The Effectiveness of Mangroves in Attenuating Cyclone- induced Waves

Abstract

A study of the effectiveness of mangroves in attenuating cyclone- induced waves was done using the SWAN 40.55MOD numerical model. Hydraulic parameters during extreme events and local mangrove vegetation parameters were estimated for the Kanika Sands mangrove island near the upcoming Dhamra Port in Orissa, India. Simplified generic analyses were first conducted to obtain insights into the characteristics and behaviour of the model and the system. These were used to select relevant scenarios for simulations of actual conditions at the case-study site. The mangroves were found to be effective in reducing wave heights at the port behind the island though the effectiveness is limited by its geometry and distance from the port. The presence of vegetation has a marked effect though the effect of a variation in vegetation density is limited. An optimum cross-shore width range for maximum protection was quantified. The required size of the mangrove patch for maximum wave attenuation under all conditions is 300 to 800 m in the cross-shore direction and around 6 km in the alongshore direction. At present the vegetation is 1.5 km cross-shore by a 4 km alongshore at a maximum with a shape that is slightly different from the optimum. Given the conditions of the area northward expansion is considered more relevant. Vegetation strips around the island seem to be an effective option though the effects of density reductions become important in this case. Model characteristics such as the sensitivity trend of hydraulic parameters and the comparative effects of emergent and submergent vegetation were also investigated. Conclusions regarding model and system characteristics observed during the study were also drawn. Finally, based on the work done recommendations were made regarding mangrove management options for the port and directions for future research in case of further numerical modeling, physical modeling and field studies.