Apparent roughness coefficient in overbank flows

João N Fernandes

doi:10.1007/s42452-021-04677-3

Abstract

Overbank flows occur in alluvial valleys during flood events when the conveyance of main channel of rivers is exceeded. Once floodplains are inundated and the so-called compound channel flow is observed, the faster flow in the main channel interacts with the slower flow in the floodplain featuring a much more pronounced 3D flow structure compared to single channel flow. These flow mechanisms comprise a shear layer near the interface, lateral momentum transfer and strong secondary currents due to the non-isotropic turbulence. This paper starts by giving an overview of the main flow mechanisms in compound channels pointing out the importance of taking into account the apparent shear stress generated between the main channel and the floodplain flows due to the interaction of these flows. A new simple model was developed to include the apparent shear stress concept as a correction of the Manning roughness coefficient of main channel and floodplains. The proposed method for predicting stage–discharge relationships was calibrated and validated by experimental data from several compound channel facilities. A significant improvement in prediction of the compound channel conveyance in comparison with the traditional methods was achieved.

Highlights

River floods are one of the most serious disasters worldwide
Taking as reference the extended research conducted in compound channels, i.e. when the floodplains are inundated and a complex flow structure is generated, this paper aims at reviewing the main flow mechanisms of overbank flows and consequences in terms of modelling
Following the work started in Fernandes [18], the paper presents a method to incorporate the concept of apparent shear stress in the Manning roughness coefficient

Summary

Introduction

River floods are one of the most serious disasters worldwide. Amongst the natural catastrophes, floods cause about half of all deaths [44] and a third of all economic losses [3]. Due to the extent of the river to be modelled, 1D simulations are chosen and conservation laws must be used For these reasons, floods have been a concern since ancient times. Taking as reference the extended research conducted in compound channels, i.e. when the floodplains are inundated and a complex flow structure is generated, this paper aims at reviewing the main flow mechanisms of overbank flows and consequences in terms of modelling. Following the work started in Fernandes [18], the paper presents a method to incorporate the concept of apparent shear stress (developed in the interface between the main channel and the floodplains) in the Manning roughness coefficient. As compared to the main channel, floodplains usually present a rougher bottom They are often covered with vegetation, submerged during floods.

Overview of the mechanisms of overbank flows

Traditional method

Proposed method

Concluding remarks