Mobile turbidity measurement as a tool for determining future volumes of dredged material in access channels to estuarine ports

摘要

Monitoring the environmental impact of dredging and relocation operations and estimating the turbidity (sediment flux) is becoming increasingly more important. Predicting the natural relocation of dredging material can lead to a better planning of the dredging activities. Of equal importance is the monitoring of the background turbidity in order to assess the relative importance of the turbidity plume created by dredging activities. Monitoring the sediment flux, caused by the action of tidal currents, waves and wind, with the help of mobile measurements at the entrance of the harbour of Zeebrugge is part of the research project "The ecological monitoring of dredging works in the Belgian coastal harbors", (MOBAG 2000) of the Ministry of the Flemish Community (Waterways and Maritime Affairs Administration, Environment and Infrastructure Department. Coastal Waterways, Oostende, Belgium). On-line mobile monitoring was performed using an acoustic Doppler profiler (model NDP, mounted on the hull of the vessel). The NDP was calibrated with backscatter turbidity sensors (mounted on a computer-controlled tow fish). Turbidity and current data were visualized and used to estimate the sediment flux. During the project 13 hours measurements took place during neap and spring tide. Data through the water column were collected along a track crossing the entrance of the Outer Harbour of Zeebrugge. The data were corrected offline for errors. Finally, the sediment flux was calculated from the corrected current and turbidity profiles. The recorded profiles made it possible to visualize flow rate and sediment flux. The profiles showed a very complex pattern of in- and outflow of current and suspension material. The amount of sediment that remains in the harbour after completion of a tidal cycle, is quite different for a neap and a spring tide. Measurements showed that after a tidal cycle during neap and spring tide, respectively 795 tons and 3200 tons of sediment remained in the harbour. The obtained results proved as well that the turbidity caused by dredging activities (in the harbour), is merely a short-time local phenomenon and mostly does not exceed background turbidity.