Beyond Design Storms: Sizing Stormwater BMPs to Maximize Multiple Benefits in Los Angeles County
The Los Angeles region rolled out its Safe Clean Water (SCW) Program in 2019, a parcel tax that collects approximately $250M annually to fund stormwater projects that accomplish a multitude of goals including water quality improvement, water supply augmentation, community benefits, and nature-based solutions. As part of the SCW Program, a scoring framework was developed to select projects for funding that increase the region’s water resiliency while advancing many of the Program’s additional priorities. While conventional design methods for stormwater controls typically involve capturing simple design storms, municipalities are finding innovative ways to maximize other benefits and nature-based solutions within the SCW Program’s scoring framework. Los Angeles County Public Works is currently developing a Green Streets Master Plan that utilizes the Program’s multi-benefit scoring metrics as targets for BMP modeling that eventually inform the design and selection of project alternatives. Through the Master Plan, hundreds of identified opportunities for green streets – green stormwater infrastructure located along roadways – were optimized to maximize scores across the range of multi-benefit categories. Rather than solely evaluating traditional design storms, over 100 green streets were also sized to meet water quality improvement metrics and concurrently adjusted to maximize augmentation of water supply using long-term continuous simulation (10 and 20-year periods). This approach ensures that concept designs are 1) based on real-world conditions instead of a single synthetic storm event and 2) sized to perform efficiently when evaluated over long periods (more similar to a project’s intended lifespan rather than on an event-based basis). Due to the multiple avenues in which projects can maximize scores across the Program’s scoring categories, the number of modeling decision variables is greater than in conventional modeling methods so an automated approach was developed to run simulations, sift through results, and compile into intuitive fact sheets to support design. ‘Scoring curves’ were produced from the model outputs for each green street to visualize how incremental changes to design details, like dimensions and design components, drive differences in water quality and water supply. These performance curves helped to identify small cost-effective design modifications that could help a water quality project achieve secondary objectives, such as water supply augmentation. The presentation will focus on key elements of the BMP sizing approach, innovative graphics that supported decision-making, the driving factors for high-scoring projects, and lessons learned for incorporating multiple objectives in a modeling approach for BMP sizing. Additionally, automated techniques for managing a large set of model scenarios will be discussed in more detail. As stormwater projects become increasingly multi-objective, managing large quantities of model output will become more of a necessity. Moving beyond design storms represents an evolution in stormwater project design that promotes well-rounded projects with a holistic focus on water sustainability. Ultimately, it is hoped that the modeling and conceptual designs from the Green Streets Master Plan will build momentum for the widespread adoption of sustainable green infrastructure in the Los Angeles region and will serve as a model for integrating multiple priorities into project design. This presentation will be of interest to a wide array of participants who will benefit from the presentation format, which will highlight key ‘lessons learned’ that will be beneficial to other green infrastructure planning efforts.