Bed material in the field often has widely spreading grain size distribution. In general, topography change is predicted by evaluating sediment transport rates using representative grain size, for example, the mean grain size. In a beach where grain size distribution is narrow, such a procedure can apply for evaluateing sediment transport rates because almost all of the particles are transported with the same mode (bed load, suspended load or sheet flow). On the other hand, in a beach where grain size distribution is wide, direction of bed material transport depends on its grain size because the mode of transport of fine bed material is not always the same as that of coarse bed material. In addition, the interaction in sediment transport between fine and coarse bed materials have to be taken into account. Some procedures have been proposed to take account of the effect of the grain size distribution on the sediment transport (Sakai et al., 2000, Sato et al., 2000, Dibajnia et al., 2001, Tanaka et al., 2001) on the basis of laboratory experiments. On the other hand, transport of bed material with widely spreading grain size distribution in the field has not been investigated yet due to the complex sediment transport. In the present study, sorting process of bed material and resulting topography change on a sloping beach where bed material is widely spreading grain size distribution were investigated through field measurements for one year.
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