Experimental investigation on a perforated semicircular breakwater in irregular waves

The spread of excessive wave energy in the coastal areas, very often, pose threats to valuable resources and properties, stability of shorelines, operations of marine facilities and socio-economy of local communities. It is for these reasons that breakwaters of various ingenious designs have been widely employed for coastal protection. Gravity-type breakwaters are most commonly used because they provide excellent surface wave attenuation. Nevertheless, the presence of these bottom mounted breakwaters in sensitive coastal regions is undesirable due to the depressing environmental impact as a result of drastic change of the marine environment. As an alternative to the bottom mounted breakwaters, other types of breakwaters, such as a floating breakwater, semi-immersed breakwater etc have been proposed.
In the present research work, a free surface semicircular breakwater (SCB) which could be supported by piles and suitable for wave protection in shallow to intermediate waters has been studied experimentally. This breakwater may also be suitable to be built on complex bathymetry and poor soil conditions.
A scaled model with perforated front wall enclosing a ‘wave chamber’ has been constructed and tested for various significant wave heights and peak wave periods in a 22-m wave flume at a water depth of 0.7m. The hydrodynamic performance of the breakwater is evaluated in the form of transmission, reflection and energy dissipation coefficients. These coefficients are presented as a function of relative submergence depth (D/d), relative breakwater width (kR) and wave steepness (Hmo/Lp), where D = the depth of immersion, d = the depth of water; k = the wave number; R = the radius of the SCB; Hmo = the significant wave height; Lp= the wavelength corresponding to the peak period of the wave.
Overall, the semicircular breakwater model performs better in greater depth of submergence due to significant water flow interactions taking place in its chamber and vicinity. It has reasonably good wave attenuation performance for large amplitude and short period waves. It is also more effective in dampening the wave energy in deeper water conditions. The behaviour of wave transformation in the upstream and within the wave chamber is also discussed in this paper. The results indicate that the SCB has the potential to be introduced as a coastal defence structure in semi-protected coasts that are predominantly exposed to steeper waves.