Predicting Sediment Flow in Coastal Vegetation

Model could help engineers design erosion-prevention strategies in marshes, wetlands, aquatic forests

Seagrass, kelp beds, mangroves, and other aquatic vegetation are often considered "ecosystem engineers" for their ability to essentially create their own habitats: Aquatic leaves and reeds slow the flow of water, encouraging sediments to settle nearby to form a foundation on which more plants can grow.

Such underwater forests provide shelter to hundreds of organisms, and can also protect shorelines from erosion. However, in the last few decades, large swaths of aquatic vegetation have disappeared around the world, including 100 million acres of wetlands, and thousands of acres of seagrass and kelp beds, in the United States.

In large part, sediment transport — how sediment flows through a region — determines the survival of coastal marshes and mangroves: Plant growth depends on the accumulation of sediment to the seafloor. When strong storms or currents carry sediment away, underwater forests can also wash away, exposing coastlines and riverbanks to erosion.

Now researchers at MIT have developed a simple model that can help scientists understand how and when sediments move through a region of aquatic vegetation, such as a wetland. The researchers say engineers may use this model to design better ways to restore seagrass, mangroves, and other underwater plant beds. For example, using the model, scientists may be able to identify locations where aquatic vegetation may be less prone to erosion.

"Wetlands are very important because they protect our coastal areas, but they are eroding," says Qingjun Yang, a graduate student in MIT's Department of Civil and Environmental Engineering. "With this, engineers can do modeling on how the stresses vary, and whether it would be helpful to plant vegetation here or there, based on the equation."

Yang and her colleagues —Heidi Nepf, the Donald and Martha Harleman Professor of Civil and Environmental Engineering at MIT, and postdoc Francois Kerger — have published their results in the journal Water Resources Research.

Catching drift

To estimate sediment transport in aquatic environments, one key factor is what's known as "bed shear stress" — the friction exerted by water at the seabed, which gives scientists an idea of how sediments move across the seafloor. Existing models and equations calculate bed shear stress for underwater environments without vegetation. However, there exist no applicable models for vegetated regions, as plants create more complicated currents and eddies, muddying the picture of sediment transport through such regions.

Yang and her colleagues sought to develop a model of bed shear stress for vegetated environments by first setting up a controlled experiment to simulate sediment transport through a simple, reed-like environment.

In a large, 10-meter recirculating water tank lined with a bottom layer of plastic, the researchers erected thousands of thin dowels to simulate sturdy, marsh-like reeds. They then deposited polymer particles in the water, and ran a pump to circulate water through the tank.

Using a technique called laser Doppler velocimetry, they aimed a pair of lasers into the tank at various depths and positions. The researchers used the lasers' backscattering, or reflected light, to calculate the particles' velocity at a particular location. As the particles were very small, their velocity was equal to that of the surrounding water parcels, or groups of water molecules. The researchersthen converted velocity measurements into estimates of friction, or stress, between water parcels, and at the bed.

Source: MIT News

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