2023 Top Projects: Stormwater project increases storage volume for California basins

After an initial project from 2012-2015, which focused on water quality improvements to Amie, Henrietta and Entradero basins, the Enhanced Watershed Management Program (EWMP) found a need to expand the work.

The program was initially implemented by Hermosa Beach, Manhattan Beach, Redondo Beach, and Torrance. The main goal of the Stormwater Basin Expansion Project was to retain, treat and increase the storage volume of the basins and surpass the 85th percentile storm event design capture volume. To this end, the project team deepened the Henriette and Entradero basins; used the excavated soils to improve the south slopes at Entradero Park and construct a walking trail; installed drywells at Henrietta Basin and extended the local trail; and adjusted pumping levels at Amie Basin.

The team conducted a pilot project at the Henrietta Basin because of its limited footprint and space to  increase the storage volume. The project drilled one drywell into the basin and simulated a 24-hour saturation period before sending measurable flows to the drywell via a fire hose. The flow rate was twice the established average flow rate. They calibrated a specific flow rate via real-time trials to determine the number of necessary drywells. The team reduced grading of the Henrietta Basin with zero impacts to the existing footprint.

“The most rewarding aspect of this project was the planning and innovation that went into the design of Henrietta Basin expansion,” Larry Tortuya, project manager for CWE, said. “Incorporating dry- wells in lieu of costly footprint expansion is something unique on this project.

Project construction was officially completed in September 2023, and the project will begin capturing and diverting runoff in the coming months.