Indexing floodplain effects for flood estimation

Combining flood estimation methodologies with hydraulic models to provide a detailed and spatially coherent representation of flood risk can be problematic. One potential difficulty is that of double-accounting the attenuating effect of floodplain storage. This occurs when effects are represented in both the flood frequency estimation of the flow and also in hydraulic modelling and can be particularly important in the context of the increasing desire to combine hydrological and hydraulic models in a manner that provides a detailed and spatially coherent representation of flood risk. This paper presents an empirically derived index that represents floodplain effects on flood magnitude. A HEC-RAS 1-D hydraulic model was used to generate downstream flow hydrographs in a generalised river reach for defined upstream hydrographs encompassing a range of flows and durations. Geometrical and resistance properties in the reach were systematically varied. Relative attenuations were determined by analysing differences in upstream and simulated downstream hydrographs. The index was derived by relating flood peak attenuations to the channel characteristics in each simulation in a multivariate regression analysis.