Morphological hysteresis in the evolution of beach profiles under sequences of wave climates - Part 1; observations

Abstract

Beach response to sea level rise is investigated experimentally with monochromatic and random waves in medium scale laboratory wave flumes. Beach profile development from initially planar profiles, and a 2/3 power law profile, exposed to wave conditions that formed barred or bermed profiles and subsequent profile development following rises in water level and the same wave conditions are presented. Experiments assess profile response to a step-change in water level as well as the influence of sediment deposition above the still water level (e.g. overwash). A continuity based profile translation model (PTM) is applied to both idealised and measured shoreface profiles, and is used to predict overwash and deposition volumes above the shoreline. Quantitative agreement with the Bruun Rule (and variants of it) is found for measured shoreline recession for both barred and bermed beach profiles. There is some variability between the profiles at equilibrium at the two different water levels. Under these idealised conditions, deviations between the original Bruun Rule, the modification by Rosati et al. (2013) and the PTM model predictions are of the order of 15% and all these model predictions are within ±30% of the observed shoreline recession. Measurements of the recession of individual contour responses, such as the shoreline, may be subject to local profile variability; therefore, a measure of the 2 mean recession of the profile is also obtained by averaging the recession of discrete contours throughout the active profile. The mean recession only requires conservation of volume, not conservation of profile shape, to be consistent with the Bruun Rule concept, and is found to be in better agreement with all three model predictions than the recession measured at the shoreline.

Hysteresis in the evolution of beach profile parameters under sequences of wave climates - Part 2; Modelling

“…previous model formulations). In part 1 of this work, Baldock et al (2017) showed a significant hysteresis could occur in the beach profile behaviour at equilibrium, both in the net bulk sediment transport and the shoreline response, during decreasing wave conditions, i.e. erosion continued despite a decrease in wave height.…”

“…However, in part 1 of this work (Baldock et al, 2017), laboratory experiments demonstrated a strong hysteresis in beach profile evolution during sequences of wave conditions with increasing and then decreasing energy. Notably, after equilibrium conditions were reached during strongly erosive wave conditions, erosion continued after a reduction in forcing.…”

“…Reduced-scale physical models are exposed to scale effects (Vellinga, 1982). The experiments have been designed to qualitatively reproduce the morphological evolution observed on natural beaches and scale effects are discussed in more detail in the previous paper (Baldock et al, 2017).…”

“…As per series M1, net offshore transport continues after the reduction in forcing, but in this case recovery is very weak. It is of note that, for both monochromatic series, sediment remains stranded in offshore bars formed under standing wave anti-nodes (see Baldock et al, 2017, for more details). As this physical artefact hampers the modelling of the morphological hysteresis, only the experiments using random waves will be considered in the rest of the paper.…”

“…Yates et al, 2009) is insufficient to model the beach behaviour, and two relationships are needed. Baldock et al (2017) proposed that this can be reconciled with the classical idea of a single beach state at equilibrium for any given forcing, as in the classical Wright et al (1985) model, through the concept of a subsequent alternate active beach state. Thus, a reduction in wave height over the existing morphology (the antecedent equilibrium beach state) may result in a more reflective beach profile, which then continues to erode as it evolves to a more dissipative equilibrium state for those new wave conditions.…”