Mathematical modelling of bed shear stress and depth averaged velocity for emergent vegetation on floodplain in compound channel

Koji Shiono, P. Rameshwaran

Monday 29 june 2015

14:05 - 14:20h at Europe 1 & 2 (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 2G. Environment – Ecohydraulic

An ecological role of riparian is very much recognised and today river engineers are encouraged to preserve vegetation found in river banks and floodplains. However the presence of vegetation on floodplains has been regarded as a problem which slows flow and increases water depth, hence flood risk. It is therefore important to predict flow capacity for flood risk management in vegetated compound channel. In the past a number of mathematical and numerical models have been proposed for predicting depth averaged velocity and bed shear stress. The prediction has been mostly performed for the depth averaged velocity but not much work on the bed shear stress. In a wide open channel uniform flow in non-vegetation case, the bed shear stress is equal to the gravitational force term (i.e _gHSo; _ is the density, g is the gravity, H is the water depth and So is the bed slope). With this analogue, the bed shear stress in the outside of shear layer or mixing layer of compound channel would be equal to _gHSo, however it is higher than _gHSo on the floodplain and lower in the main channel (see. Shiono and Knight, 1991 and Wormleanton 1996). This is in question in compound channel flow as to whether this is caused by secondary flow or momentum transfer. As for the prediction of the velocity and boundary shear stress in compound channel, the secondary flow term was adjusted to make up for those differences in the main channel and floodplain. This is noticeably seen in the results of the vegetated compound channel (Rameshwaran and Shiono, 2007). In this paper, we show further investigation into this adjustment as to whether the secondary flow term is an appropriate way or not, using mathematical solutions to Shiono and Knight Method (SKM) based on a depth averaged Navier-Stokes equation for vegetated compound channels. The mathematical solutions are for the main channel, floodplain and mixing regions and the sloping bank region. These solutions were previously obtained by Shiono and Knight,(1991) and Shiono et al, (2012).