Carson Stormwater and Runoff Capture Project

A large StormTrap® system was installed at the Carriage Crest Park as part of an innovative stormwater management project in Carson, California. This project will divert millions of gallons of stormwater per year to help the City of Carson and unincorporated areas of Los Angeles County to protect waterways, meet MS4 permit requirements, and increase water supply to the local wastewater treatment plant—all in a cost-effective manner. Carriage Crest is a collaboration between the City of Carson, County of Los Angeles, and the Los Angeles County Sanitation Districts, with design support provided by Tetra Tech.

Project Needs and Design

The Carson Stormwater and Runoff Capture Project at Carriage Crest Park was identified as a priority within the Enhanced Watershed Management Program (EWMP) for the Dominguez Channel Watershed Management Group. The goal was to improve the water quality of stormwater and urban runoff within the Machado Lake watershed and provide opportunities to increase local water supply. The project was initially identified as an infiltration project in the EWMP, but due to the poor percolating soils, an alternative plan was pursued.

A key aspect to this project is its location: Carriage Crest Park is situated next to two large storm drains, with a total drainage area of 1,146 acres, and across the street from the Joint Water Pollution Control Plant (JWPCP), the sanitation districts’ largest wastewater treatment plant. The proximity to the JWPCP makes it possible to safely send large volumes of stormwater to the sanitary sewer without jeopardizing the sanitation districts’ primary mission of safely managing wastewater.

Stormwater runoff will be diverted from the existing storm drains to a pretreatment system to remove trash, debris, and sediment. A drainage pipeline will convey the pretreated stormwater into a 13.54-acre-foot StormTrap underground basin located underneath the park’s baseball field. The captured stormwater will then be pumped to an existing 60-inch sanitary sewer trunk line for further treatment at the wastewater treatment plant.

Underground Stormwater Management System

A 14′8″ DoubleTrap® was selected for this project. The system was built with 712 modular precast concrete units and is capable of storing a total volume of 589,903 cu. ft. of stormwater. The basin was designed to provide detention as well as additional treatment through sedimentation. The sedimentation portion of the system was designed to be watertight to remove impurities before the water would spill over into the detention area.

“The system designed for Carson is unique for its height, stormwater storage capacity and application,” affirmed Faisal Khan, PE, Engineering Department Manager at StormTrap. “The basin also meets the special seismic loading criteria based on ACI 350.3 Seismic Design Provisions, which allows it to remain functional during and after a seismic event.”

Contrary to typical means of installation, StormTrap worked with the engineer and contractor on this project to allow for installation to occur in phases, thus permitting travel over the basin throughout construction.

During installation months, storm events caused delivery delays, and the StormTrap team and the City successfully rearranged the logistics to deliver the precast units within the project timeline. Julio Gonzalez, Sustainability Administrator for the City of Carson, commented, “My experience with StormTrap was very pleasant; their expertise and quick response made our installation and process easy.”

The installation of the underground StormTrap system was finished in November 2019, and the project construction is anticipated to be completed by summer 2020. The collaboration of all parties involved and the funding from many different local agencies were essential for accomplishing the project goals.

This water diversion project will make a significant impact on reducing the impurities in the local watershed, help cities meet MS4 permit requirements, and also benefit the sanitary treatment facility and community.