Longitudinal dispersion in natural channels: 3. An aggregated dead zone model applied to the River Severn, UK

An Aggregated Dead Zone (ADZ) model is presented for longitudinal dispersio n of tracer in river channels, in which the channel cross-section is divide d into two parallel regions: the bulk flow and dead zone storage. Tracer pa rticles in the bulk flow are assumed to obey plug-flow advection at the dis charge velocity U without any mixing effects. The dispersive properties of the model are completely embodied in the residence time for tracer storage in the dead zone. The model provides an excellent description and predictio n of empirical concentration-time distributions, for times t > x/U. Its phy sical realism is demonstrated by using it to describe the evolution of a tr acer cloud in the River Severn, U.K., and by comparing it with a more compl ex model which incorporates the additional effects of shear flow dispersion within the bulk flow. The ADZ model is a potentially useful tool for pract ical prediction of dispersion in natural channels.