Simulation of the effects of bank vegetation on velocity profiles in a meandering compound channel

Abstract

Advances in computing and simulation capability increase interest in using computational fluid dynamics to solve complex flow structures in meandering compound channel. Highly complex and three-dimensional flows inside the meandering compound channel with the presence of vegetation are still not fully comprehend. The aim of the research is mainly to simulate the effects of bank vegetation on velocity profiles inside the meandering compound channel using computational fluid dynamic models. An existing meandering compound channel at Hydraulics and Hydrology Laboratory, Universiti Teknologi Malaysia was modelled numerically. TELEMAC-2D (two-dimensional) model and TELEMAC-3D (three-dimensional) model were used to simulate hydrodynamics pattern inside the main channel and floodplain with and without existence of bank vegetation. Both models use the same horizontal unstructured triangular meshes of the meandering compound channel. Simulations were computed for different relative depths (DR) and vegetation spacing of 2-, 4- and 8-times the vegetation diameter (d). Models were calibrated using the roughness coefficient and validated using streamwise velocity profiles at the apex sections. Velocity components between modelled and measured were discussed at selected cross-sections inside the meandering compound channel. Significant reductions of depth-averaged streamwise velocity at the outer bend were 69.9% (DR0.30) and 71.4% (DR0.45) for 2-times diameter (2d) vegetation spacing. The three-dimensional model also shows that streamwise velocity reduction for overbank flows at the outer bend of 83.3% (DR0.30) and 72.2% (DR0.45). Depth-averaged streamwise velocity at the inner bend shows an increase of 51.4% (DR0.30) and 58.4% (DR0.45). The streamwise velocity increases 3.2 times (DR0.30) and 4 times (DR0.45) from the three-dimensional results at the same inner bend. This is because vegetation protects and reduces the velocity of overbank flows at the outer bend while it increases the velocity at the inner bend by blocking and redirect the overbank flows into the direction of the main channel. Vertically averaged velocity from TELEMAC-3D shows difference of less than 15% between simulated and measured inside the main channel. However, TELEMAC-2D gives a higher difference of up to 3.8 times than measured velocity at cross-over regions. The high percentage of differences is believed to be due to three-dimensional interactions inside the cross-over regions from the interactions between overbank and inbank flows. The presence of vegetation significantly increased the level of complexity in cross-over regions, which contributed to high difference between model and measurement. In computational model, the effects are more significant during low relative depth. As the distances between vegetation increases, velocity patterns inside the meandering channel tends to resemble non-vegetation conditions. Both two- and three-dimensional model also predicted the same velocity patterns. In conclusion, TELEMAC-2D and TELEMAC-3D show the ability to simulate flow properties inside the meandering compound channel with and without vegetation.