Abstract
Barrier beach change in directionally bi-modal wave climates presents an increasing challenge for coastal communities, both in the short-term (storm events), and decadal to centurial time scales (long-term evolution). Predicting and planning for subsequent variations requires understanding of the morphological response to changes in wave energy, along with the atmospheric forces driving the wave climate. In this paper, multi-method topo-bathymetric surveys are used to assess the morphological change of a semi-sheltered gravel barrier (Start Bay, Devon, UK). Total sediment budgets (supra- to sub-tidal), with spatially-varying uncertainty levels, indicate the embayment is closed. One third of total sediment flux occurred in the sub-tidal, establishing the importance of sub-tidal transport for this type of coastline. Our results demonstrate that under the predominance of a given wave direction, rotation first occurs within sub-embayments. Additional sustained and extreme energy levels are then required for full embayment rotation to occur, with significant headland bypassing. In this instance, 6 × 105 m3 of gravel was transported alongshore during a 3-year sustained period of dominant-southerly waves, including a 1:50 year storm season (full-embayment rotation), whilst 3 × 105 m3 was returned during a 2-year period of dominant easterly waves (sub-embayment rotation only). A novel parameter is introduced that predicts beach rotation based on the directional wave balance. In turn, winter wave direction is shown to correlate with a combination of two climate indices. Given adequate predictions of relevant climate indices, these findings constitute the basis of a generalisable method to predict and plan for future beach rotation on similar beaches globally.
Highlights
Gravel barrier beaches often act as the first line of defense from elevated water levels and wave forcing during storms, absorbing wave energy at the shoreline and providing protection from inundation for housing and infrastructure (Poate et al, 2016)
Implementing a structure-from-motion (SfM) approach (Westoby et al, 2012), overlapping aerial photographs were aligned and georeferenced using Real Time Kinematic (RTK) Global Positioning System (GPS) measured ground control points (GCP's), deriving a high-resolution (N50 pts./m2), three-dimensional point cloud, which was interpolated to a 1 m grid digital elevation model (DEM) of inter/supratidal beach topography
This study examines extreme event and decadal sub and fullembayment rotation within a headland embayed gravel coastline with a bi-directional wave climate
Summary
Gravel barrier beaches often act as the first line of defense from elevated water levels and wave forcing during storms, absorbing wave energy at the shoreline and providing protection from inundation for housing and infrastructure (Poate et al, 2016). These mobile barrier deposits are highly dynamic and respond rapidly to changes in wave climate (Ruiz de Alegria-Arzaburu and Masselink, 2010; Bergillos et al, 2016a; Bergillos et al, 2017). Where the incident wave climate is directionally bi-modal, morphological changes are often controlled by the time-integrated balance of wave power from the two directions (Ruiz de Alegria-Arzaburu and Masselink, 2010; Bergillos et al, 2016b)
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