Many sand spits are morphodynamically complex cases that are hard to quantify. Therefore, most case studies on sand spits in a specific type of environment, are descriptive or even non-existent. As is the case for spits in low-energy lake environments, like the two spits at the Marker Wadden islands. Because quantification is necessary for spits in such complex environments, sedimentation was quantified around both spits using polar coordinates and morphologically simplified elevation levels around the spit-platform. It was found that, sediment rich currents that pass over the spit-platform boundary at a certain direction drive spit-platform growth in that direction. The spit-platform growth in turn dictates the growth of the emerged spit. Spit growth quantification enables a detailed understanding of the Marker Wadden spits, which gives insights in spit behaviour in low-energy lake environments. The quantification of spits in other environments could be a powerful tool for the understanding of spit behaviour.

The Noordstrand of the Marker Wadden has been subject to much more erosion than expected. From our current understanding of this low-energy system we cannot explain what has been the cause of this. The goal of this thesis is to create a better understanding of the different processes responsible for the development of this non-equilibrium beach in a low-energy lake environment. A better understanding of the link between hydrodynamic and morphodynamic processes can be used for the maintenance of the existing Marker Wadden beaches and the design of new sandy protections in this kind of systems. By combining the results of a hydrodynamic and morphodynamic data analysis with the results of the Delft3D Marker Wadden model, it is possible to create a conceptual system description. This can be used to improve our understanding of the Noordstrand development and the responsible processes. In this system description the influence of the general Markermeer system cannot be neglected as the different systems (Markermeer, Marker Wadden and Noordstrand) cannot be regarded separately as the different hydrodynamic processes are linked.

The Houtribdijk is a dike intersecting the IJsselmeer and Markermeer, and is partly reinforced by the construction of sandy shores. The sand eliminates the wave loads before they can reach the levee. Making use of sandy shores as flood protection in a freshwater lake environment is unique. The main goal of this thesis was to find the causes of the formation of beach cusps and their morphodynamic behavior at the Houtribdijk. From a literature study, we had found that high-angle wave instabilities are the likely cause of beach cusp formation. This mechanism causes longshore transport of sediment due to differences between wave run-up when waves arrive at the beach under an acute angle. Low-energy environments like these are influenced by higher-energy (stormy) events, caused by wind forcing that produces short, steep, and erosive waves. Topography and bathymetry measurement data along two kilometer-long transects was available, as well as constant hydrodynamic measurements at two locations. Using this data, we quantified the number of cusps, their alongshore position, wavelength, amplitude, and the asymmetry of their top-view shape. By extrapolating the prevailing theory on high-angle wave instabilities, we arrived at a conceptual model for the large scale (kilometer-long) morphological changes of the beach. This conceptual model then related these changes to the hydrodynamics we thought were responsible.
The results of this analysis indicate that the number of cusps increases under the influence of high-angle waves. Also, their peaks will start moving in the same direction as the waves, causing them to ‘lean’. Most of these changes are on a time-scale in the order of days, while some larger features migrate in the order of months. We have found that environments like these are very likely to develop beach cusps.