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Field Experiments of Beach Scarp Erosion During Oblique Wave, Stormy Conditions (Normandy, France)

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Field Experiments of Beach Scarp Erosion During Oblique Wave, Stormy Conditions (Normandy, France) Geomorphology 236 (2015) 132–147 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Field experiments of beach scarp erosion during oblique wave, stormy conditions (Normandy, France) Yoann Bonte ⁎,FranckLevoy Unité Morphodynamique Continentale et Côtière (M2C), Université de Caen-Basse Normandie, 24 rue des Tilleuls, 14000 Caen Cedex, France article info abstract Article history: A field-based experimental study of beach scarp morphodynamic evolution was conducted on the shoreface of a Received 2 July 2014 macrotidal sandy beach subject to storms combined with spring tide events (Luc-sur-Mer, France). Both video Received in revised form 3 February 2015 and in-situ measurements on an artificial berm are used to understand beach scarp evolution over one tide Accepted 15 February 2015 during stormy conditions. Image time stacks are used to analyze the swash action on the beach scarp and topo- Available online 21 February 2015 graphical data of the scarp are recorded with a terrestrial scanner laser to quantify the morphodynamic response Keywords: of the beach scarp to wave action. This work provides a new and unique dataset about beach scarp changes and Beach scarp berm morphology in particular under rising tide and oblique wind-wave conditions. During one stormy event, Swash zone the berm was completely destroyed. However, contrasting alongshore changes were measured during the Berm erosion erosive phase with different crest and foot scarp retreats and eroded volumes between the west and the east Video system measurement side of the berm. The beach in front of the scarp also shows a contrasting residual evolution, indicating an evident Terrestrial laser longshore sediment transport on the study area as a consequence of incident oblique wave conditions. A strong Macrotidal beaches connection between beach evolution and beach scarp changes is clearly identified. The scarp erosion increases on the west side of the berm when the beach level is lowered and reduces when the beach surface rises on the east side. The beach slope and foreshore elevation as a result of a longshore sediment transport between east and west profiles, influence swash activity. Overall, water depth and swash activity became progressively different along the scarp during the experiment. Swash measurements indicate that the presence of the beach scarp strongly influences the swash motion. At high tide, the reflection of the uprush on the scarp front induces a collision between the reflected backwash and the following uprush dynamic. These collisions reduce and sometimes stop the motion of the following uprush, reducing the incoming swash excursion. Consequently, the scarp presence modifies the swash interaction that normally appears on a planar beach surface. With a beach scarp, the swash energy level is substantially attenuated and its spectrum is characterized by a large band. The number of uprush impacts on the scarp front calculated from video images reaches about 25 per 5 min. In spite of the swash energy attenuation due to swash/swash interactions, these impacts provoke the berm destruction in about two hours. However, the onshore migration of the swash zone induced by the rising tide appears to be important to explain scarp destruction, compensating the attenuated swash activity due to backwash-uprush interactions. © 2015 Elsevier B.V. All rights reserved. 1. Introduction et al., 2005) and cause inconvenience to beach users (Kobayashi et al., 2009; Ruiz de Alegria-Arzaburu et al., 2013). Coastal dunes protect low-lying area from flooding and must be Scarp formation has been also observed in a variety of natural sandy actively managed to prevent damage along many sandy coasts around beaches around the world (Sherman and Nordstrom, 1985; Carter, the world. The berm height constitutes an important element of the 1988; Short, 1999; Vousdoukas, 2012) and on artificial beaches natural system to control and reduce the rate of dune recession. For (Kubota et al., 1997). Scarp persistence depends of the local beach example, many beach nourishment projects include the construction conditions. A beach scarp can disappear through continued upslope of a wide berm to improve the dissipation of wave energy during migration, collapse as a result of drying of the beach, or be destroyed major storms and to limit the shoreline erosion. However, beach scarps by wave overtopping (Sherman and Nordstrom, 1985). Scarps probably after nourishment can appear unexpectedly (Nishi et al., 1994; Seymour retard post-storm recovery by waves because low steepness construc- tive waves during high water levels are reflected by the vertical face of this morphology (Nishi et al., 1994). ⁎ Corresponding author. Tel.: +33 231 565 744. This morphological feature is common on eroding beaches, as point- E-mail address: [email protected] (Y. Bonte). ed out by Carter and Stone (1989), and although there are numerous http://dx.doi.org/10.1016/j.geomorph.2015.02.014 0169-555X/© 2015 Elsevier B.V. All rights reserved. Y. Bonte, F. Levoy / Geomorphology 236 (2015) 132–147 133 studies on the morphology and physical processes affecting the scarp of impacts induced by swash, water level and beach scarp face retreat is a dune and many conceptual models describing beach scarps on investigated. These data constitute a unique set of active scarp erosion microtidal and mesotidal environments (Wright and Short, 1983; based on field measurements helpful for calibrating/validating 3D Sunamura, 1985; Short, 1999), quantitative information about beach beach erosion modeling over short timescales also accounting for a scarp formation and evolution in macrotidal environments is still longshore sediment transport gradient. lacking. Sherman and Nordstrom (1985) give a qualitative description of beach scarp formation and evolution based on field observations, 2. Equipment and methods but without a data set to analyze the conditions for the scarp initiation and evolution. 2.1. Field site Katoh and Yanagishima (1990, 1992) studied natural berm formation and erosion using a multi-year dataset of daily surveys of An instrumental field study of the beach scarp morphodynamics was beach profiles. The authors demonstrated the role of long waves near conducted in March 2012 on a beach where natural beach scarps have the shoreline and watertable level on berm erosion. Longshore often been observed. This field site is located at Luc-sur-Mer Beach, on transport and its morphological effects are also important. For instance, the south coast of the Bay of Seine (Fig. 1). The maximum spring tidal Seymour et al. (2005) observed rapid erosion of a berm fill and that range reaches 8 m and the tidal wave is dominated by the M2 semi- scarping resulted in an alongshore quasiperiodic variability. diurnal harmonic whose amphidromic point is a virtual one located Two main field experiments studied artificial sandy beach scarp onland in the southwest of England (Pingree and Griffiths, 1979). The (respective heights of about 0.3 and 1.0 m) with hydrodynamic central English Channel is an area of strong tidal current activity, mainly measurements and topographic surveys during storm events to calcu- longshore oriented and revealing a maximum 3 hours after high and late the eroded volume of the dune as a function of the swash impact. low tide. This coast is partly sheltered from the influence of the prevail- At the Army Corps of Engineers Field Research facilities at Duck, during ing western Atlantic swells by the Cherbourg peninsula, and is mainly the SUPERDUCK experiment, an artificial dune approximately 1.0 m affected by local wind waves (Larsonneur et al., 1982). The prevailing high and 1.3 m wide (classical height for a berm scarp) was built on winds are from the north to northwest, although the strongest winds the beach and allowed to erode naturally during rising tide (Fisher are directed offshore (from the southwest). The local wind induces et al., 1986; Crowson et al., 1988; Overton and Fisher, 1988). Video waves from the north-western sector with short periods ranging from camera recorded both the changes in the artificial berm scarp and the 3to7s(Monfort et al., 2000). The mean annual significant wave height swash impact sequences through one lateral face made with a clear measured on the lower part of the foreshore at Luc-sur-Mer Beach plastic (different to the other made with plywood). Vertical stakes and between 2007 and 2012 (Fig. 2) is around 0.33 m. wave gages were placed in front of the artificial berm scarp to measure The beach slopes, expressed as tan β, vary from 0.08 for the high tidal depth and velocity of the swash. These experiments concluded that it is zone to 0.03 for the upper part of the mid-tidal zone. The sediment difficult to isolate the impact of a single swash on the amount of the comprises a variable range of grain sizes from fine sands (D50, mid = sand eroded, but the total erosion during one event is linearly correlated 0.217 mm) on the mid-tidal zone to coarse sands (D50, high = with the summation of the swash force for each individual swash. 0.622 mm) on the high tidal zone. A 20 m-wide low-amplitude sandbar Another dataset originated from Hasaki beach on the Pacificcoastof with a 6–8 m wide runnel is observed across the profile, separating the Japan where several field experiments on sediment transport and pro- high and the mid-tidal zones (Fig. 3). The runnel area is composed of file evolution in the swash zone have been performed (Kubota et al., very coarse sands, broken shells and few pebbles. The low tidal zone is 1997, 1999). During some of these experiments, an artificial beach flat, planar and partially wet, due to cropping out of the watertable, face was created by accumulating sand on the beachface using a and mainly composed of a rocky platform with a slope of 0.01 and few bulldozer.
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