The spatial and temporal variability of sand bar morphology

Lippmann, Thomas; Holman, Robert A.

doi:10.1029/jc095ic07p11575

Cited by 399 publications

(296 citation statements)

References 22 publications

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“…One explanation for the anomalous results obtained from bar 2 may be bathymetric variability that was not considered in the present analysis, such as increased alongshore variability. Alongshore variability is well documented for this feature [Lippmann and Holman, 1990] Differences between consecutively surveyed profiles were assumed to result from cross-shore sediment transport patterns. These patterns were described by a transport magnitude and a cross-shore phase shift.…”

Section: Anomalous Resultsmentioning

confidence: 99%

Role of morphologic feedback in surf zone sandbar response

Plant¹,

Freilich²,

Holman³

2001

J. Geophys. Res.

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Abstract. Several aspects of feedback mechanisms associated with surf zone sandbar response have been characterized using bathymetric surveys, sampled approximately monthly over a 16-year period at the Army Corps of Engineers' Field Research Facility (North Carolina). The measured bathymetry was alongshore averaged and modeled by the superposition of two Gaussian-shaped sandbars on an underlying planar slope. A third, half-Gaussian-shaped bar represented steepening at the shoreline. The rms error between the measured bathymetry and the profile model was 0.10 m (estimated over 322 different surveys). The model explained 99% of the profile variance that remained after first removing the linear, cross-shore trend from each observed profile. Bar response, which was extracted from the modeled profiles, was compared to a local hydrodynamic forcing variable F (F was defined as the ratio of the wave height to water depth, evaluated at bar crest locations). At low values of F (i.e., nonbreaking conditions), bars migrated onshore, and their amplitude tended to decay. At high values of F (i.e., breaking conditions), bars migrated offshore, with relatively little change in amplitude. The transition between onshore and offshore migration occurred at a value of F that was consistent with the onset of wave breaking. Bar migration was associated with a stabilizing feedback mechanism, which drove bar crests toward an equilibrium position at the wave breakpoint. However, we observed that the rate of bar response showed no reduction for any nonzero choice of F, indicating that bars never reached equilibrium. Systematic bar amplitude decay was observed under nonbreaking conditions. Bar amplitude decay could drive F farther away from breaking conditions, allowing further bar amplitude decay. This is a destabilizing feedback mechanism, potentially leading to bar destruction.

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Section: Anomalous Resultsmentioning

confidence: 99%

Role of morphologic feedback in surf zone sandbar response

Plant¹,

Freilich²,

Holman³

2001

J. Geophys. Res.

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“…Part of the difficulty is that the relevant scales span a very broad range, from millimeters (individual sand grains) to kilometers (the cross-shore width of the surfzone) and tens of kilometers (alongshore extent of littoral cells). The largest spatial scales are particularly important because they contain the majority of the spatial and temporal variability of nearshore bathymetric change [Lippmann and Holman, 1990;Plant et al, 1999]. These are also the spatial and temporal scales that characterize human interactions with the coast.…”

Section: Introductionmentioning

confidence: 99%

Morphologic properties derived from a simple cross‐shore sediment transport model

Plant¹,

Ruessink²,

Wijnberg³

2001

J. Geophys. Res.

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Abstract. This paper builds on the now classical discussions by Bowen [1980] and Bailard [1981] on the applicability and implications of Bagnold's [1963] sediment transport model to nearshore profile modeling. We focus on the morphologic implications of both the strengths and weaknesses of Bagnold's model, isolating the transport terms that are well predicted (i.e., mean flow terms) from those that are not well predicted (i.e., transport due to correlations between flow and sediment load). We factor Bagnold's model into a dimensional transport magnitude and a nondimensional term. The nondimensional term describes the relative importance of transport due to undertow, gravity, and correlations between flow and sediment load. The transport magnitude largely determines the response time of nearshore profiles. For typical nearshore environments this response time was estimated to vary as a function of incident rms wave height (Hrms) from ----500 years (Hrm s "• 0.5 m) to 2 years (Hrm s '-" 3 m). The relative importance of competing transport mechanisms is shown to depend strongly on the relative wave height (defined as the ratio of the rms wave height to the local depth). Simplified nearshore transport parameterizations that are a function of this variable were derived and were interrogated for the existence and form of equilibrium profiles. Several differences from previously computed equilibrium profiles were noted. First, because the relative wave height saturates in natural surf zones, equilibrium profiles converge to a relatively flat profile near the shoreline. Second, under some situations a seaward sloping equilibrium profile may not exist. Third, the long response times combined with unknown stability of an equilibrium profile make it difficult to assess the physical connection between theoretical equilibrium profiles and profiles observed in nature. IntroductionAt present, accurate prediction of nearshore bathymetric change at all relevant scales is impossible. Part of the difficulty is that the relevant scales span a very broad range, from millimeters (individual sand grains) to kilometers (the cross-shore width of the surfzone) and tens of kilometers (alongshore extent of littoral cells). The largest spatial scales are particularly important because they contain the majority of the spatial and temporal variability of nearshore bathymetric change [Lippmann and Holman, 1990; Plant et al., 1999]. These are also the spatial and temporal scales that characterize human interactions with the coast. Unfortunately, the difficulty in modeling and prediction is acute at the largest scales, since evolution at this scale requires the integration over all smaller scales [Roelvink and BrOker, 1993].Ideally, the interaction between the large-scale morphology (e.g., surf zone sandbars or the cross-shore profile as a whole) and small-scale processes (e.g., wave-driven hydrodynamics and sediment transport) can be described in terms of param- The advantage of a purely large-scale model is its transparency, which allows di...

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“…Wave breaking is delayed in deeper rip channels, which shows up as darker regions owing to a lack of wave breaking. Long-term monitoring of nearshore morphology with high spatial and temporal resolution has become possible with the application of video imaging (Lippmann and Holman, 1990). Video "time stacks" have proven a useful means of examining the evolution of nearshore morphology and rip channels (e.g., Holland et al, 1997;van Ekenvort, 2004).…”

Section: Introductionmentioning

confidence: 99%