Wave reflection characteristics of permeable and impermeable submerged trapezoidal breakwaters.

摘要

The present study aims to quantify reflection coefficients in the vicinity of impermeable and permeable submerged trapezoidal breakwaters. Three impermeable breakwater models are tested, each with a unique side slope for the breakwater faces. Two permeable models are tested, one made of PVC pipe and the other consisting of golf balls contained within a mesh cage, to provide two separate porosities. Testing is carried out at the Clemson University Flow Physics Laboratory in a laboratory wave tank with the inclusion of a 1:20 sloping sandy beach to simulate natural environmental conditions. Wave elevations are collected during experiments by two capacitance wave gages. A dimensional analysis is conducted to identify the governing dimensionless parameters in this study. Based on experimental results, the relative submergence depth, hc/Hi, is found to be of primary importance (hc - depth of submergence, H i - incident wave height). Breakwaters remain submerged throughout testing and gathered reflection coefficient values are compared to the parameter, hc/Hi. The relationship between the reflection coefficient and the relative submergence depth of the breakwater is found to evolve through several stages as hc/H i increases. Initially, for plunging breakers occurring on the offshore breakwater face (corresponding to hc/H i values between 0.0 and roughly 0.5, depending on the breakwater face slope), the reflection coefficient is observed to strongly decay, without an explicit regard to varying breakwater slopes. From hc/H i values of roughly 0.5 to approximately 1.0, the reflection coefficient bulges in response to a shift in breaker types (from plunging to spilling), resulting in less energy dissipation from wave-breaking and additional energy available for reflection from the structure. After the bulge, reflection coefficients generally decay as hc/H i increases. This region is associated with waves that do not break over the breakwater. Detailed discussions on reflection coefficient behavior with changing wave conditions are provided based upon our experimental observations. Predictive capabilities of reflection coefficient equations available in literature were tested using a large set of experimental data. The results from this study will assist those designing breakwaters by providing additional insight to further detail the hydrodynamic processes surrounding submerged breakwaters.