Proposed Changes to Washington State Municipal Stormwater Permits May Have Far-Reaching Impacts


Research conducted throughout the Puget Sound region indicates a connection between land clearing activities and adverse impacts to stream hydrology and instream habitat quality. The inclusion of salmon on the endangered species list has led to increased stormwater management and land development regulations, with broad implications throughout the region. To further strengthen stormwater and development controls, the Washington State Department of Ecology (Ecology) is currently updating its Nationwide Pollutant Discharge Elimination System (NPDES) stormwater permitting process. The new NPDES Phase I and Western Washington Phase II permits, which go into effect August 1, 2013, take a significant step towards comprehensive stormwater management and will make Washington the first state to require low impact development (LID).

Low impact development is a comprehensive approach to land use planning intended to minimize or mitigate the impact development has on the natural hydrologic regime and associated water related ecosystems. The new permits address LID at three scales: (1) Site Development; (2) Revised Development Codes; and (3) Watershed Planning. At the site development scale, the 2012 Stormwater Manual (soon to be a requirement) provides a more restrictive list of presumptive BMPs for permit approval. These BMPs will prioritize infiltration, dispersion, and bioretention particularly for small sites. In addition, the existing flow control standard has been supplemented with an LID performance standard requiring that post-developed flow rates match eight percent of the two-year pre-developed peak flow rate; this targets much lower and more frequent storms and will apply to all sites greater than 2,000 square feet of hard surfaces.

The new permit requires that each jurisdiction review and revise development codes to make LID the “preferred and commonly used approach for site development” and references the Puget Sound Partnership Manual for guidance on revising local development codes. The permit does not clarify the extent to which revisions to development codes, rules, and standards should be made, which leaves room for uncertainty and interpretation. Lastly, Phase I permittees (incorporated cities with a population over 100,000 and unincorporated counties with populations of more than 250,000) will be required under the new permit to identify a watershed and lead a watershed planning process, intended to provide a more holistic approach to land use and stormwater planning. This is an important but substantial new effort. Phase II jurisdictions (small municipalities with separated stormwater sewer systems) located within the watersheds selected for this planning process will also be required to participate.

Washington isn’t the only state increasing regulations, recent Phase I permits issued in Oregon are requiring a hydromodification assessment to determine susceptibility and risk of stream erosion and the development of strategies to mitigate these impacts. Jurisdictions must demonstrate how LID and adaptive management are generally encouraged in developing stormwater management programs. Hydromodification Management Plans (HMPs) are also required for Phase I and many Phase II permits issued in California. These plans must also include hydromodification assessment procedures based on stream susceptibility, although susceptibility classifications and lower flow duration limits differ by permit. Permittees are also required to develop monitoring programs to assess the effectiveness of HMP implementation. For each permit, LID and adaptive management components have also been incorporated into Water Quality Improvement Plans. Eyes are on the state of Washington as these NPDES changes are developed and implemented. The impacts of these permit changes and mandatory LID development could be far-reaching.

The proposed permitting process has not been without controversy in the Pacific Northwest. Ecology has been holding workshops and considering comments from a wide range of potentially affected jurisdictions; concerns have been raised about potentially unintended impacts to land development patterns, as well as the cost of implementing and enforcing the new permits. Ecology will consider these and other comments as they work to finalize the proposed changes later in 2013.

For information about Washington’s changing stormwater regulations or help managing stormwater, contact Marjorie Wolfe at mwolfe [at] esassoc [dot] com or 503.226.8018.