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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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A
rticl
e
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.
0148-0227/07/2006JF000614$09.00
F02012 1of18

layer with a thickness of one to two times the ripple height,
the flow dynamics are dominated by these coherent periodic
vortex structures, whereas above this layer the coherent
motions break down and are replaced by random turbulence
[Davies and Villaret, 1997]. The phase of sand pick-up from
the bed during the wave cycle is linked to the phase of
vortex shedding. This has potentially important consequen-
ces for the net sand transport rate beneath asymmetrical
waves which can be negative (‘‘offshore’’) despite higher
positive (‘‘onshore’’) orbital velocities. The underlying
mechanism of this wave-related transport opposing the
direction of wave propagation in the case of asymmetrical
waves is the existence of phase differences between the
peak concentrat ions and peak velocities related to the
generation of vortices on the lee side of steep ripples.
Because of the time needed for sand settling, phase lags
are generally more important for finer sand and shorter
wave periods.
[
4] Process-based sand transport models [e.g., Hansen et
al., 1994; Andersen, 1999; Malarkey and Davies, 2002;
Eidsvik, 2004; Davies and Thorne, 2005] represent many of
the detailed physical processes involved in sand transport by
waves over rippled beds. These unsteady mode ls have
rarely been validated against data from experiments with
full-scale, mobile ripples [see van der Werf, 2006]. In order
to further develop such models and to increase our under-
standing of the complex nature of ripple reg ime sand
transport, detailed measurements of the velocity and sand
concentration field above rippled beds from controlled, full-
scale laboratory experiments are required.
[
5] Many laboratory measurements of velocity and con-
centration have been made over rippled beds in the past but
these were generally limited either by the capability of the
available instrumentation or by the experimental scale or
setup. For example, many experiments have been carried
out to measure velocities over ripples in which the rippled
bed was fixed. In some cases [e.g., Sato, 1987; Doering and
Baryla, 2002] the velocities were measured using a laser
Doppler anemometer (LDA) or an acoustic Doppler
velocity meter (ADV), which provide velocity measure-
ments at selected points only. In other cases [e.g., Earnshaw
and Greated, 1998; Marin, 2004] the full flow field was
measured using particle image velocimetry (PIV). Detailed
velocity measurements over mobile ripples are rare and only
Ahmed and Sato [2001] have measured the full flow field
over mobile ripples using PIV. However, the flow period T
was short and the ripples were small in their experiments
(T = 3 s, ripple height h = 0.02 m, ripple length l = 0.16 m)
compared with full-scale conditions.
[
6] Bosman [1982] used an optical concentration meter
(OPCON) to perform point concentration measurements
over ripples. However, also in his experiments flow periods
were short (T < 3 s) and ripples were small compared to full-
scale conditions. Point measurements of concentration for
full-scale ripples have been obtained by Clubb [2001] using
transverse suction sampling in an oscillatory flow tunnel.
Villard et al. [2000], Vincent and Hanes [2002] and Thorne
et al. [2003] performed acoustic backscatter system (ABS)
measurements of time-dependent concentrations in large
wave flumes. However, in each of these studies no
corresponding detailed velocity measurements were made.
Chen [1992] carried out point measurements of both
velocities and concentrations, but these were small-scale
experiments with short wave periods (T = 1.76 s) and small
ripples.
[
7] A new experimental study was carried out in the
Aberdeen oscillatory flow tunnel (AOFT) at the University
of Aberdeen to provide a complete set of detailed measure-
ments of velocities and concentrations over rippled beds. In
this facility oscillatory flow corresponding to the near-bed
flow induced by moderate (nonbreaking) waves can be
simulated at full scale for a wide range of relevant coastal
conditions. The objective of the experiments was to obtain
measurements of the combined time-dependent flow and
suspended sand concentration field above full-scale rippled
beds under controlled regular flow cond itions, and to
increase our understanding of the essential physical pro-
cesses involved. These new measurements distinguish
themselves from previous work by the fact that they involve
‘natural’’, mobile ripples generated by oscillatory flows
with field-scale p eriods and amplitudes, and that both
velocity and concentration measurements were carried out.
[
8] In this paper the new measurements of the detailed
time-dependent sand transport processes over rippled beds
are presented and discussed. A description of the experi-
mental facility, measuring methods and experimental con-
ditions is given in Section 2. The next section discusses
the time-dependent velocity, concentration and sand flux
measurements. More specifically, we look into the near-bed
flow along the ripple and the timing of the concentration peaks,
since these are important in the context of sand transport. The
time-averaged behavior of the sand transport processes above
rippled beds i s described in Section 4. The conclusions are
presented in the final section.
2. Experimental Setup
2.1. Experimental Facility
[
9] The Aberdeen oscillatory flow tunnel (AOFT) is a
large laboratory facility in which near-bed horizontal flows,
equivalent in period and amplitude to the near-bed flows of
full-scale waves, can be generated over sand beds. Oscilla-
tory flow tunnels such as the AOFT have a closed top and
therefore do not simulate wave effects associated with the
free water surface (e.g., vertical orbital motions). The AOFT
has an overall length of 16 m with a 10 m long, glass-sided
rectangular test section, 0.75 m high and 0.3 m wide. The
test section was filled with a 0.25 m thick sand bed leaving
0.5 m for the flow. A more detailed description of the
facility can be found in O’Donoghue and Clubb [2001].
[
10] Each experiment started with a flat bed and the
ripples were allowed to develop over time until they
reached ‘equilibrium’’, i.e., when the average ripple height,
length and shape remained more or less constant in time.
Measurements were then made of the ripple dimensions,
time-dependent velocity field, time-dependent sand concen-
trations, time-averaged sand concentrations and net sand
transport rates.
2.2. Ripple Dimensions Measurement
[
11] Ripple dimensions were measured using a laser
displacement sensor (LDS) mounted on a positioning car-
riage. The LDS made point measurements of bed elevation
with a 0.05 mm resolution in the vertical direction. The laser
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displacement system was used to measure the equilibrium
rippled bed. Six parallel profiles were measured, spaced at
40 mm intervals across the tunnel width, with heights
measured every 5 mm along each profile.
[
12] For LDS measurements the flow is stopped. To
measure the ripples while the flow is active, an acoustic
sand ripple profiler (SRP) was used. The SRP uses a
sweeping sonar beam to measure 1.6 m long profiles of
the bed with an accuracy of ±5 mm in the vertical direction;
each sonar scan of the bed takes 35 s approximately.
Further details on the SRP can be found in Doucette and
O’Donoghue [2006].
2.3. Velocity Measurement
[
13] Flow velocities over the ripples were measured using
a cross-correlation particle image velocimetry (PIV) system.
PIV is a planar measurement technique wherein a pulsed
laser light sheet is used to illuminate a flow field seeded
with tracer particles small enough to accurately follow the
flow. For the present experiments, the flow is illuminated
from above using a double pulse Yag laser with a pulse
separation of 2 ms. The positions of the partic les are
recorded on a cross-correlation camera with a resolution
of 1000 1000 pixels mounted outside the tunnel perpen-
dicular to the flow. The camera and laser are synchronized
and the camera grabs a pair of images at a rate of
approximately 13.2 Hz. The data processing consists of
determining the average displacement of the particles over a
small interrogation region in the image. Knowledge of the
time interval between the light sheet pulses then permits
computation of the flow velocity. For the present experi-
ments, measurements were made using two camera viewing
areas of 0.40 0.40 m and 0.235 0.235 m. The cross-
correlation analysis between each image pair was carried
out with inte rrogation areas of 32 32 pixels and an
overlap of 16 pixels. The spatial resolution of the resulting
velocity field measurement is 6.4 6.4 mm and 3.8 3.8 mm
for the 0.40 0.40 m and 0.235 0.235 m viewing areas
respectively; the corresponding velocity resolution is 20 mm/s
and 12 mm/s.
[
14] The suspended sand acted as the seeding agent, and
therefore mea sured velocities are velocities of the sus-
pended sand rather than the water itself. The grain size
distribution of the suspended sand was measured by taking
suction samples at elevations between 10 and 120 mm
above the ripple crest level: median grain sizes were found
to range from 0.29 to 0.37 mm. The corresponding fall
velocities computed using the formula of Van Rijn [1993]
are of the order of 0.04 0.05 m/s. This gives an indication
of the difference between the measured vertical velocity
component measured and the actual water velocity.
[
15] Many of the experiments involved asymmetric
oscillatory flow with higher ‘onshore’ than ‘offshore’
velocities. Because of the asymmetry, the ripples migrate
onshore during the experiment and may change shape
slightly as they migrate. In order to limit the effects of
migration and distortion on ensemble-averaged results, the
duration of the PIV measurements was limited to approx-
imately five flow cycles in each experiment. Depending on
the flow period, this corresponds to between approximately
200 and 500 image pairs recorded for each experiment.
2.4. Suspended Sand Concentration Measurement
[
16] Vertical profiles and point measurements of the time-
dependent suspended sand concentration were measured
using an acoustic backscatter system (ABS) and an optical
concentration meter (OPCON) respectively. A transverse
suction system (TSS) was used to measure the vertical
profiles of time-averaged concentration. Since two of these
systems are intrusive within the flow field, the measurement
of suspended sand concentration was performed separately
from the PIV measurements. The concentration and velocity
measurements could still be combined to produce sand flux
values since the flow conditions and resulting ripple size
and shape were repeatable between the experiments. The
consistency between experiments of the equilibrium ripple
size and shape for a given flow condition has been shown
by Doucette and O’Donoghue [2006]. This consistent size
and shape was maintained as the ripples migrated.
[
17] An acoustic backscatter system (ABS) was deployed
to measure time-dependent suspended sand concentrations.
The ABS used was a three-frequency AquaScat unit,
operating at 0.98, 2.52 and 4.8 MHz. These three trans-
ducers were aligned across the tunnel. Only ABS concen-
tration measurements from the middle frequency transducer
are presented here since the low-frequency transducer had
too wide a footprint while the higher-frequency transducer
had problems with attenuation.
[
18] The ABS transducers were mounted into the tunnel
lid with their heads flush with the underside of the lid at
0.5 m above the flat sand bed and looking vertically
downward. The ABS data are averaged online to give
backscatter profiles measured at 8 Hz a nd these are
subsequently converted to high-resolution concentration
profiles. The averaging is required, because of the statis-
tical nature of the backscattered signal. The system pro-
vides concentration profiles with a 0.005 m vertical spatial
resolution extending between 0.1 to 0.6 m below the
transducers. Calibration was based on concentrations mea-
sured by the OPCON and TSS.
[
19] The ABS measured continuously while six entire
equilibrium ripples migrated underneath it. The bed level
below the ABS was continuously monitored by the SRP.
The ABS concentration measurements are accurate within
approximately a factor of 2 (because of uncertainties in the
suspended sand g rain size) with an uncertainty in the
vertical position of ±5 mm.
[
20] Point measurements of time-dependent concentra-
tions were made with an optical concentration meter
(OPCON), which can measure concentrations in the range
of 0.140 g/l. The measure ment method is based on the
extinction of near-infrared radiation by suspended particles
applying Beers law. The OPCON consists of a transmitter
and a receiver, spaced 30 mm apart with dimensions of
2.6 2.6 mm, and a measuring volume of 2.6 2.6
30 mm. The orientation of the light beam between trans-
mitter and receiver is horizontal and perpendicular to the
flow. The temporal resolution of the OPCON measurements
is 50 Hz. For the present experimen ts, the OPCON was
calibrated in a stirring vessel with known concentrations for
five different sand mixtures to determine the influence of
the grain size on the calibration coefficient.
[
21] Since the OPCON is only capable of point measure-
ments and was difficult to move horizontally in the water
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tight flow tunnel, the migration of the ripples themselves
was used to facilitate measurements along the ripple length.
For each flow condition, the OPCON was positioned at a
fixed elevation above the ripple crest level. Measurements
of suspended sand concentration were then recorded as the
ripple migrated past the sensor. Once a whole ripple had
migrated past, the OPCON was raised to a new measure-
ment level and the observations recorded as another ripple
migrated past. These measurements were repeated at five
different elevations. The estimated total (random) error in
the OPCON measured concentration is 20%, and the
uncertainty in vertical position with respect to the ripple
surface is ±5 mm. This estimation does not include uncer-
tainties associated with the ripple size and shape.
[
22] A transverse suction system (TSS) was used to
measure time-averaged concentration profiles of suspended
sand. The system consists of 68 separate horizontal intake
nozzles, each with an inner diameter of 3 mm, which were
positioned perpendic ular to the flow. The nozzles were
mounted at the same elevation in a line covering the full
ripple length and spaced 35 or 70 mm apart. The duration of
suction sampling was 200 s corresponding to 40 flow cycles
for flows with a period of 5 s and resulting in approximately
5l of sample for each nozzle. The time-averaged suspended
sand concentration was determined by drying and weighing
the sampled water-sand mixture. The particle size distribu-
tion of the dried sample was determined by sieve analysis
and the calibration of the suction efficiency was determined
by the method outlined in Bo sman et al. [1987]. The
estimated total (random) error in the measured concentra-
tion is 5%, and the uncertainty in vertical position with
respect to the ripple surface ±4 mm. This estimation does
not include uncertainties associated with the ripple size and
shape.
[
23] Suspended sand samples were collected with the TSS
once equilibrium ripples were established for the given flow
condition. All nozzles of the TSS were mounted at the same
elevation relative to a flat bed. Once the samples were
collected, all the nozzles were moved to a higher position
above the ripple and sampling was repeated at a total of five
different elevations. Since the sampling took 200 s to
complete, the horizontal position of the nozzles had to be
corrected for migration of the ripple between sampling.
Nozzles were also repositioned horizontally during the
suction sampling if the ripple migrated more than 0.02 m.
This was accomplished by switching off both the AOFT
piston and the TSS in order to reposition the nozzles
horizontally before continuing with the s ampling. The
actual height of each nozzle above the rippled bed was
recorded before and after suction sampling by lowering the
sampler to the bed from the set elevation (in still water).
Elevation of the nozzle above the rippled bed was taken as
an average of these two heights to account for the changes
due to ripple migration during the sampling.
2.5. Net Sand Transport Measurement
[
24] The net sand transport rates were measured using a
mass conservation technique. After the ripples reached
equilibrium, the bed profile over the full test section was
measured using LDS and all sand was removed from the
sand traps. The flow was run again for 180 cycles for
Mr5b63 and Mr5c63, and 720 cycles for Mr5a63. Then the
bed profile over the full test section was measured again and
the sand taken from the ends was dried and weighed. Given
the sand masses in the traps at both ends of the test section
and the volume changes derived from the bed profiling
system, the net sand transport rates along the test section
were calculated by integration of the sand continuity equa-
tion from the left-hand and the right-hand side boundary.
The calculated net transport rate in (or close to) the middle
of the test section was taken as a representative value. Errors
involved in net transport measurements are due to uncer-
tainty in the value of the bed porosity and possible change
in porosity during the experiment. Repeated measurements
for the same condition show that the (random) error in the
measured net transport rate is 20%.
2.6. Experimental Conditions
[
25] The experimental conditions are listed in Table 1.
The table includes the measured equilibrium ripple height,
h, ripple length, l, ripple steepne ss, h/l, ripple migration
rate, c
r
, and net sand transport rate hq
s
i for each experiment.
The analyses below focus on the experiments denoted with
an asterisk. Note that mobility number and ripple size vary
considerably over the range of experiments. The sand used
for the experiments was well sorted with D
10
, D
30
, D
50
, D
70
and D
90
equal to 0.25 mm, 0.35 mm, 0.44 mm, 0.53 mm
and 0.66 mm respectively. Ripple dimensions and flow
velocities were measured for all of the experiments; con-
centrations and net sand transport rates were only measured
for experiments Mr5a63, Mr5b63 and Mr5c63.
[
26] The velocity function for the regular flow experi-
ments had the same form as the near-bed flow beneath
Stokes second-order waves:
utðÞ¼u
1
cos wtðÞþu
2
cos 2wtðÞ ð1Þ
where u
1
and u
2
are the first- and second-order near-bed
orbital velocity amplitude respectively, and w =2p/T is
angular frequency where T is the flow period. The velocity
values presented in Table 1 are based on the ripple-averaged
velocity measured at approximatel y three ripple heights
above the ripple crest level.
[
27] The following equations are used to compute the
parameters:
Root-mean-square orbital velocity u
rms
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
0:5u
2
1
þ 0:5u
2
2
q
ð2Þ
Orbital diameter d
o
¼
ffiffi
2
p
u
rms
T
p
ð3Þ
Degree of flow asymmetry R ¼
u
1
þ u
2
2u
1
ð4Þ
Mobility number y ¼
2u
2
rms
DgD
50
ð5Þ
where D =(r
s
r
w
)/r
w
= 1.65 is the relative sand density
with r
w
the water density and r
s
the sand density.
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Explanation of the nomenclature of the experiments: in
experiment Mr5b63, ‘M’ denotes medium sand, ‘r is
regular oscillatory flow, ‘5’ is the flow period, ‘b’ is an
indication of the orbital velocity, and ‘63’ denotes the
imposed degree of flow asymmetry.
[
28] In the following sections we look at the velocity and
concentration fields for six particular experiments: Mr5a63,
Mr5b50, Mr5b54, Mr5b5 8, Mr5b63, and Mr5c 63. The
general flow and concentration behavior is representative
of the flow and concentration measured in the other experi-
ments. The ripple geometries shown below the measured
flow, suspended sand concentration and suspended sand
flux field reflect the actual shape of ripple above which the
measurements were carried out.
3. Time-Dependent Behavior
3.1. Time-Dependent Flow Field
[
29] Figure 1 shows the measured velocity field
corresponding to eight phases within the flow period for
experiment Mr5b63. Figure 1, top, shows the free-stream
orbital velocity u
1
and also shows the phases for which the
eight velocity fields are presented. Note that the full
measurement at each phase consists of 66 vertical profiles
over the length of the ripple but that only 12 profiles are
shown for clearer presentation. The velocities have been
ensemble-averaged over four flow cycles.
[
30] Positive, ‘onshore’ flow is directed to the right.
Horizontal and vertical axes, x and z, have their origin at the
ripple crest and are normalized by the ripple length l and
ripple height h respectively. The stoss slope of the ripple is
the slope facing offshore (i.e., the left side of the ripple in
Figure 1) and the lee slope is the slope facing onshore. The
ripple lee slope is steeper than the stoss slope, giving
the ripple an asymmetric shape that is consistent with the
asymmetric flow.
[
31] With reference to Figure 1 (ah), the general flow
behavior is as follows:
[
32] 1. (a): Off-onshore flow reversal. Free-stream
velocities are close to zero but there are higher near-bed
onshore velocities on the stoss slope due to the ejection of
the stoss vortex from the stoss side of the ripple.
[
33] 2. (b): Free stream is accelerating onshore. The flow
accelerates as it rises up the stoss slope because of the
constriction of the flow from the trough to the crest. This
produces high velocity flow over the ripple crest, extending
to z/h 0.7. Flow decelerates down the lee slope, producing
very low near-bed velocities toward the ripple trough.
[
34] 3. (c): Time o f maximum onshore free-stream
velocity. Flow acceleration over the ripple crest persists.
There is strong separation in the lee of the ripple with flow
starting to reverse near the bed on the lee slope.
[
35] 4. (d): Onshore free-stream velocity is decelerating.
The region of high velocity flow over the ripple crest is no
longer present. Flow reversal has occurred at the bed in the
lee resulting in a well-defined vortex.
[
36] 5. (e): Free-stream velocity is close to on-offshore
flow reversal. The free-stream velocity is weak and the lee
vortex starts to move toward the ripple crest, producing
relatively high, offshore-directed near-bed velocities on the
lee slope. These near-bed velocities are much higher than at
times of off-onshore flow reversal (Figure 1a), since the lee
vortex is stronger than the stoss vortex because of flow
asymmetry.
[
37] 6. (f ): Free stream is accelerating offshore. Free-
stream flow reversal has occurred and the free-stream
velocity is low. However, offshore velocities at the ripple
crest are already high, cause d by the phase lead of the near-
bed flo w acceleration and the constriction of the flow from
the trough to the crest. This jet flow is responsible for the
ejection of the lee side vortices over the ripple crest. Note
that the velocity magnitude at the crest at this time is as high
as the maximum free-stream velocity.
[
38] 7. (g): Close to time of maximum offshore free-
stream velocity. This is similar to C, but the degree of flow
separation on the ripple flank is less because of flow
asymmetry.
[
39] 8. (h): Offshore free-stream velocity is decelerating.
The stoss slope vortex grows as the offshore flow deceler-
ates. This vortex is much smaller than the corresponding
vortex formed on the lee slope during onshore flow
(Figure 1d).
[
40] Figure 2 shows the measured velocity field
corresponding to the same eight phases within the flow
Table 1. Experimental Conditions and Measured Ripple Height h, Ripple Length l, Ripple Steepness h/l, Ripple Migration Rate c
r
, and
Net Sand Transport Rate
hq
s
i
a
Experiment D
50
,mm T,s u
1
,m/s u
2
,m/s u
rms
,m/s d
o
,m R y h,m l,m h/l c
r
, mm/min hq
s
i,10
6
m
2
/s
Mr5a54 0.44 5.0 0.31 0.023 0.22 0.49 0.54 14 0.065 0.29 0.22 ... ...
Mr5a58 0.44 5.0 0.32 0.044 0.23 0.51 0.57 15 0.045 0.25 0.18 ... ...
Mr5a63* 0.44 5.0 0.31 0.057 0.22 0.50 0.59 14 0.039 0.24 0.16 5 7 0.43
Mr5b50* 0.44 5.0 0.48 0.000 0.34 0.76 0.50 32 0.080 0.46 0.17 ... ...
Mr5b54* 0.44 5.0 0.56 0.025 0.40 0.89 0.52 44 0.070 0.41 0.17 ... ...
Mr5b58* 0.44 5.0 0.53 0.065 0.38 0.85 0.56 40 0.070 0.41 0.17 ... ...
Mr5b63* 0.44 5.0 0.54 0.095 0.39 0.87 0.59 42 0.076 0.41 0.19 18 3.69
Mr5c63* 0.44 5.0 0.65 0.120 0.47 1.05 0.59 61 0.081 0.49 0.17 10 15 14.0
Mr5d54 0.44 5.0 0.43 0.015 0.30 0.68 0.52 26 0.075 0.43 0.17 ... ...
Mr5d58 0.44 5.0 0.43 0.044 0.31 0.69 0.55 26 0.050 0.35 0.14 ... ...
Mr5d63 0.44 5.0 0.45 0.080 0.32 0.73 0.59 29 0.050 0.54 0.09 ... ...
Mr363 0.44 3.1 0.34 0.060 0.24 0.34 0.59 17 0.025 0.19 0.14 ... ...
Mr463 0.44 4.1 0.44 0.074 0.32 0.58 0.58 28 0.045 0.28 0.16 ... ...
Mr763 0.44 7.4 0.52 0.070 0.37 1.24 0.57 39 0.100 0.55 0.18 ... ...
a
The analyses focus on the experiments denoted with an asterisk.
F02012 VAN DER WERF ET AL.: SAND TRANSPORT PROCESSES OVER RIPPLES
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F02012

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Journal ArticleDOI
Abstract: Shoaling short gravity waves at sea approaching the shore become asymmetric and are able to generate a net resulting sand transport in cross-shore direction (on-shore-offshore transport). The wave-related sand transport is still very difficult to predict due to the complexity of its underlying processes, which mainly take place in a thin layer near the sea bed in the wave boundary layer (thickness of order centimeters). The development of models for cross-shore sand transport heavily relies on experimental lab research, especially as taking place in large oscillating water tunnels (see, e.g., Nielsen, 1992). In oscillating water tunnels the near-bed horizontal orbital velocity, as induced by short gravity waves, can be simulated above fixed or mobile sandy beds (for a detailed description, see, e.g., Ribberink and Al-Salem, 1994). It should be realized that the vertical orbital flow and relatively small wave-induced residual flows as streaming and drift are not reproduced in flow tunnels. Research aimed at their contribution to the net sediment motion under surface waves is still ongoing (see Ribberink et al., 2000).

111 citations


Journal ArticleDOI
David Hurther1, Peter D. Thorne2, Mickaël Bricault1, Ulrich Lemmin3  +1 moreInstitutions (3)
Abstract: The use of acoustics to measure sediment transport boundary layer processes has gained increasing acceptance over the past two decades. This has occurred through the development of increasingly sophisticated measuring systems and theoretical developments, which have enabled flow and suspended sediment parameters to be obtained from acoustic data with a high degree of accuracy. Until relatively recently, separate acoustic systems were used to measure flow and suspended sediment concentration. Over the past few years, however, the technology has become sufficiently advanced so that flow and sediment measurements can be integrated into a single system. This integration provides, quasi-instantaneous, non-intrusive, co-located, high temporal-spatial resolution measurements of benthic flow and sediment processes. Here the development of such an instrument, the Acoustic Concentration and Velocity Profiler (ACVP) is described. The theory underpinning its application is outlined, new approaches to velocity de-aliasing and suspended sediment inversion instabilities using multi-frequency capabilities are presented and the application of the system to sediment transport processes over a sandy ripple bed is illustrated. The observations clearly show the value of such instrumentation for studying the dynamical interaction between the bed, the flow and the sediments at and within the bottom boundary layer.

100 citations


Journal ArticleDOI
Abstract: [1] The waveshape effects on sediment transport are investigated for cross-shore beach profile changes. This study is based on experiments performed in the Laboratoire des Ecoulements Geophysiques et Industriels wave flume for irregular waves. The interest of such experiments resides in presenting complex combinations of wave skewness and asymmetry in bed load, ripple, and sheet flow regimes. Net sediment transport rates on typical beach morphodynamics are analyzed in regard to wave skewness and asymmetry, undertow, and ripple occurrence. Onshore bar migration is mainly associated with onshore-directed sediment transport, whereas terrace profile and offshore bar formation correspond to offshore sediment transport. As for natural beaches, energetic (moderate) wave climates mostly induce offshore (onshore) sediment fluxes. For a given significant wave height, an increase (decrease) in the wave climate peak period is associated with an increase (decrease) in wave skewness and leads mostly to offshore (onshore) sediment fluxes. The experiments are fully characterized by unsteady behavior. Consequently, several conditions exhibit phase-lag effects where the sediment is mobilized by the wave crest and transported by the following trough, which produces a net offshore transport even for a weak undertow. The presence of ripples clearly contributes to enhance this behavior. An original concept, due to its application to skewed asymmetric irregular waves, presents the important interaction between wave nonlinearities driving the sediment fluxes. The net sediment transport rate under strongly skewed waves is either offshore directed due to phase-lag effects or onshore directed when the wave asymmetry is large enough. Both these mechanisms probably largely contribute to bar formation and migration.

79 citations


Journal ArticleDOI
David Hurther1, Peter D. Thorne2Institutions (2)
Abstract: The present study focuses on the fine‐scale flow and sand transport processes above onshore migrating ripples below skewed surface gravity waves in the shoaling zone. A set of acoustic instruments was deployed in the shoaling region of the large‐scale wave channel at Canal d'Investigacio i Experimatacio Maritima, Universitat Poltiecnica de Catalunya, Barcelona, Spain, in order to provide high‐resolution velocity and sediment concentration profiles with an acoustic concentration and velocity profiler (ACVP). Measurements are analyzed relative to the positions of the measured nonmoving sand bed and the interface separating the suspension from the near‐bed load layer. This interface is detected here by the application of a novel acoustic bed echo detection method. Furthermore, the use of the dual‐frequency inversion proposed in the work of Hurther et al. (2011) allows for the calculation of the sediment concentration profile across both the suspension and near‐bed load layers. The sand bed was covered by quasi‐two‐dimensional suborbital ripples migrating onshore. As proposed by O'Donoghue et al. (2006), the occurrence of quasi‐two‐dimensional ripples is attributed to the fine‐size sand of D50 = 250 mm used in the present study under full‐scale forcing conditions. In order to determine the effect of shoaled wave skewness on the ripple vortex entrainment and sediment transport, the instantaneous and mean measurements of the flow, sediment concentration, and sediment flux along the ripple profile are discussed in terms of (1) the occurrence of ripple vortex entrainment on either side of the ripple crest; (2) the wave velocity phase lagging driven by the ripple vortex entrainment process and the turbulent bed friction effects in the wave boundary layer; (3) phase lagging between velocity and maximum concentration and sediment flux events; (4) the structure of bed friction and ripple‐driven turbulence across the suspension and the near‐bed load layers; and (5) the streaming components. The results on these aspects strongly support that the wave velocity skewness effect under shoaling waves is fairly similar to the one obtained in skewed oscillatory water tunnel flows. Furthermore, it is found that the onshore‐oriented net bed load sediment transport is at the origin of the onshore ripple migration. This flux is roughly twice as much as the opposite offshore‐oriented net suspension flux dominated by the ripple vortex entrainment processes.

65 citations


Journal ArticleDOI
Abstract: Many existing practical sand transport formulae for the coastal marine environment are restricted to a limited range of hydrodynamic and sand conditions This paper presents a new practical formula for net sand transport induced by non-breaking waves and currents The formula is especially developed for cross-shore sand transport under wave-dominated conditions and is based on the semi-unsteady, half wave-cycle concept, with bed shear stress as the main forcing parameter Unsteady phase-lag effects between velocities and concentrations, which are especially important for rippled bed and fine sand sheet-flow conditions, are accounted for through parameterisations Recently-recognised effects on the net transport rate related to flow acceleration skewness and progressive surface waves are also included To account for the latter, the formula includes the effects of boundary layer streaming and advection effects which occur under real waves, but not in oscillatory tunnel flows The formula is developed using a database of 226 net transport rate measurements from large-scale oscillatory flow tunnels and a large wave flume, covering a wide range of full-scale flow conditions and uniform and graded sands with median diameter ranging from 013 mm to 054 mm Good overall agreement is obtained between observed and predicted net transport rates with 78% of the predictions falling within a factor 2 of the measurements For several distinctly different conditions, the behaviour of the net transport with increasing flow strength agrees well with observations, indicating that the most important transport processes in both the rippled bed and sheet flow regime are well captured by the formula However, for some flow conditions good quantitative agreement could only be obtained by introducing separate calibration parameters The new formula has been validated against independent net transport rate data for oscillatory flow conditions and steady flow conditions

62 citations


Cites background from "Detailed measurements of velocities..."

  • ...…for fine sand sheet-flow conditions (Dohmen-Janssen et al., 2002; O’Donoghue and Wright, 2004; Van der A et al., 2009) and rippled bed conditions (Van der Werf et al., 2007), where phase lag effects can significantly affect the magnitude and sometimes even the direction of the net transport rate....

    [...]


References
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1,902 citations


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

  • ...The corresponding fall velocities computed using the formula of Van Rijn [1993] are of the order of 0.04–0.05 m/s....

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  • ...The corresponding fall velocities computed using the formula of Van Rijn [1993] are of the order of 0....

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Abstract: This book is intended as a useful handbook for professionals and researchers in the areas of Physical Oceanography, Marine Geology, Coastal Geomorphology and Coastal Engineering and as a text for graduate students in these fields. With its emphasis on boundary layer flow and basic sediment transport modelling, it is meant to help fill the gap between general hydrodynamic texts and descriptive texts on marine and coastal sedimentary processes. The book commences with a review of coastal bottom boundary layer flows including the boundary layer interaction between waves and steady currents. The concept of eddy viscosity for these flows is discussed in depth because of its relation to sediment diffusivity. The quasi-steady processes of sediment transport over flat beds are discussed. Small scale coastal bedforms and the corresponding hydraulic roughness are described. The motion of suspended sand particles is studied in detail with emphasis on the possible suspension maintaining mechanisms in coastal flows. Sediment pickup functions are provided for unsteady flows. A new combined convection-diffusion model is provided for suspended sediment distributions. Different methods of sediment transport model building are presented together with some classical models.

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Michael S. Longuet-Higgins1Institutions (1)
Abstract: It was shown by Stokes that in a water wave the particles of fluid possess, apart from their orbital motion, a steady second-order drift velocity (usually called the mass-transport velocity). Recent experiments, however, have indicated that the mass-transport velocity can be very different from that predicted by Stokes on the assumption of a perfect, non-viscous fluid. In this paper a general theory of mass transport is developed, which takes account of the viscosity, and leads to results in agreement with observation. Part I deals especially with the interior of the fluid. It is shown that the nature of the motion in the interior depends upon the ratio of the wave amplitude a to the thickness $\delta $ of the boundary layer: when a$^{2}$/$\delta ^{2}$ is small the diffusion of vorticity takes place by viscous 'conduction'; when a$^{2}$/$\delta ^{2}$ is large, by convection with the mass-transport velocity. Appropriate field equations for the stream function of the mass transport are derived. The boundary layers, however, require separate consideration. In part II special attention is given to the boundary layers, and a general theory is developed for two types of oscillating boundary: when the velocities are prescribed at the boundary, and when the stresses are prescribed. Whenever the motion is simple-harmonic the equations of motion can be integrated exactly. A general method is described for determining the mass transport throughout the fluid in the presence of an oscillating body, or with an oscillating stress at the boundary. In part III, the general method of solution described in parts I and II is applied to the cases of a progressive and a standing wave in water of uniform depth. The solutions are markedly different from the perfect-fluid solutions with irrotational motion. The chief characteristic of the progressive-wave solution is a strong forward velocity near the bottom. The predicted maximum velocity near the bottom agrees well with that observed by Bagnold.

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....

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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

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"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....

    [...]



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