Conventional breakwaters are massive in size and usually associated with large amounts of construction materials, effort and cost. The development of poorly designed and managed large breakwater projects can trigger a number of negative effects on the nearby coastal environment, e.g. significant wave reflection, alteration of beach morphology, deterioration of water quality and a change in the marine ecosystem. To alleviate the above problems, various ingenious designs of lightweight breakwaters have been proposed, tested and constructed in the past as alternatives to conventional breakwaters, including free surface breakwaters. Open surface breakwaters are essentially barriers located close to water. surface where the energy flow is maximum. The overall height of these barriers is usually much less than the depth of the water, making it easier for water to circulate around the structures. These barriers can be built on a group of piles driven into the seabed or kept afloat as floating breakwaters. Wave reflection and dissipation are the main energy damping mechanisms inherited by these barriers. Free surface breakwaters are most suitable for construction on semi-protected sites where the soil condition is poor. The present study is motivated by the construction of bottom-mounted semi-circular breakwaters in Miyazaki Port (Japan), Tianjin Port (China), and the Yangtze River. Estuary (China) for maritime defense. Extensive studies of these 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 a number of studies have been reported in the literature associated with...... middle of article...... which include a large number of discrete constituents for energy dissipation. Examples of such breakwaters are multi-layer breakwaters (Wang et al., 2006) and porous pile breakwaters (Hsiao et al., 2008). These structures are generally very porous to flowing water, resulting in wave reflection and relatively weak horizontal wave forces on the structures. One of the major concerns in the design of maritime infrastructure is the safety of navigation near breakwaters. In many cases it is important to keep wave reflection in front of the breakwater to a minimum. The majority of the breakwaters mentioned above serve primarily as wave reflectors. It is hoped that the semi-circular free surface breakwater considered for the present research will provide better performance characteristics by producing low reflection and desirable wave attenuation..