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County flood control officials detail emergency expansion at Sierra Madre Dam after post‑fire storm
Summary
Los Angeles County Flood Control officials told the Sierra Madre City Council the Sierra Madre Dam project deepened the basin, added an upstream debris barrier and replaced damaged spillway rails after the Eden fire and a Feb. 13 storm that deposited roughly 80,000 cubic yards of sediment. Officials said about 10,500 cubic yards have been removed so far and the project should wrap in early April.
Los Angeles County Flood Control engineers described the emergency sediment capture expansion at Sierra Madre Dam during the March 24 Sierra Madre City Council meeting, emphasizing that post‑fire flows and a single Feb. 13 storm deposited unusually large volumes of sediment that threatened downstream neighborhoods.
"That storm brought in over 80,000 cubic yards of sediment to Sierra Madre Dam in one day," said Scott Grigowski, principal engineer in the district’s storm water engineering division. He said sediment levels rose more than 60 feet during the peak event and caused a brief spillway flow.
Patrick Holland, area engineer for the Flood Control District, outlined the district’s work to increase basin capacity and protect the community. The district deepened the basin, removed sediment further upstream than typical maintenance, and built an 8‑foot‑tall upstream rock‑filled debris barrier to capture large materials before they reach the main basin. "We removed about 10,500 cubic yards of material between January and February," Grigowski said, adding that crews were replacing approximately 120 feet of damaged spillway handrail and installing removable rails to avoid damage in future high flows.
Council members thanked staff for rapid post‑fire work. "Between the fire and that Feb. 13 storm, you came in and removed thousands of cubic yards of debris," Council member Edward Garcia said, noting that intervention likely prevented more severe overtopping.
The Flood Control District also explained why debris basins must be cleared quickly after storms: the basins capture mud and wood but fill fast, and filled basins reduce protection until excavated and hauled away. District staff described routing sediment to managed placement sites and attempting haul routes that minimize community impacts.
Public works director Brian Dickinson said the city will coordinate with the county on smaller rail‑and‑timber structures in the foothills, including the Yucka Trail structure, where ownership and maintenance have shifted between agencies in the past. County staff said they are reviewing permitting and longer‑term policy on temporary rail‑and‑timber structures used to reduce post‑fire debris flows.
The district also reported above‑average rainfall this water year and noted local recharge benefits: the Raymond Basin spreading grounds are at about 106% of annual average recharge for the year to date, which the district said is enough water for roughly 24,000 residents for a year. The Sierra Madre project was described as scheduled for completion in early April; officials asked for resident patience while hauling and heavy equipment activities continue.
The council did not take action beyond receiving the presentation. Staff said they will provide follow‑up information on scheduled work windows, haul routes and opportunities to coordinate future maintenance with the city.
