Abstract:
Coastal zone of Bangladesh is one of the most vulnerable regions as a severe cyclone strikes Bangladesh’s coast every three years. Bangladesh is also at the receiving end of two-fifth of global storm surge impacts. Addition to death and destruction of coastal population and livelihood, coastal infrastructures, especially polders, as first and only defensive structure against storm surges, pose most threat and often fail to protect hinterland. This study investigated the impact of mangrove afforestation in attenuating storm surge impact on the embankment and economic as well as social benefits of adapting this measure. While, two of the design parameters of afforestation, width and roughness were examined in a hypothetical land, effectiveness of community based afforestation focusing on its technical efficacy, economic feasibility and social benefits were studied for polder 47/4 of Kalapara upazila of Patuakhali district. This study involved application of Delft-3D, a numerical model along with a MATLAB based tool, Delft-Dashboard, Dynamic Force Model (DFM) and a Mapping tool ArcGIS 10.3 to simulate, calculate and visualize inundation condition and thrust force distribution. Primary and secondary source information from various sources was used to validate, modify the model, determine economic feasibility and assess social benefits of afforestation. From this study it is found density and width of a forest plays a crucial role in determining the effectiveness of mangrove afforestation in attenuating surge propagation and thrust force reduction. Denser forest is more effective in reducing storm surge impacts and as the width increases, for a given value of roughness, efficacy of vegetation also increases. However mangrove afforestation has influence up to a certain region inland (termed as control area) which is a function of afforestation width. But as the control area shifts, area with relatively high inundation moves upward transferring the risk beyond the control area. Existence of polder, while very effective in reducing inundation extent and depth can not reduce thrust force on itself. Combination of both approach exhibits great efficacy in protecting hinterland from inundation and reduced threat to the polder. For polder 47/4, presence of 1.61 km2 afforestation on the foreshore protected additional 3 km2 land from inundation during cyclone SIDR. While increasing polder height to 7 m influenced inundation extent slightly by reducing it 0.3 km2, additional afforestation of same roughness as of existing helped to reduce inundation depth in the hinterland. For this polder, afforestation was found to be more economically feasible measure. Increasing polder height from 4.51 m to 7 m was estimated to cost about Tk. 35,071/m length of the polder. Whereas, cost of protecting 1 m length of polder by providing 1 km afforestation on the foreshore was only Tk.4881.50. Social benefits provided by existing forest was also considerable as local people collects fuel wood and Nipa palm regularly. Fuel wood collection pattern varied spatially (depending on distance of forest from home) and temporally (round the year). While fuel wood was used for non-commercial uses, Nipa palm, on the other hand was used to prepare mat to sell at local market by village women. From the collected data, net benefit of this forest was found to be about Tk. 5990/ha/year.
