Leading Green Street Implementation
Stormwater and urban runoff carries numerous pollutants of concern to receiving water bodies. With the Municipal Separate Storm Sewer System (MS4) Permit for Los Angeles permittees, and the adoption of the Enhanced Watershed Management Program (EWMP) Plans, the use of green streets and Low Impact Development (LID) policies have become a critical factor to meet compliance goals. The main approach to reducing runoff pollution from developed areas is to implement structural treatment control Best Management Practices (BMPs) to capture, treat, infiltrate, and/or use the “first flush” runoff. Doing so, will reduce stormwater runoff volumes, improve water quality, and assist with the compliance of TMDL standards. Additionally, utilizing runoff as a resource by infiltration replenishes the local groundwater basin with much needed water within southern California. The Laurel Canyon Boulevard Green Street project’s goal is to enhance the water quality of the Los Angeles River and serve as a functional example for future applications within the Los Angeles Region. To adhere to these important requirements, the City of Los Angeles started implementation of a multi-benefit project that aims to reconstruct a section of Laurel Canyon Boulevard to capture, retain, treat, and infiltrate stormwater runoff. The City of Los Angeles applied for and received a grant from the State of California under Proposition 84 to implement a series of bioretention areas that will treat and infiltrate stormwater runoff. Uniquely, the project area is not served by any storm drains and floods with minimal rain events. Approximately 125 acres of drainage area is tributary to the project site with the flows concentrating along northbound Laurel Canyon Boulevard. Laurel Canyon Boulevard, within the project area, has minimal longitudinal slopes and therefore even in small rain events the street runs like a river and impedes access for its residents. The presentation will discuss the soils investigation conducted and the results of the infiltration tests to develop a treatment train and infiltration criteria, and design of BMPs implemented. Additionally, the hydrologic analysis used to determine peak flow discharge rates and water quality volumes generated from various rainfall return frequencies will be presented. The presentation will expand on how this information was used to appropriately select and size BMPs that targeted a volume reduction from the 85th percentile events to meet or exceed the EWMP goals. Planting material and the soil substrate were reviewed and incorporated into the design that provided medium to high pollutant removal efficiencies. Based on the above analysis, numerous LID strategies were evaluated to configure a BMP treatment train capable of achieving the project goals. Final design included the use of a unique soil mix and infiltration techniques to enhance stormwater retention and the use of California natives to reduce irrigation. The project, as designed, handles the 90th percentile rainfall depth and exceeds the 85th percentile which is normally required for compliance. Additionally, the project recharges over 40 acre-feet of runoff annually to the local San Fernando groundwater aquifer. The design phase was completed in June 2015 and construction began in December 2015 with an estimated construction completion in the third quarter of 2016. The relevance of this topic is significant to the CASQA attendees as this presentation provides how to evaluate and use strategies to meet or exceed compliance goals within the same project footprint. The presentation will discuss the thought process of how the design was enhanced to retain the additional runoff. The presenters will discuss the analysis completed and how the changes in design were implemented to exceed compliance goals.