Section 40.385.020

40.385.020 Standards – Stormwater Control

A. General Standards.

1. The SMMWW as modified by the county’s Stormwater Manual is adopted by reference, and the recommendations and requirements contained therein will be the minimum standards for this chapter except as modified in this chapter.

2. Where provisions of this chapter conflict with other Title 40 requirements, the more stringent shall apply.

3. Stormwater facilities shall be constructed in accordance with the Standard Specifications for Road, Bridge, and Municipal Construction 2008 as prepared by the Washington Department of Transportation.

4. All urban new development and redevelopment shall comply with the following:

a. Minimum requirement No. 2 and Section 40.385.030.

b. New development and redevelopment that creates or adds two thousand (2,000) square feet or more of new, replaced, or new-plus-replaced impervious surface or which has land-disturbing activity of seven thousand (7,000) square feet or more shall comply with minimum requirements No. 1 through No. 5 for the new and replaced impervious surfaces and the land disturbed.

c. New development and redevelopment that creates or adds five thousand (5,000) square feet or more of new impervious surface, converts three-quarters (0.75) of an acre or more of native vegetation to lawn or landscaped area, or converts two-and-a-half (2.5) acres or more of native vegetation to pasture shall comply with minimum requirements No. 1 through No. 10 for the new impervious and converted pervious surfaces.

d. An off-site analysis as described in the Stormwater Manual, unless exempted by Section 40.385.010(C)(4).

e. The county may allow the minimum requirements to be met for an equivalent (flow and pollution characteristics) area within the same site. For public road projects, the equivalent area does not have to be within the same project limits but must drain to the same receiving water. For frontage improvements required within the public right-of-way, the equivalent area must be immediately adjacent to the site.

5. All rural new development and redevelopment shall comply with the following:

a. Minimum requirement No. 2 and Section 40.385.030.

b. New development and redevelopment that adds or replaces impervious area of greater than two thousand (2,000) square feet and less than five percent (5%) of a site, or is a land-disturbing activity greater than seven thousand (7,000) square feet, are subject to the minimum requirements dependent on site-specific characteristics.

(1) Minimum requirements No. 1 through No. 5 shall apply if the project meets all of the following criteria:

(a) Is outside of habitat or wetland areas or their buffers; and

(b) Does not generate runoff in channelized flow or discharge directly or indirectly to the county’s storm sewer system; and

(c) Is not located in, and does not discharge onto, steep slope hazard areas or landslide hazard areas as designated in Section 40.430.010.

(2) Projects not meeting all the criteria in Section 40.385.020(A)(5)(b)(1) shall be subject to minimum requirements No. 1 through No. 10.

c. New development and redevelopment that adds impervious area of greater than two thousand (2,000) square feet and that is more than five percent (5%) of a site shall comply with minimum requirements No. 1 through No. 10 for the new impervious surface.

d. An off-site analysis as described in the Stormwater Manual, unless exempted by Section 40.385.010(C)(4).

6. In addition, all redevelopment shall comply with the following:

a. Road-related projects that create or add five thousand (5,000) square feet or more of new impervious surface and the new impervious surface totals fifty percent (50%) or more of the existing impervious surface within the project limits, shall comply with minimum requirements No. 1 through No. 10 for new and replaced impervious surfaces. The project limits shall be defined by the physical length of the project and the width of the right-of-way.

b. Nonroad-related projects where the valuation of the proposed improvements exceeds fifty percent (50%) or more of the existing site tax assessment valuation of the existing site improvements, and the total of new plus replaced impervious surface is either five thousand (5,000) square feet or more in the urban area or five percent (5%) or more of the site in the rural area, shall comply with minimum requirements No. 1 through No. 10 for new and replaced impervious surfaces.

7. Drainage Structure Labeling and Signage. All catch basins and manholes capable of accepting stormwater shall be signed or stenciled in accordance with the Stormwater Manual.

8. Basin Plans.

a. Basin plans as addressed in minimum requirement No. 9 are strategies designed to protect and enhance surface and groundwater within a watershed.

b. A plan shall include but not be limited to recommendations for:

(1) Stormwater requirements for new development and redevelopment;

(2) Capital improvement projects;

(3) Land use management through identification and protection of critical areas, comprehensive land use and transportation plans, zoning regulations, site development standards, and conservation areas;

(4) Source control activities, to include public education and involvement, and business programs;

(5) Other targeted stormwater programs and activities, such as maintenance, inspections, and enforcement;

(6) Monitoring; and

(7) An implementation schedule and funding strategy.

c. To be valid, a basin plan must:

(1) Be stamped, signed and dated by a registered professional engineer licensed in the state of Washington;

(2) Be adopted by the board;

(3) Meet the requirements of Chapter 36.94 RCW and the SMMWW;

(4) Be formally adopted by all jurisdictions with responsibilities under the plan; and

(5) Be approved by the Department of Ecology.

In addition, all ordinances or regulations called for by the plan must be in effect.

d. The policies and standards in an adopted basin plan shall supersede the requirements of this chapter.

9. Regional Stormwater Facilities.

a. The county encourages the use of regional stormwater facilities.

b. If regional stormwater facilities are used to meet some or all of the requirements of this chapter, the following conditions shall be met:

(1) Stormwater runoff shall be transported from a project site to a regional stormwater facility through a pipe or manmade open channel conveyance system.

(2) If the regional stormwater facility does not yet exist, interim quantity control and treatment methods shall be used to meet the requirements of this chapter. All interim methods shall be approved in writing by the responsible official.

(3) The facility must have sufficient capacity to provide the treatment and quantity control specified in this chapter at the time of connection.

(4) 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.

c. Where a stormwater utility exists, a system development charge can be assessed for use of a regional stormwater facility.

10. Wetland Protection. If the county determines based on information in the preliminary stormwater plan, or information submitted for wetland review per Chapter 40.450, that the proposed project will degrade wetland function, then the applicant shall implement flow control or other measures to mitigate the adverse impacts of this alteration in accordance with the wetland hydrology protection guidelines in Volume I, Appendix 1-D of the SMMWW.

11. Off-site Analysis. If the county determines based on information in the preliminary stormwater plan that the proposed project will adversely impact off-site drainage systems, then the applicant shall implement additional flow control or other measures to mitigate those adverse impacts.

(Amended: Ord. 2009-12-01)

B. Water Quality Treatment.

1. General Standards.

a. If project 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 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.

b. Runoff treatment facilities shall be selected pursuant to Volume V, Chapter 2 of the SMMWW.

c. The stormwater treatment facilities shall be sized for the entire flow directed to them.

d. The following water quality management plans and local ordinances/regulations have been identified pursuant to Volume V, Chapter 2 of the SMMWW as having specific requirements for receiving waters:

(1) Total maximum daily load plans for Gibbons Creek and Salmon Creek; and

(2) Lake management plans for Lacamas and Round Lakes.

2. Phosphorus treatment is required in the Lacamas watershed above the dam at the south end of Round Lake for all project sites exceeding one (1) acre in size.

3. Experimental BMPs shall follow the guidelines for emerging technologies Volume V, Chapter 12 of the SMMWW.

4. Stormwater treatment facilities shall be maintained in accordance with the latest version of the Stormwater Facility Maintenance Manual.

5. Hydrologic analysis for runoff treatment design shall be in accordance with Volume III and Volume V, Chapter 4 of the SMMWW, with the following exceptions:

a. Table III-2.1, Hydrologic Soil Groups for Selected Soils in Washington State, is replaced by:

(1) The Soil Conservation Service Clark County Soil Survey published in 1972 and updated by the Natural Resources Conservation Service (NRCS);

(2) GIS soil maps of Clark County; or

(3) Washington Soil Survey data as available on the NRCS website.

In the case of conflicts, the more stringent soil group shall apply unless site-specific hydrological soil groups are developed by a registered soil scientist using criteria in the NRCS National Soils Handbook.

b. Appendix III-A, Isopluvial Maps for Design Storms, is replaced by Isopluvial Maps for Design Storms in Clark County.

C. Quantity Control.

1. General Standards.

a. No new development or redevelopment shall be allowed to materially increase or concentrate stormwater runoff onto an adjacent property or block existing drainage from adjacent lots.

b. All lots must be designed to provide positive drainage from the bottom of footings to an approved stormwater facility, unless a geotechnical report has been prepared stating that a footing drain is not required.

c. Detention facilities shall be functional prior to completion of site improvements (e.g., impervious surfaces). If permanent infiltration ponds are used for flow control during construction, these facilities shall be protected from siltation during the construction phase in accordance to the project SWPPP, including but not limited to temporary sedimentation ponds.

d. 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 for all streams and manmade channels within the county.

2. Pre-development Land Cover Requirements.

a. The pre-developed condition to be matched shall be the land cover condition existing at the time of the development application.

b. Where an approved basin plan exists, the land cover condition to be matched shall be commensurate with achieving a target flow regime identified in the study. If no land cover condition or target flow regime is identified, land cover condition to be matched shall be as required above.

c. This requirement is not applicable to project sites designed to retain all stormwater runoff on-site.

3. Design Methodology for Stormwater Infiltration Systems.

a. The design of stormwater infiltration facilities shall follow the requirements in Volume III, Section 3.3 of the SMMWW and the Stormwater Manual, except as revised herein, and the

Washington Department of Ecology Guidance for UIC Wells that Manage Stormwater. If a facility meets any part of the UIC definition in Section 40.385.010(D), the developer must register the UIC and provide proof of registration to the county prior to use.

b. The design shall follow the methodology in either the simplified or detailed approaches in Volume III, Section 3 of the SMMWW, except that the infiltration testing shall only be conducted using the methods described in the Stormwater Manual.

c. Infiltration receptor characterization shall include the installation of groundwater monitoring wells unless the highest groundwater level is demonstrated to be at least fifteen (15) feet below the proposed infiltration facility. These wells shall be installed and monitored during at least one (1) wet season within three (3) years prior to the date of final approval.

d. Test locations for performing infiltration tests as called for in the simplified method shall be performed as follows:

(1) One (1) or more infiltration tests shall be conducted at the location of each proposed infiltration facility; and

(2) At least one (1) test shall be conducted for each location where the soil characteristics significantly vary within the vicinity of proposed infiltration facilities.

e. Allowable methods for determining infiltration rates as called for in the simplified method (Section 3.3.4 of the SMMWW) are as follows:

(1) The single-ring falling head test as described in the Stormwater Manual; or

(2) A pilot infiltration test (PIT) conducted pursuant to Appendix III-D, Volume III of the SMMWW.

f. Groundwater. The base of all infiltration basins or trench systems shall be greater than five (5) feet above the seasonal high-water mark, bedrock (or hardpan) or other low permeability layer. A separation down to three (3) feet may be considered if the groundwater mounding analysis, volumetric receptor capacity, and the design of the overflow and/or bypass structures are judged by the county to be adequate to prevent overtopping and meet the site suitability criteria specified in the SMMWW.

g. Stormwater Infiltration Facility Setbacks.

(1) Stormwater infiltration facilities shall be set back according to Table 40.385.020-1.


Table 40.385.020-1. Stormwater Infiltration Facility Setbacks
Stormwater infiltration facility set back from:	Distance:
Drinking water wells1	One hundred (100) feet minimum
Building foundations	Twenty (20) feet minimum, upslope One hundred (100) feet minimum, downslope
Building foundations
Slopes equal to or greater than fifteen percent (15%)2	Fifty (50) feet minimum
Roof downspout infiltration systems	Ten (10) feet minimum from any structure or property line

1 Infiltration facilities upslope of drinking water supplies and within the one (1), five (5), and ten (10) year time of travel zones must comply with Chapter 40.410.

2 See Chapter 40.430 for steep slope and landslide hazard area setbacks.

(2) Setbacks may be reduced if a geotechnical report addresses potential impacts of trench phreatic surface on structures within twenty (20) feet of the proposed facility.

h. No permanent infiltration systems shall be allowed into service until:

(1) The entire contributing drainage area has received final stabilization; and

(2) Permanent water quality BMPs are in place and have been approved by the county.

i. Before acceptance of any infiltration facility by the county, the completed facility must be tested and monitored to demonstrate that the facility performs as designed. If the tested coefficient of permeability determined at the time of construction is at least ninety-five percent (95%) of the uncorrected coefficient of permeability used to determine the design rate, construction shall be allowed to proceed. If the tested rate does not meet this requirement, the applicant shall submit an additional testing plan to Clark County that follows the requirements in Chapter 2 of the Stormwater Manual. This plan shall address steps to correct the problem, including additional testing and/or resizing of the facility to ensure that the system complies with the provisions of this chapter.

j. A groundwater mounding analysis shall be conducted at all sites where the depth to seasonal groundwater table or low permeability stratum is less than five (5) feet or where the depth to seasonal groundwater table or low permeability stratum is less than fifteen (15) feet and the runoff to the infiltration facility is from more than one (1) acre of effective impervious surface. Groundwater modeling (mounding analysis) of the proposed infiltration facility shall be done using the design infiltration rate and the estimated maximum groundwater elevation determined for the proposed facility location.

D. Stormwater Facilities.

1. General.

a. Stormwater facilities shall be located in accordance with the county’s critical areas ordinances, Chapters 40.410 through 40.450.

b. Stormwater facilities, other than closed conveyance systems, shall be located in relation to existing and proposed on-site sewage system drainfields as follows:

(1) At least thirty (30) feet when downslope from the drainfield system.

(2) At least ten (10) feet when upslope from the drainfield system.

(3) At least one hundred (100) feet for infiltration and dispersion systems. This distance can be reduced upon submittal of a report prepared by a registered geotechnical engineer licensed in the state of Washington that provides evidence that neither system will be compromised by a closer proximity. All applicable state and federal regulations must still be followed.

c. Stormwater facilities, other than underground closed systems, 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 facility.

d. Sites used for stormwater facilities shall be owned by the applicant, county, or state.

(1) 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.

(2) 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.

e. Stormwater facilities other than underground closed systems 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.

2. Side Slopes. Side slopes of stormwater facilities shall be according to Table 40.385.020-2 and Section 40.385.020(D)(3).


Table 40.385.020-2. Stormwater Facility Side Slopes
All facilities with slopes flatter than or equal to three to one (3:1)	Allowed.
All facilities with slopes flatter than or equal to two to one (2:1)	Allowed, if:
	• Side slopes don’t need mowing; and
	• Erosion control and slope stability are provided.
Public facilities, vertical slopes	Allowed, if:
	• Perimeter has less than or equal to seventy-five percent (75%) vertical slopes;
	• Vertical slopes more than two (2) feet tall are fenced;
	• Maintenance access is adequate; and
	• Side slopes in a biofiltration treatment area are three to one (3:1) or flatter.
Private facilities, slopes steeper than three to one (3:1)	Allowed if:
	• Perimeter has less than or equal to seventy-five percent (75%) vertical slopes;
	• Vertical slopes more than two (2) feet tall are fenced;
	• Long-term erosion control is provided;
	• Side slopes in a biofiltration treatment area are three to one (3:1) or flatter; and
	• It is demonstrated that the facility can be adequately maintained.

3. Fencing.

a. Public stormwater treatment and runoff control facilities shall be fenced in accordance with Volume III of the SMMWW.

b. Fences are not required for private stormwater facilities, provided a hold-harmless agreement is provided to the county.

c. If the facility is not enclosed by a fence, the covers for all control structures, manholes, and catch basins shall be bolted in place.

d. A gate or lockable bollards shall be provided across any access road.

e. Wood board fences are not allowed.

E. Maintenance and Ownership.

1. County Ownership of Stormwater Facilities. 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. Stormwater facilities that 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. 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 properly maintained and in satisfactory condition. The responsible official shall 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) 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, or 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 the county’s Stormwater Facility Maintenance Manual pursuant to Section 13.26A.040.

(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) Following final acceptance for county ownership, the county shall maintain stormwater facilities.

b. Privately Owned Facilities.

(1) For stormwater facilities for which the county will not provide 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 standards in the county’s Stormwater Facility Maintenance Manual pursuant to Section 13.26A.040. The responsible official shall approve such arrangements prior to county approval of the final stormwater plan. Final plats shall include a note specifying the party(ies) responsible for long-term maintenance of stormwater facilities.

(2) The county shall 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.

(3) Easements or a covenant acceptable to the responsible official shall be provided to the county for purposes of inspection and maintenance of all privately maintained facilities. The minimum dimensions of easements are listed in the Stormwater Manual, and shall allow for access to all areas within the pond and drainage structures by standard maintenance equipment vehicles.

4. Recovering Costs of Stormwater Facilities.

a. The following costs associated with stormwater facilities may be recoverable through latecomers’ agreements (RCW 35.91.010):

(1) Over-sizing on-site facilities above their existing capacity or the capacity required for the proposed development; and

(2) A proportionate share of the total cost of off-site facilities.

b. If a stormwater utility exists, the costs for building or over-sizing a stormwater facility may be eligible as a credit against applicable system development charges.

5. Bonds and Insurance.

a. Performance Security. In lieu of completing required stormwater facilities within a preliminary plat prior to recording, the applicant may, with the approval of the county, 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 40.385.020(E)(3) 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.

b. Maintenance Security. In cases identified in Section 40.385.020(E)(3), a maintenance bond or other security acceptable to the responsible official, in the amount of ten percent (10%) of the project engineer’s construction cost, shall be posted and maintained throughout the two (2) year initial maintenance period for a stormwater facility.

(Amended: Ord. 2009-01-01)