Stormwater Management Standards

The City of Mercer Island operates 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. 

National Pollutant Discharge Elimination System (NPDES) Permit

The NPDES permit requires jurisdictions to update their stormwater control standards in the following ways:

• Updated Stormwater Manual - Adopt the 2019 Stormwater Management Manual for Western Washington.
• 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 Management Practices (BMPs).  The intent is to make LID the preferred and commonly used approach where feasible.
  • Learn why LID is important.
  • Learn more about common LID practices.

Construction Stormwater General Permit (CSWGP)

A Construction Stormwater General Permit from the Washington State Department of Ecology is required prior to any construction activities.

1. If the construction site results in the disturbance of one or more acres (including off-site disturbance acreage related to construction-support activity) and discharges stormwater to surface waters of the state; or
2. Clearing, grading and/or excavation on sites smaller than one acre that are part of a larger common plan of development, if the common plan of development will ultimately disturb one acre or more and discharge stormwater to surface waters of the state.

For more information, refer to the Washington State Department of Ecology’s website.

Overview of Stormwater Requirements

The following generally describes the stormwater requirements. The full regulations can be found in Mercer Island City Code, Chapter 15.09. For more information on stormwater requirements for small projects creating < 5,000 square feet of new plus replaced hard surface, see the Tip Sheet for Small Projects Stormwater Requirements.

Minimum Requirements

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:

• Runoff Treatment (>5,000 sf PGHS***)
• Wetland Protection
• 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, landslide 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.