Flood modelling in river basins with highly variable runoff

In the Mediterranean area numerous small to medium-scale river basins are characterized by highly-variable runoff, intermittent or ephemeral. This is due to both the climatic regime and the geomorphological and physiographic peculiarities of the hydrological system itself. Typically, these basins are affected by flash floods, for which effective modelling can be more difficult than in the case of large basins with permanent runoff. In this study we compare different modelling approaches in two representative catchments (one in Greece and one in Cyprus), on the basis of a number of observed flood events. Initially, we employ the well-known SCS-CN method, combined with a synthetic unit hydrograph (SUH) approach, whose parameters (namely, the curve number, the initial abstraction ratio and the time-to-peak of the SUH) are calibrated against each individual flood event. Yet, even with calibrated parameters, the above method, which is widespread among flood engineers, generally fails to reproduce the observed hydrographs. Next, we test different modelling structures, all of which use elementary hydraulic analogues (by means of interconnected tanks) to represent the storage processes, which are dominant in such types of basins. For each event we run different settings of the calibration problem, thus obtaining a large set of alternative optimal parameter values. The significant variability of the parameter values reflects the complexity of the involved hydrological processes. In addition, it reveals the crucial role of flood measurements, in order to build realistic models and provide consistent estimations of the related uncertainties.