In tidal saltwater marshes, research has shown that vegetation enhances sedimentation. But scientists know less about how vegetation affects sedimentation in freshwater marshes that are common in river deltas. Nardin and Edmonds used sophisticated computer modeling to study how marsh vegetation influences the transport and deposition of sediment in river deltas. They conducted 75 simulations involving varying scenarios of vegetation height and density and rates of water flow.
They found that vegetation of intermediate height and density results in the greatest deposition of sand and mud. However, if the plants are too tall or densely packed, sediment tends to remain in the river channel, bypassing marshes and being carried directly to the sea.
The researchers also analyzed remote-sensing data collected from Wax Lake Delta in Louisiana. The analysis showed the delta exhibits a hydrodynamic response to the presence of vegetation that corresponds with what the researchers found with their model.
The rate at which sediment is delivered to marshes in river deltas is strongly influenced by storms and flooding, as well as the construction of engineered diversions. The research suggests the resilience of river deltas will depend on the timing of floods and storms relative to the seasonal growth of vegetation. And of course, scientists can't control or even predict when floods will occur.
"That unpredictability adds an interesting twist to the problem," Edmonds said. "The time that the flood wave arrives does not always coincide with the timing of vegetation growth and decay. Those two have to line up just right for vegetation to really enhance deposition of sand and mud."
|Contact: Steve Hinnefeld|