Assessment of Tsunami and storm surge impact mitigation by coastal vegetation

Abstract

The devastation caused by the Indian Ocean tsunami in 2004 and subsequent tsunami alerts in 2005 and 2007 as well as the historical records of possible tsunamis in the past have highlighted the vulnerability of the country to such major hazards. More than two third of the 1600 km long coastline in the Northern, Eastern, Southern as well as part of relatively sheltered Western regions suffered with a widespread damage in the event. The damage caused by the tsunami varied significantly with the local near shore wave height, topography and the resistance offered to the overland flow. In addition to the ground surface resistance the resistance offered by the various features in the coastal zone is among the factors contributing to such hydraulic resistance. These features include coastal infrastructure such as buildings as well as natural features such as coastal green belts. The study on the effect of such contributory features is important in identifying and planning possible tsunami impact mitigation measures. In this study, attention is focused on the resistance offered by coastal green belts to overland flow and their effectiveness as a possible tsunami impact mitigation measure. Such a measure would also have the dual advantages of being environmentally friendly and cost effective, which could be a more suitable method for developing countries like SriLanka.In this study investigation conducted to assess the influence of coastal vegetation in resisting the overland flow. The resistance would depend on characteristics of individual plant structure and the vegetation as a whole, as well as the ability of plants to resist the flow without being damaged or destroyed. The factors affecting the drag force were related to the characteristics describing a vegetation, namely, plant type, size, extent, spacing & pattern. Experimental & numerical studies were conducted to assess the influence of these characteristics on energy dissipation of the flow under steady condition and reduction in inundation distance under unsteady flow. Small scale physical model tests were carried out in a hydraulic flume (length 10 m, width 30 cm and depth 30 cm). Coastal vegetation was represented by geometrically similar small scale models and energy dissipation characteristics were observed under different discharge conditions. Experiment results indicate up to about 48% of energy loss through the vegetation and it highlight the contribution of different patterns and densities of vegetation in effective impact mitigation.