Mangroves thrive in the intertidal areas (interface between land and sea) of tropical and subtropical belt and play an important role in overall attenuation of nearshore waves. Multiple interactions of waves with mangrove trunks and roots and bottom friction are the two primary mechanisms responsible for wave attenuation in mangrove forests. Earlier studies, comprising both analytical and experimental, reported an exponential decay in wave height for waves propagating over vegetation with idealized bottom topography and a few on sloping bottom. But hardly studies have attempted to characterize the wave attenuation by vegetation over varying seabed slopes since mangroves generally grow luxuriantly on gradual topography having large tidal amplitudes. Nowadays, several studies are being carried out on development of artificial mangroves to reduce the coastal hazard risks; thenceforth, there is an imperative requirement to study the wave damping characteristics of mangroves on varying seabed slopes. Consequently, this study performs sensitivity experiments to analyze the wave attenuation over mangroves with different sea-bottom slopes using a thirdgeneration wave model. The study exposes sensitivity of wave attenuation characteristics to different beach slopes in the presence of mangroves and aims at understanding how the wave attenuation characteristics by mangroves differ with varying bottom slopes. The total percentage energy reduction for waves reaching the shoreline after propagating through mangroves on mild slope (1:80, 1:40) is observed to be 93%-98%, nearly 84% for 1:20 slope, and 67% for steep slope (1:10). The study reveals that the wave height decays exponentially for the mild slope and found to be consistent with the earlier studies, but as the degree of bottom steepness increases, the wave height reduction becomes gradual, and this can be attributed to the water depth variation, shoaling, breaking, and reflection characteristics associated with different slopes, in the presence of mangroves.

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Nearshore wave transformation is a complex coastal process of shoaling, refraction, diffraction, reflection, and energy dissipation due to bed friction and breaking contributing variations in the wave height, period and direction. A well defined sediment cell of about 45 km extending from Kovalam headland to Varkala cliff which forms a part of Thiruvananthapuram coast along the southwest coast of India, is selected for the wave transformation studies. In the present study MIKE 21 Spectral Wave model (DHI, 2011) was used. The model simulates the growth, decay and transformation of wind generated waves and swells both in offshore and coastal areas. Providing MIKE 21 SW with a suitable bathymetry is essential for obtaining reliable results from the model. Usually the offshore bathymetry is derived from C-MAP, ETOPO, GEBCO etc. and the nearshore bathymetry is generated from close grid bathymetric surveys. In this study offshore bathymetry was generated from GEBCO-08 grid which is a freely available software with 30 arc (~ 1 km) resolution. In the nearshore zone, surveyed close grid bathymetric data were used. The other inputs such as wave measurements and wind data provided in the model were from observations in Lakshadweep Sea. Model result is calibrated with field observations along this sector. The model has efficiently simulated the process of shoaling and refraction along the coast. The percentage of observed shoaling is 12.7% at a distance of 24 km from the shoreline at a depth of 70m and it was seen to be increasing to 27.9% when it reached around 2.4 km from the shore at a depth of 10m. The model result also shows that the wave is almost aligned parallel to the coast as wave approaches the coast. This model result can be used for further applications in designing along this coast.

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The mutual interaction between surface gravity waves and vegetation such as mangroves using numerical wave models can provide better location specific wave forecasts having practical implications in the overall development of better near-shore wave climatology. Sundarban delta bordering the head Bay of Bengal is the largest mangrove forest in the world, and the present study reports on inter-seasonal variability of wave damping in presence of mangroves that thrive on a region dominated by seasonal reversing wind field. It aims to understand relative wave attenuation characteristics during a period of one year over this region that experiences a seasonal reversal of wind system. The study considers a multi-scale nested modelling approach to account for distant swells that propagate from Southern Ocean reaching various destinations in the North Indian Ocean region. The model was forced using 0.25 ∘ × 0.25 ∘ spatial resolution 6-hourly ECMWF ERA-Interim winds for coarse grid WAM and SWAN runs, and NCDC blended winds with a similar spatial and temporal resolution was used to force SWAN model executed in a flexible unstructured mesh. To avoid model uncertainties of providing wind from two different sources, the wind data of same temporal and spatial resolution has been adopted. The present study quantifies relative rate of wave energy dissipation for monthly and seasonal scales such as pre-monsoon, monsoon and post-monsoon periods by analysing characteristic variation of wave parameters and their spectral energy in presence of mangroves. The study has put forth an effort to validate model results against altimeter measurements and ERA-Interim data, as no actual measurements are available to validate model results simulated in the presence of mangroves. This study solely emphasizes on role of reversing wind system on relative wave attenuation characteristics and highlights the importance of a more detailed investigation to ascertain impact of varying vegetation parameters on wave attenuation characteristics.