Post-storm recovery assessment of urbanized versus natural sandy macro-tidal beaches and their geomorphic variability
Introduction
Sandy beaches play an important role in protecting the coastline from single high impact storms or sequences of storms. With main challenges such as climate change, rising sea levels and growing populations in coastal areas, an understanding of how beaches respond to and recover from high magnitude events has become essential for effective coastal management (Zhang et al., 2004).
The morphological consequences of storm and beach response to storms have been well documented during the last decades. Highly variable morphological impacts have been reported, depending on the forcing factors, such as storm surge intensity and magnitude (Haerens et al., 2012) with respect to the existing coastal state (Morton et al., 1994; Houser et al., 2008). Storm-induced extreme waves and high water levels are key forcing factors in erosion (e.g. Wang et al., 2006; Suanez et al., 2015). A single storm can immediately erode a large amount of sediment within hours, but a cluster of storms, for instance during the winter season, may cause more extensive beach erosion over a short period (e.g. Coco et al., 2014; Castelle et al., 2015; Masselink et al., 2016; Scott et al., 2016). Although storm energy may be sustained at high levels during an extended storm phase, erosion tends to decrease in magnitude and to reach equilibrium over the event period (Aagaard et al., 2012).
A general consensus is that beaches commonly recover more or less from storm-induced erosion (Davidson-Arnott, 2009). However, in contrast to beach erosion response, literature about the ability of beach recovery is scarce, sometimes even neglected in studies (e.g. Splinter et al., 2014). In opposite to beach erosion that takes place within a few hours during a storm, the post-storm beach recovery is a much slower process. The initial recovery can be extremely fast and commence immediately after the storm (Morton et al., 1994), but there are also reported cases where eroded beach can last for over a year, and foredune erosion even persist over several years (e.g. Wang et al., 2006). In addition to a lack of reported data on the rapidity of beach recovery following storm erosion, the morphological context and features that signal how beach recovers after storm events are also not well studied. Beach recovery has been shown to be dependent on a wide range of parameters in addition to wave climate. These include antecedent beach morphology (Wright and Short, 1984), geological setting (Anthony, 2013), local availability of sediment (e.g. Forbes et al., 2004), beach type (Masselink and Van Heteren, 2013) and the propensity for sand transfer from the beach to foredunes by aeolian transport (Anthony et al., 2006). Nevertheless, most attention has been on natural beach in micro to mesotidal settings and that understanding post-storm beach recovery processes is still limited along macro-tidal coasts (Héquette et al., 2019) where large fluctuations of water level may significantly result in variations of sediment supply and dynamics. Also, it is to the authors' knowledge that there is little beach recovery studies along urbanized beach where hard and soft protection are present, and where morphological response and recovery to the forcings may be different to natural beaches.
This study aims to assess and compare the response and subsequent recovery to storm Dieter (14–15th January 2017) of two sandy macro-tidal beaches in Belgium. Both beaches, one natural and the other periodically nourished and in an urbanized setting, are exposed to the same southern North Sea wave and storm climate. The natural beach has experienced long-term accretion, while the urbanized beach has been periodically nourished and armoured with hard structures such as groynes and a seawall to ward off mild but chronic erosion. Also, they are characterized by different morphological features such as intertidal bars, dunes and berms across the beach. Given these differing environmental settings, two specific objectives are identified: (1) determining the rates of morphological recovery within different zones across the beach; and (2) identifying stages of post-storm beach recovery and the driving mechanisms. Analyzing the post-storm response of beaches with large tidal ranges is, from an initial research standpoint, interesting, as such beaches would seem to be resilient by being capable of dissipating storm waves. Since large stretches of the western European coast are characterized by macro-tidal conditions with a large tide range exceeding 3.5 m, it presents a challenge in our current knowledge of post-storm beach-dune recovery. Given the post-storm recovery beach behaviour, this study provides useful knowledge to design appropriate coastal management strategy.
Section snippets
Regional setting
The Belgian coast is situated in the Southern North Sea basin and is about 67 km long (Fig. 1A). The coastline is oriented SW-NE and stretches from the French border in the west to the small tidal inlet of Zwin at the Dutch border. The coast consists of broad, gently sloping sandy beaches that commonly exhibit multiple bar-through systems, and are up to 600 m at low tide (Haerens et al., 2012). Numerous sandbanks (the Flemish Banks) are present in the shallow continental shelf. In general, the
Methodology
To assess how the beach morphological systems response and recover after the storm, three high resolution airborne LiDAR (Light Detection and Ranging) surveys and monthly cross-shore field topographic profiles using real-time kinematic global positioning system (RTK-GPS) were performed and analyzed over the study period.
Storm surge in January 2017
Based on an extended classification of storms defined as a water level exceeding 5 m TAW by Haerens et al. (2012), the January 2017 storm surge, named Dieter, is rated as one of the top ten of the most extreme events along the Belgian coast since 1984. The storm, which came from NNW, occurred on 14–15th January with a depression located above Northern Europe. Fig. 3 illustrates time series of wind, water level and wave conditions near the study sites during the course of the storm. The maximum
Comparison of the beach response and recovery between the study sites
The beach response and rebuilding processes are investigated within 5 months of a severe storm surge on a natural beach at Groenendijk and urbanized beach at Mariakerke along the Belgian coast. The period from January (post-Dieter) to June 2017 was characterized by typical low to moderate wave height (<1 m) coming from SW-W sector. Two relatively energy events with a lifespan of <2 h occurred in February and March but they did not reach the storm threshold of significant morphological change.
Conclusions
- 1)
Severe beach erosion during Dieter storm on 14–15th January 2017 was generated by high wave conditions accompanied by an extreme water level, causing a sand loss in the entire beach at both the natural and urbanized beaches. Despite site-specific environmental settings, erosion dominated in the intertidal zone, and at lesser extent, sand was removed from the backshore with a similar net rate at both study sites.
- 2)
The initiation of beach rebuilding was evident 1.1 month after the storm and the
Acknowledgements
This research is part of the CREST project, funded by the Strategic Basic Research (SBO) program of Instituut voor Innovatie door Wetenschap en Technologie (IWT). The authors acknowledge Coastal Division of the Flemish Authority and the Flemish Banks Monitoring Network of the Flemish Government for access to data.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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