Post-Nourishment Beach Scarp Existence at the Sand Engine

Abstract

A survey dataset taken at the Sand Engine mega-nourishment on the southwest coast of Holland was analyzed in an attempt to locate beach scarps and characterize their development. A beach scarp is defined as a vertical discontinuity in the upper foreshore slope generated through the removal of sediment by natural processes. More specifically, this report defines a scarp as a feature with a minimum height of 30 cm and a slope of 0.15. A few studies on beach scarp behavior have been investigated in recent years, but there is still quantitative ambiguity regarding the parameters that determine their post-nourishment generation, which can partially be attributed to their ephemeral nature. In turn, this lack of knowledge has directly hindered our ability to predict and/or prevent their existence in future nourishment projects. Scarps were identified by use of an automated tool created to locate the steepest sections in the beach profile. This tool identifies the scarp crest (top) and toe (bottom) by the local minimum and maximum of the second order derivative of the profile elevations. Scarps were also identified by means of manual transect analysis to validate this automated process. These features were observed during 15 of the 33 measurement periods, with 8/15 being summer months and 3/15 being winter months. Thus, this scarping phenomena yields a strong seasonal signal with the majority of removal periods occurring during winter months when the wave climates were more energetic; however, it was also observed that calmer periods interrupted by storm events were also capable of altering scarp geometries. Scarping at the Sand Engine consistently occurred between +3 & +2 m NAP, with the average scarp height at the southern flank, head, and point being 0.85, 0.78 & 1.0 m respectively. In general, scarps follow an overall pattern of periodic variability at the Sand Engine depending on the original profile geometry, water levels (storm surge & tidal elevation), and wave runup events. Observations showed that already developed scarps were only affected when the maximum runup levels (RHI) exceeded the scarp toe (SLO), which occurs during the collision, overtopping, and inundation regimes. The collision regime is responsible for the landward migration of the scarp without destroying the entire feature; the runup elevation is able to reach the scarp base inducing an undercutting effect which leads to slumping of the scarp face, but not necessarily to complete removal. Furthermore, scarps were completely removed only upon entering the overtopping and/or inundation regimes. It appeared the swash regime had no effect on the scarps at all considering the geometric characteristics remained approximately constant between measurement periods, insinuating the water levels were not high enough for the resulting runup to reach the scarp base. Thus, storm surge and tidal elevations have a strong influence on scarp generation/degradation at the Sand Engine, by exposing a greater area of the coast to wave attack.