Stormwater Capture to Enhance Water Resiliency and Sustainability in Southern California

While achieving water resiliency and sustainability in the semi-arid climate of Southern California is of great importance, a large portion of stormwater runoff that picks up pollutants along the way is directed and discharged into the Pacific Ocean. Capturing and treating such runoff not only can help address water quality objectives in a watershed, it also is a huge untapped potential for providing local water supply and diversifying the water supply portfolio in a region that imports over 50% of its water supply. The imported water is mainly sourced from the State Water Project, Los Angeles Aqueduct, and Colorado River (Figure 1). All these sources have been affected by recent droughts and increasing water demands. Yet, more water supply interruptions are anticipated in the future, accentuating the need for increasing the resiliency and sustainability of local water supply. On the other hand, the existing stormwater infrastructure in Southern California has been built approximately 100 years ago with the main objective of addressing concerns about flooding impacts by channelizing main rivers, creeks, and other waterways to convey stormwater runoff to the Ocean faster. However, today's water supply deficit in the presence of infrequent and intense precipitation patterns is dictating a dual mission for water resources managers/decision-makers to create flood protection, while supporting local water supply. In 2018, Los Angeles County residents voted for passage of Measure W, which is a multi-benefit measure that provides cities and watershed areas within the County with funds to capture, treat, and recycle stormwater. Funding is provided through a parcel tax of 2.5 cents per square foot of impermeable land area, which provides approximately $280 million per year for multi-benefit stormwater capture projects. These projects are planned to enhance water supply, water quality, and community investment benefits through nature-based solutions such as installation of bioswales, native vegetation, constructed wetlands, etc. Additionally, projects funded by Measure W usually support addressing a major river's Total Maximum Daily Load objectives, while providing local water supply. These projects also require development and implementation of a Community Outreach and Engagement Program for the public, community, and key stakeholders to maintain awareness of project development and to gather their input in key design elements from the beginning of the pre-design phase to the end of the construction. This presentation will highlight two projects with objectives aligned with or funded by Measure W. Westwood Neighborhood Greenway Project: This project provides for urban runoff treatment, green space, access to public transit, and educational and recreational needs. The design treats dry- and wet-weather stormwater flows from drainage areas surrounding the project site. Dry-weather flow is diverted from the stormdrain to capture runoff from 2,400 acres of drainage area. The runoff flow is then captured and lifted to the simulated stream for treatment by flowing through plant communities, soil medium, and exposure to sunlight. The flow is diverted through an existing culvert and transported via a lift station and finally back to the stormdrain system. The main challenge during the project design phase achieving consensus among several stakeholders. The project team conducted frequent workshops to involve neighborhood and tribal communities, stakeholders, and the public to help them understand the benefits of the project's implementation and the beautification of the site that over time led to a consensus. Wilmington Q Street Local Area Urban Flow Management Project: This Measure W-funded project will use BMPs to capture the 85th percentile, 24-hour storm event runoff and treat it using flowthrough infiltration systems to improve water quality, remove pollutants affecting local water bodies, and modernize stormwater infrastructure. To capture and infiltrate approximately 17.2 acre-feet (AF) of runoff per year, various new nature-based stormwater infrastructure improvement BMPs will be installed. These include installation of drywells to capture stormwater flow, catch basins with diversion structures, permeable concrete sidewalks, and infiltration planters (Figure 2). These BMPs could remove pollutants of concern while providing a beneficial water supply to the local groundwater basin. The Project also incorporates drought-tolerant landscaping into stormwater treatment processes and reduces heat island effect. One of the main challenges we have anticipated through our initial investigations to be encountered during the design phase is that the soil infiltration rate assumed in the project's feasibility study might not be confirmed through further geotechnical analysis. This could highly influence the total load of pollutants removed, volume of captured stormwater, and consequently groundwater recharge volume that augments local water supply. In this case, we will modify the number, size, and/or depth of the drywells to ensure adequate stormwater is captured. Through this presentation, we will provide further details about these projects and share challenges and opportunities our investigators faced, solutions we found, and lessons learned.