T. Moramarco and F. Melone (Italy)
Watershed hydrology, flooding,distributed modelling, steep wave, floodplain hydraulics
The reliability of a procedure for flooding investigation of an ungauged river reach close to an urban area is investigated. The approach is based on the application of a semi-distributed rainfall-runoff model for a gauged basin including the flooding prone area, and that furnishes the inlet flow conditions for a two-dimensional hydraulic model, whose computational domain is the urban area. The flooding event occurred on October 1998 in the Upper Tiber river basin and that caused high damages in the town of Pieve S.Stefano is used for testing the approach. The built-up area, often inundated, is included into the gauged basin of the Montedoglio dam (275km2 ), for which the rainfall-runoff model was adapted. The hydrological model was found fair accurate in simulating the discharge hydrograph, observed at the dam, of three selected flood event without over-bank flow. Whereas the flooding event occurred on October 1998 was simulated poorly, with an error on peak discharge and time to peak at the dam, of –58% and 20%, respectively. The main reason of this discrepancy is quite probably due to the combined effects of the rainfall that might be mainly occurred in some portions of the basin, and of the flood resulting from the sudden opening of the bridge obstruction in Pieve S. Stefano. Under this hypothesis, the hydraulic model was able to estimate with a fair accuracy the flow depths in Pieve S.Stefano. Moreover, the discharge hydrograph at the basin outlet was adequately represented if the flow resulting from the sudden opening is considered as contribution of the basin at Pieve S.Stefano. In this case the error on peak discharge was -7% while the time to peak was estimated correctly. Results show that the proposed procedure can be conveniently adopted for flooding risk evaluation in ungauged river reach where a built-up area is located. The need of a more detailed analysis regarding the processes of runoff generation and flood routing is also pointed out.
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