Numerical simulation of mass exchange processes in a dead zone of a river

The presence of dead zones in streams and rivers significantly affects the characteristics of mass transport. In a river, dead zones can be due to geometrical irregularities in the riverbanks and riverbed and/or to spur dikes and groyne fields. Dead zones produce a difference between the concentration curves measured and modeled by the classical 1D advection-diffusion equation with sharper front and longer tails. In a dead zone, the mean flow velocity in the main stream direction is essentially zero and the main transport mechanism is transverse turbulent diffusion which controls the exchange processes of solutes with the main stream. This Chapter presents the results of 3D steady-state and time-variable numerical simulations carried out with Multiphysics 3.5™ in a rectangular channel with a lateral square cavity representing a dead zone. This geometry was previously experimentally studied by Muto et al. . The exchange coefficient between the main flow and the dead zones was calculated both from the transverse velocity data along the dead zone-main channel interface and from the temporal decay of the concentration of a tracer that was homogeneously injected in the dead zone.