(1) 
All surface and stormwater runoff from a proposed project to construct new, or to modify existing drainage facilities must discharge at the natural or otherwise legally existing location so as to not affect adjacent downstream properties. Discharge of runoff shall be of the similar quality, flow rate, and velocity which flowed from the site under existing conditions.
(2) 
Proposals which do not discharge at the natural location in order to address an existing or anticipated problem will be considered as a technical accommodation.
(3) 
Discharge from the development must produce no significant adverse impacts to existing, downstream drainage systems. If no conveyance system exists at the adjoining property line and the discharge was previously unconcentrated flow, then any concentrated flow must be conveyed to an acceptable discharge point. If existing topography does not provide for the natural downstream flow of surface waters, drainage easements must be secured from downstream property owners prior to Approval.
(Ord. 1161 § 6, 1998)
The Stormwater Plan submittal shall include an analysis of off-site conditions that may affect or be impacted by the new drainage system. These conditions may include, but are not limited to, ponding water, high and low flows, siltation, and erosion. The Applicant shall demonstrate that the proposed project will not aggravate an existing drainage problem or create a new drainage problem. An analysis of a minimum distance of ¼ mile downstream or upstream from the site shall be considered. If conditions warrant, the Administrator may require a more extensive analysis.
(Ord. 1161 § 6, 1998)
(1) 
Drainage easements shall be provided in a proposed development for all stormwater conveyance systems that are not located in public rights-of-way or tracts. Said drainage easements shall be granted to the parties responsible for providing on-going maintenance of the systems.
(2) 
Drainage easements through structures are not permitted.
(3) 
Stormwater facilities that are to be maintained by the County shall be located in public right-of-way, or separate tracts dedicated to the County. Approval and acceptance of these stormwater facilities must be granted by the County before dedication to the County is made.
(4) 
All runoff from impervious surfaces, roof drains, and yard drains shall be directed so as to not create a nuisance nor damage adjacent properties. Wording to this effect shall appear on the face of all final plats/Planned Unit Developments/binding site plans, and shall be contained in any required covenants.
(Ord. 1161 § 6, 1998)
(1) 
Stormwater runoff shall be limited to the peak rate of runoff under existing site conditions as follows:
(a) 
2 year, 24-hour duration design storm event.
(b) 
10 year, 24-hour duration design storm event.
(c) 
25 year, 24-hour duration design storm event.
(2) 
Detention shall not be required when discharge is directly to a lake of greater than 300 acres or directly to a major river listed in Figure 2-6.1 of the Washington State Department of Transportation Highway Runoff Manual, February 1995. The Project Engineer shall apply sound engineering judgment in recognizing instances where the risk of damages is such that a higher mean recurrence interval storm should be detained.
(Ord. 1161 § 6, 1998)
(1) 
The determination of existing and post-development runoff quantities can be analyzed using the following methods.
(a) 
The TR-55 method can be used to determine peak runoff flows for drainage basins less than 100 acres. This method is not acceptable for estimating detention storage volumes and designing detention facilities.
(b) 
The Santa Barbara Urban Hydrograph method can be used to determine peak runoff flows and quantities for detention, retention, and water quality systems.
(Ord. 1161 § 6, 1998)
(1) 
Identify and delineate the overall drainage basin for each discharge point from the project site under existing conditions.
(a) 
Identify existing land uses.
(b) 
Identify existing soil types using SCS soil survey, or soils analysis.
(c) 
Identify existing drainage features such as streams, conveyance systems, ponds, detention facilities, wetlands, ponding areas, depressions, and such.
(2) 
Select and delineate pertinent sub basins under existing conditions.
(a) 
Select homogeneous areas which have generally consistent soils, topography and land use.
(b) 
Select separate sub basin areas for on-site and off-site drainage.
(c) 
Select separate sub basin areas for major drainage features.
(3) 
Determine runoff parameters for each sub basin under existing conditions.
(a) 
Identify pervious and impervious areas.
(b) 
Select SCS curve numbers.
(c) 
Compute time of concentration.
(4) 
Determine allowable release rates for the appropriate design storms.
(a) 
Compute runoff hydrographs for each design storm from each sub basin under existing conditions.
(b) 
For sub basins which drain into an existing storage facility, route the hydrographs for each design storm through the existing facility. This computation will yield the attenuated version of the hydrograph which will be representative for the sub basin under existing conditions.
(c) 
Sum the appropriate sub basin hydrographs to obtain the total hydrograph for the design storm under existing conditions.
(5) 
Repeat Steps 2 through 4 for the proposed development conditions.
(6) 
Determine whether peak flow rate and velocity surface runoff under developed conditions meet those under existing conditions.
(a) 
If the peak runoff rate under proposed development conditions for the 25-year, 24-hour design storm is less than or equal to 0.2 cfs of the peak runoff rate under existing site conditions, then the concentrated runoff may be discharged through a dispersal trough or other dispersal system provided the Applicant can demonstrate that there will be no significant adverse impact to downhill properties or drainage systems.
(b) 
If the peak runoff rate under proposed conditions for the 25-year, 24-hour design storm exceeds 0.2 cfs of the peak runoff rate under existing conditions, then detention shall be required.
(7) 
Design and size the detention facility using level pool routing.
(a) 
Use the developed runoff hydrograph and an iterative process to size the detention facility to meet the required discharge requirements. To meet performance, the developed runoff hydrograph when routed through the new detention facility must not yield an outflow rate greater than the discharge rate for that design storm under existing conditions.
(8) 
Design and size the new conveyance system for the proposed development. At minimum, conveyance systems shall be designed for the 25 year, 24-hour storm.
(Ord. 1161 § 6, 1998)
(1) 
Stormwater discharges must be treated and controlled by a suitable combination of BMP's. These BMP's are outlined in DOE's Manual.
(2) 
The following technical requirements apply to the construction of all stormwater facilities:
(a) 
Stormwater detention or retention facilities shall be sized according to computational requirements in Sections 6.31 and 6.32. Berm embankments greater than 6 feet in height, as measured through the center of the berm, shall require design by a qualified Geotechnical Engineer licensed in the State of Washington.
(b) 
Emergency overflows shall be provided at all stormwater storage facilities. The impacts of a system failure at the 100 year, 24-hr storm under developed conditions shall be analyzed to determine on-site and off-site effects. Most likely, an overflow spillway will require the analysis of a broad crested weir. The overflow spillway shall be armored and located to direct overflows safely toward the downstream conveyance system.
(c) 
All detention or retention ponds, or infiltration ponds should be placed out of the 100-year flood plain or special flood hazards area referenced in Ch. 15.35 LCC, and shall have a minimum freeboard of one foot above the maximum design water surface elevation of the 25 year event under post development conditions.
(d) 
All required detention or retention facilities must be constructed and in operation prior to building construction and paving.
(e) 
Access roads shall have a minimum easement of 20 feet. The minimum road width shall be 10 feet, surfaced with at least six inches of gravel, and grades shall not exceed 15% without the Administrator's approval.
(f) 
No facilities shall be located in the public right-of-way without the written consent of the Administrator.
(g) 
Adequate measures shall be incorporated into the design facilities to protect the public from the inherent hazards of such facilities including health risks associated with a failure of the system. All facilities shall be designed in such a manner that system failure will not cause flooding that threatens the safety of the public. Special protective measures may be required by the Administrator to safeguard public health and safety.
(h) 
Detention and retention facilities may include, but are not limited to, open basins, wet ponds, constructed wetlands, parking lot ponding, underground storage vaults, gravel filled trench storage, infiltration trenches, combinations of the above, or other facilities as outlined in DOE's Manual.
(i) 
Side slopes for earth lined ponds should not exceed 3 horizontal to one vertical (3:1). Flatter slopes are encouraged. All pond slopes shall be stabilized to prevent slope failure. Applicants are encouraged to install fencing, impenetrable vegetation, or other determents for slopes steeper than 3:1 to provide a barrier to entry of small children or others who may be incapable of climbing out of the facility.
(j) 
Constructed and graded detention facilities shall be smoothly or evenly sloped no less than 0.005 ft/ft (0.5% slope) toward the outlet.
(k) 
The facilities should be designed and constructed to minimize the frequency and difficulty of future maintenance. The Applicant should evaluate the range of potential problems that may occur with the system and determine the corrective action. An Operation and Maintenance (O&M) Manual to be prepared by the Applicant's engineer is required. The responsibility to implement the O&M manual shall be identified by the Applicant and recorded. All facilities shall be accessible for maintenance and operation.
(Ord. 1161 § 6, 1998)
(1) 
Infiltration basins may be selected, designed, and maintained according to the latest edition of DOE's Manual. Suggested minimum standards shall be Lewis County's minimum standard. Texture class shall be based on the SCS classified soil type or by field analysis. Soils analysis, including test borings, are necessary for infiltration facilities.
(2) 
The Applicant shall ensure that the soils information utilized for design is consistent with the actual soil conditions upon completion of construction, prior to final project approval.
(Ord. 1161 § 6, 1998)
(1) 
Lewis County requires that such permitted activities address the development's impact on stormwater quality as well as quantity. The objectives of providing stormwater treatment BMP's are, for both surface and ground waters, to protect the quality and quantity, and to allow attainment of the designated uses.
(2) 
Stormwater BMP's are required as part of all development activity. A selection process to determine if a BMP is appropriate for a given use and site outlined in DOE's Manual should be used to determine appropriate water quality BMP's. BMP's which are specified in DOE's Manual, and those set forth in this Section have been pre-approved for use.
(3) 
The County reserves the right to require an engineering analysis of any proposed practice or design before Approval. Plans not specifically pre-approved are subject to the review and approval by the Administrator.
(Ord. 1161 § 6, 1998)
(1) 
The stormwater system of Lewis County may only be used to convey stormwater runoff. A stormwater system shall mean all natural and man-made systems which function together or independently to collect, store, purify, discharge, and convey stormwater. All stormwater facilities and natural systems such as streams and creeks which convey, store, infiltrate, or divert stormwater are included.
(2) 
Illicit discharges to the stormwater system are prohibited. No person shall use this system, directly or indirectly, to dispose of any solid or liquid matter. No person shall make any connection to the stormwater system which could result in the discharge of polluting matter. Connections to the stormwater system from the interiors of structures; and for any purpose other than to convey stormwater or groundwater are prohibited and shall be eliminated.
(Ord. 1161 § 6, 1998)
Owners and operators of oil/water separators, wet ponds, biofiltration/biofilter facilities, sediment and erosion control systems, infiltration systems, and any other pollution control devices shall operate and maintain such control devices to assure that performance meets the intended level of pollutant removal.
(Ord. 1161 § 6, 1998)
The following discharges are exempt from the provisions of this section:
(1) 
The regulated effluent from any commercial or municipal facility holding a valid state or federal wastewater discharge permit for such effluent.
(2) 
Acts of nature not compounded by human negligence.
(3) 
Properly operating on-site permitted domestic sewerage systems.
(4) 
Properly applied agricultural chemicals and materials.
(Ord. 1161 § 6, 1998)
(1) 
Runoff treatment BMP's shall be sized to intercept and treat the water quality storm, identified as the 6-month, 24-hour storm event. A high priority for treatment shall be to infiltrate as much as practicable of the design storm runoff. Ground water recharge is only recommended when site conditions are appropriate and ground water quality will not be impaired.
(2) 
Direct discharge of polluted, untreated stormwater to ground water is prohibited.
(3) 
Runoff treatment BMP's shall be selected, designed, and maintained according to the latest edition of DOE's Manual.
(Ord. 1161 § 6, 1998)
(1) 
Streambank erosion control BMP's are designed to prevent or control excessive erosion to streams. When an adopted plan or completed engineering study indicates a receiving water body is experiencing chronic streambank erosion problems, then BMP's to reduce streambank erosion shall be applied.
(2) 
BMP's for peak runoff rates shall be selected, designed, and maintained according to the latest edition of appropriate technical references in LCC § 15.45.120.
(Ord. 1161 § 6, 1998)
Erosion and sedimentation control BMPs are designed to prevent, minimize, or capture sediments and pollutants released during construction or under post project conditions. Some of these BMP's may be to trap sediments before it reaches the storm drainage detention system; to prevent or minimize vehicular tracking away from the project site; to address effects of seepage and slope failure; to construct energy dissipaters, bank reinforcement, and retaining walls; to landscape; or to stabilize exposed soils. These BMP's may be either source control, runoff treatment, or streambank control BMP's which can be selected, designed, and maintained according to the appropriate technical references in LCC § 15.45.120.
(Ord. 1161 § 6, 1998)
(1) 
Pursuant to the Critical Areas Chapter as set forth under Ch. 17.38 LCC, wetlands shall not be used for the treatment of stormwater except when the functions and values of the existing wetland are enhanced by the creation of a constructed wetland and not materially degraded by its use in stormwater management.
(2) 
Wetland quality control BMP's are designed to prevent or control the degradation of water quality entering regulated wetland eco-systems. The requirements of this Section apply only to situations where stormwater discharges through a conveyance system into a wetland.
(3) 
Discharges to wetlands shall approximate the saturation period and flows of existing site conditions to the extent necessary to protect the important characteristic uses of the wetland.
(4) 
Prior to discharging to a wetland, alternative discharge locations shall be evaluated. Natural water storage and infiltration opportunities outside the wetland shall be optimized to the extent practicable. Proposed discharges to wetlands shall be subject to review and compliance under the applicable provisions of Ch. 17.38 LCC.
(Ord. 1161 § 6, 1998)