INTRODUCTION

Coastal vegetation aids in shoreline protection by damping the incoming waves and dissipating the energy. Vegetation characteristics, wave conditions, and the water depth are the function of wave attenuation by vegetation. The vegetation characteristics include stem density, leaf structure, geometry, stiffness and the wave conditions include wave period, wave height, water depth and direction. The variability of wave damping is very large and it is difficult to define a generalized behaviour of the "plant-induced dissipation" as wave attenuation depends on the characteristics of the plant (geometry, buoyancy, density, stiffness, degrees of freedom and spatial configuration) as well as wave parameters (mainly wave height, period and direction).

One such structure that helps in wave attenuation is vegetation breakwater. Vegetation breakwater is usually located away from the beach and it attenuates wave energy due to the turbulence caused by the wave structure interaction. They allow the incoming waves to pass through the gaps and hence reflection decreases, and also water circulation occurs within the protected area with no blocking or obstructing of any moving objects / organisms. Due to these reasons they are constructed for the protection of coast, without spoiling the beauty of the natural beach.

For solving the above problems, Computational Fluid Dynamics (CFD) is introduced, wherein full scaled, three dimensional perspective of the interactions can be obtained using a three dimensional numerical simulation. The concept of CFD is nothing but utilisation of advanced computing technology along with the development of accurate numerical algorithms for solving physical problems, as numerical simulations provide better insight into the physical processes which cannot be attained through experimental approach.

Wave-structure interaction on structures has been analytically evaluated, using different theories and formulae. In order to get accurate results, these are applicable only in ideal cases, where all the assumptions are fulfilled. In this scenario, it is beneficial to adopt numerical simulations. Interaction of waves with non-porous vegetation breakwater is simulated in a three dimensional numerical wave tank using REEF3D and resulted transmission co-efficient is validated using the physical model studies. Also, the efficiency of porous piles over non-porous piles are studied by simulating wave conditions by varying wave height, Wave period, water depth and percentage porosity of the piles.

The usage of hard solution like breakwaters, seawalls etc will dissipate and reflect wave energy and thereby protect that part of the region. When such methods are adopted they alter the near-shore hydrodynamics and regional sediment transport characteristics, sometimes resulting worse scenario in its surroundings. In this scenario, coastal vegetation is used as a soft solution for protecting the coast from the action of waves by attenuating the wave height and reducing the energy of the waves.

An artificial, rigid emerged vegetation for a length of 2m is developed in a numerical wave tank of REEF3D. The model is tested for regular waves of height 0.08m, 0.12m and 0.16m and different wave periods of 1.8 sec and 2 sec in different water depths of 0.40m and 0.45m, wave heights at different locations along the vegetation at 0.5m intervals is recorded. The numerically obtained results are checked for concurrency with the experimental results.