Abstract
Abstract. We investigate the links between the drainage density of a river basin and selected flood statistics, namely, mean, standard deviation, coefficient of variation and coefficient of skewness of annual maximum series of peak flows. The investigation is carried out through a three-stage analysis. First, a numerical simulation is performed by using a spatially distributed hydrological model in order to highlight how flood statistics change with varying drainage density. Second, a conceptual hydrological model is used in order to analytically derive the dependence of flood statistics on drainage density. Third, real world data from 44 watersheds located in northern Italy were analysed. The three-level analysis seems to suggest that a critical value of the drainage density exists for which a minimum is attained in both the coefficient of variation and the absolute value of the skewness coefficient. Such minima in the flood statistics correspond to a minimum of the flood quantile for a given exceedance probability (i.e., recurrence interval). Therefore, the results of this study may provide useful indications for flood risk assessment in ungauged basins.
Highlights
Drainage density (Dd ) was defined by Horton (1945) as the ratio of the total length of streams in a watershed over its contributing area
The analytical derivation confirms the presence of an optimal value for Dd with regard to coefficient of variation (CV), which is due to the interplay between the depth-duration-frequency curve for rainfall and Horton’s infiltration equation
Drainage density is a classical descriptor of catchment morphology which is known to control the formation of river flows
Summary
Woodyer and Brookfield, 1966). Dd is higher in highly branched drainage basins with a relatively rapid hydrologic response (Melton, 1957). With reference to the Riarbero River basin, which is a right tributary to the Secchia River (Italy) with a drainage area of 17 km, we simulate many 100-year long time series of hourly river flows for different Dd values, setting all of the other external forcings and model parameters constant. AFFDEF is based on the application of a conceptual model for the continuous time simulation of the infiltration and runoff formation processes at local scale, as well as energy and mass balance concepts for simulating runoff concentration and propagation It is capable of fully simulating the direct controls of Dd on flood statistics. In order to make the analytical computation possible, simplifying assumptions need to be introduced in the schematisation of the rainfall-runoff transformation
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