Resistance computation in overbank flows

Abstract

There is a recognized need for a reliable method of determining the stage discharge curve for compound channels, of which a closely studied example is the floodplain-channel-floodplain combinations of the type investigated by the UK Flood Channel Facility (FCF). This paper re-analyses the coherence approach to use subsection weighting based on area instead of perimeter. According to textbook analysis of channel conveyance, this corresponds with the weighting of the local driving gradients balanced by the whole-section friction slope, so avoiding unrepresentative overweighting of floodplain subsections in areas which are barely inundated. The reanalysis leads to a general treatment which is verified within a tolerance of well under 5% by reference to FCF results. Practicalities of model implementation are discussed for a general-purpose compound channel model, and result validations from the AULOS hydraulic design software package are presented.