T.J. van Veelen1*, H. Karunarathna1, D.E. Reeve1
1 Zienkiewicz Centre for Computational Engineering, Swansea university, United Kingdom
Nature based coastal defences in the form of vegetated foreshores are increasingly preferred over traditional hard defences. Salt marshes are a common vegetated foreshore in temperate climate zones. Their vegetation dampens incoming waves to reduce the load on coastal vegetation structures. However, salt marshes exhibit a wide variety of species, ranging from rigid shrubs to flexible grasses. These interact differently with waves and provides a different level of protection. This is often included in numerical model through the calibration of a drag coefficient. This is not suitable for planned nature-based defences or when vegetation conditions are subject to change. Here, we provide a new modelling approach that links wave damping directly wave and vegetation properties, including plant flexibility.
Based on combined experimental investigations of wave heights, orbital velocities and plant motion in the Swansea University Coastal Engineering Laboratory, we have developed a physics-based model that includes the effect of plant motion on wave damping. The modelled wave height is directly linked to wave and vegetation properties, including flexibility. Our model is validated for five vegetation species that differ in flexural rigidity under a wide range of wave conditions. We combined the validated model with Delft3D to study wave damping by salt marshes in the Taf estuary under five vegetation conditions.
The salt marshes in the Taf estuary contribute significantly to coastal protection. Wave heights reduce by up to 75% in the first 100 metres. The impact of vegetation flexibility can be significant and strongly relates to the local conditions. It will be shown how vegetation flexibility can be introduced in numerical models, such that model predictions of wave height are less dependent on local calibration.
Figure 1 Modelled wave damping under a mixed vegetation scenario (top) and for five scenarios on selected transect T1.