Flash flood hydrology in karstic terrain: Flumineddu Canyon, central-east Sardinia

In the last five winters (2004–2008) several exceptional meteorological events producing flash floods have been registered in central-east Sardinia. The first of these (December 2004) was the most severe and caused important geomorphic changes in the Riu Flumineddu watershed where the influence of human activity is limited. The hydrological characterisation of this flood is extremely difficult because of the lack of streamflow gauges and the relative paucity of meteorological stations in the region. Peak discharge of the fluviokarstic Riu Flumineddu Canyon has been estimated based on a distributed hydrological model (TOPKAPI) and on empirical methods based on geomorphic and sedimentological observations. The comparison between the results derived from these independent methods allows us to obtain the best possible estimate of peak discharge. Differences between modelled and measured peak flows can be attributed to water losses and/or gains along the river channel from interactions with the underground karst drainage network. The December 2004 flood, with an estimated recurrence interval of at least 65 years, generated overbank flow and destroyed several bridges in upstream reaches, caused important changes in channel morphology and sediment distribution and was able to move boulders up to 1 m in diameter in downstream reaches.