Noncohesive bed particles enter motion as soon as the shear stress applied on the bed material exceeds the critical shear stress. Generally, silt and clay particles enter suspension (see Chapter 10), and sand and gravel particles roll and slide in a thin layer near the bed called the bed layer. Bedload, or contact load, refers to the transport of sediment particles that frequently maintain contact with the bed. Bedload transport can be treated either as a deterministic or as a probabilistic problem. Deterministic methods have been proposed by Duboys and by Meyer-Peter and Müller; probabilistic methods were developed by Kalinske and Einstein. Satisfactory estimates of bedload discharge are obtained from empirical methods presented in Section 9.1. Bed-material sampling is discussed in Section 9.2, and bed sediment discharge measurement techniques are summarized in Section 9.3.

The unit contact sediment discharge, or unit bedload discharge, refers to the flux of sediment per unit width per unit time that is in motion within the bed layer. The bed layer thickness is approximately twice the particle diameter, which is typically less than 1 mm in sand-bed channels and up to tens of centimeters in gravel-bed streams. Note that the bed layer thickness should not be mistaken for the laminar sublayer thickness defined in Chapter 6. The unit bedload discharge can be measured by weight qbw with dimension M/T3, by mass qbm with dimension M/LT, or by volume qbv with dimension L2/T.