Friday, January 15, 2010

River deltas -- my sediments exactly

Down to Earth: Deltas around the world are either eroding or have stopped progressing. Climate change, sea level rise, flood management measures take the blame. Since a delta is formed where a river meets a standing body of water, there is talk about diverting more water and sediments into the bay to save the ones drowning, such as the Mississippi delta.

So far deltas were classified into three kinds depending on whether the river was the dominating force shaping it or the standing body of water: river-dominated (bird’s foot shaped), wave-dominated (fan-shaped), or tide-dominated (funnel-shaped). Geologists Douglas Edmonds and Rudy Slingerland have now proposed changing the nature of the sediments that make a delta. And the stickier the sediments, the better.

A delta composed of more cohesive sediments creates stable features that in turn retain sediment. If the sediment is non-cohesive, the delta is easily eroded. The study published online in Nature Geoscience on December 20 said that the stickiness of sediment is as important as rivers, waves and tides in controlling deltas. “This has interesting implications for the restoration of deltas, because our research suggests that the shape and morphology of the delta can be controlled on all types of coastlines. For example, in the case of Mississippi delta, a more cohesive sediment mixture should be considered,” said Douglas Edmonds, at the University of Minnesota in usa and lead researcher.

The authors of the study generated computer-simulated models of a rectangular standing body of water. On one of its borders they placed a river and specified the amount of water and sediment the river carries. Thirty simulations of discharge of 1000 cubic metre per second of water carried equal concentrations of cohesive and non-cohesive sediment into the standing body of water. All other factors like waves, tides and buoyancy were kept constant.

The results showed the relative stickiness of the sediments determined whether the deltas developed into bird’s foot deltas with rough shorelines and complex floodplains (cohesive delta), such as the Mississippi River Delta, or fan-shaped deltas (non-cohesive delta) with smooth shorelines and flat floodplains such as the Nile River Delta….

Nile delta view by NASA's Landsat 7

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