Nature-based flood protection: The efficiency of vegetated foreshores for reducing wave loads on coastal dikes

Abstract

This paper analyses the effect of vegetation on wave damping under severe storm conditions, based on a combination of field measurements and numerical modelling. The field measurements of wave attenuation by vegetation were performed on two salt marshes with two representative but contrasting coastal wetland vegetation types: cordgrass (Spartina anglica) and grassweed (Scirpus maritimus). The former is found in salty environments, whereas the latter is found in brackish environments. The measurements have added to the range with the highest water depths and wave heights presented in the literature so far. A numerical wave model (SWAN) has been calibrated and validated using the new field data. It appeared that the model was well capable of reproducing the observed decay in wave height over the salt marsh. The model has been applied to compute the reduction of the incident wave height on a dike for various realistic foreshore configurations and hydraulic loading conditions. Additionally, the efficiency of vegetated foreshores in reducing wave loads on the dike has been investigated, where wave loads were quantified using a computed wave run-up height and wave overtopping discharge. The outcomes show that vegetated foreshores reduce wave loads on coastal dikes significantly, also for the large inundation depths that occur during storms and with the vegetation being in winter state. The effect of the foreshore on the wave loads varies with wave height to water depth ratio on the foreshore. The presence of vegetation on the foreshore extends the range of water depths for which a foreshore can be applied for effective reduction of wave loads, and prevents intense wave breaking on the foreshore to occur. This research demonstrates that vegetated foreshores can be considered as a promising supplement to conventional engineering methods for dike reinforcement.

Risk reduction by combining nature values with flood protection?

“…simulations with the SWAN wave model for a study site in the Dutch Wadden Sea suggested that under storm conditions with a frequency of 5-10 times/year a vegetated foreland of some 90 m in width will dampen the waves more than 80%, whereas under extreme conditions (1/2000 year -1 ) a foreland covered with dense vegetation will dampen the wave height up to 50% [28]. These modelled values were in line with field measurements of wave damping by a salt-marsh zone under a 1/10 year -1 storm event [30].…”

“…However, especially during extreme events, when flood protection is needed most, wave damping is less then under less extreme circumstances due to the high water level above the marsh surface. Although modelling [28] as well as flume experiments [29] and field observation [30] indicate that even under storm conditions salt marshes are able to substantially attenuate waves, there are still question about their performance. Furthermore, the marsh areal can be affected by lateral erosion during extreme events.…”

“…The exact reductions under different scenarios are not yet known and require the calibration and refining of existing models with field observations during storm events. This work is currently undertaken as part of a recently launched Dutch research programme [30]. …”

“…30,[35][36][37][38], experiments under controlled conditions in a laboratory [e.g. 29,39], or modelling work [e.g.…”

Editorial: Promoting coastal resilience using green infrastructure

“…The article usefully adds to the growing evidence-base on the effectiveness of coastal vegetation in attenuating the effects of wave energy, which to date, has largely been focused on the role of emergent vegetation (e.g. Vuik et al, 2016;Rupprecht et al, 2017;Anderson and Smith, 2014).…”