Conventional breakwaters are enormous in size and generally associated with large amounts of construction materials, effort and cost. The development of large breakwater systems with inadequate design and management can trigger a number of negative effects on the neighboring coastal environment, for example a large amount of wave reflection, alteration of beach morphology, deterioration of beach quality water and the change of the marine ecosystem. To alleviate the above problems, various ingenious lightweight breakwater designs have been proposed, tested and constructed in the past as alternatives to conventional breakwaters, one of which is free surface breakwaters. Free surface breakwaters are essentially barriers located near the water surface where energy flow is greatest. The total height of such barriers is usually much lower than the water depth, which helps the circulation of water around the structures. These barriers can be built on a group of piles driven into the seabed or kept floating as floating breakwaters. Wave reflection and dissipation are the main energy damping mechanisms inherited from these barriers. Clear surface breakwaters are best suited to be constructed on semi-protected sites where soil conditions are poor. The present investigation is motivated by the construction of semicircular bottom-mounted breakwaters in Miyazaki Port (Japan), Tianjin Port (China), and Yangtze River Estuary (China) for sea defense. In-depth studies on such breakwaters have been undertaken by several researchers from Japan, China and India, for example Tanimoto et al. (1989), Sasajima et al. (1994), Xie (1999), Dhinakaran et al. (2002), Yuan and Tao (2003) and Zhang et al. (2005). Although numerous studies have been reported in the literature associated with… half of the paper… which include a large number of discrete components for energy dissipation. Examples of such breakwaters are multilayer breakwaters (Wang et al., 2006) and porous pile breakwaters (Hsiao et al., 2008). These structures are usually highly porous to water flow, resulting in wave reflection and horizontal wave forces on the relatively small structures. A major concern in marine infrastructure design is the safety of navigation adjacent to breakwaters. In many cases it is important to keep the amount of wave reflection in front of the breakwater to a minimum. Most of the aforementioned breakwaters serve primarily as wave reflectors. It is hoped that the semicircular free surface breakwater considered for the present research will provide improved performance characteristics producing low reflection and desirable wave attenuation.