Catalogus

To evaluate the success of this new disposal strategy an extensive monitoring programme was set up and several criteria were defined. Frequent (2-weekly, monthly) topo-bathymetric surveys using multibeam echo sounding are performed in combination with seasonal sedimentation-erosion measurements using both RTK (intertidal) and singlebeam (subtidal). Also flow velocities on 10 transects, each consisting of 4 measurements points, in the shallow subtidal and the intertidal zone near the disposal locations are monitored. In the shallow subtidal area one RDI ADCP (1200 kHz) is used as measurement device, in the intertidal zone Nortek AquaDopp (2 MHz) devices are used. All devices look upward, allowing maximal vertical resolution when flooded during the tidal cycle. The RDI ADCP is mounted on a frame; the Nortek AquaDopps are dug in the sandbar.

Before the start of the deepening, the reference situation was monitored for all 40 locations, measuring continuously over 2 spring-neap tidal cycles. Since the start of the works, new measuring campaigns at all locations have been executed to evaluate the effect of the disposal activities. Preliminary results show different effects near different sandbars, from almost no effect to a significant reduction or increment of current velocities. During the next 6 years this monitoring will continue, allowing the evaluation of the new disposal strategy on the longer term.