Changes in the physical characteristics of the water column at the mouth of a torrent during an extreme rainfall event

The city of Genoa (Italy) was hit by a severe flash flood on the 4th November, 2011. The effects of this event on the water column at the mouth of the Polcevera Torrent, the main water course flowing into the Port of Genoa, are presented in this paper. The hydrological characteristics were measured with two conductivity–temperature–depth probes equipped with a turbidimeter, one fixed on the port breakwater and one used at mobile stations around the mouth of the torrent. The dynamics were measured with a horizontal acoustic Doppler current profiler (H-ADCP) fixed on the breakwater. Data collected before, during and after the flash flood were analysed to quantify the changes due to the event. The weather conditions during the event showed extremely heavy rain associated with strong weather instability, the convergence of a low-level southerly flow and the persistence of a squall line over a restricted area. The temperature, salinity, turbidity and dissolved oxygen measurements taken during the event showed the strong influence of the weather conditions and the fresh water input of the torrent itself on the water column at its mouth, an influence that dissipated during the following days. Instead, the dynamics measured at the mouth of the torrent were affected more by the strong south-easterly wind and the sea than the flow of fresh water.