Diverting Stormwater to Subsurface Infiltration Facilities to Reduce Flooding, Manage Water Quality, and Provide Community Benefits in Lomita, California

The purpose of this presentation is to discuss alternatives to managing stormwater flows through natural systems that can benefit separate and combined sewer systems. The benefits of the presentation will include informing participants on the benefit of the application and will discuss key lessons learned that should be considered early in the planning phase to improve outcomes and reduce costs. The City of Lomita, located in Los Angeles County, is implementing a stormwater capture project that diverts flows from the 85th percentile rainfall event to a subsurface infiltration gallery located beneath the City Hall lawn, which equates to a design storm volume of 5.4 acre-feet. The project provides a series of benefits to the City and to residents by removing the volume of flow from the storm drain system. This project will alleviate flooding, remove urban watershed pollutants from being conveyed to the receiving waterbody, and provide groundwater recharge, an important aspect in areas prone to drought. By managing the stormwater in a fully buried system, it allows for full use of the site on the surface. While this project involves diverting flows from a municipal separate storm sewer system, the concept could provide similar benefits to combined sewer systems by diverting stormwater at a point prior to comingling with sanitary sewer flows, thereby alleviating CSOs during high flow events. Subsurface infiltration provides an effective means of managing stormwater, though there are several factors that must be considered prior to implementation. The City of Lomita's project encountered several obstacles which were resolved during the preliminary design phase, including identification of a past land use at the original site that posed a potential water quality risk to the groundwater basin as well as the results of geotechnical investigations that necessitated a shift in the type of subsurface infiltration being designed. Lomita remained flexible to modifications, and the revised design accommodated these conditions, and the lessons learned may improve the outcome of future projects. The project has been partially funded through the County of Los Angeles' Safe, Clean Water Program, a voter approved measure designed to fund stormwater and water supply projects throughout the County. This presentation will describe the project's intended outcomes, lessons learned, and key takeaways for those embarking on similar projects. It would appeal to municipalities managing stormwater and wastewater conveyance systems and those responsible for compliance measures. The project is currently in the design phase, which will be complete or near completion at the time of the conference.