Over the last years the Netherlands has often had to deal with droughts and water shortages during summer. This problem is caused by long periods without rain but with high evaporation rates and is enhanced by groundwater extraction for drinking water. Due to climate change, these droughts are expected to occur more often and become more severe. A possible strategy to mitigate drought in the eastern part of the Netherlands is to scale down the groundwater extraction, thereby limiting the groundwater depletion. In this case however, an alternative drinking water source has to be created. This research explores the option to use the former river bed of the Rhine near the Dutch-German border, the Rijnstrangen, to create this alternative drinking water source, answering the following question: How can retention of Rhine water in the Rijnstrangen contribute to drought mitigation in the eastern part of the Netherlands?

With this study, it is shown that the Rijnstrangen realistically can contribute up to 100 Mm3/y to the drinking water production in its region. This is up to 75% of the drinking water production of the Dutch province Gelderland, in which the Rijnstrangen is located. The exact maximum extraction volume from the Rijnstrangen depends on policy choices such as the maximum accepted water level in the Rijnstrangen and the maximum accepted average extraction from the region around the Rijnstrangen.

From a water quantity point of view, the maximum extraction volume of up to 100 Mm3/y indicates that utilizing the Rijnstrangen as a retention reservoir is a promising option to contribute to drought mitigation in the eastern part of the Netherlands. Therefore, further investigation of this idea is relevant.

Many low-lying densely populated areas and important economical regions in Europe are threatened by sea level rise. \cite{groeskamp2020need} suggest an international cooperation to be able to protect these areas if climate change mitigation fails: the construction of the Northern European Enclosure Dam (NEED). In this research first order calculations are used to show the effects the construction of the NEED has inside the enclosed North Sea basin. The topics covered are: monthly water level variations; hydrodynamics (tides, waves and currents); salinity; temperature and lastly sediment transport. At first these topics are treated independently, after which a first step is made with studying their dependencies. Besides the dependencies, the implications of the NEED on the environment, economy and society are explored. The implications are also shown in a case study looking into the Wadden Sea, which is Natural UNESCO World Heritage.

It is found that, with constant pumping, the water level variability stays within a range that is an order of magnitude smaller than the tidal amplitudes that prevail nowadays. The salinity of the top layer drops with an order of magnitude as well, from 35 PSU to 3.5 PSU in 50 years, while the deep, stratified part of the basin stays salt. The surface temperature only changes slightly with a drop of 0.3 $^{\circ}$C.
The hydrodynamic processes in the enclosed North Sea basin together generate a weak anti-clockwise circulation that replaces the stronger anti-clockwise circulation imposed by the tides that prevailed before the enclosure of the basin. Due to a drop in average flow velocities, the sediment transport in the basin decreases and transforms from tide-dominated to a system where tides and wind are equally important. It is found that all of the above described changes have major implications on environment, economy and society and that much more research is needed to fully understand the changing processes and their effects.