The present paper outlines the numerical calculation of wave interactions with a pair of thin permeable vertical slotted barriers extending from the water surface to some distance above the seabed. The numerical model is based on an eigen function expansion method and utilizes a boundary condition at the surface of each barrier which accounts for energy dissipation within the barrier. Relevant numerical results which are presented relate to the transmission and reflection coefficients and the wave force on the upwave and downwave barriers. The influence of different permeabilities in the upwave and downwave barriers is described and the practical implications of the results are discussed.
Breakwaters have been used extensively to provide economical protection from waves in harbours, marinas and other coastal facilities. In certain cases, breakwaters in the form of thin, rigid, pile-supported vertical barriers which extend some distance down from the water surface have been considered or adopted. These have the advantages of allowing water circulation and sediment transport beneath the breakwater, and of being relatively economical by providing protection closer to the water surface where wave action is most pronounced. In some instances, a permeable barrier, such as a slotted vertical barrier made from timber planks, may be preferred. This has the advantage of reducing wave reflection on the upwave side of the barrier. However, in order to reduce the wave transmission to an acceptable level, it may be necessary to use a pair of such barriers in practical applications. In such cases, the influence of each barrier's permeability needs to be understood. Although previous studies have addressed various situations relating to single and double barriers, permeable and impermeable barriers, and barriers extending fully or partially to the seabed.
