Abstract: Most experimental studies on overtopping breaching of embankment dams use small-sized sediment grains and little differences in the diameter profile of sand specimens.Aiming at the issues of sediment sizes,two distinct types of sediments are used in this experimental study.The experiment reveals that the breaching processes of two types of dams are basically identical under relatively strong erosion conditions.During the overtopping event,the down cutting phenomena are most likely observed and the backward erosion phenomena occur only near the crest of the dam.The entire breaching process can be divided into three stages.In the first stage,a headcut is formed near the lower margin of the dam crest.In the second stage,the headcut collapses and the erosion actions become violent.In the third stage,antidunes occur and the erosion actions terminate.Experiments also detect that the downstream slop of the dam has a significant effect on the breaching process.The steeper the slope is,the faster the degradation rate of the crest is and the larger the flood peak discharge is.Moreover,a flat hydrograph is produced by the embankment dam-breach flood,which contains coarse sand particles.The corresponding value of the flood peak tends to be smaller compared to that of resulting from small-sized materials.However,it is worth mentioning that a large difference in sediment sizes will have a limited effect on the peak discharge.