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Attenuation of tides and storm surges in coastal mangroves

Mangrove forests have been shown to provide coastal protection by reducing waves, moderating currents, stabilizing sediments, and lessening storm winds. However, the interaction between storm surge and mangroves is less understood with some studies showing substantive water level reductions across mangrove forests and other work demonstrating negligible flood reduction. Here, the relationship between mangrove forests and storm surge is investigated using a variety of techniques including a simplified analytic solution, water level observations in two contrasting mangrove forests (with and without channels), and a numerical model used to explore the importance of variations in vegetation density and complex bathymetry. The analytic solution to flow through vegetation was based on the shallow water approximation to the equations of motion and predicts water level across a simplified mangrove forest during a flood event. The solution accurately reproduced observed peak water levels of a 10-year flood event in the Firth of Thames, New Zealand and in a forest during Hurricane Charley in Ten Thousand Island, Florida. The analytic solution demonstrated that in a simplified scenario vegetation density, forest size, flood amplitude, and flood duration determine peak water level reduction. Mangroves reduce flood levels by limiting the landward flow of water and acting as a water storage mechanism. The variability in flood protection provided by coastal mangroves is demonstrated through the comparison of inundation events in two contrasting forests in New Zealand, channelized mangroves in Tauranga and a more homogenous forest in the Firth of Thames. New observations from Tauranga were collected to complement existing observations from the Firth of Thames. Both forests are ~1 km wide and populated by mono-specific cultures of grey mangroves and subjected to inundation events that reached a depth of ~0.6 m at the seaward edge of the forest. However, no reduction of water level occurred across the mangroves in Tauranga and the forest in the Firth of Thames provided quantifiable flood protection. The influence of channels, variations in vegetation density, and forest slope on the flood protective services provided by mangroves was explored with a series of numerical “experiments” based on a depth integrated 2-D numerical model of a mangrove forest in Tauranga, New Zealand. Modelling results demonstrated that channelization and large-scale distribution of vegetation are more important to determining flood attenuation than detailed characterization of the vegetation. Population densities in coastal areas are rapidly expanding; simultaneously, intertidal ecosystems that provide coastal protection are degrading resulting in an increased reliance on traditionally engineered solutions. Sea-level-rise and a projected increase in storm frequency and severity requires modifications to existing coastal defense strategies. Natural ecosystems, such as mangroves, may both protect coastlines and help shorelines adapt to climate change. Integrating natural ecosystems into coastal defense schemes requires a thorough understanding of the interaction between the coastal hazard and ecosystem. Here we show that the flood defense provided by mangroves is non-linear and strongly site specific and that mangroves provide effective flood protection if vegetation properties, local bathymetry, and storm characteristics allow for vegetation to reduce the landward flow of water.
Type of thesis
Montgomery, J. (2021). Attenuation of tides and storm surges in coastal mangroves (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14608
The University of Waikato
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