Effet des courants secondaires sur la diffusion des rejets en canaux et rivières

The effect of secondary currents on the dispersion of sewage discharging into canals and rivers

Ingénieur à SOGREAH.

Résumé

The velocity distribution in a now due to wall friction causes transverse forces which generate secondary currents tending to give the main or primary flow a helicoidal appearance. Attempts to explain the formation of such currents in straight canals and rivers are still very unsatisfactory. There is a considerable difference between flows on a solid bed (Fig. 1) and on a loose one (Fig. 4). In the latter case, the combined secondary current and sediment bed load result in meandering flow, becoming more marked with increasing bank roughness. In river or canal sections featuring a change in direction (e.g. a bend, converging section, fork, etc.) formation of the secondary current is readily explained by the fact that the high-velocity flow nucleus or core tends to keep to its direction for as long as possible by displacing any lower-velocity streamlines on the bottom and by the banks towards the inside of the change in direction. The effect of these currents on sewage dispersion is discussed while making a distinction between floating bodies, suspended solutions and solids, and bed load. The discharge positions most conducive to dispersion are stated for each case.