Equilibrium shoreline response: Observations and modeling

Abstract

[1] Shoreline location and incident wave energy, observed for almost 5 years at Torrey Pines beach, show seasonal fluctuations characteristic of southern California beaches. The shoreline location, defined as the cross-shore position of the mean sea level contour, retreats by almost 40 m in response to energetic winter waves and gradually recovers during low-energy summer waves. Hourly estimates of incident wave energy and weekly to monthly surveys of the shoreline location are used to develop and calibrate an equilibrium-type shoreline change model. By hypothesis, the shoreline change rate depends on both the wave energy and the wave energy disequilibrium with the shoreline location. Using calibrated values of four model free parameters, observed and modeled shoreline location are well correlated at Torrey Pines and two additional survey sites. Model free parameters can be estimated with as little as 2 years of monthly observations or with about 5 years of ideally timed, biannual observations. Wave energy time series used to calibrate and test the model must resolve individual storms, and model performance is substantially degraded by using weekly to monthly averaged wave energy. Variations of free parameter values between sites may be associated with variations in sand grain size, sediment availability, and other factors. The model successfully reproduces shoreline location for time periods not used in tuning and can be used to predict beach response to past or hypothetical future wave climates. However, the model will fail when neglected geologic factors are important (e.g., underlying bedrock limits erosion or sand availability limits accretion).

Beach response to a sequence of extreme storms

“…Despite having a detailed set of data with morphological and hydrodynamic characteristics, it remains difficult to provide a conclusive statement on the theoretical effect(s) of storm clustering. One might expect a depleted beach to be more prone to erosion but recent work (Yates et al, 2009) indicates the opposite: since beaches always tend to an equilibrium shape and since a storm tends to push the beach far from equilibrium, sequences of storms should be less and less effective in driving the beach away from equilibrium. Our results have elements that are consistent with the simple model of Yates et al (2009) although we also suggest that other characteristics of the 'storm chronology' such as tidal stage and surfzone characteristics are at least as important as wave height and are needed in such a model in order for it to be more universally applicable.…”

“…Both hypotheses are viable and in this context recent data-driven models have shown that shoreline erosion tends to diminish if erosive conditions are maintained (Yates et al, 2009). At the same time, because of the difficulty in collecting an appropriate dataset, little is known about the alongshore variability in beach response to storms.…”

Response of a fringing reef coastline to the direct impact of a tropical cyclone

“…4). Although the magnitude of beach response in reef environments is much less than sandy environments, the underlying forcing mechanism appears to be similar, with the magnitude of divergence (or disequilibrium) of nearshore wave heights from typical nearshore wave heights a key factor determining storm-induced beach response (e.g., Yates et al 2009). …”

Role of wave forcing, storms and NAO in outer bar dynamics on a high-energy, macro-tidal beach

“…It may be suggested that the change in wave conditions is more relevant in driving bar morphological change than the absolute values of the wave parameters: e.g., all offshore migration events occurred following an increase in H/L, whereas all onshore migration events coincided with falling H/L values. If changes in the wave conditions are more relevant in driving bar behaviour than actual wave parameters, an equilibrium type of response model, involving deviations from antecedent wave conditions is worth pursuing (e.g., Yates et al, 2009;Davidson et al, 2013;Castelle et al, 2014). Following Davidson et al (2013), bar dynamics are related to the product of the wave energy flux P and a disequilibrium term (Ω m -Ω), where Ω m is the background antecedent value for Ω, and Ω represents the instantaneous value.…”