Predicting Effects of Dead Zones on Stream Mixing

Longitudinal mixing in open-channel flow is described by two coupled partial differential equations, the solution of which, in the frequency domain, is a function of four parameters: mean residence time; Peclet number or dimensionless dispersion coefficient; fraction of stream volume which is dead zone; and mean residence time in a dead zone. The solution is given and methods of estimating the parameters from hydraulic data are described. The fraction of stream volume which is dead zone is a function of the friction factor, and the ratio of mean dead zone residence time to mean residence time in a reach is essentially a constant. The effects of errors in estimating values of the parameters are discussed and illustrated.