This research focuses on a new system for nourishing the Dutch coast with sand, which utilizes semi-autonomous sailing barges and an electrical crawler to continuously replenish the beach. Yearly, the Dutch government grants contracts to nourish the Dutch coast with volumes around 10.000.000 m$^3$. The Dutch dune coast protects the low part of the Netherlands against inundation from the North sea. This sand is dredged by Trailer Suction Hopper Dredgers (TSHD) on a depth of 20 m and transported to the coast. This transport causes a CO2 emission of about 190 kilo tonne a year. As emissions are expected to grow with increasing sea levels the Dutch government looks for ways to improve the high-energy demanding strategy that is currently used. Sweco introduced the Slow Sailing, and believes in the energy reducing concept and therefore initiates this further research. The general idea of this concept is to reduce the energy demand and emissions associated with traditional sand nourishment practices by using multiple barges that sail slowly and continuously nourish the Dutch coast. The aim of this research is to concretize this concept, in which feasibility of the system compared with energy, emission and cost calculations are central. To achieve this goal, the Slow Sailing system is compared to the traditional trailer suction hopper dredger, currently used for sand nourishment in the Netherlands. The comparison is based on a scenario in the Northsea. This leads to the main research question: What is the gain in energy consumption, emissions and cost for the Slow Sailing concept compared to classical nourishment strategies? In order to achieve this goal a review of current sand nourishment strategies was conducted. Next, the OpenTNSim model was adapted to perform energy calculations and enable a comparison of energy demands and emissions between the new system and the traditional trailer suction hopper dredger. Finally, a cost evaluation was conducted to compare the cost per cubic meter of sand for a 1.000.000 cubic meter nourishment project at the Egmond aan Zee nearshore in the Netherlands.

Compared to the traditional trailer suction hopper dredger, the Slow Sailing system showed a significant factor 15 decrease in energy demand in the sailing component of the dredging cycle. The Slow Sailing system with barges of 1500 m$^3$ was found to be the most energy-efficient and cost-effective option, with an estimated project cost of 3.00 euro/m$^3$, while the TSHD with 4280 m$^3$ was found to be the cheapest option, with an estimated project cost of 2.55 euro/m$^3$. In terms of CO2 emissions, the Slow Sailing system emitted significantly less CO2 compared to the TSHD, with a difference of 1000 tonnes CO2+ emission in a one million cubic meter dredging project. Overall, the results suggest that the Slow Sailing system is a more energy-efficient and environmentally-friendly option for sand nourishment of the Dutch coast.

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.

The Sandwindmill system could decarbonize the Dutch coastal protection whilst harmonizing with the building with nature approach. The system consists of a pipeline connecting an offshore borrow location to a nearshore area, where the mined sediment is discharged. The equipment is powered by wind turbines. As long as the wind blows sufficiently, sediment is pumped through the system, leading to a near-continuous nourishment. After the sediment has left the pipes, natural forces should distribute the material. Despite the considerable environmental benefit, one substantial challenge is the financial competitiveness with the traditional hoppers. This research aims to provide insights that can enable the system to be also financially competitive to the hoppers. In the first part, an integral analysis of the Sandwindmill is carried out. The second part of this thesis focuses on the optimization of the dispersion of the nourished sediment.

In the first part, all sub-systems of the Sandwindmill concept are treated separately. Wind data, theoretical formulations, and an exploration of the mining options are used to identify optimization opportunities. From the assessment, it becomes apparent that the interdependency of these sub-systems complicates the cost-optimization. Hence, a competitive system design requires an accurate harmonization of these sub-systems. Three main conclusions are drawn. First of all, the costs per cubic meter decrease with an increasing nourishment volume. Finding the marginal costs is essential in determining the feasibility of the system for certain volumes. Secondly, it is concluded that - given an annual nourishment volume - the pump capacity and windmill size should be attuned. Their cost-optimum is found at a set-up that leads to a yearly operational time of approximately 70%. Lastly, the analysis shows that the application of batteries to support the system in case of lower wind velocities can contribute to a more economical system. This is mainly the case if the system has wave-induced limited operational times.

Due to the costs of the displacement of the pipe outlet and the financial benefits of nourishing large volumes, the second part of the research aims at generating guidelines to designing a dispersive nourishment. Both wave- and tide-dominated systems are a potential field of application for the Sandwindmill. North-Holland is selected as an appropriate case study, containing the Marsdiep tidal inlet and a wave-dominated closed coastal section. A process-based coastal area model is set up to determine the sensitivity of different nourishment strategies on the dispersion at a one-year timescale. A tidal channel wall nourishment at the Marsdiep and a shoreface nourishment at Callantsoog are assessed.

One of the key findings of the second part is that the strong tidal velocities at the Marsdiep inlet are more capable of transporting the nourishment than the conditions at the wave-dominated coast. After one year, the distance travelled by a significant part of the nourished sediment is a factor four higher for the Marsdiep nourishment. Secondly, the research shows that the grain size plays a vital role in the local and regional dispersion. Therefore, the borrowed grain size should be part of the system design. Under the evaluated scenarios, the nourishment application under forcing conditions with a higher transporting potential has a smaller effect in the longer term. This implies that the optimal operational time could be determined solely based on the cost-optimization and should hence be around 70%.

The growing awareness of climate change asks for innovation in the methods of applying nourishments at the Dutch coastline. The concept Zandwindmolen has therefore been developed. Using wind mills at sea, sediment is dredged offshore and transported towards the shore, from where hydrodynamic and morphodynamic processes redistribute sediment alongshore. A continuous supply of sediment is assumed. The physical behavior of the disposed sediment is yet unknown. Research has been performed to study the behavior of sediment around the nourishment location, as well as the effects on the initial behavior of the coastline. The research has given insight in the optimal depth, discharge rate and spreading configuration of a continuous nourishment, for which the longshore redistribution of sediment was the major requirement. These three boundary conditions have formed a basis for further research into this topic. The concept itself is feasible, as it shows desirable predictions of nourishment formation and coastal strengthening.