Wave energy dissipation in the mangrove vegetation off Mumbai, India

Abstract. Coastal regions of India are prone to sea level rise, cyclones, storm surges and human induced activities, resulting in flood, erosion and inundation. The primary aim of the study is to estimate wave energy attenuation by mangrove vegetation using SWAN model, and validate the model results with measurements for the Mumbai coastal region. Wave measurements were carried out during 5–8 August 2015 at 3 locations in a transect normal to the coast using surface mounted pressure level sensors in spring tide conditions. The measured data presents wave height attenuation of the order of 52 %. The study shows a linear relationship between wave height attenuation and gradual changes in water level in the nearshore region, in phase with the tides. Model set-up and sensitivity analyses were conducted to understand the model performance to vegetation parameters. It was observed that wave attenuation increases with an increase in drag coefficient, vegetation density and stem diameter. For a typical set-up for Mumbai coastal region having vegetation density of 0.175 per m², stem diameter of 0.3 m and drag coefficient varying from 0.4 to 1.5, the model reproduced attenuation, ranging from 49 to 55 %, which matches well with the measured data. Spectral analysis performed for the cases with and without vegetation very clearly portrays energy dissipation in the vegetation area. This study has the potential of improving the quality of wave prediction in vegetation areas, especially during monsoon season and extreme weather events.