The application of a continuous nourishment in a tidal inlet system

Abstract

There is a demand by Rijkswaterstaat for CO2-neutral sustainable methods and technologies to maintain the Dutch coastline. The Zandwindmolen is a fixed CO2-neutral dredge, transport and nourishment concept that nourishes sediment continuously. The purpose of using a Zandwindmolen is to provide the entire surrounding area where the Zandwindmolen nourishes with sediment and allowing it to grow with the (accelerated) sea level rise. With the traditional nourishment method, it is often desired that the sand remains at the vulnerable site for as long as possible. Whereas, in a nourishment with the Zandwindmolen, it is often desirable for the sand to move to other nearby parts of the coast as quickly as possible.
Between 2011 and 2020, 27.5 million m3 of sand has been nourished at the Ameland Inlet. Therefore, it might be considered as an erosion hotspot. Because of the fixed nourishment method of the Zandwindmolen and the continuous sand shortage at the Ameland Inlet, there is a match. However, the usefulness of a Zandwindmolen depends on the extent to which the nourished sand volume is dispersed by natural processes in the short and long term.
In this study, both the short- and long-term effect of a continuous point nourishment in a tidal channel is investigated for four sediment fractions (100, 200, 300 and 400 μm). The goal is that the sediment is dispersed by nature in order to allow the entire Ameland Inlet (adjacent coastlines, coastal foundation, outer delta and Wadden Sea basin) to grow along with sea level rise.
For optimal dispersion, the nourishment must be carried out by means of a spreader pontoon with a sediment mixture concentration of 1-2%. By doing so, a mixing plume is created in which sediment settles according to their individual settling velocity. This makes the mixing plume prone to the tidal ebb and flood current and can therefore be steered towards either the North- or Wadden Sea.
In the short term (time scale: instantaneous to a few weeks), it appears that the dispersion of sediment is mainly determined by the initial sedimentation process of sediment and by re-suspension of settling sediment particles. The dispersion by natural sediment processes is minimal. In the longer term (timescale: half a year) the sediment spreads further after it has settled to the seabed. Based on half a year, the total dispersion is mainly determined by the short term dispersion.
It is found that a continuous nourishment with the Zandwindmolen is expected to ensure that the intertidal areas in the eastern Wadden Sea can continue to grow naturally with (accelerated) sea level rise. Moreover, the natural distribution of the nourished sand will, through sediment connectivity, compensate for the erosion (autonomous and due to sea level rise) of the inlet (especially the outer delta) and is also expected to contribute to keeping the North Sea coast of Terschelling and Ameland safe. Therefore, the Zandwindmolen could be a useful new sustainable nourishment method in a tidal inlet.