Cashing in on Offshore Sand Deposits

Beach replenishment discussion kicks off Coastal State Series

 

Wind and waves crashing on a beach gradually move the shoreline back. At Napatree Point in Westerly, where this is allowed to happen naturally, as there are no structures to contend with, the entire barrier (including the beach) has moved 200 feet north since the late 1930s as storms overwashed the barrier and deposited washover fans.

But where there are streets and buildings blocking the natural flow of sand, beaches don’t move – they erode. Replenishing those beaches, whether to protect the structures behind them or to widen the beach itself for tourism and recreation, is both costly and—if it is to be effective—ongoing. To have any enduring effect, significant amounts of sand must be added to the beach and replenished every 2 to 10 years, said Bryan Oakley, Eastern Connecticut State University assistant professor and geologist, at the Coastal State Discussion Series in February.

That sand can be added to dunes, the upland area of the beach. Dune replenishment typically involves planting grass as well. Oakley showed a slide of a dune replenishment project where all the grass appeared to have died, highlighting one challenge of trying to build up dunes, which are often replenished to protect adjacent development. Another challenge with such projects is that the height of the dunes needed to meet the Federal Emergency Management Agency’s threshold to survive a significant storm would often be so high as to obscure the view of the ocean from the buildings being protected.

Another form of beach replenishment is adding sand to the berm, or the flat area of the beach where people typically recreate. This approach adds protection for the dunes as well as increasing the recreational area. This is often accomplished using dump trucks to deposit and grade the sand on the beach. Placing sand offshore, using vessels that carry multiple dump trucks’ worth of sand at a time, allows waves to carry the sand to the beach. While some sand is lost to the system in the process, what does end up on the beach is placed in a more natural pattern than what can be done with dump trucks.

Oakley’s talk highlighted the lack of entirely neat solutions to the challenges of preserving beaches.

Dredging accumulated silt from the salt ponds, some people suggest, could be a source for replenishing nearby beaches. True restoration, however, of a beach like Misquamicut requires far more sand than could be dredged out of Winnapaug Pond behind it, and the presence of other materials in the sand can pose problems as well, as a project to dredge Point Judith Harbor and use the sand for offshore beach replenishment demonstrated.

“They moved somewhere in the neighborhood of 80,000 cubic meters or so out of Point Judith Harbor and brought it over to two different placement areas on the Matunuck shoreface,” Oakley said, “The infamous part of this is that there was a lot of debris in the Point Judith sand—lobster rubber bands, pieces of netting, deck boots, and all kinds of stuff you’d [find in] a commercial fishing harbor. All that stuff started washing up on the beach.”

In 2014, with $3.1 million in Superstorm Sandy relief funds, the state replenished 65,000 cubic meters of sand to the berm area of the beach. To put that number in perspective, that much sand required approximately 3,000 dump trucks to cart it to the beach.

That project cost $47 per cubic meter of sand, which was acquired from a land-based source. Sand that can be obtained offshore and pumped directly onto a beach can cost in the range of $20 per cubic meter, but you have to know where to find it, as was the case for New Jersey, which was heavily damaged by Sandy and replenished its beaches using offshore sand. John King, University of Rhode Island oceanographer and ocean geologist, described at the lecture work that was funded by the Bureau of Ocean Energy Management (BOEM) to identify sites off the coast of Rhode Island to determine whether they contain enough suitable sand to replenish state beaches for years to come.

Since the project was funded by a federal agency, the work focused on federal waters that are beyond the 3-mile limit of state waters and where the bottom is less than 70 feet deep, the depth limit beyond which extracting the sand is impractical.

The researchers targeted areas they expected to find quality sand—sand of the right grain size and with minimal organic material that would make it unsuitable to use—areas that were left behind by retreating glaciers 20,000 years ago.

 

Using a bubble gun—sonar equipment whose sound waves can both penetrate the seafloor to show what types of sediment are present and minimally affect any nearby marine mammals—they surveyed those areas to determine where the sand deposits are and how thick they are.

They then ground-truthed those areas by extracting sand from them.

“You not only need to know if it’s sand, you need to know if it’s high-quality sand,” King said, “If it’s full of debris or it’s slightly muddy or it smells bad, people don’t like that being dumped on their beach.” The team used a vibrating pipe known as a vibracore, King said, to reach deep into the sand for samples, which capitalizes on the property of sand that causes it to spontaneously liquefy, allowing the core to sink in easily.

After all these surveys and samples, did they find enough sand?

“We need a total volume of 6,100,000 meters to do the projects Bryan [Oakley] was talking about,” King said, “The offshore resource we actually have out there [is] an order of magnitude more sand out there than we need anytime soon.”

Though there is ample sand for beach replenishment, the process of removing that sand from the seafloor will have consequences.

While some of the deposits are several meters thick, “You can’t go out there and do anything to the bottom without there being some cost, so there have to be some pretty strict constraints on what you can actually do,” King said, “If you … remove 20 meters of material, you’ve fundamentally changed the environment, so what we’re really talking about is probably removing a meter and a half to three meters, so 5 – 10 feet of material could get scooped off the top of this … but the question is, can you go out there and remove 5 to 10 feet of this material without having a really devastating impact to the bottom? That’s a question we don’t have an answer to yet.”

“We need to do more characterization; that comes in the second phase” of research BOEM has agreed to fund, King added, saying that once the area is more thoroughly analyzed, including to locate and avoid paleocultural resources such as shipwrecks and Native American villages, the next step would be to convene stakeholders, such as fishermen, and look at tradeoffs.

But King is convinced the sand mining is inevitable.

Even though the project would be expensive, “You can, I think, make the case that the economic value of the beach to Rhode Island has to exceed what we’re talking about … I don’t think it’s going to come down to whether we’re going to do this or not … I think the answer is we’re going to do it, it’s just a matter of what are the impacts, what are we willing to trade off for the fact that we’re going to do this.”

[info]The mapping work John King talked about in his presentation grew from a Rhode Island Sea Grant-funded project, BayMap, and work done as part of the R.I. Ocean Special Area Management Plan.

This talk was the first in the 2017 Coastal State Discussion Series sponsored by Rhode Island Sea Grant and supported by the University of Rhode Island’s Coastal Institute, College of Environment and Life Sciences, and the Graduate School of Oceanography. For information on upcoming events, visit the Coastal State webpage.[/info]

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– Monica Allard Cox | Rhode Island Sea Grant Communications

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