Practical application of numerical modelling to overbank flows in a compound river channel

Abstract

Compound sections formed by a river channel and floodplains, are used in river channels design to provide additional conveyance capacity during high discharge periods. When the overbank flow occurs, the flow in the river channel is affected by the momentum transfer between the main channel and floodplains, which modifies water levels and velocity distributions given by traditional methods. One-dimensional (1D) models using the Single Channel Method (SCM) and the Divided Channel method (DCM) have been proven to be not accurate enough in compound channel flows. New more advanced models have been developed in order to accurately estimate discharge flows and depth-averaged velocity distributions. The quasi-two dimensional model Conveyance Estimation System (CES) estimates discharges and velocities in a cross-section based on the Lateral Distribution Method (LDM). Two-dimensional (2D) modelling solves the depth-averaged Navier-Stokes equations in a discretized reach of a river. In this work, published field measurements of the River Main (UK) are analyzed via 1D, CES and 2D modelling, in order to find a practical solution to give good predictions of water levels and velocity distributions in overbank flows in river channels with floodplains. The results show that 1D modelling combined with CES gives reasonable accurate values and is a complementary tool for advanced 2D models in real conditions.