APPLICATION OF MULTIPLE SURVEYING TECHNIQUES AT A TO-BE-GAUGED RIVER SECTION

In the need to perform (i) a field analysis of sediment transport and (ii) risk assessment for a small torrent in Northern Italy, two sections where discharge measurement would be feasible by indirect methods were individuated. This manuscript presents one of these sections where a 6-m bridge crosses the stream. Topographic reconstruction was achieved by a total station and an Unmanned Aerial Vehicle (UAV, commonly named as drone) equipped with a metric camera. The flow velocity was measured by a propeller and, on the same days, movies were taken from above the bridge in order to obtain the surface velocity field by Large-Scale Particle Image Velocimetry (LS-PIV). The objective of the work was to calibrate a correspondence between the surface velocity measured by LS-PIV and the discharge. The chosen site is appropriate for such an intent as a check-dam with a fall of several meters is present downstream of the bridge, ensuring a relative stability of the bed elevation at the bridge section. The calibrated relationship enabled quick and safe measurements of the surface velocity (and, in turn, of the flow rate) to be taken also in the occasion of recent moderate-flow events.