Longitudinal dispersion in natural channels: 2. The roles of shear flow dispersion and dead zones in the River Severn, U.K.

摘要

The classical one-dimensional advection-diffusion equation (ADE) gives aninadequate description of tracer cloud evolution in the River Severn, U.K. Asolute transport model incorporating the effects of tracer storage in dead zonesis presented in which the channel is conceived as being divided into twoparallel regions. The bulk flow region occurs in the central part. Itslongitudinal dispersive properties are described by the ADE. Adjacent to this,an additional cross-sectional area is defined in which tracer can be storedtemporarily in regions of slowly moving water called dead zones. Exchangebetween the two regions follows a first order rate equation. Applying the modelto the River Severn shows that a dispersing cloud’s evolution occurs in twodistinct stages with a rapid transitional phase. Initially, shear-dispersion isdominant while the tracer particles mix fully over the bulk flow. Once this hasoccurred, dead zone storage accounts well for the non-Fickian evolution of thecloud. After the transitional phase the dead zone storage mechanism clearlydominates over shear-dispersion. Overall, the combined shear flow dispersion –dead zone model (D-DZM) provides a much better, physically consistentdescription of the tracer cloud’s evolution than the simple classical ADEapproach can do alone. style="line-height: 20px;">Keywords: Channels; dispersion; deadzones; tracers; River Severn