Modelling effects of mangroves on tsunamis

Abstract

The hydrodynamic effects of estuarine mangrove trees on tsunami currents have been studied in this paper. An existing two-dimensional depth-integrated numerical model, with both the effects of drag force induced by mangrove trees and the blockage effects on the water flow through mangrove swamps, have been included. To investigate how these trees affect the attenuation of the tsunami current at the head of the estuary, one idealised case has been considered with mangrove trees distributed along the whole channel. This model has been used to simulate the water level variations and velocities induced by tsunami currents at three locations along the centre of the channel. Recorded data of tide elevation during a historical tsunami have been used to set up the open seaward boundary condition in this model. Three cases of different diameters and densities of idealised mangrove trees have been studied to examine the significance of the vegetation on the current attenuation. It was found that mangrove trees have a significant impact on the hydrodynamic processes of tsunami waves. The model has also been set to simulate these tsunami currents in a real estuary, namely the Merbok estuary, Kedah, Malaysia. Simulations have been undertaken for the swamp area and main river, for both conditions of with and without mangrove trees, with the aim of studying the effects of mangrove trees on the hydrodynamic processes during a tsunami. Comparisons of velocity profiles and water elevations for both cases have been undertaken. These results show that the mangroves help in reducing the effects of a tsunami in the main river and swamp areas.