Monday, May 16, 2005

The Dredge Story, Part 1

Dredging. Some folks have walked the beach and said “gee, this beach re-nourishment thing sure was sort of a mess.” Folks up by the Port of Mansfield are is a high dither because their 20-foot deep channel is only seven feet deep now. A few charter boats have had to be hauled and have their propellers and shafts straightened already. The Port of Brownsville and the Intra-Coastal need a hundred million bucks to widen, deepen, and straighten the shipping lanes but all the Corps of Engineers money is going to the War on Iraq. One of the contributing factors to the tugboat collision with the Queen Isabelle Causeway was because of shifting sandbars on a dangerous 70-degree curve.

Dredging is one of the nastiest professions around, although it made US ports so important for this global economy. It is a relatively new technology, with steam dredges being used after the Civil War, many until the 1960’s when they were converted to diesel. The main technologies are the crane, sand-sucker, and dust-pan. Most folks understand cranes OK, which may be clamshell buckets, drag-lines, or hydraulic excavators. Ladder bucket cranes were used in construction of the Panama and Suez canals but had the same idea. The sand suckers were a little different, where spoils (the stuff on the bottom) was sucked into a pipe and deposited elsewhere. This rapidly evolved into the cutter-suction dredge we know and love today. The “cutter” snout loosens the spoils but basically it is still a pipe; its origins date back to the Gold Rush days such as the Klondike gold boats. The dustpan is pretty new stuff, evolving in about 1930 and invented for use on the Mississippi River. It is essentially a big vacuum cleaner, about as wide as the boat, maybe 30-40 feet, which is lowered to the bottom and pulled like a shrimp net. Water jets at the edges loosen the spoils. A huge pipe in the middle sucks the loosened spoils up to the ship. Now days, the dustpans are called “hopper dredges” that more resemble ships.

All three have been used at or around SPI. The cranes were used to deepen the small craft channel running from the Causeway past Louie’s up nearly the Convention Center. The cutter-suction dredge was used to maintain the Brownsville Ship Channel, especially Brazos Santiagos Pass. This is where we got the spoils from the recent “beach re-nourishment.” Dustpans were used in the past to reduce the sandbar off the jetties. Interestingly, a study was conducted ands they found that the offshore sands were much better for beach re-nourishment. A case study showed that dustpan dredges could dump their loads close to the shore, in about 14 feet of water, and that these underwater berms actually reduced erosion rates over several years, by providing a nice stockpile of high-quality marine sand a quarter-mile off the beach.

The US Army Corps of Engineers is mandated to keep commercial ports open, which is why Brazos Channel gets dredged but Mansfield does not – the latter only supports recreational fishing. Unfortunately, one of the persistent things about Mama Nature is that the channels will silt up. This silt can be similar to beach sand but will contain higher amounts of clay and organic fines. In addition, the local geology shows that there is a stratum of Rio Mud in the area of Brazos Channel. This was why I jokingly called the dredge a “clay turd machine” in a web forum. However, the SPI area is blessed because there is little or no hazardous material, except maybe for some way up by Brownsville about 17 miles up the Ship Channel. Hazardous material such as dioxin and PCB contaminated areas near Houston, New York, and Providence RI must be dumped in special landfills on land.

Most ecological studies do show major disturbances when dredge scoop up the spoils and then dump it somewhere else. How could it not? However, after about 4 months the sites usually restore themselves. In some cases there may be positive impacts. For example, the “spoil islands” along the channel create an ideal site for birds, new eel grass beds, and in some cases, oysters. As to beach re-nourishment, if sand was not periodically emplaced on the beaches, the surf might be up to the bulkheads, such as at the Iverness. The northern end of the city beach is where the beach sand is needed.

For this reason, I recommend using hopper dredges to shoot clean marine sand on just the northern parts of the city beach, maybe from north of La Quinta to somewhere near Suntide II. For a small amount of money, Texas A&M or UT could be hired to investigate if the sand is needed, and if so when the sand would be needed, and finally where the sand is actually needed. I’m a proponent of creating offshore sandbars, although the hopper dredges can actually pipe sand to the beach itself, where it can be used to create dunes or flattened to make the beach wider. Or try all three and see which works best!

Benefits would be a less invasive technique having higher quality sand that is more consistent with the native beach sands – and no need for seven miles of pipe on the beach. I am not sure about the costs, which is why you want a reputable study before making future decisions.

There is some anecdotal information that beach re-nourishment over the years has changed the beach ecosystem. The conventional method using Brazos Channel spoils seems to affect the ghost crabs on the beach, the blind shrimp in the surf zone, and near-shore the crustaceans, crabs, and baitfish. I am not sure if these patterns are part of a larger decline which might be common to the entire Gulf coastline, but there sure seems to be less shell on the beach.

At some time in the future, something will have to be done because of the high erosion and depletion rates at the northern end of the island’s city beaches. A few good tropical storms and it may become a necessity.

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