Sediment traps won’t do squat for the coast!
by Len Bahr, PhD*
On March 18 Mark Waller reported in NOLA.com that Webster Pierce, Jr., an inventor from Cutoff, Louisiana has obtained a patent for a plastic device he calls “Wave Robber” that he believes could trap coastal sediments to help rebuild the coast. I share Mr. Pierce’s commitment to the great coastal cause but, with all due respect, he’s wasting his time on sediment traps.
Waller’s article was updated on March 19 to announce that the Wave Robber won the New Orleans Ideas Village Entrepreneur Week Water Challenge award, carrying a prize of $50,000 from the Greater New Orleans Foundation. Kari Dequine Harden covered the story for The Advocate. This award is a big deal, as evidenced by the fact that both Harry Shearer and Walter Isaacson showed up to encourage the contestants.
Back in the day, while directing the Governor’s Office of Coastal Activities, I was frequently approached by folks anxious to demonstrate a new coastal ‘mousetrap,’ a sediment-trapping device that could revolutionize our efforts to slow the annual loss of 16 square miles of coastal landscape. The sediment trapping concept typically involved installing linear rows of “immovable” objects along the gulf shoreline that would, in theory, absorb hydraulic wave energy and induce suspended clay and silt particles to settle out of the water column, elevating the substrate and offsetting subsidence.
Proposals for sediment traps along the coast are inevitably based on two basic fallacies about why and how the coast is disappearing. First they assume that the major problem is shoreline retreat from south to north, when shoreline erosion is not the primary issue. The entire landscape is in fact breaking up from within. Second, sediment trap proponents assume that if we could capture much of the suspended mud that turns the water brown from Grand Isle to Holly Beach and induce it to settle in place, land loss would decline.
Trapping suspended coastal sediments is a fool’s errand, sort of like bailing a sinking sailboat with a spoon.
Noted Tulane geographer Rich Campanella recently estimated that the Mississippi River Delta is currently running a nearly insurmountable annual sediment deficit of 1 billion tons (200 Mercedes-Benz Superdome Equivalents or SDEs). The only net sediment input to the delta comes from the Mississippi River, which carries only half the sediment it once did. Suspended sediments in our nearshore waters are not new to the system; they are primarily recycled river sediments, dislodged from the local landscape. In other words, trapping them wouldn’t change the annual net loss.
Be that as it may; let’s imagine hypothetically that ten thousand wave robbers were installed along the coast for a year, each successfully trapping 100 lbs. of silt per day (a huge leap of faith). At that rate, at year’s end the coast would be about 0.37% of one SDE closer to balancing its sediment deficit of 200 SDEs. Whoop de do.
Finally, the proposed sediment traps, no matter how well they’re anchored, are inevitably vulnerable to the ignominious fate of being ripped loose and scattered across the coast during storms, creating labor intensive recovery issues. Mr. Pierce boasts that his Wave Robbers are buoyant, which makes this circumstance more likely, even with relatively small storms.
Failed demonstration projects have included mats of recycled tires cabled together, concrete rings designed by a Tulane engineering professor, devices resembling short slotted staircases or shelves and many other versions too numerous to recall. Mr. Pierce’s plastic traps remind me of these efforts and I strongly argue against wasting our collective time and money.
I’ve been told that the Wave Robber is undergoing field testing along an open shoreline near Cutoff. I hope that the test extends through the 2013 hurricane season.
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