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City of Torrance Stormwater Basin and Treatment Wetlands Enhancement

CWE designed two treatment wetlands, two infiltration basins, and 300 feet of pressured storm drain piping. This was designed to retain, treat, and infiltrate stormwater runoff to help the City comply with the Santa Monica Bay Beaches Bacteria Total Maximum Daily Load (TMDL).

CWE performed a topographic and boundary survey of each basin. This included coordinating a geotechnical investigation for the design of earthwork, pipelines, structures, other site improvements, and infiltration capabilities of three basins.

A wetlands sustainability analysis was performed to evaluate dry-weather inflows, evaporation, evapotranspiration, and infiltration to ensure the long-term functionality of the wetlands system. CWE conducted a hydrologic analysis using the Modified Rational Method (MODRAT) in Watershed Modeling System (WMS). This evaluated the 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 estimated annual pollutant load reduction for this project is 91%. Using the stormwater for irrigation reduces the potable water demand by approximately 2,800,000 gallons annually.

The project included designing and preparing construction documents for the proposed improvements.

Wildlife Habitats
The project also included placement of new trash sweeping signage and catch basin screens throughout the basin subwatersheds. The project included coordination with the Los Angeles County Flood Control District (LACFCD). Special consideration was given to protect the burrowing owl and legless lizard habitat. CWE performed legless lizard surveys prior to and during construction. Specifications per Greenbook and special provisions and estimates were prepared. CWE also provided construction management and support services.

Project Tasks

Treatment wetlands
Infiltration basins
Storm drain piping
Construction documents
Topographic and boundary surveys
Legless lizard surveys
Coordinating geotechnical investigation
Wetlands sustainability analysis
Hydrologic analysis
Water quality modeling
Placing trash sweeping signage and catch basin screens
Coordinating with LACFCD
Preparing specifications and estimates
Providing construction management and support services