MODELING AND APPLICATION OF THE GEOMORPHIC AND ENVIRONMENTAL CONTROLSON FLASH-FLOOD FLOW

The surface water runoff (sheet wash) during simulated heavy rainfall of between 25 and 100 mm/h (rainfall durations of between 1 and 6 hour s, on plots of between 2.5 and 3.5 m(2)) on soils of Pliocene and Quat ernary sediments and on cherts from Ordovician sediments in nearly-nat ural environments was dependent on inclination of slope (32%) and on k inetic energy of rainfall (30%). When using vegetation cover as an add itional variable for nearly-natural and human influenced environments, the vegetation cover increases R(2) from 0.62 to 0.74. The degree of slope controls between 25 and 30% of topsoil characteristics. The resu lts of model simulations were confirmed during natural heavy rainfalls on different field plots. Because simulated rainfalls were based on r ecorded intensities and durations, results compared with historic reco rds and estimations imply also that specific intensity of between 25 a nd 100 mm/h and duration of between 1 and 6 hours art: not so importan t, relative to geomorphic-environmental impact, in flash flood generat ion in a Mediterranean climate area like the Roussillon area (SE-Franc e). On 26 September 1992 the rainfall intensity of a four-hour heavy r ainfall event was to a large extent influenced by the topography of th e catchment of River Reart/Canterrane (Roussillon, SE-France), increas ing with altitude. As an example of the model application: the flash f lood of 26/27 September 1991 was simulated. The peak flash-flood flow at the river mouth of River Reart/Canterrane was 1100 m(3)/s or 7 (m(3 )/km(2))/s. Runoff conditions for the natural or nearly natural catchm ent would have accounted for 43.6% of this, agricultural impact fur 9. 1% and building areas and construction sites for 5.5%. A further 41.8% was accounted for by the effects of breaching of stored floodwater. I n absence of breaching of stored floodwater, the nearly-natural part o f the peak flash-flood flow would have been about 480 m(3)/s (75 degre es C) and the part caused by human influence about 160 m(3)/s (25%).