Graduation of Tom van Meurs

10 november 2020 12:00 t/m 13:00 - Locatie: Lecture Hall G, Faculty of Civil Engineering and Geoscience - Door: Webredactie

Beach cusps in a low-energy lake environment

Relating hydrodynamics to coastal feature formation at the Houtribdijk

  • Professor of graduation: Prof. dr. ir. S.G.J. (Stefan) Aarninkhof

  • Supervisors of graduation: Ir. A.M. (Anne) Ton (TU Delft), prof. dr. ir. A.J.H.M. (Ad) Reniers (TU Delft), prof. dr. ir. J.A. (Dano) Roelvink (UNESCO-IHE).

Background
The Houtribdijk is a dike intersecting the IJsselmeer and Markermeer, and is partly reinforce 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 is to find the causes and effects of the formation of beach cusps at the Houtribdijk.

Summary
From a literature study, we have 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 the two locations. Using this data, we quantified the number of cusps, their alongshore position (xc), wavelength (Lc), amplitude (Ac), and the asymmetry of their top-view shape (As), as shown in the figure below. By extrapolating the prevailing theory on high-angle wave instabilities, we arrived at a conceptual model for the large scale (kilometer-wide) morphological changes of the beach. This conceptual model then relates these changes to the hydrodynamics we think are responsible. The results of this analysis indicate that high-angle waves are an important process in the formation of beach cusps. In most cases, 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.