Gravity Wave Damping by Stratified Porous Structures

Abstract

In the present study, wave transformation due to multiple porous structures in the presence and absence of vertical rigid wall, barrier-rock breakwaters of various configurations, multiple horizontally stratified porous absorbers, vertically stratified porous structure lying on flat seabed, elevated seabed and stepped seabed is analysed under the oblique wave incidence. The eigenfunction expansion method using the continuity of pressure and velocity along with mode-coupling relation is adopted based on linearized wave theory. The direct analytical relations are derived for finding the wave reflection and transmission coefficients due to porous breakwaters of various structural configurations. In the preliminary stage, the analytical results are validated with numerical and physical model results available in the literature. As a special case, a comparative study is performed between the vertical rigid wall, permeable wall and stepped wall away from the double horizontally stratified wave absorbers. The vertical and stepped wall shows almost similar values of wave reflection at each of the resonating crests, but minimal values of the resonating trough in wave reflection is obtained from the stepped wall. A comparative study is performed between single and multiple porous structures of fixed structural width and depth. The 42% reduction in wave transmission is achieved with double porous structures as compared with single porous structure for uniform structural width, which may be due to wave damping in the free spacing available between the two structures. The distribution of incident wave energy in the form of wave reflection and transmission is effective in the case of horizontally stratified porous structure as compared with other structures. The vertically stratified porous structures performance is partially dependent upon structural width. Higher structural width effectively reduces the wave transmission as compared with conventional porous structures. The effect of each layer porosity, friction factor, structural width, incident wavelength, number of structures, angle of incidence, free spacing and trapping chamber effect on wave reflection, transmission damping, fluid force on seaward/leeward sides of breakwater and force on vertical wall is analysed for various types of porous structures. The critical angle due to standing waves, fluid resonance in the free spacing and clapotis has an efficient role in the design of porous structures.