Flow distribution and mass removal in a floating treatment island with different depths in a channel cavity

Abstract

In open rivers and canals, lateral cavities are adjacent regions that affect flow, creating a water recirculation zone and reducing flow energy and with that they provide the retention of sediments, the protection of riverbanks and refuge for organisms that develop in calmer waters. The presence of vegetation in these structures offers additional resistance to flow which affects the magnitude of flow- related variables, for example, intensifying the deceleration inside the cavity and impacting mass exchange processes between the main channel and the dead zone. This study evaluated the hydrodynamic interactions effect of floating vegetation within the lateral cavity on the circulation system using Computational Fluid Dynamics (CFD) and laboratory experiments. In the simulations, the anisotropic resistance generated by the vegetation was represented by a porous zone, this is a simple, way to represent this resistance exerted to the flow, and yet effective in capturing the effects on hydrodynamics. Vegetation density remained constant and had 5 different vegetation depths studied in the simulations and for initial practical tests only one vegetated case was used. In all simulated cases, the presence of floating vegetation reduced the magnitude of the velocity in the innermost part of the cavity compared to areas without vegetation, while in the laboratory cases there was almost absolute deceleration in most of the vegetated region with points of acceleration. Increasing the depth of vegetation decreased the flow velocity in the lateral cavity, reaching its maximum effect in a case of a fully filled cavity.