Wave-Induced Currents Within Mangrove Forest

Abstract

Mangroves are tidal trees commonly observed along the sheltered shorelines of most tropical (from equator to 23.5° North and South latitude) and few subtropical (23.5° to 40° North and South latitude) countries. These plants are adapted to loose wet soils, saline habitats and periodic tidal submergence. With more attention paid into the approach of building with nature, natural coastal defence strategies are gaining more importance as an asset in addressing the coastal squeeze that is prevalent not only in urban areas, but also in agriculture and industrial areas that are located along the coastline. Mangroves are receiving more attention due to their coastal protective role against wave and hydrodynamic forcings as well as their ability to adapt to sea level rise. Mangrove vegetation attenuates and damps the hydrodynamics forcings by providing obstacles to the flows and creating drag. To date and to the knowledge of the author, no study has been conducted on interaction of the wave-induced currents with mangrove vegetation. This lack of relevant studies may be due to the fact that mangrove forests and the foreshore in front of the mangroves are usually of very gently sloping bed (varying in order of 1:300 to 1:1500). This means that in order to conduct physical model experiments to study wave-induced current within a mangrove forest, a very large wave basin is required in order to conduct modelling without using a very large scale factor difference between prototype and model. This is to ensure that the relevant processes are representing prototype as closely as possible, as well as to be measureable. Numerical modelling of the interaction of wave-induced current with mangrove vegetation is yet to be conducted due to the lack of measured data for validation, both field as well as experimental measurements. An experiment by Hulsbergen (1973) was selected as validation data for current study. The main objective of the study is to understand the difference of nearshore processes for (stationary) tidal gradient-driven and oblique wave-driven current for both with and without mimic mangrove vegetation. The scope of the study involves desktop analysis of the main validation data and other relevant and similar experiments, assessment of reliability of Delft3D for the study, validation against measured data, and simulation of various hydraulic conditions for condition with mangrove forest. Among questions answered in this study are the extent of wave-induced longshore current damping within mangrove forest, the significance of wave-induced longshore current within mangrove forest, the effects of bed slope and mangrove density on wave-induced current and the extent of model’s reliability for current study. It was shown that the damping of wave-induced longshore current is more than 80% and the contribution of waveinduced current to the total velocity can be more than 70%. Of course, both of the above was specific to the bathymetry, mangrove properties and hydraulic conditions specified within current study. Furthermore, it was shown that bed slope and mangrove density affect wave-induced longshore current within the mangrove forest. It was also found that current model setup has its limitations.