Runoff thresholds in derived flood frequency distributions

摘要

In general, different mechanisms may be identified as responsible of runoffgeneration during ordinary events or extraordinary events at the basin scale.In a simplified scheme these mechanisms may be represented by differentrunoff thresholds. In this context, the derived flood frequency model, basedon the effect of partial contributing areas on peak flow, proposed byIacobellis and Fiorentino (2000), was generalized by providing a newformulation of the derived distribution where two runoff components areexplicitly considered. The model was tested on a group of basins in SouthernItaly characterized by annual maximum flood distributions highly skewed. Theapplication of the proposed model provided good results in terms ofdescriptive ability. Model parameters were also found to be well correlatedwith geomorphological basin descriptors. Two different threshold mechanisms,associated respectively to ordinary and extraordinary events, wereidentified. In fact, we found that ordinary floods are mostly due to rainfallevents exceeding a threshold infiltration rate in a small source area, whilethe so-called outlier events, responsible of the high skewness of flooddistributions, are triggered when severe rainfalls exceed a threshold storagein a large portion of the basin.