A comparative study of hydraulic geometry between sand- and gravel-bed rivers based on a large database

Hydraulic geometry is an important subject in the study of alluvial rivers. So far, little research has been devoted to the comparison of hydraulic geometries between sand- and gravel-bed rivers. Based on a large database composed of the data from 270 rivers, this comparative study has been made. The bankfull width (B) - discharge (Q(bf)) relationship of sand-bed rivers shows basically no difference from that of gravel-bed rivers, and the grain size of bed material has little influence on the B-Q(bf) relationship. When the discharge is given, the bankfull depth of sand-bed rivers is significantly larger than that of gavel-bed rivers, the bankfull slope is gentler, and the flow velocity at bankfull discharge is lower. When all data are put together without differentiating sand- and gavel-bed rivers, the relationship between flow velocity and bankfull discharge shows some non-linear variation. The flow velocity increases with the increased discharge to some peak, followed by a decline. This non-linearity may be attributed to the different manners for the sand- and gavel-bed rivers in which the flow velocity is adjusted to the changing discharge