Storyline Jana Cox

Key message

In the lower delta, the construction of a dam has created large differences in water level between the northern and southern outlets. As a result, strong currents erode the beds of the channels that connect these outlets, leading to a net sediment loss each year.

The hydrodynamics of the lower delta near Rotterdam are complex. The combined discharge of multiple rivers flow into the North Sea at two outlets. The one to the south, the Haringvliet, is almost fully closed/blocked by a dam most of the time; the sluices in this dam are only fully open when the river discharge is high and the tide is low. Sediments typically settle in these more-or-less stagnant waters. The outlet to the north, the Nieuwe Waterweg, is the main and open outlet of the rivers Meuse and Rhine. A ‘nautically guaranteed depth’ is maintained here with intensive dredging. The two outlets are connected by smaller rivers. Because the tides only enter the delta area through the northern outlet, the difference in water level between the northern and southern outlet is large. As a result, strong currents develop in the connecting river branches eroding the riverbeds.

The available data indicates that the sediment budget is negative: more sediment is dredged annually from this lower delta section than is being supplied from rivers and the sea. Part of the sediment dredged in the Nieuwe Waterweg comes from the eroding north‐south flowing channels. With the current situation of an open outlet with intensive dredging for shipping to the north and a closed one to the south, the erosion of north‐south flowing channels and the negative sediment budget will persist. The net loss of sediments from the area may be overestimated however, as the sediments influx from the North Sea is probably larger than the available measurements/data indicate.