Natural Drainage System Partnering in Seattle: A Portfolio of Innovations to Address Challenges and Opportunities in Retrofitting Informal Right-of-way

Since 1998, Seattle Public Utilities (SPU) has innovated with retrofits of public right-of-way to improve stormwater using natural drainage systems (NDS). Based on lessons learned from prior projects and to address regulatory commitments, SPU launched the NDS Partnering program in 2017 to develop retrofits in the three major creek watersheds within the City: Longfellow Creek, Thornton Creek and Piper's Creek, see Figure 1. Due to topography, historical development patterns and standards and economic considerations much of this part of the city consists of informally developed right-of-way without curb or formal drainage infrastructure. This presentation will share the program goals, a summary of the site selection process and more importantly lessons learned in addressing the challenges and opportunities of partnering to retrofit underdeveloped urban right-of-way. The Plan to Protect Seattle's Waterways is a comprehensive strategy that embodies SPU's commitments that were documented in the July 2013 Consent Decree to address combined sewer overflows. It meets Seattle's federal commitments to create a Long-Term Control Plan (LTCP) that will make water quality improvements to Seattle's receiving waters, e.g., Puget Sound, Lake Washington, and the urban creeks that feed them. The final Plan received final approval in August 2015. Within the Plan to Protect Seattle's Waterways, SPU recommended the Integrated Plan (IP) alternative, which addresses water pollution from both combined sewer overflows and stormwater-only runoff into waterways. As part of the IP, SPU is committed to deliver high-value stormwater-only projects/programs ahead of a set of smaller-volume combined sewer overflow (CSO) projects for a greater overall water quality benefit. NDS Partnering is one of the three stormwater-only projects/programs recommended in the IP for pollutant reductions. In addition, the NDS Partnering program fits within SPU's Strategic Business Plan Focus Area No. 1, 'Better Protecting Your Health and Environment.' The mission of the NDS Partnering Program is to achieve the water quality goals identified in the City's Plan to Protect Seattle's Waterways. The NDS Partnering program emphasizes working with sister agencies, concurrent SPU programs and community partners to deliver high-value neighborhood improvements, including bioretention systems that will capture and treat stormwater before it drains to creeks feeding Puget Sound and Lake Washington. The purpose of the partnering in the NDS program is to develop shared projects within Seattle Public Utilities (SPU) programs and between SPU and other City agencies, such as Seattle Department of Transportation (SDOT), to offer multiple benefits to neighborhoods and ecosystems, including: greener, more attractive neighborhoods, lower risk of flooding, additional natural habitat for native plants and animal species, healthier creek ecosystems, calmer traffic patterns, and more street trees. For SPU, the NDS projects provide water quality treatment for street runoff that drains to urban creeks by retrofitting the roadsides with NDS (also referred to as bioretention cells) and addressing localized flooding issues. The City of Seattle has a long history in developing retrofits of right-of-way in urban creek basins beginning with the SEA Streets project in 1998. This presentation will focus on the current NDS program consisting of the Longfellow Creek Basin (due to be complete with construction in 2023), South Thornton Basin (due to begin construction in Spring 2023) and North Thornton and Pipers Creek Basins (under development and design). This program leverages innovations developed under some of the most recently constructed retrofit projects including the Delridge, Ballard (Phases 1 and 2) and Venema NDS projects to enhance the viability of green infrastructure in challenging site constraints and soils. These innovations include the use of weirs, underdrains, structural soil cells and underground injection control (UIC) wells. Working in underdeveloped or informally-drained right-of-way presents opportunities to address infrastructure gaps that will benefit the local community, specifically providing traffic calming, improving pedestrian access, and correcting nuisance drainage. This presentation will share the approach and response to community outreach to communicate impacts and receive feedback on tradeoffs on design of improvements. Additionally, the co-location of improvements with partner agencies presents opportunities to reduce overall project cost for the City balanced with increased coordination needs. Key issues addressed by the project include how to negotiate space constraints to co-locate new sidewalks with bioretention facilities in existing right-of-way. Another key issue commonly expressed by the community is minimizing impacts to automobile access and parking where historical use is informal and haphazard and existing driveways do not conform to code. Public investment in infrastructure presents the opportunity to improve community spaces and services. In addition to considering equity and deep connection with the community through the design process, the program also has had opportunities to incorporate art and create community spaces. This presentation will include examples where the projects have included a range of scale of art from small touches to enhance the individual elements of the green infrastructure to more significant efforts to incorporate public art in new pocket community spaces that are co-located and funded with the NDS projects. The project also specifically targets addressing local drainage issues where informal drainage is inadequate to collect and convey runoff from the neighborhood streets creating nuisance flooding. While addressing these local drainage issues is a significant benefit for the neighboring property owners, SPU was also challenged to incorporate NDS and drainage improvements in a manner to avoid conveying the additional runoff to create or aggravate existing issues downstream. The program addressed these challenges by evaluating the downstream system capacity, maximizing stormwater retention within the project and providing compensatory storage where required to minimized downstream impacts. In summary, this presentation will provide the opportunity to share lessons learned at a unique point in time that represents past, present and future of a program that is delivering green infrastructure improvements to over 60 blocks of neighborhood streets in the City of Seattle's urban creek watersheds. This program and presentation deliver unique and innovative techniques to balance the needs of the right-of-way to improve the quality of stormwater runoff. SPU strives for continual program improvement and delivering community-centered projects that achieve improved service with reduced costs.