Modeling cross‐shore sandbar behavior on the timescale of weeks

Abstract

[1] We compare predictions of a coupled, wave-averaged, cross-shore wavescurrents-bathymetric evolution model to observations of onshore and offshore nearshore sandbar migration. The observations span a 10-and 44-day period with onshore/ offshore bar migration at Duck, North Carolina, and at Hasaki, Kashima Coast, Japan, respectively, a 3.5-month period of onshore bar migration at Duck, and a 22-day period of offshore bar migration at Egmond, Netherlands. With best fit parameter values the modeled temporal evolution of the cross-shore bed profiles agrees well with the observations. Model skill, defined as 1 minus the ratio of prediction to no-change error variances, ranges from 0.50 at Egmond to 0.88 for the prolonged onshore bar migration at Duck. Localized (in time and space) reductions in model skill coincide with alongshore variations in the observed morphology. Consistent with earlier observations, simulated offshore bar migration takes place during storms when large waves break on the bar and is due to the feedback between waves, undertow, suspended sediment transport, and the sandbar. Simulated onshore bar migration is predicted for energetic, weakly to nonbreaking conditions and is due to the feedback between near-bed wave skewness, bedload transport, and the sandbar, with negligible to small effects of bound infragravity waves and near-bed streaming. Under small waves and conditions, when breaking and nonbreaking conditions alternate with the tide, the sandbar is predicted to remain stationary. The intersite differences in the optimum parameter values are, at least partly, induced by insensitivity to parameter variations, parameter interdependence, and errors in the offshore wave forcing.

Morphological coupling in multiple sandbar systems – a review

“…Process-based models that focus on cross-shore migration (e.g. Hoefel and Elgar, 2003;Ruessink et al, 2007bRuessink et al, , 2012 or on alongshore variability (e.g. Reniers et al, 2004;Calvete et al, 2005;Drønen and Deigaard, 2007;Castelle and Coco, 2012) alone, have become quite mature.…”

Numerical predictability experiments of cross‐shore sandbar migration

“…[5] To simulate the temporal evolution of cross-shore bathymetry, we use the deterministic wave-averaged crossshore profile model detailed by Ruessink et al [2007]. The model, which is one of only few operational models that accurately predicts onshore and offshore sandbar migration on the time scale of days to weeks, uses an initial bed profile, the median bed material grain size, and time series of offshore wave parameters (height, period, direction) and water levels to evolve the bed profile through coupled hydrodynamic (waves and currents) and sediment transport (bed load and suspended load) equations.…”

“…[4] Here, we employ a coupled hydrodynamic/sediment transport model [Ruessink et al, 2007] in an ensemble prediction scheme [Hoffman and Kalnay, 1983] using a multi-year data set of daily bed profile surveys [Kuriyama, 2002] to examine the time scales on which our model might reflect reality and to address the question as to whether the unpredictability of cross-shore sandbar migration results from model inadequacy or deterministic chaos. First, we discuss the model, data, and ensemble scheme; next we examine the temporal evolution of ensemble spread and model skill; and finally we consider the implications of the results for surf zone research and modeling.…”

Numerical modelling of pronounced sloping beach profile evolution: comparison with the large-scale BARDEX II experiment

“…In this paper, we discuss the model configuration and associated results, as well as the need of intensive high-frequency full-scale data to further develop and improve process-based models. beach face, is usually handled the same way as in the surf zone or, more commonly, ignored (Ruessink et al, 2007). This results in large errors in the intertidal domain (e.g., Ruessink, 2005).…”

“…In parallel, the development of numerical, process-based, beach profile models have recently succeeded in simulating surfzone sandbar evolution on timescales of weeks (Ruessink et al, 2007) to years (Walstra et al, 2012;Kuriyama, 2012) on sandy beaches with reasonable skill. However a number of limitation remain, for instance, the bottom evolution in the swash zone, and hence the Barriers and sandbars are ubiquitous natural coastal features, whose variability often determines nearshore morphological evolution.…”