Effect of Longitudinal Dispersion on Dynamic Water Quality Response of Streams and Rivers

The amplitude and phase characteristics of a river subjected to a time variable waste input are computed for two cases: zero dispersion with no mixing and dispersion levels representative of streams and large rivers. The amplitude characteristic provides a guideline on the efficacy of using a no-dispersion model for river water quality studies. The frequency response depends on a set of dimensionless numbers that characterize the reactive and dispersive nature of the river. The results show that for upland streams and rivers small amounts of dispersion may be important when the input varies with periods of about 7 days or less. For large, deep rivers the effect of dispersion can generally not be neglected when the input is time variable.