Building with Nature: Common cordgrass and grassweed help dikes
Vegetated foreshores can strongly reduce wave loads on coastal dikes, meaning these dikes may not have to be built so high. On Wednesday 27 March, Vincent Vuik will be awarded his PhD at TU Delft for his work on this subject.
‘Protecting coastal areas from flooding has traditionally taken a hydraulic engineering approach, using dikes and dams. However, in recent times more attention is being paid to natural solutions for water safety’, says Vincent Vuik. ‘Such solutions utilise the properties of coastal ecosystems to mitigate the impact of extreme weather conditions. Ecosystems such as salt marshes, mangrove woods, coral reefs and dunes are protected, reinforced or even created from scratch, to reduce the risk of flooding. Natural solutions such as dunes can hold back water themselves but they can also work in conjunction with constructed barriers, for example in the form of vegetated foreshores that limit the wave load on the dike or dam behind them.’
Vincent Vuik's PhD research is focused on such hybrid solutions in which wave load that impacts a dike or dam can be limited by a salt marsh – a vegetated foreshore – in front of the dike. ‘Knowledge about the working and stability of vegetated foreshores in severe storms is still limited. Moreover, these natural systems fluctuate relatively strongly in space and time. So we are still relatively unsure about the extent to which vegetated foreshores can contribute to water safety in comparison to constructed barriers.’
The research was aimed at developing methods for determining how and to what extent natural solutions can reduce flood risks. ‘Among other things we investigated the influence of vegetated foreshores on wave height, wave run-up and wave overtopping discharge during storms. For a period of around three years (2014-2017) we took continuous measurements on salt marshes in the Western Scheldt and Wadden Sea. We recorded a strong reduction in wave height during storms, with more than 50% decay across a 300-metre wide section of salt marsh. During extreme storms it will be primarily the higher bed of the foreshores that ensure that the wave height at the dike is lower than without such a foreshore. And the roots of the salt-marsh vegetation, such as common cordgrass and grassweed, reinforce the soil so that there is virtually no erosion. This study enables us to demonstrate that vegetated foreshores can significantly reduce the wave load on dikes.’
This research enables hybrid defences to be assessed according to state-of-the-art safety standards, based on failure frequencies. We expect this research to contribute to vegetated foreshores and similar natural solutions for water safety being used more often, because their effectiveness and costs over the entire life-cycle can now be directly compared to those of traditional hydrological engineering structures.
This research is part of project BE-SAFE van Building with Nature.
Karlijn Spoor, Communication Advisor Civil Engineering and Geoscience TU Delft, +31(0)15 2789885, K.Spoor@tudelft.nl