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9611 SE 36th Street
Mercer Island, WA 98040
Phone: 206.275.7605
Email: epermittech@mercer. . .

Open to the public:
8:30 AM - 4:00 PM
on Mon, Tue, Thu, & Fri

10:00 AM - 4:00 PM on Wed

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Stormwater Management Standards


The City of Mercer Island has operated under a stormwater discharge permit (called NPDES permit) from the Washington State Department of Ecology similar to other cities in Washington with populations greater than 10,000.  The NPDES permit is authorized by the Federal Clean Water Act and implemented by the Washington State Department of Ecology (DOE).  The NPDES permit requires cities and counties to adopt stormwater management control standards.  Mercer Island's standards were last updated in 2017.  Mercer Island is one of 100 cities that must comply with the regulations. 

The NPDES Permit

The NPDES permit requires jurisdictions to update their stormwater control standards in the following ways:
  1. Updated Stormwater Manual- Adopt the 2014 Stormwater Management Manual for Western Washington.
  2. Low Impact Development- Review, revise and make effective, local development-related codes, rules, standards, or other enforceable documents to incorporate and require Low Impact Development (LID) principles and LID best mangement practices (BMPs).  The intent is to make LID the preferred and commonly used approach where feasible.

Overview of Stormwater Requirements

The following generally describes the stormwater requirements.  The full regulations (MICC 15.09) are available here.
Click here for a tip sheet on stormwater requirements for small projects creating < 5,000 square feet of new plus replaced hard surface.

Minimum Requirements
Rain garden- one LID technique
  1. Preparation of Stormwater Site Plans
    • Additional requirements for soil/infiltration testing and stormwater site plan submittals
  2. Construction Stormwater Pollution Prevention (SWPP/TESC)
    • Protect LID BMPs
  3. Source Control of Pollutants
  4. Preservation of Natural Drainage Systems and Outfalls
  5. On-Site Stormwater Management
    • LID is required where feasible (starting at a threshold of 2,000 sf new plus replaced hard surfaces or a net increase of 500 sf or more of impervious surface.)
    • Provide stormwater detention where LID is not feasible and the downstream drainage system includes a watercourse or there is a capacity problem identified in the conveyance system.  Refer to the detention sizing table.  A fee-in-lieu of detention may be allowed by the City Engineer in some cases.
Projects that create > 5,000 sf of new plus replaced impervious surface, must meet requirements 1 through 5 above and the following:
  1. Runoff Treatment (>5,000 sf PGHS***)
  2. Flow Control (using 2014 DOE Manual)
  3. Wetland Protection
  4. Operation and Maintenance
***PGHS= pollution generating hard surfaces (e.g driveways/parking)

LID is not feasible in many cases on Mercer Island given its many steep slopes, geologic hazards, shallow groundwater, and clay soils.  The Infeasibility Map considers steep slopes, landsilde hazard areas, erosion hazard areas, shallow groundwater and setbacks from landslide and erosion hazards.  The map depicts areas where LID is deemed not feasible.  Other appropriate stormwater management BMPs must be implemented to properly manage stormwater runoff. 


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