Water and Solid Transport Estimates Through the Venetian Lagoon Inlets Using Acoustic Doppler Current Profilers

The present work reports the preliminary results of the analysis based on the time series from the ADCPs (Acoustic Doppler Current Profilers), employed in the inlets of the Venetian Lagoon. Basically, in situ data originate from two types of measurements, namely, point measurements from the bed-mounted sensors and periodic measurements along the entire cross section carried out by means of a vessel-mounted instrument. Both types of instrumental installation acquire current velocity and backscattered acoustic signal, which is then calibrated for estimating the suspended sediment concentration. Discharge is obtained from the time series of current velocity in the inlet (index velocity method). Solid flux is then a product of the discharge and suspended sediment concentration. The time series of the water and sediment fluxes are updated for the Lido inlet, which has the longest time record, while for the other two inlets data are available for shorter time intervals. Simultaneous data from all the three inlets are available only for a time period of about three months (February - April 2006). The present analyses lead to the following considerations: the linear relationship between index velocity and discharge has changed in the Chioggia inlet, and the possible cause could be the narrowing of the cross section due to the construction of a navigation lock in correspondence to the ADCP location; the estimates of the solid flux at the Lido inlet, originating from the calibration of the backscattered signal from bed-mounted and vessel-mounted sensors are, prevalently, in a good agreement; exceptionally high discrepancies in several cases could be due to the asymmetry of the solid load distribution over the section, which increases with the action of the waves during particular weather conditions. A preliminary budget of the sediment transport between the lagoon and the open sea for the period February - April 2006 (without exceptional meteorological events) highlights an export of about 60000 tons of suspended materials from the lagoon toward the open sea, with a predominant contribution through the Lido inlet.