Density Induced Return Currents in Outlet Channels

When a channel discharges fresh water into a basin filled with salt water an arrested saltwater wedge will develop. If the fresh water is discharged by an outlet-structure that is situated close to the basin, the salt wedge cannot develop its full length. The salt water is then mixed up with the discharged water close to the outlet-structure. A return current of salt water develops from the basin to the mixing region. The theory of two-layer flows is applied to determine the maximum return current. It is shown that this happens when both in the mouth and in the mixing region the flow pattern is internal critical, i.e., when one of the velocities of propagation of internal waves is zero. Nondimensional graphs are given and measurements in nature and in models show good agreement. Conclusions are drawn for the design of outlet channels, cooling water channels, and pneumatic barriers against saltwater intrusion.