A Distributed Hydrological Model of Flash-Floods

Abstract

The flash-floods of small watersheds are characterized by a low predictability. The monitoring of these events is in fact very often problematic, due to the size of the phenomenon and to the lack or failure of appropriate real-time survey networks. Furthermore the forecasting potentiality is implicitly limited either by the fast response of the catchment area or the uncertainty in the description of the dynamic and spatial variability of rainfall fields and the hydrological soil properties. The relevance of physically based reconstruction of past events is therefore recognized and the increasing knowledge of the complex hydrological processes has stressed the need to analyze the phenomena at quite fine spatial resolution. Two sub-models compose the distributed hydrological model described here. The first one describes the various soil and subsoil processes, which contribute to the total runoff by using the hydrological budget equation on a small-scale (less than or equal to 1 km) discretization or equispaced grid. Inside each grid-cell, considered as a unitary system, the internal variance is studied through a stochastic approach, which assumes locally stationary, in space, distribution functions of all the quantities involved. The water mass transfer among adjacent cells is considered on geomorphological basis. The second submodel addresses the flood routing. To evaluate the first order dynamics of the flood event each stream-branch, considered as a group of cells, is modeled as a linear reservoir. The discharge to the control stream section is compiled, taking into account the hydraulic characteristics of the upstream branches. Different identification procedures of local precipitation dynamic and spatial structures have been experimented in the applications on three small watersheds located in Tuscany (Italy).