The role of mangroves in coastal flood protection: The importance of channelization

The historical, present, and projected future risk of coastal flooding necessitates measures to protect coastal communities and environments, including using natural defences such as mangroves. Estimates of extent of flood protection provided by mangrove forests range widely, probably because of differences in growth characteristic of trees combined with variations in local geomorphology in each case study site. Here we use a Delft3D model of a mangrove forest in Tauranga, Aotearoa New Zealand, with vegetation flow resistance parameterized by frontal area, as a basis of idealized scenarios designed to explore and generalize the impact of channels, vegetation density, and forest slope on long wave (surge/tide) attenuation. Previous models often parameterise the frictional effect of mangroves with high bottom friction, which we show using theoretical arguments, becomes increasingly problematic with larger amplitude flood events. However, the contorted forms of the shrubby Avicenna mangroves at the field site necessitate new ways to quantify vegetation density more accurately for modelling. Nevertheless, modelling experiments show that vegetation distribution and channelization have a greater impact on flood attenuation than vegetation density, with large events resulting in less attenuation than smaller events due to the increased hydraulic efficiency associated with increased water depth. Additionally, a forest with sloping topography reduces the rate at which long wave attenuation occurs. Implications are that mangroves can only be effective coastal flood protection if the larger scale (forest-scale) distribution of vegetation also contributes to reducing the landward flow of water.