Low-energy, non-tidal lake beaches are known to be subject to longshore morphodynamics, but little is known about how they are driven by wind and wave-driven currents. Lake Markermeer is a shallow (∼4 m deep), wind-dominated lake, of approximately 700 km². A gradient in wind-induced water level set-up at the leeward shore induces a flow from the shallower to the deeper parts of the lake, thereby generating a large-scale, horizontal circulation. Flow measurements and results from a numerical Delft3D model of the lake show that these circulations impact the nearshore currents greatly, even more than wave-driven longshore currents for most wind conditions. From nearshore measurements at the first study site in lake Markermeer, we found a clear relation between longshore sediment transport capacity and the measured longshore volume flux. The model numerical can predict flow direction and magnitude for any wind condition. Using wind statistics, the net transport capacity for a short period or a long term mean can be predicted. The relation is confirmed for a second study site, which shows a distinct net transport capacity that could not be explained from wave-driven longshore flow alone. Concluding, large-scale lake circulations are of great significance for the morphological development of low-energy, non-tidal beaches in shallow, wind-driven water bodies. Knowledge of these circulations and their dependence on wind characteristics is a crucial factor to better understand and predict sediment losses of lake beaches.

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Many sand spits are morphodynamically complex landforms, that are either analysed with complex and expensive computational models or at a conceptual level. Therefore, most case studies on spits in different environments are descriptive. A novel method based on the use of polar coordinates was devised to quantitatively analyse spit morphodynamics in a non-tidal, wind-dominated lake environment, using the Marker Wadden islands in Lake Markermeer, the Netherlands, as a case study. A high-resolution morphological data set allowed for the quantification of sedimentation processes around two spits, in two distinctive depth zones. Spit-platform growth is governed by alongshore currents that transport sediment over the spit-platform into deeper waters; the size of the spit-platform in turn affects the growth of the spit around the mean water level. Insight in this complex interplay of processes is crucial to understand spit behaviour in low-energy lake environments. At the Marker Wadden the submerged spit-platform grows during high energy wind events while the emerged spit part grows under mild to moderate energy conditions. With this new method we can quantitatively explore the role of different wave and flow conditions and predict spit growth direction in non-tidal, wind-dominated environments, beyond the level of conceptual descriptions.

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Sandy foreshores, beaches and dunes play an eminent role in flood risk reduction in coastal areas, reducing the impact of wind waves and storm surges on the hinterland. In some areas, sandy protection is naturally present. In other coastal areas, engineering solutions are needed to provide safety. “Soft” sediment-based solutions often serve multiple objectives, including flood safety, but also provide other ecosystem services. Knowledge of morphodynamics of these “soft” solutions (i.e. beaches) is crucial for protecting and managing coastal areas prone to flood risk. The aim of this thesis is to understand and quantify how hydrodynamic processes drive morphological development of low-energy, non-tidal, sandy beaches.@en

The morphodynamic behaviour of low-energy beaches is poorly understood, compared to that of exposed coasts. This study analyses the morphological development of sandy, low-energy beaches and the steering hydrodynamic processes. Four densely-monitored study sites in the non-tidal lake Markermeer in the Netherlands offered a unique opportunity to examine the relation between their hydraulic boundary conditions and morphodynamics. Regular bathymetric surveys were executed at all locations. Furthermore, the wave climate was monitored at one of these four sites. All four sites exhibit a commonly found low-energy beach morphology, with a narrow beach face and a low-gradient, subaqueous platform. This platform reaches an equilibrium depth quickly and then stays relatively stable. The stable elevation of the platform is located near Hallermeier's depth of closure. A sediment budget analysis over time demonstrates that the beach faces at all study sites have eroded during more energetic periods, and sediment accumulated offshore. During the monitoring periods of 2 to 4 years, the elevation of the platforms reached an equilibrium, but other morphological dimensions are still developing. The new insights gained from this study enable the prediction of platform elevations along sandy beaches in low-energy, non-tidal environments, and have contributed to our insight in the underlying processes driving the morphological evolution.

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Monitoring of beach and nearshore environments is essential for obtaining better insights into the functioning of the coastal zone. It has driven the understanding of these environments and worked beneficially alongside modelling studies. Hydrodynamics, water quality, and sedimentological and morphological processes can be observed and quantified through field measurements. A successful monitoring programme has a well-considered design, reflecting the interests of all parties involved and balancing scientific requirements (such as measuring scales and resolutions in time and space) against available budgets and resources. The key to utilizing the monitoring result is a data management system that accommodates the FAIR principles – Findable, Accessible, Interoperable and Reusable – for data handling. For the future of coastal monitoring we foresee that recent technological developments will help define the way; particularly miniaturized sensors, data transmission advances, and remote sensing techniques. These developments, especially if embedded in high-profile, open-access coastal observatories, can pave the way towards now-casting of coastal systems.@en