Dredging operation has excavated some 400,000 cubic yards of sand

By Martin L. Jacobs

The clamshell dredge in Marina del Rey harbor, operating 24/7 since December, is guided by sonar images of the sea floor

You could do a lot with 400,000 cubic yards of sand, like sculpt a sandcastle bigger than Disney Hall or freshen 18 million kitty toilets. That much sand is also enough to impede the draft of marine vessels passing in and out of Marina del Rey Harbor, thus the current dredging operation at the harbor’s mouth.

Seasonal currents move the sand, which doesn’t have the common sense to know where it’s not wanted. Oblivious to the constructs of man, sand continually drifts in, making the channel shallower. Periodic dredging ensures an established depth of 20 feet MLLW (nautical speak for “mean lower low water,” the average depth of lowest tide) is maintained.

The U.S. Army Corps of Engineers planned and is managing the dredging project, which began last year and is expected to wrap up as early as Monday, March 20, weather conditions permitting. The L.A. County Dept. of Beaches and Harbors has been coordinating the agencies involved, tracking progress and warning boaters to use extra vigilance while work is underway.

The J.E. McAmis company of Chico, Calif., is the contractor undertaking the work, and the massive clamshell dredge and associated scow at the north side of the harbor’s entrance belong to them.

It is heavy, methodical work. The clamshell dredge holds itself in position via spuds (poles) instead of chains or cables, which causes less interference with boat traffic. The crane arm positions the grab (scoop), and it methodically makes its way along the area to be cleared via charts and sonar images of the sea floor. Once the area in its range is done, the dredge spuds are raised and the dredge is repositioned.

The constraints of the project preclude it from cleaning the entirety of the harbor. Instead, a priority list is generated to target specific areas where sediment accumulation is worst. The current effort foregoes the south end of the harbor entrance, which was dredged a few years back.

The north side of the harbor’s entrance is a key location for regular dredging. Not far from the end of the jetty, at a depth far below the standard specification, is a massive sand trap — a previously excavated hole designed to collect sand and prevent it from traveling further into the harbor. More than 250,000 cubic yards of sand will be removed from this location alone.

The dredge runs 24/7, unless sea conditions prevent it. The grab dumps the captured sand onto a scow, which is flat-bottomed, like a barge. The scow holds about 1,500 cubic yards of material, and makes, on average, four runs a day to Dockweiler State Beach to deposit the material about 100 yards off-shore.

“The deposit sites are pinpointed by GPS,” explained Greg Fuderer of the Army Corps of Engineers, “and are designed to re-nourish the beach.”

Currents gently redistribute the sand over time. The USACE calls this “beneficial use of waste materials.”

Personally, I would stick with “re-nourish.” It sounds a lot better, and it works for shampoo ads.

Martin L. Jacobs is on Twitter as @ML_Jacobs_Venice.

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