In coastal protection, groins are usually used to stabilize shoreline, which will finally reach an equilibrium state corresponding to the incoming wave characteristics. This investigation aims at the design criteria of groins for an equilibrium shoreline. A set of shorelines under oblique wave attacks has been studied in the wave basin of Hydraulic and Coastal Model Laboratory, the Department of Water Resources Engineering, Chulalongkorn University. The length and spacing of groins are the design variables while wave height, wave period, wave length and wave direction represent the design condition. The tools used in this study consists of a wave basin, groin models, wave generator, wave height meter, and an IBM-PC computer with an analog to digital conversion card for measuring wave data. The shoreline was made of fine sand with an average size of 0.25 mm and was adjusted with the angle of 15, 30, and 45 to the wave front. For each experiment, wave height and wave period were varied to give a wave steepness of 0.004-0.050 in the wave basin and the spacing of groins set of 1,2, 3 and 4 m. About 60 case studies were experimented, and the obtained equilibrium shoreline was analyzed. It was found that the recession of a shoreline depended on the wave direction and the spacing of groins. The wave steepness had little effect on the shoreline shape but it affected the wave breaking distance, which was one of the important factors to design length of groin. The recession distance of a shoreline and the wave breaking distance were related to the wave climate condition at deepwater, intermediate depth and shallow water. Finally a set of design criteria was then formulated using the wave, climate at deepwater condition. If the characteristics of deepwater wave were known, the groin length, the groin spacing and the recession distance could be determined.
