High‐efficiency gravel longshore sediment transport and headland bypassing over an extreme wave event

Abstract

Gravel beaches are common throughout the high latitudes, but few studies have examined gravel transport rates, in particular at high energy levels, and no studies have quantified gravel transport around headlands. Here, we present the first complete sediment budget, including supra‐, inter‐ and sub‐tidal regions of the beach, across multiple headland‐separated gravel embayments, combined with hydrodynamic observations, over an extreme storm sequence, representing at least a 1‐in‐50‐year event. Unprecedented erosion was observed (~400 m3 m−1, −6 m vertical), with alongshore flux of 2 × 105 m3, equivalent to annual rates. Total system volume change was determined to the depth of closure and then used to calculate alongshore flux rates. Alongshore wave power was obtained from a wave transformation model. For an open section of coastline, we derive a transport coefficient (CERC formula) of KHs = 0.255 ± 0.05, exceeding estimates in lower‐energy conditions by a factor of 5 or more. We apply this coefficient to rocky segments of the shoreline, determining rates of headland bypass from 0 to 31% of potential flux, controlled by headland extent and toe depth. Our results support the hypothesis that gravel is transported more efficiently at higher energy levels and that a variable rate or threshold approach may be required. Complete coverage and varying morphology make this dataset uniquely suited to improving model predictions of gravel shoreline change. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

A rules-based shoreface translation and sediment budgeting tool for estimating coastal change: ShoreTrans

“…13c) may have contributed to the destruction of an old fishing village [58,59], in addition to aggregate dredging of the shoreface. Short-term changes in longshore flux, forced by variations in the bi-directional wave climate, result in rapid changes in beach width due to rotation [47,48], particularly near headlands ( Fig. 13b, d).…”

“…The method shown in Fig. 7 is suitable for estimating long-term trends and short-term variability in longshore transport within an embayment, e.g., at Start Bay in the UK, where multi-decadal unidirectional flux is overlain by short-term oscillations [47,48]. However, for cross-shore dominated embayments, where rotation is insignificant, such as Perranporth [49], a different approach is required to account for alongshore sediment redistribution.…”

“…SLR was projected at 1 ± 0.5 m by 2120, as for the Perranporth application (Section 3.2). Upper depth of closure was obtained from observations around an extreme event [48], taking the observed depth of morphological change as the shallow bound, with a range of 2 m, increasing from south (<10 m depth) to north (up to 12 m depth). As stated above, an assumption of zero-transport is applied between the upper-and lower shoreface, due to the presence of the gravel-sand transition.…”

“…Shortterm variability due to longshore transport (i.e., rotation), was estimated as a bulk volume based on observations ( Fig. 13 b-d; [47,48] and (iii) the barrier will be stationary in the absence of SLR or longshore flux gradients. The validity of these assumption will be discussed in Section 5.…”

“…Volume change at each profile was multiplied by the distance alongshore represented by each profile. To maintain simplicity and brevity, the uncertainty introduced by using widely spaced profiles to represent alongshore variable morphology was not accounted for in this instance (see [48] for a more detailed approach).…”