Experimental study on hydrodynamic performance of vertical thick porous breakwater

Abstract

This research work investigated hydrodynamic performance of rectangular porous breakwater(s) both submerged and emerged as a shore protection structure. Experimental studies were carried out in a two-dimensional flume (21.3 m long, 0.76 m wide and 0.74 m deep) of the Hydraulics and River Engineering Laboratory of Bangladesh University of Engineering and Technology. A set of experiments were carried out at h= 50 cm water depth with fixed vertical porous breakwaters of three different porosity (n= 0.45, 0.51 and 0.7) and with three different heights (hb= 40 cm, 50 cm and 60 cm). Total thirty six runs were conducted with monochromatic waves having wave periods (T= 1.5 sec, 1.6 sec, 1.8 sec and 2.0 sec). Wave heights were measured manually by providing a vertical scale on the flume side made of glass. Four different locations, two in the front, one over the breakwater and one behind the breakwater were selected for data collection. At each measuring location, water surface data were recorded for 1 min, at 5 sec time interval. Different hydrodynamic co-efficient like transmission co-efficient (Kt), reflection coefficient (Kr) and wave energy loss co-efficient (KL) were determined from the measuring water surface data for various run conditions. These co-efficient values were then, analyzed with respect to relative submergence (hb/h), relative breakwater width (k×B), [where, k = wave number (2π/L), B = breakwater width] and porosity of breakwater. Experimental results reveal that, for transmitting smaller part of wave energy through the breakwater, minimum transmission co-efficient, Kt = 0.261 was obtained for breakwater with lowest porosity (n= 0.45) and with emerged condition (when hb/h= 1.2) for short wave, i.e. when T = 1.5 sec. Minimum reflection coefficient, Kr = 0.089 was obtained for breakwater with highest porosity (n=0.7) and with minimum submerged (hb/h=0.8) condition. It is also noticed that the effect of increasing relative breakwater width (k×B) on the transmission coefficient (Kt) and the reflection co-efficient (Kr) is less when the breakwater height is kept fixed. Porosity has effect on the wave energy loss co-efficient (KL). It is seen that wave energy loss co-efficient (KL) increases with increasing porosity. This study can provide engineers with useful information for designing coastal structures.