Seventy per cent of the world’s megacities (cities with more than 1.6 million inhabitants) are located in the coastal zone. The anticipated rise in sea levels and more frequent extreme weather events make it all the more essential to protect these areas against the sea. This is traditionally done with dykes and sea walls, but, according to Willemsen, these defences can be made even more effective and sustainable if salt marshes can develop in front of them. “These tidal marshes and the vegetation they support will make these coastal defences more sustainable,” says Willemsen.

Keeping pace with rising sea levels

Because they will be able to keep pace with rising sea levels, both natural and artificially created salt marshes could contribute to sustainable coastal protection. “As the sea level rises, it deposits more sand and silt on the salt marsh, causing it to expand at the same pace as the sea level rises,” explains Willemsen. “The salt marshes in turn dampen the incoming waves, even as the sea level continues to rise.” For his PhD research, Willemsen studied the effectiveness and predictability of wave attenuation by salt marshes during the average lifetime of traditional coastal defences (around 50 years).

Model and field measurements

To be able to predict the width of a salt marsh over this period of 50 years, Willemsen developed a computer model with which he could vary the average wave height and the amount of available sediment in the water. The results of his model corresponded with what he observed on old maps and photographs of salt marshes, and with his measurements of waves in existing salt marshes. The model predicts if the edge of a salt marsh will expand seawards or in fact shrink landwards and how this will affect the wave attenuation capacity of the marsh.

Further information

Pim Willemsen carried out his research at the Department of Water Engineering and Management (WEM; Faculty of Engineering Technology) of the University of Twente and the Netherlands Institute for Sea Research (NIOZ). His supervisors were Prof. Suzanne Hulscher (University of Twente), Prof. Tjeerd Bouma (NIOZ) and Dr Bas Borsje (University of Twente).