Graduation of Cas van Bemmelen

26 February 2018 08:30 - Location: Room 2.99, Faculty of Civil Engineering and Geosciences - By: Webmaster Hydraulic Engineering

"Beach Scarp Morphodynamics (Formation, Migration, and Destruction)" | Professor of graduation: Prof. dr. ir. S.G.J. (Stefan) Aarninkhof, supervisors: Dr. ir. M.A. (Matthieu) de Schipper (TU Delft), Ir. M.H.P. (Maarten) Jansen (Witteveen+Bos), Prof. P.L.F. (Philip) Liu (NUS), Dr. P. (Phil) Vardon (TU Delft)

Beach scarps are nearly vertical seaward facing walls within the cross-shore beach profile. These features are often associated with eroding (nourished) coastlines and are capable of reaching heights of O(2-3 m), leading to serious hazards to beach users and negatively impacting local ecosystems. New insights into beach scarp morphodynamics related to geometrical, geotechnical and hydrodynamic parameters are presented in this thesis. Aimed at increasing our general understanding of these features, these insights are provided by means of analysing beach scarp presence at (large scale) nourishments and conducting field experiments.

Beach scarp presence at the Sand Engine between 2011 and 2017 has shown that the formation is linked to mildly erosive (summer storm) conditions , whereas destruction is related to both extremely erosive (winter storm) conditions (overwash or inundation, ~50%) and non-hydrodynamic controls (drying collapse or aeolian transport, ~50%). Newly obtained measurements of beach scarps at this nourishment indicate that the toe elevation is `fixed' around the maximum runup elevation, providing a direct relation between the final scarp height, nourishment platform and hydrodynamic conditions. The associated beach scarp slope can be derived from a Culmann-type stability analysis, in which the matric suction provides the apparent cohesion necessary for the stability of scarps.

Field experiments were carried out, which consisted of monitoring the formation, migration, and destruction of scarps from artificially constructed linear slopes. Video observations show that the formation of beach scarps takes place between the 15% and 2% exceedance runup elevation (R15%) and (R2%) and can be influenced by geometrical controls. High platform nourishments will lead to the formation of beach scarps, as overwash is required for a diffuse beach profile. These field experiments have furthermore shown that steep initial slopes are more susceptible to beach scarp formation. Beach scarp migration will take place when the maximum swash elevation exceeds the scarp toe, and is inversely related to the beach scarp height. The observed beach scarps reported in these experiments are in accordance to the definition presented; a non-vegetated, subaerial beach feature with a slope larger than the critical angle of repose of 32° and a minimum height of 0.30 m.

Based on these findings, a conceptual model relating beach scarp morphodynamics to geometrical, geotechnical and hydrodynamic parameters is presented. In general, the formation of beach scarps is preceded by a continuous steepening of the beach slope (between R15% and R2%) until a small vertical discontinuity of O(10 cm) is present. Upon landward migration of this small scale feature, the scarp height changes depending on the backshore topography. Beach scarp destruction can be initiated by four mechanisms; hydrodynamically controlled overwash (1), drying collapse (2), burying by aeolian transport (3) and swash deposition (4).

The findings presented in this study provide a better understanding of beach scarp morphodynamics and their relation to various geometrical, geotechnical and hydrodynamic parameters. Furthermore, the design of beach nourishments can be adjusted to limit the formation of beach scarps and to increase the natural destruction of these beach features.