Discharge, stream flow & channel shape

Discharge Measures the volume of water passing a given point over a set time period (usually measured in Cumecs (cubic metres per second).

Velocity River velocity is influenced by three factors: Channel shape in cross section Roughness of the bed and banks Channel slope.

Channel shape Described by the term Hydraulic radius Hydraulic radius = cross-sectional area / length of wetted perimeter. The larger the hydraulic radius, the lower the friction acting on the water allowing for higher velocities.

Roughness of bed & banks The smoother the bed and banks, the less friction, the higher the velocity. Higher roughness leads to greater turbulence, which in turn leads to material on the bed being picked up and transported by the river. Roughness is measured using Manning’s n. The greater the roughness, the higher the value.

Channel slope Steeper slopes lead to higher velocities due to the increased effect of gravity. However, steeper slopes, often found in the upper parts of rivers, usually have turbulent flow and high roughness. The bedload is usually large which increases friction.

Summary Lower parts of rivers usually have greater velocities and discharge than upper parts. Velocity increases with increases in channel depth & width. Increased roughness reduces velocity, increases turbulent flow, and leads to more sediments and load being transported by the river.