Sediment Removal and Sites
Configuration A — The Project
Removal Method
Historically, as storm events have deposited sediment in the reservoir, native and non-native vegetation have become established in the sediment. During subsequent storm events some of the vegetation and trees have been washed out by storm flows or submerged when the reservoir level rises, or buried under sedimentation. Despite the dynamic changes to water elevation and flows in the reservoir, mature black willow trees, Riversidean Alluvial Fan Sage Scrub, Mule Fat Scrub, and riparian vegetation have thrived in the reservoir. During storm events following the 2009 Station Fire, a large portion of the reservoir vegetation was buried in sediment; however, significant amounts of vegetation, including numerous mature willow trees, remain intact.
In order to remove the sediment from the reservoir, trees and vegetation growing within the excavation areas will need to be removed. The accumulated sediment will be excavated with construction equipment including but not limited to approximately four front loaders with 4-cubic-yard buckets, two bulldozers, one excavator, one grader, one water truck, and two tender trucks (for fuel and maintenance). Vegetation and organic debris will be separated from the sediment. Coarse material may need to be processed through sorters and crushers to be hauled offsite. Depending on the moisture content of the sediment removed, the sediment may need to be stockpiled to allow the sediment to dry. Stockpiling of the sediment will occur onsite within Devil’s Gate Reservoir.
Project Site
The Proposed Project site covers approximately 120 acres. The Proposed Project site has a General Plan Land Use designation of Open Space and is zoned as Open Space under the City of Pasadena General Plan (City of Pasadena 2004).
Through easements granted in May of 1919 and March of 1965, the City of Pasadena granted the LACFCD, under a perpetual easement, the right to construct, reconstruct, inspect, maintain, repair, and operate Devil’s Gate Dam, its spillway, bypasses, tunnels, and other support facilities as may be necessary for the construction and maintenance of a reservoir capable of impounding the waters of the Arroyo Seco for purposes of storage and control, and to control such waters as may be necessary in the prevention of damage by flood (City of Pasadena 1919/1965).
Project Goal
The Proposed Project will remove sediment from Devil’s Gate Reservoir to restore the design capacity (volume for two DDEs below the spillway elevation of 1,040.5 feet) and establish a reservoir management system to maintain the flood control capacity of the reservoir. This will include removal of approximately 2.9 million cy of current excess sediment in the reservoir in addition to any additional sediment received during the project sediment removal phase.