigating off-site impacts:
(a) Chapter III-4.2, Runoff Treatment and Streambank Erosion Control;
(b) Section III-4.2.1, Background;
(c) Section III-4.2.2, Mechanisms of Pollutant Removal;
(d) Section III-4.2.3, Classification of Detention BMPs;
(e) Chapter III-4.3, General Design Criteria;
(f) Section III-4.3.1, Hydrologic Analysis;
(g) Section III-4.3.2, Sizing Detention BMPs for Runoff Treatment;
(h) Section III-4.3.3, Sizing Detention BMPs for Streambank Erosion Control.
d. Sand filtration BMPs (RF.05 and RF.10) are not allowed on commercial or industrial sites where the effluent from the treatment systems will drain to groundwater.
e. For biofiltration swales and vegetative filter strips, the hydraulic residence used for design shall be no less than nine (9) minutes. Swale slopes, however, may be no less than one percent (1%) unless underdrains are provided. Swales shall have a free discharge. When placed within a detention basin, calculations shall be provided that demonstrate that the peak stage during the water quality design storm is lower than the minimum swale elevation.
f. Permanent infiltration BMPs shall not be used as temporary erosion control devices.
g. Alternative roof downspout systems that provide an equivalent level of performance to the system in the BMP manual (R1.15) may be approved by the responsible official. Roof downspout systems can be constructed without observation wells.
5. Advanced BMPs for Nutrient Control.
a. Advanced control of nutrients is required in the Lacamas watershed above the dam at the south end of Round Lake, for all development sites exceeding one (1) acre in size. All water leaving the site during the water quality design storm shall be treated.
b. Acceptable BMPs for achieving advanced nutrient control may, depending upon circumstances and site characteristics, include the following from the BMP manual (Chapters III-3 and III-4):
(1) R1.05 – WQ Infiltration basin;
(2) R1.10 – WQ Infiltration trench;
(3) R1.15 – Roof downspout system;
(4) RD.09 – Constructed wetland;
(5) RD.06 – Wet pond with marsh;
6. Source Control BMPs. In addition to the other water quality treatment requirements in this section, commercial, industrial and public works development activities and redevelopment shall, to the maximum practicable, be designed in accordance with Chapter IV of the BMP manual and utilize BMPs specified in Chapters IV-2, IV-3 and IV-4 of the BMP manual.
7. Oil/Water Separators.
a. Development activities or redevelopment creating the following facilities require API or CPS-type oil/water separators:
(1) Industrial machinery and equipment, trucks and trailer aircraft, parts and aerospace, railroad equipment;
(2) Log storage and sorting yards;
(3) Airfields and aircraft maintenance;
(4) Fleet vehicle yards;
(5) Railroads;
(6) Gas stations;
(7) Retail/wholesale vehicle and equipment dealers;
(8) Vehicle maintenance and repair;
(9) Construction businesses such as paving, heavy equipment storage and maintenance, storage of petroleum products. (This does not include construction sites);
(10) Other activities that exhibit a significant risk of high oil loading in runoff.
b. Development activities and redevelopment creating the following facilities shall require spill control (SC) type oil/water separators:
(1) Restaurants;
(2) Duplex or multifamily residential development activities creating parking spaces for twenty-five (25) or more vehicles;
(3) Other activities where the risk of oil spills or illegal dumping of oil or grease is significant.
c. For development activities and redevelopment cited in Section 40.380.040(B)(7)(a) and B)(7)(b), oil/water separators shall not be required on portions of a site where the risk of oil or grease spills or dumping is minimal.
d. Oil/water separators shall be designed in accordance with Chapter III, Section III-7 of the BMP manual.
8. Infiltration BMPs on Industrial and Commercial Sites.
a. Infiltration of stormwater runoff shall not be allowed on commercial industrial sites that, due to location or the proposed use, pose a significant threat of contamination to groundwater.
b. Approval for use of infiltration BMPs (R1.05-30 in the BMP manual) on industrial and commercial sites, including gas stations, shall be conditioned on all the following criteria, unless found inappropriate by the responsible official:
(1) Analysis of the potential for groundwater contamination from the site. This analysis shall include a soils and groundwater evaluation if deemed appropriate by the responsible official;
(2) Demonstration that no other feasible alternative exists for disposing of stormwater from the site;
(3) A “state waste discharge permit,” as described in Chapter 173-216 WAC, obtained from the Washington Department of Ecology, where required by the state, and other state permits and approvals as appropriate.
c. The requirements of Section 40.380.040(B)(8)(a) shall not apply to runoff from portions of a site where the risk of groundwater contamination is no greater than single-family residential sites. Examples of these areas include rooftop drainage, runoff from undeveloped portions of a site, and drainage from portions of parking lots where the risk of illegal dumping is minimal.
d. In cases where infiltration is allowed on commercial and industrial sites and a significant risk of groundwater contamination exists, the responsible official may require groundwater monitoring to ensure against groundwater contamination. The responsible official may also require an agreement from the applicant for full mitigation in the event of groundwater contamination.
e. The provisions of this subsection do not apply to non-industrial and noncommercial sites that are defined under the NPDES permit system as industrial due to temporary construction activity.
9. Experimental BMPs.
a. Experimental best management practices are those which have not been fully tested and evaluated by the county or the Washington Department of Ecology and are not included as accepted practices in this code or the BMP Manual. Experimental BMPs that are adequately tested and proven effective shall be incorporated into this section as standard or accepted BMPs in the future.
b. Experimental BMPs may be allowed if all the following conditions are met:
(1) The experimental BMP usage is part of a Washington Department of Ecology or Clark County research project;
(2) Monitoring of the effluent quality produced by the BMP, as well as influent quality, will be conducted for at least two (2) years;
(3) Results of the research will be published;
(4) Financing is available to construct the BMP, conduct the testing and publish the results.
c. The responsible official may approve use of alternative water quantity and/or water quality treatment devices that are acceptable for projects meeting the eligibility requirements of Section 40.260.110 with evidence from the applicant that water quality, water quantity control, and maintainability are not affected.
10. Drainage Structure Labeling and Signage.
a. All catch-basins and manholes capable of accepting stormwater shall be stenciled. For infiltration systems stenciling shall read: “Please protect – Drains to Drinking Water.”
b. For facilities draining to surface waters the stenciling shall read: “Please protect – Drains to (name of water body).”
c. Signs shall be installed along water quality biofiltration systems that read: “Water Quality Filter – Please Leave Vegetated.”
d. Fenced detention and retention basins shall be marked with a sign that reads “[Public/Private] Stormwater Control Facility.”
(Amended: Ord. 2006-11-07)
C. Quantity Control.
1. General Standards.
a. All development activities and redevelopment, unless exempted in Section 40.380.030, shall provide quantity control of stormwater runoff in accordance with the requirements of this section.
b. Natural drainage flow routes to streams and wetlands shall be maintained, and discharges from the site shall occur at the natural location and elevation, to the maximum extent practicable.
c. Transfer of runoff from one (1) basin to another shall not be allowed.
d. Surface water exiting a parcel shall be discharged with adequate energy dissipaters within the development site to prevent downstream damage.
e. In addition to the requirements of Chapter 40.420, no reduction of existing conveyance capacity and no net loss of existing storage capacity for the one hundred (100) year storm is permitted in special flood hazard areas as defined in Section 40.420.010(C). This requirement shall also apply to all areas within the limits of the existing one hundred (100) year floodplain, as determined by hydrologic/hydraulic computations in accordance with this section, for all streams and manmade channels within Clark County.
f. Where provisions of this section conflict with the BMP manual or other cited design guidance, this section shall take precedence.
g. No development within an urban growth area shall be allowed to materially increase or concentrate stormwater runoff onto an adjacent property or block existing drainage from adjacent lots. This requirement shall not apply to existing drainageways. This shall apply to all new residential lots less than twenty thousand (20,000) square feet in size and all nonresidential developments within the urban growth area created after September 10, 1996. Alterations or remodels that increase the building footprint by less than fifty percent (50%) are exempt from this provision.
h. All lots within the urban growth area must be designed to provide positive drainage from bottom of footings to an approved stormwater system. Positive drainage may be accomplished by swales, drywells, french drains, laterals to the street, laterals behind the curb or within a public utility easement, an approved system in the side or rear setback, or some other method acceptable to the responsible official.
2. Hydrologic and Hydraulic Analysis.
a. Hydrologic and hydraulic analysis shall be in accordance with Chapters III-1 and III-2 of the BMP Manual, with the following exceptions:
(1) Table III-1.6, Hydrologic Soil Groups for Soils in the Puget Sound Basin, is replaced by Hydrologic Soil Groups for Soils in Clark County. (Source: SCS TR-55, Second Edition, June 1986, Exhibit A-1. Revisions made from SCS, Soils Interpretation Record, Form #5, September 1988.) Alternatively, hydrological soil groups can be developed by registered soil scientist using criteria set in the USDA, SCS National Soils Handbook.
(2) Appendix AIII-1.1, Isopluvial Maps for Design Storms, is replaced by Isopluvial Maps for Design Storms in Clark County. (Source: NOAA Atlas 2, Precipitation – Frequency Atlas for the Western United States, Volume IX – Washington.)
(3) The HEC-1 Flood Hydrograph Package computer program, developed by the Hydrologic Engineering Center, U.S. Army Corps of Engineers is an acceptable hydrologic computation program for use in Clark County.
(4) Design of stormwater collection systems shall be in accordance with Hydraulic Engineering Circular #12, Drainage of Highway Pavements, 1984 Edition, published by the United States Department of Transportation, Federal Highway Administration (FHWA).
b. Table III-1.3, SCS Western Washington Runoff Curve Numbers of the BMP Manual shall be used to calculate predevelopment and post-development runoff with the following constraints:
(1) Predevelopment land use shall be established as the use over the last thirty (30) years which results in the least amount of site runoff, as demonstrated by evidence acceptable to the responsible official. Acceptable evidence may include, but not be limited to thirty (30) year old aerial photos, crop history or tax assessor records.
(2) Redevelopment of existing sites less than ten thousand (10,000) square feet in area can assume predevelopment land use equivalent to the facility being redeveloped.
3. Design Methodology for Quantity Control Facilities.
a. Except as limited by Section 40.380.040(B)(8) for commercial and industrial sites, infiltration of the one hundred (100) year storm is the preferred method for all stormwater disposal from development sites where local soil types and groundwater conditions are suitable (in general, soils classified as A-1-a, A-1-b, A-3, A-2-4, and A-2-5 as defined in AASHTO Specification M145); provided, that water quality treatment as detailed in Section 40.380.040 is provided prior to infiltration. Soil suitability for infiltration shall be determined by a qualified geotechnical engineer through both approved field testing and laboratory testing.
b. The design infiltration rate for infiltration systems shall be limited to one-half (1/2) the measured infiltration rate. Infiltration rates shall be tested on-site for all soils. The responsible official may require a representative drywell be tested after completion of the stormwater improvements to verify design infiltration rates. The test results shall be submitted to the county by the project engineer prior to completing construction of the stormwater facilities. Redesign may be required if tested rates are less than those utilized in the design. The tests may be waived during preparation of the preliminary plans if the engineer presents credible test results from properties within one-quarter (1/4) mile distance from the development.
c. The responsible official may allow the base of infiltration facilities to be less than three (3) feet above seasonal high water or an impermeable layer if the quality and quantity control requirements of this chapter can be met and a groundwater mounding analysis verifies that the facility will function as required.
d. For surface runoff leaving a development site, the following criteria shall be met:
(1) The peak release rate for the two (2) year design storm after development shall not exceed one-half (1/2) the pre-developed two (2) year design storm peak runoff rate.
(2) The peak release rate for the ten (10) and one hundred (100) year design storms after development shall not exceed the respective predevelopment design storm peak rates.
(3) After meeting the requirements of Section 40.380.040(C)(3)(d)(1) and (C)(3)(d)(2), the pond volume shall be increased by either the following multiplication factor F: F = (composite curve number/46)-0.6 or by using Figure III-1.1 in section III of the BMP Manual. This correction factor is to be applied to the volume of the pond without changing its depth or the design of its outlet structure, which shall result in an increase in surface area.
e. To ensure the standards in this section are met, the volume available for storing runoff in a stormwater facility shall be increased to allow for storage of high seasonal groundwater and/or ordinary high water.
f. Residential and commercial structures shall be designed to direct roof runoff to downspout roof systems in areas that contain soils of AASHTO M145 types A-1-a, A-1-b, A-3, A-2-4, and A-2-5 where the measured infiltration rate is equal to or greater than eight (8) inches per hour. The system shall be designed to discharge a minimum two (2) year twenty-four (24) hour design storm into the ground. Runoff from roofs during the ten (10) and one hundred (100) year storms shall be included in the postdevelopment design flow of the site facility(ies) unless provided for in the roof system. Infiltration tests shall be provided for all proposed roof downspout systems prior to final stormwater plan approval. Infiltration is not required in the rural area, or in erosion hazard areas as defined in Chapter 40.100.
g. The responsible official may waive this requirement upon written findings by a qualified geotechnical engineer demonstrating that such infiltration is unsuitable and roof runoff is conveyed to an approved water quantity control facility.
h. Design of stormwater control facilities shall be in accordance with the following methods from the BMP Manual (Chapters III-1 and III-3):
(1) Section III-1.4.4 – Hydrograph Routing;
(2) Section III-1.4.5 – Hydrograph Summation and Phasing;
(3) Section III-1.4.6 – Computer Applications;
(4) Section III-1.5 – Closed Depression Analysis;
(5) Section III-3.3 – Feasibility Analysis and General Limitations for Infiltration BMPs;
(6) Section III-3.4 – General Design Criteria for Infiltration and Filtration BMPs;
(7) Section III-3.5 – Construction and Maintenance;
(8) Section III-4.3 – General Design Criteria;
(9) Section III-4.4 – Standards and Specifications for Detention Ponds.
4. Conveyance Systems.
a. Open channel conveyance systems incorporating water quality treatment, habitat improvement and emergency overland flood relief routes shall be utilized to the maximum extent practicable.
b. Stormwater conveyance elements to transport water within and from a development activity site shall be sized to carry flows from the “design storm” from the contributing drainage area based upon the projected full build-out of that contributing drainage area, and be fully compatible with existing downstream conveyance elements and flow conditions.
c. For stormwater conveyance design, the “design storms” shall be as follows:
(1) Ten (10) year storm: contributing drainage areas less than forty (40) acres;
(2) Twenty-five (25) year storm: contributing drainage areas of forty (40) acres or more;
(3) One hundred (100) year storm: culverts with contributing drainage areas greater than two hundred (200) acres, culverts in areas of special flood hazard as described in Federal Emergency Management Agency Flood Insurance Rate Maps (FIRM) and reports for Clark County, culverts where upsizing in order to meet design requirements for the one hundred (100) year storm is required.
d. Development sites shall be planned to be able to pass a one hundred (100) year storm through the site.
e. Closed conveyance system elements shall be designed to operate in an open flow, not pressure flow regime except during the one hundred (100) year storm.
f. Runoff from the one hundred (100) year storm may leave pipes and channels but shall not rise to elevations more than two (2) feet below that of the lowest finished floor of buildings.
g. For the ten (10) year storm, street ponding shall be limited to one-half (1/2) of the roadway area and shall not exceed the capacity of the inlet or produce a flow depth of greater than 0.12 feet at the edge of the travel lane.
h. For roadway flooding conditions during the one hundred (100) year storm, one (1) travel lane in either direction shall remain open to emergency vehicles at all times. A travel lane will be considered to be open to emergency vehicles if the maximum depth of flow in the travel lane does not exceed 0.5 feet.
i. For parking lot flooding conditions during the one hundred (100) year storm, the maximum depth of ponding shall not exceed one and one-half (1.5) feet. Storage volumes resulting from ponding in street and parking lot areas may be used to meet the storage requirements of Section 40.380.040(C)(3)(d)(2) of this section for the one hundred (100) year storm.
j. Design of conveyance systems shall be in accordance with Chapter III-2 of the BMP Manual.
k. Design of bridge stormwater systems shall be in accordance with the Washington Department of Transportation Bridge Design Standards, 1991 Edition.
l. Stormwater easements shall be provided to the county for access and maintenance of all conveyance systems (including streams, if utilized) within the development site, which are to be maintained by the county. The minimum widths of easements shall be as follows, although the responsible official may require increased widths when necessary to ensure adequate area for equipment access and maintenance:
(1) Pipes with an inside diameter less than or equal to thirty-six (36) inches: twenty (20) feet;
(2) Pipes with an inside diameter greater than thirty-six (36) inches: twenty (20) feet plus the pipe’s inside diameter;
(3) Pipes shall be located with their center line no closer than one-quarter (1/4) the easement width from an abutting property line;
(4) Channels: top width of channel plus fifteen (15) feet on one (1) side.
m. No buildings or other structures that prevent access are permitted within easements. Fences crossing easements shall provide gates of sufficient width over the easement for access by maintenance vehicles.
5. Discharge to Large Water Bodies. Development activities and redevelopment meeting all the following criteria are exempt from the quantity control requirements of Section 40.380.040(C)(3)(d):
a. The runoff from the development activity or redevelopment directly enters one (1) of the following water bodies through a pipe or other approved discharge structure:
(1) Columbia River,
(2) Lacamas and Round Lakes,
(3) North Fork Lewis River,
(4) Vancouver Lake,
(5) Lake River;
b. Runoff is treated in accordance with the requirements of Section 40.380.040;
c. The discharge structure is designed to avoid erosion during all storms up to the one hundred (100) year storm;
d. If an existing discharge structure is used:
(1) The structure must meet the requirements of Section 40.380.040(C)(5)(c); and
(2) The discharge structure and conveyance system leading to the discharge must have adequate capacity to meet the requirements of this chapter.
(Amended: Ord. 2007-06-05)
D. Location of Stormwater Facilities.
1. Treatment, runoff control and recharge facilities shall be located prior to the point of discharge into a stream, lake or fish-bearing water or prior to discharge to groundwater.
2. Locations of stormwater facilities in relation to wetlands are specified in Chapter 40.450.
3. Stormwater facilities, other than closed conveyance systems, shall be located at least one hundred (100) feet from existing and proposed on-site sewage system drainfields.
4. Infiltration systems used for stormwater disposal shall be located at least one hundred (100) feet from domestic water supply wells.
5. Swales and other stormwater treatment facilities using biofiltration shall be located outside easements and corridors used by phone, electric, water, natural gas, and other utilities unless the utilities are installed prior to construction of the biofiltration system.
6. Sites used for stormwater treatment and runoff control facilities shall be owned by the applicant, county or state, and:
a. If the county or state owns the site, a letter from the responsible agency allowing use of the site for stormwater control shall be submitted with the preliminary stormwater plan; or
b. If the county or state does not own the site, the ownership shall be included for consideration with the land use application for the development activity.
7. Publicly owned and maintained stormwater treatment and control facilities in urban residential subdivisions and short plats shall be located on separate tracts which are recommended, but not required, to meet minimum zoning lot size requirements. The plat or other dedication instrument shall indicate tract disposition in the event of county abandonment or vacation.
Privately owned and maintained facilities may be within easements. In addition, easements shall be provided to the county for access and inspection of the facilities.
(Amended: Ord. 2007-06-05)
E. Protection of Infiltration Systems from Erosion.
Stormwater infiltration systems shall be isolated and protected from sedimentation due to erosion during the construction phase of a development activity or drainage project. Furthermore, use of infiltration systems shall be minimized until the erodible parts of a site are stabilized with adequate vegetation.
F. Fencing of Stormwater Facilities.
1. Stormwater treatment and runoff control facilities located in or adjoining residential areas shall be fenced unless these facilities are constructed as part of a development amenity such as a park or the responsible official waives the fencing requirement due to special circumstances.
2. Stormwater treatment and runoff control facilities, other than those described in Section 40.380.040(F), shall be fenced if they pose safety risks to the public.
3. The size and type of fence shall be determined by the responsible official.
G. Side Slopes of Stormwater Facilities.
1. For maintenance, safety, and stability reasons, side slopes of stormwater facilities normally shall be no steeper than three to one (3:1) within the area of submergence.
2. For facilities to be maintained by the county, vertical slopes are allowed if all the following conditions are met:
a. No more than seventy-five percent (75%) of the perimeter of the stormwater facility shall have vertical sides;
b. Vertical sides more than three (3) feet high shall be fenced;
c. Access for maintenance of facilities satisfactory to the responsible official shall be provided; and
d. Side slopes in a biofiltration treatment area shall be no steeper than three to one (3:1).
3. For facilities that will not be maintained by the county, slopes steeper than three to one (3:1) are allowed if all the following conditions are met:
a. Side slopes in a biofiltration treatment area shall be no steeper than three to one (3:1);
b. Adequate long-term erosion control is provided;
c. No more than seventy-five percent (75%) of the perimeter of the stormwater facility shall have vertical sides;
d. Vertical sides more than three (3) feet high shall be fenced; and
e. The maintenance and operations manual for the facility shall demonstrate that the facility can be maintained.
4. Side slopes steeper than two to one (2:1) may be allowed by the responsible official for specialized development activity, such as streambank reconstruction, where all the following conditions are met:
a. Side slopes do not need to be mowed; and
b. Adequate long-term erosion control and slope stability is provided.
H. Maintenance and Ownership.
1. County Ownership of Stormwater Facilities – When Required. County ownership of stormwater facilities is required for all such facilities that are to be located within a public right-of-way or for which arrangements for private long-term maintenance which are acceptable to the responsible official have not been made.
2. Acceptance of Ownership by the County.
a. Provisional Acceptance. Stormwater facilities, which are to be owned by the county, will be provisionally accepted for ownership upon the approval of the record drawings and approval of an inspection of the facilities by the county. Provisional acceptance of the facilities shall not relieve the applicant from any obligation to undertake any remedial measures to correct deficiencies in the design, construction, maintenance or operation of the facilities.
b. Final Acceptance of Ownership by the County. No sooner than eighteen (18) months following the provisional acceptance of the facilities, the applicant shall notify the responsible official that the facilities are eligible for final acceptance of ownership by the county. Prior to their final acceptance for ownership, the facilities shall be inspected to determine that they are in satisfactory condition. The responsible official may require the applicant to conduct tests of the facilities to reasonably demonstrate that they are operating as designed and to the county standards for quality and quantity control as a condition of final acceptance. Upon approval of the facilities by the responsible official and all necessary ownerships and easements entitling the county to properly access and maintain the facilities have been conveyed to the county and recorded with the County Auditor, they will be finally accepted for ownership by the county.
3. Maintenance of Stormwater Facilities.
a. County-Owned Facilities.
(1) Initial Maintenance and Repair. For a period of at least two (2) years following the provisional acceptance of stormwater facilities or thereafter until the facilities are finally accepted by the county, the developer constructing the facilities shall maintain, repair, redesign, reconstruct the facilities to ensure that they operate as designed and to the county standards for quality and quantity control. This obligation shall extend to remedying any damage caused to the facilities by builders or other third parties during the initial maintenance period. The required maintenance shall be performed according to county’s Stormwater Facilities Maintenance Manual as adopted by Chapter 13.26A.
(2) During the initial maintenance period, remedial work to correct deficiencies shall be the responsibility of the developer and shall be completed prior to final acceptance. Required remedial work to correct maintenance and construction deficiencies shall be completed by the applicant prior to final acceptance.
(3) Long-Term Maintenance. Following their final acceptance for county ownership, the county shall maintain stormwater facilities.
b. Privately Owned Facilities.
(1) Responsibility for Maintenance. For stormwater facilities for which the county will not provide long-term maintenance, the developer shall make arrangements with the existing or future (as appropriate) occupants or owners of the subject property for assumption of maintenance to the county’s Stormwater Facilities Maintenance Manual as adopted by Chapter 13.26A. The responsible official prior to county approval of the final stormwater plan shall approve such arrangements. Final plats shall specify the party(ies) responsible for long-term maintenance of stormwater facilities within the Developer Covenants to Clark County.
The county may inspect privately maintained facilities for compliance with the requirements of this chapter. If the parties responsible for long-term maintenance fail to maintain their facilities to acceptable standards, the county shall issue a written notice specifying required actions to be taken in order to bring the facilities into compliance. If these actions are not performed in a timely manner, the county shall take enforcement action and recover from parties responsible for the maintenance in accordance with Section 32.04.060.
(2) Easements Required. Easements or a covenant acceptable to the responsible official shall be provided to the county for purposes of inspection of privately maintained facilities. The minimum dimensions of easements for stormwater facilities are as follows:
(i) Pond design and easements shall allow access to all areas within the pond and drainage structures by standard maintenance equipment vehicles;
(ii) Widths of easements for conveyance facilities shall be as detailed in Section 40.380.040(C)(4).
(Amended: Ord. 2005-04-12; Ord. 2007-06-05)
I. Recovering Costs of Stormwater Facilities.
1. The following costs associated with stormwater facilities may be recoverable through latecomers agreements (RCW 35.91.010):
a. Oversizing on-site facilities above their existing capacity or the capacity required for the proposed development;
b. A proportionate share of the total cost of off-site facilities.
2. If a stormwater utility exists, the costs for building or oversizing a stormwater facility may be eligible as a credit against applicable system development charges.
J. Bonds and Insurance.
1. Performance Security. In lieu of completing required stormwater facilities within a preliminary plat prior to recording, the applicant shall post a performance bond or other security acceptable to the responsible official in the amount of one hundred fifty percent (150%) of the estimated cost (prepared by the project engineer) of completing construction per the approved stormwater plan. After determination by the responsible official that all facilities are constructed in compliance with the approved plan, are performing their intended functions in a satisfactory manner, and that the maintenance bonding requirements of Section 40.380.040(H) are met, the performance bond or security shall be released. No building permits shall be issued until the stormwater facilities are completed and provisionally accepted.
2. Maintenance Security. In cases identified in Section 40.380.040(H)(2), a maintenance bond or other security acceptable to the responsible official shall be posted and maintained throughout the two (2) year initial maintenance period for a stormwater facility.
K. Basin Plans.
1. Basin plans are strategies for a watershed designed to protect and enhance surface and groundwater within a watershed.
2. Where conflicts occur, the policies and standards in an adopted basin plan shall supersede the other requirements of this section, which shall be equal to or exceed the requirements of this section.
3. To be valid, basin plans must be stamped by a registered professional engineer, adopted by the board, meet the requirements of Chapter 36.94 RCW, and incorporated into this section.
L. Regional and Subregional Facilities.
1. If regional or subregional facilities are used to meet some or all of the standard requirements of this section, the following conditions shall be met:
a. Stormwater runoff shall be transported from a development site to a regional/subregional facility through a pipe or manmade open channel conveyance system.
b. If the regional/subregional facility does not yet exist, interim quantity control and treatment methods shall be used to meet the standard requirements of this section. All interim methods shall be reviewed and shall require written approval by the responsible official.
c. The facility must have sufficient capacity to provide the treatment and quantity control specified in this section.
d. A written commitment from the owner of the facility, or the responsible official in the case of county-owned facilities, shall be provided that allows use of the facility by the applicant.
e. The county encourages the use of regional and subregional stormwater facilities. Review of designs of these types of facilities shall be expedited by the county and receive priority review.
2. Where a stormwater utility exists, a system development charge can be assessed for use of a regional/subregional facility.
M. Record Drawings.
1. Record drawings which accurately represent the development site as constructed shall be provided to Clark County prior to: the issuance of building permits for single-family/duplex residential subdivisions; the issuance of occupancy permits for development activities subject to site plan review; and within sixty (60) days following completion of construction of other development activities.
2. The record drawings shall include corrected engineering plans for the stormwater system, showing constructed dimensions and elevations. In addition, revisions to the final stormwater plan shall be submitted with the record drawings where changes which take place during construction significantly alter the calculations and assumptions contained in the plan.
3. All plans submitted shall be reproducible and on Mylar.
4. The record drawing submittal shall be stamped, signed and dated by a licensed professional engineer, registered in the state of Washington.
5. Record drawings are requested to be submitted on computer disk in a format determined by the county, upon notice to do so.
N. Exceptions for Single-Family/Duplex Residential Subdivisions.
1. The responsible official may approve the issuance of building permits for up to fifty percent (50%) of the lots after the stormwater and road improvements are substantially complete. Substantial completion is defined as:
a. Following inspection, stormwater facilities are operational and constructed to county standards;
b. Streets are constructed and at least one (1) lift of asphalt is installed when paving is required;
2. The development activity is in full compliance with this chapter.
3. Building permits for “model homes” may be approved subject to the special use provisions of Section 40.260.145.
(Amended: Ord. 2007-11-13)