Channel Shape and Turbulence Issues in Flood Flow Hydraulics

This paper reports the results of some further reflections by the writer on the nature of the flow physics in flooded channels where the floodplain flow overtops that in the main channel. A lot of the early conclusions drawn on this issue come from results of the Flood Channel Facility (FCF) program carried in the United Kingdom in the 1990s, but also from numerical work such as that of the writer. This paper takes this work further and reports that the FCF geometry may have led to conclusions that are dependent on a channel layout that is not fully representative of nature. It indicates that the flow structure evolves as a function of the channel width-to-depth ratio, and requires different turbulence model approaches to be computed accurately as this happens. In particular the standard k– is shown to become less adequate. It is also shown that the bank slope is influential in determining the flow structure, and that the flatter the slope the more likely it is to present increasing difficulties for modeling.