Delta Life 10

DELTARES, September 2018 1515 A s the sea level rises, the land is expected to subside in the decades to come. In some places by up to 15 to 20 centimetres a year, for example due to the extraction of oil, gas or water. 'Obviously, that makes the country difficult to defend against flooding,' is how Peter Herman (Senior Advisor at Deltares and, since May, Professor of Ecological Hydraulic Engi- neering at Delft University of Technolo- gy) describes the threat of these two simultaneous developments. 'Particu- larly in areas where the sea defences are not so strong, or where there are no defences at all.' Fortunately, there are ways to tackle the threat. One is to create a coastal system that rises with the sea level: you use plants or trees to collect silt so that the land sort of 'grows back'. This can sometimes be a better solution than a hard dike alone: if the land continues to subside as the water level rises, the dike height and stability will no longer be adequate over time. But systems like this rise in line with the water level. Peter Herman: 'We can often make dike designs much lighter by providing protection in the shape of a foreland: an overgrown strip between the dike and the water.' Woods versus waves Deltares is looking at the best way of using nature in the battle against water. For example, we conduct trials in the Delta Flume, a 300-metre-long, 5-metre-wide and 9-metre-deep flume through which researchers can send waves up to 5 metres high. As part of the Woods versus Waves project, wil- lows were positioned in the flume to look at how they can break large waves. The aim is to find out to what extent willows on riverbanks can reduce the impact on the dikes behind them. This major trial is, to a large extent, the 'baby' of Deltares researcher Bregje van Wesenbeeck. She managed to obtain the 500,000 euros that were needed from a range of sources including Rijkswaterstaat and a crowd-funding campaign. She is now planning similar trials with mangroves, explains Peter. 'These trees play an important role in some of the largest river deltas in the world.' In order to really integrate adaptive coastal systems in coastal protection systems, more research is needed. 'It's not just a question of knowing what plants do to waves. You also have to be able to predict whether the vegetation will still be there in twenty years and whether they capture enough silt.' So Deltares is collaborating with partners in the field and using models to study the development of forelands between dikes and the water. 'What is the mini- mum width needed for a strip to prevent erosion by the sea? And if you have a very wide strip, how much lower can you make the dikes behind it?' Miracle solutions Obviously, solutions to water problems of this kind sound fantastic: willows that protect dikes from waves, mangroves that prevent subsidence in river deltas, plants on the coast that capture silt... But these are not miracle solutions that make other approaches redundant, emphasises Peter. 'For example, dams in rivers can drastically reduce the amount of silt that reaches the coast. When that happens, vegetation in the river delta to capture silt and raise the bed will be pretty pointless. And if the land is subsiding ten centimetres a year as local people extract drinking water, you could be better off installing a cen- tral drinking water system. It's still important to work out what is actually causing the problem.' For more information: peter.herman@deltares.nl 'Using plants and trees to capture silt allows the land to grow back.' WATER CHALLENGES DOSSIER Self-sustaining dunes A large part of the Dutch coast is protec- ted by dunes, not dikes. They are a good example of an 'adaptive coastal system'; a natural system that keeps pace with the water level. Thanks to Rijkswaterstaat, that ensures the dunes are always supplied with enough new sand to sustain themselves. In addition, Rijkswaterstaat is currently investigating how this can be done as environmentally friendly as possible.