City of Torrance: Multibenefit Approach to Improving Water Quality
Stormwater and urban runoff carries numerous pollutants of concern to receiving water bodies. The City of Torrance is subject to the Santa Monica Bay Beaches Bacteria Total Maximum Daily Load (TMDL), which requires zero bacteria exceedance days for the dry season and requires limited days for the wet weather season.
In order to adhere to these important requirements, the City of Torrance is implementing a multibenefit project that will reconstruct the Amie, Henrietta, and Entradero Basins to retain, treat, and infiltrate stormwater runoff. Once these wetlands are constructed, they will provide passive wetlands treatment, additional retention and infiltration, enhanced groundwater recharge, improved public trails and recreational opportunities, and habitat restoration.
A topographic and boundary survey of each basin was performed, along with a geotechnical and infiltration investigation for the design of earthwork, pipelines, structures, and other site improvements. A wetlands sustainability analysis was then conducted to evaluate dry-weather inflows, evaporation, and infiltration and ensure the long-term functionality of the wetlands system. A hydrologic analysis was carried out using MODRAT in Watershed Modeling System (WMS) to evaluate the wet weather retention and reservoir routing and pumping from the interconnected wetlands/detention basin system under Capital Flood and Standard Urban Stormwater Mitigation Plan (SUSMP) conditions. Water quality modeling determined the reduction in pollutant loads. The project included design and preparation of construction documents for the proposed improvements, including storm drain design, site, grading, paving, and piping plans; landscaping and irrigation details; inlet and outlet structures; a pump station and other control structure electrical and mechanical plans; and SCADA plans.
The design phase was completed in January 2014 and a construction contractor has been selected to begin construction in May 2014 for an estimated beginning of next year completion.