3.2
For minimum requirement purposes, runoff leaving the project site is classified based on the type of receiving water and/or drainage system into which the project site discharges. The minimum requirements vary considerably based on these classifications. The applicant must thus determine the receiving water or natural discharge point for the stormwater runoff from the project site (e.g., wetland, lake, creek, salt water, or Underground Injection Control [UIC]) and the type of downstream conveyance. In addition, the applicant must also note the sequence of the discharge. For example, projects discharging to a drainage system within a creek basin that then discharge to a designated receiving water must meet the requirements applicable to creek basins.
An overview of the types of receiving waters and drainage systems is provided below:
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Wetlands are designated under Chapter 19.200 KCC.
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Flow Control-Exempt Receiving Waters listed in Appendix I‑A of the Ecology Manual are approved by Ecology as having sufficient capacity to receive discharges of drainage water. Stormwater discharges to salt water bodies, including Puget Sound, and other large rivers and lakes listed in Appendix I‑A of the Ecology Manual are exempt from the flow control requirement provided that the project meets all restrictions included in Appendix I‑A of the Ecology Manual.
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Non-Flow Control-Exempt Receiving Waters include creeks, lakes, or other receiving water bodies not listed in Appendix I‑A of the Ecology Manual.
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Critical Drainage Areas refers to those areas designated in Chapter 12.28 KCC (Critical Drainage Areas), which have a high potential for stormwater quantity or quality problems. In order to mitigate or eliminate potential drainage-related impacts on critical drainage areas, the director may require drainage improvements in excess of those required by the minimum requirements.
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Conveyance Systems are both natural and manmade systems that collect, contain, and provide for the flow of surface and stormwater from the highest points on the land down to a receiving water. The natural elements of the conveyance system include swales and small drainage courses, streams, rivers, lakes, and wetlands. The human-made elements of the conveyance system include gutters, ditches, pipes, channels, and most retention/detention BMPs. While downstream conveyance systems do not affect minimum requirements applicability, the applicant must demonstrate that the proposed project would not aggravate existing problems or create new problems for those systems (See Vol II–4.7 Downstream Analysis).
Receiving waters may also have specific management plans that have established specific requirements. Such management plans could potentially affect how the minimum requirements shall be applied to a given project (see Vol I–3.7 Step 7 – Determine Which Minimum Requirements Apply). Examples of plans to be aware of include:
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Watershed or Basin Plans can be developed to cover a wide variety of geographic scales (e.g., Water Resource Inventory Areas, or subbasins of a few square miles), and can be focused solely on establishing stormwater requirements (e.g., “Stormwater Basin Plans”), or can address a number of pollution and water quantity issues, including urban stormwater (e.g., Puget Sound Non-Point Action Plans).
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Water Clean-Up Plans establish a Total Maximum Daily Load (TMDL) of a pollutant or pollutants in a specific receiving water or basin, and to identify actions necessary to remain below that maximum loading. The plans may identify discharge limitations or management actions (e.g., use of specific treatment facilities) for stormwater discharges from new and redevelopment projects.
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Groundwater Management Plans (Wellhead Protection Plans) protect ground water quality and/or quantity, these plans may identify actions required of stormwater discharges.
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Lake Management Plans are developed to protect lakes from eutrophication due to inputs of phosphorus from the drainage basin. Control of phosphorus from new development is a likely requirement in any such plans.