The generation of runoff through ephemeral streams

Ephemeral streams are morphological valleys, which typically exist in arid or semi-arid regions. They are normally dry and occasionally drain runoff during extreme rainfall events and potentially may drain high discharges, which can flood the areas close to the streams. For this reason, it is of paramount importance to understand how these streams activate and which is the difference between rivers and ephemeral streams in terms of response to rainfall. A problem affecting the analysis and prediction of flood events in karst semi-arid regions is the almost complete absence of discharge time-series, measured at the outlets of the ephemeral streams. This is due to the extreme rare occurrence of flood events, generated by the ephemeral streams. This prevents from the identification of accurate statistics of flood events and on the determination of rainfall events, which may potentially generate floods. The definition of rainfall threshold related to the activation of ephemeral streams is a complex problem, since the dynamics of the catchment drained by these streams in highly non-linear and biased by multiple variables (e.g. urbanization, land use, etc.). The key issue for the analysis of ephemeral streams is to understand how the flow originates and the shape of the hydrograph. Due to the peculiarity of the flow dynamic in these streams, the use of traditional rainfall-runoff modelling cannot accurately describe the hydrograph, thus leading to biased evaluations. Then, it is necessary to change the approach to ephemeral stream modelling, by integrating the hydrological and hydraulic modelling. This work presents an analysis of the ephemeral stream activation in karst catchment located in Apulia (southeast of Italy). The analysis is based on a comparison of the simulations of hydraulic behaviour of the ephemeral streams obtained by a 2D simulation and a traditional lumped model based on the Instantaneous Unitary Hydrograph (IUH). The 2D approach integrates the hydrological and hydraulic models, in order to account first for the dynamic of catchment response to rainfall and activation of the streams, and then the hydraulic behaviour of the streams for rainfall of different intensity. The work will show the differences between the hydrographs returned by the IUH and by the integrated approach, for different intensity rainfall events. In particular, peak discharges and lags of discharge generation through the ephemeral streams will be carefully analysed.