Towards saving deltas

Marine ecology

Towards saving deltas

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 recently 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 1,000 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.

The sediment stickiness is controlled in part by the amount and type of vegetation surrounding the river.

The Sunderbans delta is a tide-dominated, cohesive delta. It has had no appreciable growth of new land along its coast during the last three centuries. This despite it getting a sediment load of 1.67 billion tonnes per year from the Ganga, the Brahmaputra and the Meghna rivers. And it is eroding. Why?

“Delta-building is impeded by the action of destructive waves continuously eroding the land,” explained Kalyan Rudra, member of the National Flood Disaster Management core group. “Increasing sea-level rise would drown deltas if the rate of sediment deposition on the delta is less than the rate of sea-level rise,” explained Goutam Ghosh, director of Geological Survey of India in Kolkata.

How then does the theory of changing the nature of an already cohesive delta work? Edmonds stands by his study, “What my theory really says is a cohesive delta is better at retaining sediment and therefore should be less likely to drown than non-cohesive deltas. It does not mean that it will go on retaining sediments when other erosive forces are on the rise. Even adding cohesive sediment to a non-cohesive delta would not necessarily stop it from drowning. But my theory allows for the possibility that if sediment is added to the delta over time, it might be saved.”

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