Spatially correlated depth changes in the nearshore zone during autumn storms

Hay, Alex E.; Bowen, Anthony

doi:10.1029/93jc00452

Cited by 29 publications

(31 citation statements)

References 38 publications

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“…Likewise, the correlation term (i.e., R•øtuher in equation (18b)) is affected by the grain size. Grains with long response times will be advected back and forth many times by the near-bed velocities [Hay and Bowen, 1993], and, potentially, the sediment load fluctuations will become uncorrelated to the velocity fluctuations. On the other hand, the movement of coarse grains should be more strongly correlated to the near-bed velocities.…”

Section: Profile Response Timementioning

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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Section: Profile Response Timementioning

confidence: 99%

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

Plant¹,

Ruessink²,

Wijnberg³

2001

J. Geophys. Res.

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“…One solution to the problem of making high-resolution observations of the bed conditions while the instruments are submerged lies in the use of high-frequency acoustic instruments [Greenwood et al, 1993]. Although such acoustics are theoretically susceptible to noise, since sound waves may be reflected as a result of differences in mass density resulting from air bubbles or suspended sediment in the water column [Orr and Hess, 1978;Hay, 1983], in practice a number of studies have successfully used acoustic instrumentation to measure distance to the seabed [e.g., Dingler and Inman, 1976;Vincent and Green, 1990;Vincent et al, 1991;Hay and Bowen, 1993;Gallagher et al, 1996;Traykovski et al, 1999;Crawford and Hay, 2001]. This is because the extremely strong returns that result from acoustic reflection at the bed generally allow for a differentiation to be made between returns emanating from the bed and those from higher in the water column.…”

Section: Introductionmentioning

confidence: 99%

Observations of bed level change in a saturated surf zone

Saulter¹

2003

J. Geophys. Res.

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[1] Sonar altimeter measurements of bed level change were made in conjunction with instantaneous measurements of sea surface elevation and currents at a fixed position in a saturated surf zone (water depths less than 1 m). Two types of bed behavior were observed, classified as ''progressive,'' where mean bed level showed a steady increase or decrease with time, and ''oscillatory,'' where the bed level took a periodic motion (amplitudes 0.5-1.5 cm, periods 10-30 min). ''Mixed'' combinations of the two behaviors were also observed. Comparisons between bed behavior and a wave-based mobility number parameter (Ψ) observed oscillatory beds for Ψ < 40 and mixed behavior for 40 < Ψ < 80, suggesting that bed level oscillations were due to the presence of smallscale migratory bedforms (ripples). Bed oscillations were suppressed with increased wave energy, such that progressive bed behavior was observed when 80 < Ψ. The increase in mobility numbers associated with progressive beds was due to the presence of an energetic infragravity component (Eu ig /Eu i > 1 and Eu ig > 0.05 m 2 s À2 ) and strong mean current (>0.1 ms À1 ). A fractal analysis of the bed level time series was used to test the hypothesis that the process of ripple migration is nonlinear (''self-organised'') rather than a direct response to variations in the forcing component. For low wave energy conditions, when ripples were observable (oscillatory and mixed time series), values of the Local Hurst Exponent (0 < H L < 1) suggested fractal behavior at ripple migration timescales, indicating that the process by which ripples migrate is nonlinear and is allowed to flourish when the hydrodynamic forcing is weak.INDEX TERMS: 3022 Marine Geology and Geophysics: Marine sediments-processes and transport; 4546 Oceanography: Physical: Nearshore processes; 4558 Oceanography: Physical: Sediment transport; KEYWORDS: marine, sediments, processes, hydrodynamics, bedforms, surf zone Citation: Saulter, A. N., P. E. Russell, E. L. Gallagher, and J. R. Miles, Observations of bed level change in a saturated surf zone,

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“…It is vital also to note that the ripple migration itself constitutes an integral part of near-bed sediment transport and is commonly related to bedload transport (e.g. Hay and Bowen, 1993;Traykovski et al, 1999;Doucette, 2002;Hoekstra et al, 2004;Masselink et al, 2007;van der Werf et al, 2007van der Werf et al, , 2008.…”

Section: Introductionmentioning

confidence: 99%