Modelling turbulence and solute transport in a square dead zone

In a river, dead zones can be due to geometrical irregularities in the riverbanks and riverbed and/or to spur dikes and groyne fields. The presence of dead zones in streams and rivers significantly affects the characteristics of mass transport. Dead zones produce a difference between the concentration curves measured and modelled 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 paper presents the results of 2D steady-state and time-variable numerical simulations carried out in a rectangular geometry with a lateral square cavity representing a dead zone reproducing the case experimentally studied by Muto et al. in 2000. Two different approaches were applied to simulate both flow and concentration fields. One based upon the standard k-ε model, one using the more advanced Reynolds stresses model (RSM). The latter was found to better reproduce the observed flow patterns around the interface between the main channel and the square dead zone. The analysis of concentration field finally pointed out the rapid exchange between the main channel and the square cavity.