Comparison of modelling approaches used in practical flood extent modelling

Abstract

Discharge predictions using four codes suitable to modelling flood extent are compared to predictions derived using theoretical and empirical methods. The model codes vary in complexity but represent the types of code used in practical application due to their easy integration with available GIS data formats. Common to the codes is that turbulent momentum interaction between main channel and over-bank floodplain flow is not explicitly taken into account. It is shown for different configurations of a hypothetical channel that the codes significantly overestimate discharge capacity as a result. Applying these codes to flood extent modelling therefore requires calibration of roughness values to account for this additional momentum transfer, resulting in roughness factors derived no longer representing physical friction values only, but combined friction and momentum correction factors. As a consequence of calibration being required even in more complex models where turbulence is explicitly modelled, these simpler models that are easier in application may, however, be equally viable.