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Sediment Management

Cogswell Dam
 
PROJECT SUMMARY
   
Cogswell Dam is a vital component of the Los Angeles County Flood Control System and is critical to water conservation efforts, serving multiple beneficial uses including sustaining the downstream habitat and groundwater recharge. During the August 2009 Station fire, approximately 90 percent of the watershed tributary to Cogswell Dam was burned. The increased rate of debris production from the denuded watershed has hindered the operability of the Dam's valves and reduced the Reservoir's storage capacity.

The Cogswell Reservoir Sediment Removal Project proposes to remove the accumulated sediment from Cogswell Reservoir up-to approximately 2.6 million cubic yards. The Station Fire resulted in an additional 1.1 million cubic yards of debris. This brings the total debris and sediment in the reservoir to 3.1 million cubic yards. This material must be removed as soon as possible to comply with United States Forest Service permitting requirements and to maintain the Dam as a fully functional component of Los Angeles County Flood Control System.

Removed debris and sediment from the reservoir will be placed at the adjacent permitted Cogswell Sediment Placement Site (SPS) located at the southern edge of the reservoir. The use of the SPS for the maintenance of Cogswell Reservoir was evaluated in an Environmental Impact Report (EIR) pursuant to the California Environmental Quality Act (CEQA). The EIR was certified by the County Board of Supervisors in 1993. The Cogswell Reservoir Sediment Removal Project will pursue necessary permitting from Regional Water Quality Control Board, California Department of Fish and Wildlife, and The United States Army Corps of Engineers.

Construction is currently planned for Summer 2019.
Cogswell Sediment Placement Site, Details Cogswell Sediment Placement Site, Details
 
Cogswell Sediment Placement Site, Artistic Rendering Cogswell Sediment Placement Site, Artistic Rendering Cogswell Sediment Placement Site, Artistic Rendering 2 Cogswell Sediment Placement Site, Artistic Rendering 2 Cogswell Reservoir Sediment Removal Project, Scope of Work Cogswell Reservoir Sediment Removal Project, Scope of Work
 
DETAILS
 
Location Angeles National Forest along the west fork of the San Gabriel River and off of Devil's Canyon Dam Truck Trail. (Thomas Guide Page 508, A5)
Fire Event(s)* 2009 Station Fire
Watershed Burned 90%
Amount of Sediment to be Removed Up to 2.6 MCY
Removal Method Mechanical Excavation
Disposal Location Cogswell Sediment Placement Site (adjacent to site)
Required Permits U.S. Army Corps of Engineers
California Dept. of Fish and Wildlife
Regional Water Quality Control Board
U.S. Forest Service
Environmental Documents Addendum to 1993 EIR. Project is Phase 2B
Community Meeting Dates None currently scheduled
*Heavy sedimentation is expected for the five years following the fire event. With 7-10 years expected for full recovery under current drought conditions.
 
CONTACTS
 
For more information on the project or to be added to the project mailing list, please contact reservoircleanouts@dpw.lacounty.gov
 
Updated on: 10/30/2017
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