Known for its low-lying and flood-prone geography, the Netherlands has a long history of flooding. Flood events in the 1990s, which led to large-scale evacuations, shifted the country's focus from simply strengthening dikes to creating more space for rivers to flow and storing water to prevent similar disasters. With changing climate conditions, rising water levels, land subsidence and sediment supply issues, the safety of primary flood defences needs to be regularly reassessed. The latest safety assessment, using a new methodology that assesses dikes based on the probability of flooding, found that many dikes do not meet the required safety standards. These dikes will need to be redesigned and reinforced, and there are opportunities for innovative and environmentally friendly solutions.
Previous research has extensively investigated the potential of different ecosystems, such as salt marshes, mangroves and willow forests, to reduce the impact of waves on coastal and riverine structures. Studies have shown that vegetation can significantly dissipate wave energy, thereby reducing the load on flood defences. However, the dynamic and unpredictable nature of vegetation has made it difficult to incorporate it reliably into defences. While there is general agreement on the benefits of vegetation for wave attenuation, its reliability in reducing failure probabilities requires further investigation.
This thesis tests the potential of riparian forests for wave attenuation and dike design optimization. The main objective is to investigate how wave attenuation by riparian forests can improve dike safety and to identify the critical variables that most significantly affect the probability of flooding. Key research questions include how wave attenuation can contribute to dike design and safety assessment, and which variables most influence the probability of flooding.
Vegetated foreshores can reduce the height of incoming waves, with positive implications for dike design and safety assessment. A model was developed to test this by combining current wave impact and run-up models with a wave energy balance that includes energy dissipation by vegetation. The model was subjected to a Monte Carlo simulation to determine the probability of dike failure due to erosion of the outer slope by wave impact and run-up. The model analyzed a dike section near Kampen and showed that riparian forests can significantly reduce the impact of waves on dikes. The results showed that the forest length, frontal-surface area and stiffness of the forest must be large enough to be effective. Strategically placing trees with the largest frontal-surface area at critical points can significantly reduce the probability of dike failure.
In conclusion, this thesis highlights the potential of riparian forests to improve dike safety and contribute to a more resilient and environmentally friendly flood protection strategy in the Netherlands.