Beach cusps in a low-energy lake environment

Relating hydrodynamics to coastal feature formation at the Houtribdijk

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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.