An analysis of the influence of river channel properties on flood frequency

Abstract

A one-dimensional flood routing model (DAMBRK) is used to explore the influence of some channel and floodplain hydraulic and geometric properties on the frequency distribution of peak flood flow rates as flood waves propagate downstream. Peak flood inflows for selected return periods are derived from a known flood frequency distribution and the effects of channel and floodplain properties on the associated hydrographs and peak flow rates are examined as the floods propagate down a single river reach; geometric and hydraulic properties of the reach are varied over a wide range. With no lateral inflow to the reach, a one-to-one mapping of computed peak flows and return periods of the routed hydrographs is used to determine the flood frequency distribution at downstream locations. For an upstream peak flood flow regime defined by an extreme value type I distribution (EV I) with coefficient of variation 0.6, the form of the downstream peak flow distribution appears to change from EV I to either EV II or EV III whenever inundation of the floodplain occurs, and the peak flow distribution becomes less variable with the amount of reduction in variance depending on the effective storage of the floodplain-channel reach. The findings are important when selecting data for use in regional flood frequency estimation.