The wave motions in front of breakwaters can be classified into three forms; standing wave, breaking clapotis and broken clapotis. Under the action of the three wave motions, the sea bottom in front of a breakwater is scoured, and this will further affect the stability of the breakwater. Our experiments show that the scouring of sandy seabed in front of a breakwater by broken clapotis is more serious than that by the other two wave motions. In this paper, first the scouring and depositing patterns of sandy seabed in front of a vertical breakwater under the action of broken clapotis are investigated experimentally to be classified into three types; DSD, SDSD and DSDS types, i.e. within one wavelength from a breakwater, the scouring (S) and deposited (D) locations are different. Secondly, formation processes of scouring and depositing pattern are probed for those three types. Thirdly, criteria for distinguishing scouring and depositing patterns are given based on the analyses of the scouring and depositing mechanisms and experimental data.