Journal ArticleDOI

# Detailed measurements of velocities and suspended sand concentrations over full-scale ripples in regular oscillatory flow

01 Jun 2007-Journal of Geophysical Research (American Geophysical Union)-Vol. 112

Abstract: The knowledge and modeling of wave-induced sand transport over rippled beds still has significant shortcomings, which is partly related to a lack of measurements of the detailed processes from controlled laboratory experiments. We have carried out new measurements of the detailed time-dependent velocity and suspended sand concentration field around vortex ripples for regular oscillatory flow conditions. The fact that the ripples were mobile and the flow conditions were full-scale makes these measurements unique. We made velocity measurements for 14 different flows and concentration measurements for three of these flows. The velocity and concentration field above ripples are dominated by the generation and ejection of vortices on the ripple flanks around the time of flow reversal. Vortex formation results in near-ripple flow reversals ahead of free-stream reversals and velocity maxima near the ripple crest that are much higher than the free-stream maxima. Asymmetry in the free stream produces steady circulation cells with dominant offshore mean flow up the ripple lee slope, balanced by weaker onshore streaming up the ripple stoss slope as well as higher up in the flow. The time- and bed-averaged horizontal velocity profile comprises an offshore streaming near the bed and an onshore drift higher up in the flow. The vortices are responsible for three main concentration peaks: one just after on-offshore flow reversal associated with the passage of a sand-laden vortex followed by two smaller peaks due to advected suspension clouds generated by vortex action at the neighboring onshore ripples. The sand flux field measured for one typical asymmetric flow condition is dominated by an offshore flux associated with the suspended sand cloud generated by vortex shedding from the ripple's lee slope around the time of on-offshore flow reversal. The net (time-averaged) current-related and wave-related horizontal sand fluxes are generally offshore directed and mostly contained within 1.5 ripple heights above the ripple crest. The wave-related suspended transport component is larger, but the contribution of the current-related suspended sand transport cannot be neglected. In addition to the measured offshore net transport of suspended sand, there is an onshore-directed transport very close to the ripple surface. The total net transport is in the offshore direction for this specific asymmetric flow condition.
Topics: Vortex shedding (55%), Vortex (54%), Sediment transport (53%), Ripple (53%), Mean flow (52%)

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Detailed measurements of velocities and suspended sand
concentrations over full-scale ripples in regular oscillatory flow
J. J. van der Werf,
1
J. S. Doucette,
2
T. O’Donoghue,
2
and J. S. Ribberink
1
Received 23 June 2006; revised 19 October 2006; accepted 8 December 2006; published 9 May 2007.
[1] The knowledge and modeling of wave-induced sand transport over rippled beds still
has significant shortcomings, which is partly related to a lack of measurements of the
detailed processes from controlled laboratory experiments. We have carried out new
measurements of the detailed time-dependent velocity and suspended sand concentration
field around vortex ripples for regular oscillatory flow conditions. The fact that the
ripples were mobile and the flow conditions were full-scale makes these measurements
unique. We made velocity measurements for 14 different flows and concentration
measurements for three of these flows. The velocity and concentration field above ripples
are dominated by the generation and ejection of vortices on the ripple flanks around
the time of flow reversal. Vortex formation results in near-ripple flow reversals ahead of
free-stream reversals and velocity maxima near the ripple crest that are much higher
than the free-stream maxima. Asymmetry in the free stream produces steady circulation
cells with dominant offshore mean flow up the ripple lee slope, balanced by weaker
onshore streaming up the ripple stoss slope as well as higher up in the flow. The time-
and bed-averaged horizontal velocity profile comprises an offshore streaming near the bed
and an onshore drift higher up in the flow. The vortices are responsible for three main
concentration peaks: one just after on-offshore flow reversal associated with the passage of
a sand-laden vortex followed by two smaller peaks due to advected suspension clouds
generated by vortex action at the neighboring onshore ripples. The sand flux field
measured for one typical asymmetric flow condition is dominated by an offshore flux
associated with the suspended sand cloud generated by vortex shedding from the ripple’s
lee slope around the time of on-offshore flow reversal. The net (time-averaged) current-
related and wave-related horizontal sand fluxes are generally offshore directed and mostly
contained within 1.5 ripple heights above the ripple crest. The wave-related suspended
transport component is larger, but the contribution of the current-related suspended sand
transport cannot be neglected. In addition to the measured offshore net transport of
suspended sand, there is an onshore-directed transport very close to the ripple surface. The
total net transport is in the offshore direction for this specific asymmetric flow condition.
Citation: van der Werf, J. J., J. S. Doucette, T. O’Donoghue, and J. S. Ribberink (2007), Detailed measurements of velocities and
suspended sand concentrations over full-scale ripples in regular oscillatory flow, J. Geophys. Res., 112, F02012,
doi:10.1029/2006JF000614.
1. Introduction
[2] Knowledge of sand transport processes induced by
waves and currents is of crucial importance in order to
understand the morphological behavior of the coastal
system and to be able to predict future changes as a result
of natural processes and human interferences. One of the
largest knowledge gaps in sand transport processes is in the
understanding of wave-induced sand transport over ripples,
typically 0.01 0.1 m high and 0.11.0 m long. This is
reflected in the inability of models to accurately predict the
net transport rate under these conditions. An intercompar-
ison of different transport models and a comparison of
model predictions with field and laboratory data by Davies
et al. [1997] and Davies et al. [2002] have shown that the
current-related transport is generally wel l predicted for
rippled bed conditions but agreement is poor for the
wave-related transport component.
[
3] Above rippled beds, momentum transfer and the
associated sand dynamics in the near-bed layer are domi-
nated by coherent motions, specifically by the process of
vortex formation above ripple slopes and the shedding of
these vortices aro und flow reversal. Above steep long-
crested ripples, this well-organized vortex process is highly
effective in entraining sand into suspension. In a near-bed
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, F02012, doi:10.1029/2006JF000614, 2007
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1
Water Engineering and Management, University of Twente, Enschede,
Netherlands.
2
Department of Engineering, King’s College, University of Aberdeen,
Aberdeen, UK.
Copyright 2007 by the American Geophysical Union.

1,139 citations

### "Detailed measurements of velocities..." refers background in this paper

• ...Therefore an onshore streaming component associated with the interaction between the horizontal and vertical orbital flow field [see Longuet-Higgins, 1953; Davies and Villaret, 1999] is not present in flow tunnels....

[...]

Book
01 Jan 1997
Abstract: Dynamics of marine sands' specifically deals with coastal and offshore sea areas, as well as rivers and estuaries, for sand and gravel sediments The book presents a convenient and useable introduction to sediment processes in a form that is accessible to a wide readership Contents: Introduction, including sections on a general procedure and errors and sensitivities; Properties of water and sand; Currents; Waves; Combined waves and currents; Threshold of motion; Bed features; Suspended sediment; Bedload transport; Total load transport; Morphodynamics and scour; Handling the wave-current climate; Case studies

1,082 citations

### "Detailed measurements of velocities..." refers methods in this paper

• ...The u* value was calculated using the friction formula of Swart [1974] with a roughness of 2....

[...]

• ...5D50 and wss was calculated using the formula of Soulsby [1997] with D = 0....

[...]

Book
18 Dec 1974

424 citations

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