The potential of dune vegetation during storm conditions

Abstract

Coastal protection is required to keep coastal areas protected during storm conditions. Coastal dunes are a natural defense against flooding and erosion from the sea against storms. Many insights have been obtained regarding the positive effects of coastal dune vegetation on dune formation and growth in the past. Several recent studies have demonstrated the ability of both aboveground and belowground dune vegetation to reduce dune erosion during storms.

However, the impacts of dune vegetation during collision regime storm conditions are not taken into consideration in morphological numerical modeling. Consequently, it is unclear how to evaluate vegetation impacts during design assessments. Therefore, the goal of this research is to investigate the potential effect of dune vegetation on dune erosion during collision regime storm conditions and subsequently link this with dune rehabilitation projects. This research is divided into two parts.

First, the capability of the numerical model XBeach to simulate the potential effects of dune vegetation during the collision regime is investigated. Four vegetation approaches are identified in the model, which could possibly represent different dune vegetation effects. Using beach-dune profiles and erosion volumes obtained from two wave flume experiments, the performance and sensitivity of the vegetation approaches are tested in XBeach. Thereby, a distinction is made between the effect of aboveground vegetation: hydrodynamic altering and belowground vegetation: soil stabilization. The model results show that XBeach is capable of simulating dune erosion with vegetation during collision regime storm conditions. This can be primarily attributed to the increase of the critical slope in the avalanching module. This approach represents the soil stabilization effects of belowground vegetation. The application of the root model was added for even better simulations. This approach accounts for the additional root cohesion provided by belowground vegetation by increasing the critical velocity for sediment pickup. The values to be used for both vegetation approaches could not be defined systematically yet due to the small number of experiments assessed and little research found in the literature. It is recommended to obtain more information about the effect of dune vegetation on the stabilizing effects and the critical slope for avalanching. The application of a higher roughness value and the vegetation module, which both represent aboveground vegetation by altering the hydrodynamics, have shown to contribute insignificantly to erosion reduction in the examined cases.

The belowground soil stabilization approaches are applied in a case study to give a first indication of the applicability of XBeach at large-scale and in dune rehabilitation projects. An XBeach model of Beira, Mozambique was set up. A higher critical slope demonstrated profile evolution and erosion reduction in line with observations in the literature. During wave impact, the avalanching module is the most suitable approach to account for the erosion-reducing effect of belowground vegetation. The root model appears to be a more appropriate strategy for accounting for belowground vegetation during milder conditions and shorter storm duration. It was proven that XBeach has very good potential to evaluate the effectiveness of dune vegetation in the design phase. The case study illustrates that vegetation will significantly increase the erosion resistance of the flood dunes. The dunes with vegetation can handle an additional 20-30 centimeters design water level or 0.5-0.8 meter storm wave height without breaching compared to the same dune without vegetation. The design with vegetation has shown to be more robust. However, the approaches are based on different assumptions and limitations and therefore the results should be considered carefully. It is strongly advised to conduct more research in this relatively new study area.

This study investigates and proposes a method for quantifying the advantages of vegetation on dune erosion reduction. The relevance and added value of mature and robust dune vegetation for a resilient coastal dune system and protection against erosion and floods are confirmed. As a result, the incorporation of vegetation in dune rehabilitation projects is promoted.