Development of a stormwater retrofit plan for Water Resources Inventory Area (WRIA) 9: Comprehensive needs and cost assessment and extrapolation to Puget Sound

A 2014 King County report projects the capital and maintenance costs of the stormwater treatment facilities that would be needed, within WRIA 9 and the Puget Sound region, to fully comply with the Clean Water Act. 

Executive summary

Stormwater from developed landscapes is one of the biggest threats to water quality and ecological health of the waters of Puget Sound, both fresh and marine. The goal of this project is to estimate the numbers of different types of stormwater treatment facilities, and their costs, necessary to rehabilitate stream flows and water quality to near-pre‐development conditions within the Water Resource Inventory Area (WRIA) 9. This report summarizes the tasks undertaken and the results of this effort, and also present a strategic framework for developing a stormwater retrofit program for WRIA 9.

The study area covers 278 square miles of the Green/Duwamish watershed and portions of the Central Puget Sound watershed that comprise WRIA 9, excluding the areas upstream of the Howard Hanson Dam and the city of Seattle. Within this area, stream flow and water quality were measured, and watershed hydrology and water quality models were developed. Using this information, stormwater facilities required to improve stream flow and water quality were modeled using a relatively new stormwater Best Management Practices (BMP) modeling and planning tool developed by the U.S. EPA – the SUSTAIN model (System for Urban Stormwater Treatment and Analysis Integration). Additional analyses were made on the impacts of population growth and economic activity on stormwater facility construction, the uncertainty associated with climate change impacts on stormwater facility sizing, and the presence of existing facilities.

This study was found that to improve stream flow and water quality to near-pre-development conditions in the study area, stormwater treatment facilities providing an average of 2.5 inches of storage for stormwater runoff generated from developed areas are needed in the project area. Relatively more storage would be needed in urban areas, ad relatively less storage would be needed in rural areas. The facilities include a combination of rain garden, roadside bioretention, cisterns and detention ponds. The storage needed and reflects the estimated 0.5 inches of storage provided from existing facilities already in the study area as well as additional estimated 10% storage need to accommodate impacts from climate change.

As required by existing stormwater regulations, many of the facilities needed to mitigate stormwater runoff from nearly one-half of the landscape are projected to be constructed as part of new and redevelopment over the next 30 years. However, is unlikely that many of the larger structures, such as regional detention ponds, would be constructed as part of a single family residential redevelopment due to the size of the thresholds required to be exceeded during redevelopment and due to site feasibility considerations. Another uncertainty to relying on new and redevelopment for stormwater facility construction is associated with potential future changes in stormwater requirements for redevelopment in urban centers. To address concerns raised about the costs of stormwater requirements relative to the environmental benefits in the context of overall urban planning goals for the Puget Sound region, the Washington State Department of Commerce has assembled a committee to identify approaches to managing stormwater in infill areas (Gates 2013).

Building all stormwater facilities required to improve stream health in WRIA 9 within 30 years would necessitate public programs taking aggressive action to

  1. Strengthen stormwater requirements during new and redevelopment to lower thresholds requiring stormwater facilities and to require fee-in-lieu if complete stormwater mitigation is not achieved onsite,
  2. Build regional facilities,
  3. Retrofit all roads and highways,
  4. Retrofit all other non-forested lands not redeveloped within the next 30 years, and
  5. Operate and maintain public facilities and inspect private facilities.

Construction of these facilities would cost a public program about $210M per year in capital costs (2013 dollars). Public program operating costs would increase annually as more facilities are built. If all anticipated facilities were built in 2013, the annual operating cost would be up to $650M per year. The annual operating costs are based on the estimated number of units needed; if smaller numbers of larger facilities are constructed than was modeled, then the annual operating costs may be lower than estimated. A large fraction of these operating costs would be associated with inspection and enforcement for private facilities. Given these costs, review of the inspection requirements may be justified.

If redevelopment requirements are strengthened and longer time horizons for completion are targeted, a larger percentage of total capital costs would be covered by new and redevelopment, reaching nearly 100 percent of the landscape with about 100 years. The public program can then focus on building 1/100th of the needed regional facilities and road and highway facilities per year. A 100-year approach would cost about $46M per year (2013 dollars) in capital costs. If all anticipated facilities were built in 2013, annual operating costs would be up to $540M per year. The long-term operation and maintenance costs would be lower than the 30-year approach as a lower number of facilities are maintained through a public program. On the other hand, the long-term inspection costs would be similar to the 30-year approach, but take 100 years to increase as facilities get built more slowly.

Constructing stormwater facilities within 30 years throughout the Puget Sound region would require the same four actions, though on a larger scale, as would be required in WRIA 9. Assuming 1/30th of the stormwater facility needs are constructed each year, the annual public capital cost to contract cost-effective stormwater facilities across the Puget Sound basin would cost approximately $4.2 to $4.4 billion. If all facilities were being operated and maintained and private facilities inspected and appropriate enforcement action taken, in 2013, the annual public cost would range from $12 to $14 billion per year.

Assuming a 100-year timeframe, public capital costs for constructing stormwater facilities across the Puget Sound basin would be about $650M per year (2013 dollars). Public program operating costs would increase annually as more facilities are built. Use of a 100-year planning horizon would extend completion beyond the 2055 completion date for the Chinook salmon recovery plans, but would be comparable to the length of time over which development occurred and altered the regions hydrology. To align a 100-year stormwater retrofit plan and the Chinook salmon recovery plan, the stormwater retrofit plan could be augmented with stormwater facility construction in sensitive Chinook salmon areas.-- Summary description from report. 


King County. (2014). Development of a Stormwater Retrofit Plan for Water Resources Inventory Area (WRIA) 9: Comprehensive Needs and Cost Assessment and Extrapolation to Puget Sound. Prepared by Jim Simmonds and Olivia Wright, Water and Land Resources Division. Seattle, Washington.

Download the full report (PDF)


Supplemental documents

Final Grant Close-Out Project Report.pdf