EISPIRATEN

https://www.karmactive.com/the-santa-an ... structure/
".....The county’s Director of Public Works, Mark Pestrella, states that they must adapt to these environmental changes and challenges, but this will not be easy or cheap. The cost of removing an estimated 15 million cubic yards of debris and mud from all five reservoirs and transporting it to sediment placement sites across the foothills is around $550 million. This money is expected to come from county funds, state and federal grants, and possibly an assessment agreed to by property owners. Pestrella notes that they do not have a limitless budget and that property owners in the county are currently charged $28 annually for these kinds of improvements, which hasn’t been changed since the 1980s.

The task at hand includes the removal of a substantial amount of debris and mud from four different facilities, including Big Tujunga Dam, Cogswell Dam, Pacoima Dam, and San Gabriel Dam. Specifically, 3 million cubic yards will be removed from Big Tujunga Dam, which protects the area of Tujunga; 2 million cubic yards will be removed from Cogswell Dam, responsible for controlling flooding in the West Fork of the San Gabriel River; 4.5 million cubic yards will be removed from Pacoima Dam, which safeguards Olive View-UCLA Medical Center, Sun Valley, and Sunland; and a total of 5 million cubic yards will be taken away from San Gabriel Dam, which manages outflow from the 200-square-mile watershed of San Gabriel Canyon.
The water from San Gabriel Dam flows down the canyon to Morris Dam, a facility a few miles above Azusa that was dedicated in honor of Los Angeles consulting engineer Sam Morris by former President Hoover in 1934. The Bobcat fire has caused a significant loss of vegetation, increasing the risk of flooding, debris flows, and mudslides, which may prevent the dams from making controlled releases. It took about four years to remove 330,000 cubic yards of sediment from behind Santa Anita Dam in the aftermath of the massive 2009 Station fire."

There is the deferred maintenance cost of $550 million per the LA Times as well.....although laist reported less..."..A lot of the reservoirs are overdue for restoration, like the Pacoima Dam which was built in 1929. Another example: The Santa Anita Dam is holding 600,000 cubic yards of muck and silt is blocking two of three valves that release the stormwater. Cleaning up the reservoirs will take three years and cost (gulp) $225 million. "

Make it an even billion to cover the next fire.

At some point they might realize they had the solution all the time. Each dam has a sluice gate at lowest level of the dam. The sluice gate is used to clean out the dam using water flowing though the lake bottom mud and out the sluice gate. The same process is required for each dam or facility all the way to the ocean. It's messy, but it does restore the natural movement of sand from the mountains to the beach. A slurry pipeline is an alternative, but it's probably easier, politically, to move the material up canyon to backfill a canyon like Burro Canyon. Moving all that material take a crap'ton of diesel and electricity for conveyer belts.

So they let the sluice gates get buried and now they can't remove enough sediment to make them work again? Is that what's going on?

Sean wrote: ↑So they let the sluice gates get buried and now they can't remove enough sediment to make them work again? Is that what's going on?

No, that would help them operate, but they are usually hydraulically operated using a remote pump system. The government cannot stand the uproar of silting up river and creek bottoms. If done more frequently there would be less silt to deal with and the impact would be minimal. IT is more complicated when there are a series of dams, e.g., on the San Gabriel.

It would be great if the environmental defenders, the Fish & Games folks too, would visit some of fast flowing mountain streams during a good downpour. They would never forget the grinding of boulders moving downstream along with entire trees and car sized rocks moving downstream. It is amazing that trout and other stream dwellers survive storm flows, but they have for thousands of years.

That isn't practical since all the sediment and debris would flow through the city of LA. The city is set up to flush the water as quickly as possible out of the LA Basin. Allowing debris in those channels would be a recipe for disaster with blockages and major flooding. We are restricted by the down stream system. In addition the sluices would not be enough to deal with all the debris, there would have to be significant investment in larger drainages typically under the reservoir to remove the amount of debris that we receive.

Typically, debris, i.e., other than silt and sand, are not included in the sluice gate flow. Sluice gates have a set of bars, called a trash rack, that catch the debris for removal during a slicing operation. The silt would be slushed out of the LA Basin into the ocean, where it actually belongs.