Chapter 12.20
STORM WATER MANAGEMENT
Sections:
12.20.010 Redevelopment activities.
12.20.020 Approved hydrological methods for design.
12.20.030 Storm water quantity control – Engineered flow control.
12.20.035 Storm water quantity control – Prescriptive flow control.
12.20.040 Storm water quality control.
12.20.050 Illicit discharges.
12.20.060 Experimental best management practices.
12.20.070 Incorporation into storm water quantity control facilities.
12.20.080 Minimum requirements – Major developments.
12.20.090 Storm water conveyance facilities.
12.20.100 Easements, tracts and covenants.
12.20.110 Wetlands.
12.20.120 Regional facilities.
12.20.130 Basin planning.
12.20.010 Redevelopment activities.
Where redevelopment activities meet the definition of a major development, the requirements of this chapter shall apply to that portion of the site that is being redeveloped. In addition, where one or more of the following conditions exist, the requirements of this chapter shall apply, to the maximum extent practicable, for the entire site, including adjoining parcels, if they are part of the project:
(1) Existing sites greater than one acre in size with fifty percent or more impervious surface;
(2) Sites that discharge to a receiving water that has a documented water quality problem. Subject to local priorities, a documented water quality problem includes, but is not limited to, water bodies:
(A) Listed in reports required under Section 305(b) of the Clean Water Act, and designated as not supporting beneficial uses,
(B) Listed under Section 304(1)(1)(A)(i), 304(1)(1)(A)(ii), or 304(1)(1)(B) of the Clean Water Act as not expected to meet water quality standards or water quality goals,
(C) Listed in Washington State’s Nonpoint Source Assessment required under Section 316(a) of the Clean Water Act that, without additional action to control nonpoint sources of pollution, cannot reasonably be expected to attain or maintain water quality standards;
(3) Sites where the need for additional storm water control measures have been identified through a basin plan, watershed ranking process, or through Growth Management Act planning.
(Ord. 199 (1996) § 7.05, 1996)
12.20.020 Approved hydrological methods for design.
Estimation of peak storm water runoff rates used in the design of storm water quantity control facilities shall utilize hydrograph methods of analysis approved by the director. The design of storage facilities that are a part of storm water quantity control facilities shall be designed using methods approved by the director.
(Ord. 199 (1996) § 7.10, 1996)
12.20.030 Storm water quantity control – Engineered flow control.
The following minimum requirements for storm water quantity control shall apply to all land developments that meet the definition of a major development and create ten thousand square feet or greater of new impervious surface:
(1) All surface water and storm water entering the development site in its pre-development state shall be received at the naturally occurring or otherwise legally existing locations. All surface water and storm water leaving the development site shall be discharged at all times during and after development at the naturally occurring or otherwise legally existing locations so as not to be diverted onto or away from adjacent downstream properties, except, diversion which will correct an existing manmade downstream problem may be permitted by the director. For the purposes of this title, “naturally occurring location” means the location of those channels, swales and pre-existing and established systems as defined by the first documented topographic contours existing for the subject property, either from maps or photographs, site inspections, decisions of a court of law, or other means determined appropriate by the director.
(2) The post-development peak storm water discharge rates from the development site for the two-, ten- and one-hundred-year, twenty-four-hour duration storm events shall at no time exceed the pre-development peak storm water runoff rates for the same design storm events, except as expressly permitted by this title. Also, where storm water directly or indirectly discharges to open channels or streams, streambank erosion protection is required; the post-development peak storm water discharge rate from the development site for the two-year, twenty-four-hour duration storm event shall not exceed fifty percent of the pre-development peak storm water runoff rate for the same design storm event. The director may require that runoff from a development site be controlled for additional design storm events.
(3) Closed depressions shall be analyzed using hydrograph routing methods. Infiltration shall be addressed where appropriate. If a proposed project will discharge runoff to an existing closed depression that has greater than five thousand square feet of water surface area at overflow elevation, the following requirements must be met:
(A) Case 1: The pre-development one-hundred-year, seven-day and twenty-four-hour duration design storms from the drainage basin tributary to the closed depression are routed into the closed depression using only infiltration as outflow. If the design storms do not overflow the closed depression, no runoff may leave the site for the same storm events following development of a proposed project. This may be accomplished by excavating additional volume in the closed depression subject to all applicable requirements. If a portion of the depression is located off the project site, impacts to adjacent properties shall be evaluated.
(B) Case 2: The pre-development one-hundred-year, seven-day and twenty-four-hour duration design storm events from the drainage basin tributary to the closed depression are routed to the closed depression using only infiltration as outflow, and overflow occurs. The closed depression shall then be analyzed as a detention/infiltration pond. The required performance, therefore, shall not exceed the pre-development runoff rates for fifty percent of the two-year and one hundred percent of the ten-year and one-hundred-year, twenty-four-hour duration and one-hundred-year, seven-day duration design storms. This will require that a control structure, emergency overflow spillway, access road, and other applicable design criteria be met. If the facility will be maintained by Kitsap County, the closed depression shall be placed in a dedicated tract. If the facility will be privately maintained, the tract shall be located within a drainage easement. If a portion of the depression is located off the project site, impacts to adjacent properties shall be evaluated.
(C) Case 3: When a proposed project is contributory to a closed depression located off-site, the volume of runoff discharged may not be increased for the two-, ten- and one-hundred-year, twenty-four-hour duration, and the one-hundred-year, seven-day duration storm events. The exception to this requirement is in the case where discharge would not result in an increase in water surface elevation of greater than 0.01-foot for the one-hundred-year storm events.
(4) Land developments shall provide storm water quantity control facilities designed to meet, as a minimum performance standard, the requirements of this chapter, except in the following circumstances:
(A) The development site discharges directly into Puget Sound, or directly into the tidally influenced areas of rivers and streams discharging into Puget Sound, where runoff quantity control is not required by other governmental agencies and streambank or shoreline erosion will not occur.
(B) The development site discharges to a regional storm water facility approved by the director to receive the developed site runoff.
(C) The development site discharges to a receiving body of water (lake, wetland, etc.) where it can be demonstrated by the applicant, to the satisfaction of the director, that storm water quantity control is not warranted.
(5) In the event that conditions downstream from a proposed development site are determined by the director to be exceptionally sensitive to potential storm water discharges from the subject site, the director may require a factor of safety be applied to the total retention/detention storage volume and/or a reduction of allowable storm water release rates.
(6) Submittals for all proposed development projects shall include an analysis of downstream water quantity impacts resulting from the project and shall provide for mitigation of these impacts. The analysis shall extend a minimum of one-fourth of a mile downstream from the project. The existing or potential impacts to be evaluated and mitigated shall include, but not be limited to, excessive streambank erosion, flooding, surcharging of existing closed drainage conveyance facilities, discharge to closed depressions, and discharge to existing off-site runoff control facilities.
(7) Retention facilities and open storm water quantity control facilities shall not be located in dedicated public road rights-of-way.
(8) Reasonable access for maintenance, as determined by the director, shall be provided to all storm water facilities.
(9) As the first priority, streambank erosion control BMP’s shall utilize infiltration to the fullest extent practicable, only if site conditions are appropriate and ground water quality is protected. Streambank erosion control BMP’s shall be selected, designed and maintained according to the manual. Streambank erosion control BMP’s shall not be built within a natural vegetated buffer, except for necessary conveyance systems as approved by the Kitsap County department of community development.
(10) Where storm water detention is proposed to meet storm water quantity controls, volume correction factors as outlined in the manual shall be applied to increase pond size.
(Ord. 375 (2007) § 2, 2007: Ord. 199 (1996) § 7.20, 1996)
12.20.035 Storm water quantity control – Prescriptive flow control.
All minor developments creating greater than 2,000 square feet of new impervious surface and major developments creating less than 10,000 square feet of new impervious surface shall:
(1) Implement either individual downspout infiltration or roof downspout dispersion per the Kitsap County Stormwater Design Manual to the maximum extent possible; or
(2) Discharge to a regional water quantity control facility designed to receive the developed site runoff.
(Ord. 375 (2007) § 4, 2007)
12.20.040 Storm water quality control.
Water quality best management practices (BMP’s) shall be used to the maximum extent practicable to control pollution in storm water. Water quality BMP’s shall be used to comply with the standards of this title, including those contained in the manual. Construction and post-development water quality BMP’s shall be utilized for all major development activities. Said water quality BMP’s shall provide runoff water quality treatment for all storm events with intensities less than or equal to the water quality design storm event, as defined in Section 12.20.080(2).
(Ord. 199 (1996) § 7.30, 1996)
12.20.050 Illicit discharges.
Illicit discharges, as described in Section 12.30.020, or illicit connections to a storm water drainage system, as described in Section 12.30.030, are prohibited.
(Ord. 199 (1996) § 7.32, 1996)
12.20.060 Experimental best management practices.
In those instances where appropriate best management practices are not in the manual, experimental BMP’s may be considered. In an effort to improve storm water quality technology, experimental BMP’s are encouraged as a means of solving problems in a manner not addressed by the manual. Experimental BMP’s must be approved by the director. The director may require that the performance of experimental BMP’s be monitored to document their effectiveness for future use.
(Ord. 199 (1996) § 7.33, 1996)
12.20.070 Incorporation into storm water quantity control facilities.
Water quality BMP’s may be incorporated into the design of storm water quantity control facilities where appropriate.
(Ord. 199 (1996) § 7.34, 1996)
12.20.080 Minimum requirements – Major developments.
The following minimum requirements for storm water quality control shall apply to all land developments that meet the definition of a major development:
(1) Source Control of Pollution. Source control BMP’s shall be applied to all projects to the maximum extent practicable.
(2) Storm Water Treatment BMP’s. Treatment BMP’s shall be sized to capture and treat developed runoff from the water quality design storm, defined as the six-month, twenty-four-hour duration storm event. For the purpose of this title, the precipitation from a six-month, twenty-four-hour storm event shall be considered equivalent to sixty-four percent of the precipitation from a two-year, twenty-four-hour storm event. All treatment BMP’s shall be selected, designed and maintained according to the manual.
Storm water treatment BMP’s shall not be built within a natural vegetated buffer, except for necessary conveyance systems as approved by the Kitsap County department of community development.
All major developments shall provide treatment of storm water discharge utilizing wetponds, bioretention facilities, dispersion, and/or biofiltration BMP’s. Other water quality BMP’s may only be substituted subject to the granting by the director subject to the provisions of the Stormwater Design Manual.
(3) Wetponds or bioretention facilities shall be required for development sites with greater than five acres of new impervious surface subject to motor vehicle use, which: (a) discharges directly to a regional facility, receiving body of water, or closed depression without providing on-site storm water quantity control; or (b) discharges directly or indirectly to a Class 1, 2 or 3 stream, or a Class 1 or 2 wetland within one mile downstream of the site.
(4) Presettling Basin. All storm water, prior to discharge to a facility designed to utilize infiltration, shall pass through an appropriate storm water treatment BMP designed to remove suspended solids.
(5) Water Quality Sensitive Areas. Where the director determines that these major development minimum requirements do not provide adequate protection of water quality sensitive areas, either on-site or within the drainage basin in which the development is located, more stringent controls shall be required to protect water quality.
An adopted and implemented basin plan may be used to develop requirements for specific water quality sensitive areas.
(6) Downstream Analysis and Mitigation. All major developments shall conduct an analysis of downstream water quality impacts resulting from the project and shall provide for mitigation of these impacts. The analysis shall extend a minimum of one-fourth of a mile downstream from the project. The existing or potential impacts to be evaluated and mitigated shall include excessive sedimentation, streambank erosion, discharges to ground water contributing or recharge zones, violations of water quality standards, and spills and discharges of priority pollutants.
(7) Oil/Water Separators. All storm water from paved areas subject to motor vehicle traffic shall flow through a spill-containment type oil/water separator prior to discharge.
Development sites that include use, storage or maintenance of heavy equipment, and those development sites that include storage or transfer of petroleum products, shall utilize appropriately sized API or CPS-type oil/water separators or a Washington State Department of Ecology approved equivalent.
(Ord. 375 (2007) § 3, 2007: Ord. 199 (1996) § 7.35, 1996)
12.20.090 Storm water conveyance facilities.
(a) All proposed developments must provide on-site storm water conveyance facilities having sufficient capacity to convey, without flooding or otherwise damaging existing or proposed structures, the post-development peak storm water runoff rate resulting from a one-hundred-year, twenty-four-hour duration storm event, plus any existing upstream runoff that will be conveyed through the development site.
(b) Estimation of peak storm water runoff rates used in the design of water conveyance facilities shall use either the rational method or a hydrograph method of analysis accepted by the director.
(c) Existing drainage ways and/or other conveyance facilities downstream from proposed developments that are identified within the scope of the downstream portion of the off-site drainage analysis, shall have sufficient capacity to convey, without flooding or otherwise damaging existing or proposed structures, the post-development peak storm water discharge for the twenty-four-year storm event. All newly constructed downstream drainageways and/or conveyance facilities shall have sufficient capacity to convey the post-development peak storm water discharge for the one-hundred-year storm event. Downstream improvements or additional on-site storm water quantity control measures shall be provided to eliminate any potential downstream flooding or other damage that may occur following completion of the proposed development. The director has the authority to waive the requirement for downstream improvements.
(d) Drainage through closed conveyance structures such as pipes shall not discharge directly onto the surface of a public road.
(Ord. 199 (1996) § 7.40, 1996)
12.20.100 Easements, tracts and covenants.
(a) Drainage easements shall be provided in a proposed development for all storm water conveyance systems that are not located in public rights-of-way or tracts. The drainage easements shall be granted to the parties responsible for providing on-going maintenance of the systems.
Drainage easements through structures is not permitted.
(b) Storm water facilities that are to be maintained by Kitsap County, together with maintenance access roads to the facilities, shall be located in public right-of-way, separate tracts dedicated to Kitsap County, or drainage easements located in designated open space. The exception is for storm water conveyance pipes that may be located within easements on private property, provided that all catch basins can be accessed without entering private property.
(c) All runoff from impervious surfaces, roof drains and yard drains shall be directed so as not to adversely affect adjacent properties. Wording to this effect shall appear on the face of all final plats/PUD’s, and shall be contained in any covenants required for a development.
(Ord. 199 (1996) § 7.50, 1996)
12.20.110 Wetlands.
The following requirements apply only to situations where storm water discharges directly or indirectly into a wetland, and must be met in addition to meeting the requirements in major development minimum requirement, Section 12.20.080(2), Storm Water Treatment BMP’s:
(1) Storm water discharges to wetlands must be controlled and treated to the same extent as all other discharges, with the goal of meeting State Water Quality and Groundwater Quality Standards.
(2) Discharges to wetlands shall maintain the hydroperiod and flows of pre-development site conditions to the extent necessary to protect the characteristic functions of the wetland. Prior to discharging to a wetland, alternative discharge locations shall be evaluated, and natural water storage and infiltration opportunities outside the wetland shall be maximized.
(3) Created wetlands that are intended to mitigate for loss of wetland acreage, function and value shall not be designed to also treat storm water.
(4) In order for constructed wetlands to be considered treatment systems, they must be constructed in areas which are not designated as wetland or wetland buffer or in other areas which are not in conflict with designated critical areas and associated buffers, and they must be managed for storm water treatment. If these systems are not managed and maintained in accordance with the manual for a period exceeding three years, these systems may no longer be considered constructed wetlands.
(5) Wetland BMP’s shall not be built within a natural vegetated buffer, except for necessary conveyance systems as approved by the Kitsap County department of community development.
(Ord. 199 (1996) § 7.60, 1996)
12.20.120 Regional facilities.
When the director has determined that the public would benefit by the establishment of a regional storm water facility which would serve as an alternative to the construction of separate on-site drainage facilities, the director may recommend to the board that a regional storm water facility be constructed which would serve more than one development in providing storm water quantity and/or quality control. In the event that a regional storm water facility is required by the board, such a regional storm water facility shall be located outside of fish-bearing streams, unless otherwise accepted by the Washington State Department of Fish and Wildlife. All future developments constructed on lands designated by the board to be served by the regional facility shall, at the time of issuance of a site development activity permit for a development, be required to contribute a fair share to the cost of land purchase, design and construction of the regional facility. In the event that a proposed regional storm water facility is not yet in operation at the time of completion of construction of a development that is to be served by the regional facility, the applicant for the development shall be required to provide temporary storm water quantity and quality controls. Temporary quantity and quality controls may be constructed in temporary easements, rather than in separate tracts.
(Ord. 199 (1996) § 7.70, 1996)
12.20.130 Basin planning.
An adopted and implemented basin plan tailored to a specific basin may be used to develop requirements for source control, storm water treatment, streambank erosion control, wetlands and water quality sensitive areas. Adopted and implemented watershed-based basin plans may be used to modify any or all of the minimum requirements for storm water quantity or quality control addressed in this title, provided, that the level of protection for surface or ground water achieved by the basin plan will equal or exceed that which would otherwise be achieved by implementation of the provisions of this title in the absence of a basin plan. Basin plans shall evaluate and include, as necessary, retrofitting of BMP’s for existing development and/or redevelopment in order to achieve watershed-wide pollutant reduction goals. Standards developed from basin plans shall not modify any of the above requirements until the basin plan is formally adopted and fully implemented by Kitsap County.
(Ord. 199 (1996) § 7.80, 1996)