Illustration: Rijkswaterstaat / Joop van Houdt

Building dams and dikes is usually considered the most efficient means of reducing the risk of flooding in river deltas. But this isn’t entirely true, claims Prof Stijn Temmerman (UAntwerp) in the journal Science: “In the long term, dikes actually increase the risk of flooding. Wetlands offer the best protection.”

Climate change has led to a rise in sea levels and extreme weather phenomena such as hurricanes, leaving world cities like London, New York and Shanghai – all located in river deltas – very vulnerable to flooding. The Low Countries, too, are high risk areas.

Coastal and riverside areas typically invest in dams and dikes. “In the short term, those measures do offer some protection”, says Prof Stijn Temmerman, a physical geographer affiliated with the Department of Biology. “But the structures disturb the natural processes taking place in river deltas. In the long term, they actually lead to a drop in the land level and thus cause local sea levels to rise more rapidly. This creates an even bigger risk of flooding.”

Temmerman and his American colleague have named America’s Mississippi delta and that of the Rhine, Meuse and Scheldt in the Netherlands and Flanders as the world’s biggest risk zones. In the Flemish Scheldt the water level has risen by up to 130 cm since 1930, five times the rise seen on the coast.

Renewal
“Coastal wetlands like salt marshes and mangrove forests play a very important role in how river deltas swell with the rising sea level. But humans have taken over that space for agriculture, industry and cities, halting the natural process of sand and silt deposits which normally ensure that the land rises with the sea level. And with dire consequences: in the Mississippi delta, for example, 100 square kilometres of land disappears into the sea every year, and in the Ganges delta, home to 170 million Indian and Bengali people, the inhabited land reclaimed from the water with dikes is now 1 to 1.5 metres lower than the remaining wetlands.”

It seems that human interventions are unintentionally creating even higher flood risks. Temmerman and colleague are calling for new, more natural solutions. “The renewal of large wetlands is extremely important. After all, a lot of water can be ‘stored’ there. Wetlands also trap sand and silt, creating a natural rise in the level of the land and thus ensuring we can better cope with rising sea levels.”

What’s more, these natural processes are often more cost efficient than building dams and dikes, especially seeing as energy costs are only set to rise in future. The world’s most ambitious plan at present involves the Mississippi delta, where 500 000 hectares of wetlands are to be renewed to give New Orleans and the surrounding area some protection from strong storms. Flood zones are also due to be restored in the Low Countries, albeit on a much smaller scale.

Source: The University of Antwerp, Belgium