Dilution‐Mixing Zones and Design Flows

The possibility that a pollutant will be discharged accidentally or intentionally into a stream/river is a matter of constant concern to those diverting and using water from streams and rivers. It is essential for water resources planners and managers to understand the physical processes involved in a stream/river prior to the implementation of any water resources projects. When a pollutant is discharged into a stream/river, it is subjected to initial dilution immediately due to the density difference between the receiving water and the pollutant. The longitudinal, vertical and two-dimensional profiles of pollutant concentration indicate the density gradient between the pollutant and the receiving water. After initial dilution, the processes are governed by advection, reaction, and dispersion phenomena that tend to modify the initial pollutant concentration. Advection phenomena represent the downstream transport of a discrete element of the waste load by the stream flow. The reaction phenomena represent the decay of biodegradable materials in the waste under the action of naturally occurring bacteria in the stream. The dispersion phenomena represent that under the influence of turbulence, eddy currents, and similar mixing forces, a discrete element of the waste load tends not to remain intact, but mixes with adjacent upstream and downstream elements. In rivers and streams, the influence of dispersion phenomena is usually relatively small compared with advection and reaction phenomena; however, it can be important in some circumstances. Keywords: advection; biochemical oxygen demand; dispersion; dilution; design flows; dissolved oxygen; mixing zones; reaction; self-purification; waste load; water quality