Wave-induced stem breakage in a vegetated foreshore and its implication on probability of flooding

Abstract

Various flood protection measures are studied across the globe, and nature-friendly and environmentally resilient methods are gaining more attention. As part of the building with nature initiative, the project BE SAFE (Bio-Engineering for SAFEty) studies the effects of a vegetated foreshore as a flood protection measure which is found to be very effective. Vegetation helps to reduce wave energy as the stems and canopy work as small hurdles and obstacles that the waves need to pass. In this process, the waves lose much of its energy and the wave height reduces. As a result, the wave height is lower at the shore and less force acts on the coastal dike. From previous research, vegetation is known to be a dominant measure of wave energy dissipation, but the detailed processes of how it interacts with waves is not well known. Until now, vegetation in the foreshore has either been completely ignored (by coastal dike managers) or considered healthy and abundant (by ecologists). This research acknowledges that vegetation exists, but its strength and stem density may vary depending on the location and time of the year. The focus of this research is understanding the interaction between vegetation and waves, as well as its implications to the probability of dike overtopping and flooding. As waves pass through vegetation, stems break from the wave forcing which results in a variation of stem density in time (season) and space (foreshore). In this research, the mechanical interaction between vegetation stems and wave force is assessed to understand the point of stem breakage. Further, the vegetation stem breakage is implemented into the wave energy balance and in the probabilistic model of V. Vuik to quantify the probability of overtopping and dike failure. Stem strength is quantified by the three-point bending test results from NIOZ (Royal Netherlands Institute for Sea Research), which is used to calculate the maximum allowable stress of the stem. This stem strength is compared to the wave-induced stress which is formulated by taking the Morison-type equation to quantify a uniform wave load acting over the submerged length of the stem. In this mechanical analysis, stems are assumed to break when the wave load exceeds stem strength. Stem breakage is then implemented into the wave energy balance formulas, through which the stem density variation affects the amount of wave energy dissipation by influencing the wave height transformation. A correction factor is introduced to take into account leaning stems, but the correction factor could also include other simplifications that are not accounted for. Further, the performance of the wave energy balance is assessed through a sensitivity analysis of incoming wave height and seasonal vegetation data. Seasonal vegetation data and stem breakage is implemented to the probabilistic model of V. Vuik which quantifies the probability of flooding (due to overtopping). Vegetation and correlation scenarios are tested to find the optimum approach and address the uncertainty in the result. Of the different vegetation scenarios, the most realistic approach is the percent stem breakage which evaluates wave load to the normal cumulative distribution function of stem strength. Further, the uncertainty of model results is reduced by using the correlation scenario with characteristic relations between vegetation parameters. Including vegetation stem breakage in the probabilistic model produces reasonable results, yet further research to calibrate the correction factor and to better define characteristic relations would strengthen the model result.