Sea dikes have long been one of the main coastal defense infrastructures in the Netherlands, protecting against flooding and storm surges. As climate change intensifies severe phenomena and sea level rises, dikes must be heightened to protect the coast efficiently against future threats. Meanwhile, nature-based solutions are emerging as viable alternatives. One of these solutions is the use of salt marshes as coastal protection in front of dikes instead of heightening the dike, which could potentially lead to a more cost-effective solution. A salt marsh vegetated foreshore not only dampens the incoming waves but also adapts to the advancing waters of the sea level rise by trapping sediment.
This study’s main objective is to assess the efficacy of salt marshes in mitigating wave run-up and overtopping on a real-scale sea dike with a vegetated foreshore.
To carry out these large-scale experiments were carried out at the Deltares Delta Flume. Experiments were carried out for low (0.75 m), medium (1.5m) and high (2.5m) water depths above the foreshore, designed to simulate the extreme storm scenarios experienced by the Friesland dikes in the northern Netherlands. In addition to these conditions, the tests were performed for three different qualities of vegetation, good damaged, and mowed vegetation. First, the experiments ran for a fully vegetated foreshore, then after several runs the vegetation was assumed damaged as an important part of the biomass had eroded. Finally, the vegetation is mowed to leave a bare foreshore which is the baseline scenario.
The acquired run-up results are compared with empirical equations from literature. In addition, the results were compared with those of similar research on salt marshes. Finally, the other important objective of this study is to quantify the reduction in wave loads according to vegetation conditions by introducing certain damping ratio parameters.
To measure the important parameters for the completion of this study, three wave gauges offshore and three wave gauges close to the toe of the dike were used for the measurement of the wave characteristics. For the wave run-up a camera was placed above the dike slope to capture the run-up events over time, and at the same time a LIDAR laser scanner recorded the same slope to collect data on run-up and overtopping.
From the wave analysis, the spectral parameters offshore and at the toe of the dike were calculated. From this, it was possible to have a first estimate of the wave attenuation by comparing the incident significant wave height at these two locations. The results reveal a 14 to 30 % decrease in wave height for low conditions. For medium and high storm conditions the attenuation was decreased to a range between 5 to 15% and 4 to 19% respectively.
The video process run-up measurements were in good agreement with the literature with RMSE = 0.18 - 0.19 m. An additional method was used to validate the results of the run-up. The detection of the run-up was completed visually, by tracking the waves that exceed the markers on the slope, for part of three experiments to compare their signal with the signal derived from the video process. The bias between the signals ranged from 0.017 to 0.022 m, indicating a strong agreement between the methods. One of the most important findings from this study is that the reduction in run-up is not directly attributed to the presence of vegetation itself, but rather to the role played by the significant wave height at the toe of the dike. The run-up was reduced between 2 and 16% for low storm conditions and a reduction between 4 and 24% for medium storm conditions, with an average value of 10.3%. At the same time, the reduction of run-up from damaged vegetation to fully vegetated foreshore is up to 9% with an average of 7.7%. The laser scanner was also used to measured the wave run-up. The signal obtained from the laser is compared with the signal of the visual detection method and reveals a bias between 0.032 and 0.037 m. The run-up 2% results were also compared with the same results from the video camera, revealing a 0.14 m (7%) deviation which may be due to the accuracy of the laser.
The laser scanner also obtained the overtopping results using the virtual overtopping method. From this method, the virtual volume and the virtual overtopping discharge can be calculated and then compared with the equations from the literature. The comparison reveals a strong agreement between the calculated and predicted values, which proves that the equations predict accurately the overtopping discharges for the case of a living dike. For low storm conditions, a decrease of 2 to 50 % of the maximum virtual overtopping volumes is possible, while for medium conditions a maximum volume decrease of 5 to 54% according to the crest height. For the highest storm conditions, this volume decrease is calculated to be between 20 to 28%.