Effect of nonlocal transverse mixing on river flows dispersion: A numerical study

A series of Lagrangian numerical simulations is performed to examine the dispersion process in open channel flows and to explain through a simple conceptual model its systematic deviations from the ideal long‐term one‐dimensional regime even in the case of straight axis and fixed impervious bed. The starting point is represented by a suitably depth‐averaged transport equation, with generally nonlocal turbulent mixing, solved in terms of particles trajectories and their first‐ and second‐order moments. Input data refer to six rivers of southern Italy, for which the irregular section morphology is known from a field survey. As the governing equation predicts, and according to experimental observations, in the case of variable cross‐sectional diffusion the tracer cloud exhibits a permanent and asymmetric peripheral drift toward the shallow boundary zones, with a remarkable deceleration along the main flow direction. Longitudinal inertia moments tend to become linear after a period equal to twice the single river diffusive time and sometimes show an early anomalous change of slope. Transverse inertia moments experience an initial very fast increment as compared to the theoretical value corresponding to the uniform transverse concentration and, after a peak, tend to stabilize about a smaller constant. Centrifugal moments remain, in any case, relatively limited, indicating that longitudinal and transverse axes practically identify with the plume principal directions, no matter how irregular and asymmetric the river section is.