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Last Updated: Wednesday, February 7, 2018 10:24 AM


linear radial drain exterior inclined draininclined drain

In 2001, the Los Angeles Regional Water Quality Control Board adopted a Total Maximum Daily Load (TMDL) regulation limiting the amount of litter that can be discharged into the Los Angeles River, which drains about 830 square miles of urban land.

The Board subsequently assigned Caltrans a Waste Load Allocation (WLA) of zero for trash within the watershed. WLAs assigned to all of the NPDES permittees in the watershed are based on phased reductions from the estimated current discharges over a 12-year period until the final load allocation is met.

The Phase 1 Gross Solids Removal Device (GSRD) Pilot Study was conducted to evaluate the performance of selected non-proprietary devices that can capture gross solids and that can be incorporated into the existing highway drainage system. Gross solids include naturally occurring material, like vegetation, rocks, and soil, as well as litter.

Eight GSRDs were constructed and monitored during the 2000-01 and 2001-02 storm seasons at sites within Caltrans District 7 and the Los Angeles River watershed. The devices include two types of linear radial devices, two types of inclined screens, and one type of baffle box. All of the devices were designed to convey the peak discharge from the 25-year design storm, and with the capacity to hold one year's estimated accumulation of gross solids. They were also designed to drain quickly to prevent mosquito breeding, and for easy access and maintenance. Mesh bags were installed at the outlets of the drainage systems downstream of each GSRD in order to determine the capture efficiency of each device.

The GSRDs captured multiple storms throughout both rainy seasons, including two major storms in January 2001. The captured gross solids were measured by weight and volume.

One of the linear radial devices (LR1) located along I-10 and one of the inclined screen devices (IS1) located along SR-170 met the criteria of the TMDL as well as Caltrans criteria and goals. The remaining GSRDs were presumed to have met some TMDL criterion if routine maintenance were performed. The remaining GSRDS did not perform as well overall compared to LR1 I-10 and IS1 SR-170.

The GSRDs Pilot Study demonstrated positive proof of concept, although some were more efficient or easier to maintain than others. The linear radial screen and inclined screen devices worked well in capturing gross solids. Removal efficiencies ranged from 83 to 100 percent by weight. Very little debris and litter was captured downstream of the GSRDs.

Design modifications and further hydraulic lab testing are ongoing.

To view reports on this subject CLICK HERE



Division of Environmental Analysis, Stormwater Unit
California Department of Transportation
P.O. Box 942874, MS-27
Sacramento, CA 94274-0001
Email: Division of Environmental Analysis, Stormwater Unit