Measurements of Oscillatory Drag on Sand Ripples

Lofquist, Karl E. B.

doi:10.1061/9780872622647.186

Cited by 12 publications

(13 citation statements)

References 10 publications

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“…Jonsson & Carlsen (1976), for example, indicated shifts in phase angle of + 18" and + 3 l o for turbulent oscillatory boundary layers. Lofquist (1981Lofquist ( , 1986 has documented similar but rather more complex relationships between bottom shear and and the free stream velocity over various rippled beds; within a single waved cycle there may be 1-3 peaks in the average bottom stress, indicative of a lack of a simple proportionality between the near-bed velocity and the instantaneous bottom stress. These relationships provide a link between the magnitudes of the concentrations of suspended sediment, the vertical convection velocity of sediment particles and a first approximation to the applied bottom stress.…”

Section: Horizontal Oscillatory Velocity (Us)mentioning

confidence: 99%

Sediment suspension under waves and currents: time scales and vertical structure

Osborne

Greenwood

1993

Sedimentology

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Field measurements of the vertical structure of near‐bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time‐dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 3 and 8 s) and in water depths of 0·5–5·0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near‐bed oscillatory velocity. Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind‐wave forcing with distance away from the bed as a result of two factors: (1) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interact with the near‐bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near‐bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevations within the fluid.

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Section: Horizontal Oscillatory Velocity (Us)mentioning

confidence: 99%

Sediment suspension under waves and currents: time scales and vertical structure

Osborne

Greenwood

1993

Sedimentology

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“…[33] Nikora et al [2001] have distinguished some layers in order to clarify the structure of the mean flow in the context of the spatially averaged equations introduced in the [1955] (triangles); Kennedy and Falcon [1965], Horikawa and Watanabe [1967], Carstens et al [1969], Mogridge and Kamphuis [1972], Nielsen [1979], and Lofquist [1980] (crosses); Ribberink and Al-Salem [1994], Rankin and Hires [2000], Faraci and Foti [2001], and O'Donoghue and Clubb [2001] (circles). The solid line represents the limiting height as given by equation (13) with b = 1 and f from equation (8).…”

Section: Appendix A: On the Structure Of Rough Flowsmentioning

confidence: 99%

Highest natural bed forms

Giménez‐Curto

Corniero

2003

J. Geophys. Res.

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[1] Fluid flow interacts with sedimentary beds forming waves of different kinds, which are of considerable practical importance since they influence significantly the near-bed flow, both over and below the bed, sediment transport, and wave height attenuation. We focus here on steep bed forms capable of producing flow separation. In this case, the largescale vorticity generated in the phenomenon of separation rules the process of friction, which appears to be practically unaffected by sediment motion. Under the crests of the bed forms, the mean shear force due to friction is balanced by the force that bed forms exert on the flow via pressure, which can be calculated from the work of Giménez-Curto and Corniero [2002]. Bed forms grow until they have a height such that friction at their troughs is negligible, thus ceasing the motion of fluid and sediment. This condition leads to a very simple expression for the limiting steepness, which compares favorably with existing observations on bed form geometry under steady open channel flow as well as under oscillatory flow.

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“…As a consequence we can express the condition under which interaction can be disregarded as ,k > 2.2r//e 2 (46) This can be rewritten as X > 6.1h/e or X/a > 2.2e, but not X/a > 1 as conjectured by Baghold [1946]. Finally, in Figure 4 we represent the measurements by Carstens et al [1969] and Lofquist [1980] corresponding to natural sand ripples generated in the laboratory, together with (45). Although there is some scatter, the general trend is in good agreement with the theory.…”

Section: Besides Test 1 By Jonsson and Carlsenmentioning

confidence: 99%

Oscillating turbulent flow over very rough surfaces

Giménez‐Curto¹,

Lera²

1996

J. Geophys. Res.

101

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The forces on the seabed in shallow water under waves influence near‐shore transport processes. However, the actual nature of these forces is not yet fully understood. Sleath [1987] simultaneously measured horizontal shear force per unit area and Reynolds stress in oscillating turbulent flow over granular beds with the striking result that maximum Reynolds stress was significantly less than total shear force per unit area of bed. Trying to explain these measurements, we use a formulation which considers two kinds of flow perturbations, namely turbulent fluctuations and some disturbances due to boundary irregularities. The resulting spatially averaged Reynolds equations contain in particular two terms which do not appear in the smooth bed case: the force due to the mean momentum flux for boundary disturbances, here called “form‐induced stress,” which owes its existence to the voracity of the disturbed motion, and the force exerted by the roughness elements on the fluid. The “jet regime” as introduced by Giménez‐Curto and Corniero [1993] for steady flow is extended to oscillatory flow. In this regime, pressure drag on roughness elements is the fundamental force acting on the boundary, and form‐induced stress due to vorticity generated by flow separation from bed irregularities becomes the leading stress, thus providing an explanation for Sleath's measurements by means of a physical mechanism which was already envisaged by Longuet‐Higgins [1981] for two‐dimensional rippled beds. A simple expression is derived for the friction coefficient which is subsequently compared with extensive series of measurements in the laboratory for granular beds as well as for rippled surfaces, showing an excellent agreement.

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Measurements of Oscillatory Drag on Sand Ripples

Cited by 12 publications

References 10 publications

Sediment suspension under waves and currents: time scales and vertical structure

Sediment suspension under waves and currents: time scales and vertical structure

Highest natural bed forms

Oscillating turbulent flow over very rough surfaces

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