Beach scarp dynamics at nourished beaches

C.W.T Van Bemmelen,M.A De Schipper,J Darnall,S.G.J Aarninkhof

doi:10.1016/j.coastaleng.2020.103725

C.W.T Van Bemmelen, M.A De Schipper + Show 2 more

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https://doi.org/10.1016/j.coastaleng.2020.103725

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Journal: Coastal Engineering	Publication Date: May 18, 2020
Citations: 20	License type: cc-by

Affiliation: Delft University of Technology

Abstract

Beach scarps are nearly vertical seaward facing sandy cliffs within the cross-shore beach profile. These features are often associated with eroding (nourished) coastlines and can reach heights of O(2–3 m). An analysis of a six-year dataset of beach scarp presence at the nourished Sand Engine beach shows that the formation of beach scarps at the nourishment is linked to mildly erosive (summer storm) conditions, whereas destruction is often related to extremely erosive (winter storm) conditions. Additional experiments were carried out showing the formation, migration, and destruction of scarps from artificially constructed mounds with linear slopes. The field experiments show that steep initial slopes are more susceptible to beach scarp formation. Video observations at these experiments show that the scarp toe level, where the vertical slope meets the gently sloping beach, is related to the high wave runup events. The commonly used 2% exceedance wave runup estimates can therefore be used to predict the final elevation of the scarp toe. The strong connection of maximum runup elevation with the scarp toe elevation provides a direct relation between the final scarp height through nourishment platform height and hydrodynamic conditions. High platform nourishments will promote the formation of beach scarps and steep initial profiles increase the speed at which scarps form. This study suggests that by adjusting the design of beach nourishments, beach scarp formation and persistency can be limited by regulating the natural destruction of these features.

Highlights

Scarps are nearly vertical seaward-facing cliffs of sand often associ ated with coastal erosion
This study suggests that by adjusting the design of beach nourishments, beach scarp formation and persistency can be limited by regulating the natural destruction of these features
The comparison shows a strong relation between the beach scarp toe elevation and the computed wave runup, which indicates that the scarp toe elevation can be predicted

Summary

Introduction

Scarps are nearly vertical seaward-facing cliffs of sand often associ ated with coastal erosion. After beach scarps are formed, they can remain small and shortlived, but are at times persistent features in the coastal profile capable of reaching heights of up to 2–3 m (Sherman and Nordstrom, 1985; De Alegria-Arzaburu et al, 2013). Beach scarps are observed along natural coastlines, but have been reported shortly after implementation of beach nourishments Erosion of man-made nourishments is often expected, but the formation of beach scarps can come as a surprise to coastal practitioners Beach scarps have been reported after large scale nourishments, and on natural beaches linked to cusp horn erosion Bonte and Levoy (2015) show that beach scarps may migrate landward over several tidal cycles while their toe elevation, where the vertical slope meets the gently sloping beach, increases. After a beach nourishment had been completed to restore the beach, another typhoon again resulted in the formation of massive beach scarps of O(3 m), which was attributed to the steep nearshore beach profile present after the nourishment

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