Treasures of the Valley: Worst-case flood scenarios – alarming, they can happen

Posted by on Mar 4th, 2010 and filed under Viewpoints. You can follow any responses to this entry through the RSS 2.0. You can leave a response or trackback to this entry

By Mike Lawler

Our history is punctuated by disaster – fire and flood, followed by bigger fires and bigger floods. The cobbles and boulders we dig up in our gardens were deposited there by debris flows that have crossed the Crescenta Valley in the past, and the undulations in our roads are channels and levees left by the passage of these flows.

At a recent meeting to update residents on flood prevention status, an official from the L.A. County Department of Public Works said the current parade of trucks will go on for another month and a half – if there is no more rain this winter. But it will rain again this year, so we can expect this activity to last well into the summer, and to pick up again next winter, and the winter after that. They haul the debris out of the debris basins and pile it at “sediment placement sites” such as the one at Deukmejian Wilderness Park, in an ever-growing mountain.

The public works official told us that, at the current rate, the park site will be full in about a year and a half. When questioned about the safety of this pile of dirt to residents below, the county said the finished fill site will be compacted and terraced to modern standards and that drainage will be adequate.

But let’s get scary for a moment, shall we? I don’t mean to be alarmist, but following are some scenarios for potential disaster.

Let’s start with a look at the Google maps aerial view of the Crescenta Valley – specifically, two sediment placement sites at Deukmejian Park (which most of us have seen) and Eagle Canyon (off La Crescenta Avenue above Harmony Place). These mountains of dirt loom above neighborhoods; in both cases, flood channels and debris basins are situated directly next to the piles, as you can see on the aerial view.

At Eagle, a flood channel runs across the site just uphill from the pile. It’s conceivable that the channel might become blocked by a boulder, as it did in the 1978 Pinecrest flood and recently at Mullally Canyon. If the blockage happens anywhere near that dirt pile or if the debris basin fills, any later flows could begin to erode the debris piles and turn the sediment mountain into the consistency of a Slurpee, inundating the neighborhoods below.

Similarly, at the Deukmejian Park site, the flood channel and the debris basin are next to and slightly up-canyon from the dirt pile. A filled debris basin or blockage might divert the flow to the west and directly into the sediment placement site. The neighborhoods below wouldn’t stand a chance.

Here are some numbers I dug up that don’t bode well for our debris basin capacity.

The total volume of mud and debris deposited in Montrose and La Crescenta from Pickens and Hall-Beckley canyons in the deadly 1934 flood was estimated at 500,000-700,000 cubic yards. But the basins built for those two canyons in response to that disaster have a total capacity of 250,000 cubic yards, as they were subsequently redesigned for a lesser 50-year flood event.

These are pretty alarming “what if” scenarios, but they’re also real possibilities. Residents should be asking the county, FEMA, the Army Corps of Engineers, and the U.S. Geological Survey some hard questions about their own safety. The flood control system that has so far protected us was designed 75 years ago, and we need to continue to improve and modify that system as we learn more about the debris flow risk.

The mud and debris comes down the mountain, and we scoop and haul it back up. It’s the only tactic we are currently using to hang onto the precarious ledges we’ve carved out on the slippery slopes of this alluvial fan we call home.

But, does it really make sense? No, because it will eventually come back down the mountain.

1 Response for “Treasures of the Valley: Worst-case flood scenarios – alarming, they can happen”

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