Modeling the Effects of Encroachments on Floodplain Flow in Straight and Meandering Channels

Abstract

Floodplain obstruction for various development works is a common human interference to natural river system. Intense hydrodynamic and morphological processes occur at or above bankfull conditions in alluvial rivers. The velocity structure, turbulence properties and boundary shear stresses in high flow conditions are significantly different compared to those at inbank and bankfull conditions. Given that real field data collection is difficult during floods, a number of studies have been performed in the laboratory channels in the past (e.g., Knight and Hamed, 1984; Kiely, 1990; Knight and Shiono, 1990; Tominaga and Nezu, 1991; Wormleaton et al., 1996; Wormleaton et al. 2004) to understand the complexity of overbank flows and related turbulence properties. The present study investigates the effects of floodplain encroachment on flow processes in compound channels. Considering the importance of the floodplain flow, a series of experiments were carried out in the Flood Channel Facility (FCF) at HR Wallingford, UK. A large volume of data was collected on 2D/3D velocity. Laboratory experiments have also been carried out in the Hydraulics and River Engineering laboratory of the Department of Water Resources Engineering, BUET. 2D flow velocities have been collected with Electro-Magnetic Currentmeter (EMC) from 14 experimental runs of the straight channel. The length, width and the shape of the encroached area were varied in different tests. In the second part of the study, the flow pattern in the channel was simulated with a program code, which solves the Navier-Stokes equations in 3D. Computed results are compared with laboratory measurements to verify the capability of the model and to investigate the detailed flow structure. The results show good matches of simulated flow pattern and experimental observations. The study has implication for understanding flow mixing, mass transport and habitat hydraulics in case of encroached floodplains.