WAVE TRANSFORMATION DUE TO A SUBMERGED POROUS BLOCK ASSOCIATED WITH A VERTICAL BARRIER K.R. Athul Krishna 1 , V. Venkateswarlu 1 and D. Karmakar 1 ABSTRACT: In the present study, the combination of vertical porous barrier along with the porous block is proposed for wave energy damping. Three types of vertical barriers such as (a) fully extended barrier (b) bottom-standing barrier and (c) surface piercing barrier away from the porous structure are analysed for wave trapping. The finite spacing in between vertical barrier and the porous structure is proposed for better wave trapping. The continuity of velocity and pressure at the interfaces of vertical barrier and porous structure are considered and the eigenfunction expansion method is adopted to determine the wave transformation characteristics due to the presence of submerged vertical barrier and porous block. The resistance and reactance offered by the porous structure are taken into account using the complex dispersion relation proposed by Sollitt and Cross (1972). The effect of structural porosity, width of the structure and angle of incidence on wave transformation due to the vertical barrier away from the porous structure are examined in detail. The results are compared and validated with the available literature for specific configurations as in Sollitt and Cross (1972) and Mallayachari and Sundar (1994). The study suggests that the increase in the structural porosity enhances the wave energy damping and global minima is achieved in the wave reflection coefficient due to the formation of standing waves by the breakwater system. The proposed structure can be adopted in leeward, port and harbour regions to achieve the tranquillity condition. Keywords: Eigenfunction expansion method, Wave reflection coefficient, Wave transmission coefficient, Porosity, Friction factor. 1. INTRODUCTION The wave energy concentration, sea level rise, global warming, melting of Artic-Antarctic ice glaciers etc. causes coastline changes due to constant erosion accretion processes. The conventional hard engineering used for coastal protection affects the aesthetical appearance of beaches. Moreover, the partial wave transmission is essential to take place the natural beach processes which maintains the beach profile. Several researches have investigated the performance of permeable plates, and porous structures of various shapes are found to be good wave energy absorbers in the presence and absence of the leeward end wall. Most of the studies are conducted by researchers on wave reflection and transmission characteristics of oblique waves. Sollitt and Cross (1972) analysed the effect of porous structures on accounting the reactance and resistance offered by the breakwater in the conventional dispersion relation. The study presented the wave reflection and transmission coefficient due to the porous structure and validated with the experimental results. The study of wave reflection and transmission on a multi- layered trapezoidal breakwater is performed by Sulisz (1985) using Boundary Element Method (BEM). The comparative study is performed between the numerical and Department of Applied Mechanics and Hydraulics, National Institute of Technology Karnataka, Surathkal, Mangalore – 575025, India. Email: dkarmakar@nitk.edu.in experimental results. The wave transmission coefficient using numerical approach is observed exactly coincide with the experimental results but in the case of wave reflection coefficient from the numerical results shows little high estimation as compared with the experimental results. Dalrymple et al. (1991) used eigenfunction expansion method to study the wave scattering due to the presence of porous structure. The study proposed the direct analytical equations for finding the wave reflection and transmission coefficients for the plane-wave and long- wave approximations considering porous structure in various configurations. Mani (2009) studied the wave reflection, transmission characteristics and wave forces on zig zag porous screen breakwater using the hydraulic tests. The wave reflection and transmission coefficients is observed to reach minor values due to the presence of submerged zig zag porous structure and the experimental results are validated with numerical results. In order to reduce the difficulties pertaining to the solution procedure of finding the roots of the complex dispersion relation for porous structure region, Liu and Li (2012) introduced a new-analytical method to examine the wave scattering due to porous breakwater. The porous structure dispersion relation is inessential in the new analytical solution. The wave scattering due to the presence of porous breakwater using analytical solution is presented Proceedings of the 10 th International Conference on Asian and Pacific Coasts (APAC 2019) Hanoi, Vietnam, September 25-28, 2019 © Springer Nature Singapore Pte Ltd. 2020 N. Trung Viet et al. (eds.), APAC 2019, https://doi.org/10.1007/978-981-15-0291-0_98 717