Determination of mangrove forest performance in reducing tsunami run-up using physical models

Abstract

Coastal ecosystems such as mangroves fringing tropical coastlines have been recognized as natural protectors of the coastal areas against destructive attack of a tsunami. In this paper, the authors aim to investigate the interaction of a tsunami wave on a typical mangrove forest and to determine its performance in reducing the run-up. A laboratory experiment using a hydraulic flume with a mangrove forest model was carried out in which tests were conducted by varying the vegetation widths of 0, 1, 2 and 3 m and average densities of 8, 6 and 4 trees per 100 cm2 using a scale ratio of 1:100. Two conditions of water levels were considered in the experiments at several tsunami wave heights between 2.4 and 14 cm. The dam break method used in the experiments produced two types of waves. At low water condition, a bore was developed and subsequently, a solitary wave was produced during high water. The results of the experiments showed that in general, vegetation widths and densities demonstrate a dampening effect on tsunami run-up. A larger vegetation width was found to be more effective in dissipating the wave energy. The first 1 m width of mangrove forest could reduce 23–32 % during high water and 31–36 % during low water. Increasing the mangrove forest width to 2 and 3 m could further increase the average percentage of run-up reduction by 39–50 % during high water and 34–41 % during low water condition. It was also observed that densities of the mangrove forest do not influence the run-up reduction as significantly as the forest widths. For mangrove forest densities to be significantly enough to reduce more tsunami run-up, an additional density of 4 trees/100 m2 needs to be provided. The experiments also showed that mangrove roots are more effective in reducing the run-up compared to the trunks and canopies. The experiments managed to compare and present the usefulness of mangrove forests in dissipating wave energy and results produced are beneficial for initiating design guidelines in determining setback limits or buffer zones for development projects in mangrove areas.