Graduation of Tom Vromen

The application of a continuous nourishment in a tidal inlet basin - Case study Ameland Inlet

• Professor of graduation: Prof. Dr. Ir. S. Aarninkhof 

• Supervisors: Dr. Ir. S.A. Miedema (TU Delft), Dr. B. van Maren (TU Delft), Ir. J. Kollen (Sweco) 

Sea level has been rising since the end of the last ice age (10,000 years ago). Initially, the water rose rapidly, but in the last millennia it decreased to about 2 mm/year. Currently, anthropogenic climate change due to CO2 emissions is expected to accelerate sea level rise again. The most recent report of the "KNMI Climate Signal ’21" expects a sea level rise of 14 - 47 cm by 2050 and of 30 - 121 cm by 2100 (KNMI, 2021). To keep up with sea level rise and to keep the coastline in place, the Dutch sandy coast has been nourished with sand every year since 1990 (Van Koningsveld and Mulder, 2004). Between 1990-2000 this was about 6 million m3 of sand, then the policy was scaled up to 12 million m3 of sand to include the foreshore (Rijkswaterstaat, 2020a). This is now done with trailing suction hopper dredgers. These dredge the sand at a depth of 20m from the bottom of the North Sea and place it near the shore. Depending on the nourishment method, tides, waves and wind spread the sand further towards and along the coast. Trailing suction hopper dredgers use fossil fuels, which release large quantities of CO2. In doing so, they themselves contribute to the problem of rising sea levels. There is therefore a demand for new CO2-neutral methods of maintaining the coast.

One of these new methodologies could be the use of the Zandwindmolen. 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 remain 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. An erosion hotspot on the Dutch coastline is the Ameland Inlet. The Ameland Inlet is a tidal inlet system, where all tidal inlet components jointly strive for an equilibrium state. The Ameland inlet is growing along with the current sea level rise and imports 1.2 Mm3 / year of sediment from the surrounding coastline, the outer delta and the coastal foundation (Elias, 2020b). Between 2011 and 2020, 27.5 million m3 of sand has been nourished in the Ameland Inlet (Rijkswaterstaat et al., 2020). 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. Moreover, little is yet known about the dispersion behavior of continuous point nourishment in general, and near Ameland in particular. In addition, there is a demand for new methodologies to nourish sediment in the Wadden Sea. Based on the research by (Rutteman, 2021), it can be concluded that a continuous nourishment in a tidal inlet has potential to be effective already in the short term (order of magnitude months / years).