The impact of channel deepening and dredging on the suspended sediment concentration in the Ems Estuary
CEDA Dredging Days 2015 - Innovative Dredging Solutions for Ports, Rotterdam
Van Maren DS and van Kessel T - Deltares, the Netherlands
Abstract: Many estuaries worldwide are becoming more urbanised with heavier traffic in the waterways, requiring regular channel deepening and larger ports. In many of these estuaries, the suspended sediment concentration (SSC) appears to be increasing as well, partly reflected in ever-increasing dredging requirements. An example of a heavily impacted estuary where SSC levels are becoming larger is the Ems Estuary, located between the Netherlands and Germany. In order to provide large ships access to three ports and a shipyard, the tidal channels in the Ems Estuary have been substantially deepened by dredging over the past decades. This has led to tidal amplification and hyper-concentrated sediment conditions in the upstream tidal river. In the middle and outer reaches of the Ems Estuary, the tidal amplification is limited, and mechanisms responsible for increasing SSC are poorly understood. Possibly, channel and port deepening lead to larger SSC levels because of an increase in maintenance dredging. Additionally, channel deepening may increase up-estuary suspended sediment transport due to enhanced salinity-induced estuarine circulation. The effect of channel deepening and port construction on SSC levels is therefore investigated using a numerical model of suspended sediment transport forced by tides, waves and salinity. The model satisfactorily reproduces observed water level, velocity, sediment concentration and mud deposition in the harbours, and is subsequently applied to test the impact of channel deepening, historical dredging strategy and port construction on SSC in the Estuary. These model scenarios suggest that: (1) an estuary-wide change in suspended sediment concentrations may be caused by deepening of the tidal channels, but especially by sediment sinks (sediment extraction from ports and loss of intertidal areas); (2) dredging and disposal primarily redistributes sediments: the sediment concentration increases near the disposal sites, but decreases elsewhere in the estuary; (3) the impact of dredging on SSC is easily overestimated in a turbid estuary.
Key words: dredging, turbidity, channel deepening, sediment extraction