Journal of Coastal Research
22
2
247–259
West Palm Beach, Florida
March 2006
Hydrodynamics and Sediment Fluxes across an Onshore
Migrating Intertidal Bar
Troels Aagaard†, Michael Hughes§, Regin Møller-Sørensen†, and Steffen Andersen†
†
Institute of Geography
University of Copenhagen
Oster Voldgade 10
DK-1350 Copenhagen,
Denmark
[email protected]
School of Geosciences
University of Sydney
Sydney NSW 2006, Australia
§
ABSTRACT
AAGAARD, T.; HUGHES, M.; MØLLER-SØRENSEN, R., and ANDERSEN, S., 2006. Hydrodynamics and sediment
fluxes across an onshore migrating intertidal bar. Journal of Coastal Research, 22(2), 247–259. West Palm Beach
(Florida), ISSN 0749-0208.
Detailed hydrodynamic and morphological data are presented from a field deployment spanning 2 days (four tide
cycles). The data include bed-elevation changes measured at each low tide and continuous records of water-surface
elevation, cross-shore and long-shore current velocities, and suspended sediment concentrations all measured within
20 cm of the bed. During the deployment, an intertidal bar migrated onshore and infilled a runnel on its landward
side. The depth of this runnel was initially 0.6 m. During the migration of the bar, the significant wave height in
deep water was ca. 2 m and wave period was 7 seconds. The significant wave height over the intertidal bar crest was
about 0.25 m. Suspended sediment fluxes were estimated (product of current velocity and suspended sediment concentration profile) and partitioned between mean and oscillatory components with the latter further partitioned between short and long wave contributions. When the bar was migrating shoreward and infilling the runnel, estimated
suspended sediment flux for all components was directed landward on the bar crest. Once the migrating bar had
infilled the runnel, however, the suspended sediment fluxes for the mean component were directed seaward, whereas
the short wave-driven flux was still directed landward. These results represent a clear example of morphodynamic
interactions—(a) as waves cross the intertidal bar the onshore mean and oscillatory components transport sediment
shoreward, (b) the presence of the runnel reduces the offshore component of oscillatory transport by channeling the
flow alongshore, (c) the runnel rapidly infills due to the strong transport asymmetry, (d) once the runnel has infilled,
the mean cross-shore current and mean sediment flux reverse direction. When the runnel is present, the general
intertidal circulation is a horizontal cell circulation with rip currents, whereas it becomes a vertical undertow circulation when the runnel has infilled.
ADDITIONAL INDEX WORDS: Swash bar, ridge and runnel, cell circulation, morphodynamics.
INTRODUCTION
The intertidal zone of micro/mesotidal beaches in semienclosed seas are often characterized by the existence of one or
more intertidal bars. Such intertidal bars can take on various
forms and display different types of dynamic behavior. Following GREENWOOD and DAVIDSON-ARNOTT (1979), WIJNBERG and KROON (2002) distinguished between two main
types of intertidal bars, (a) slip-face ridges that are asymmetric forms and relatively mobile and (b) low-amplitude
ridges that are more symmetric in form and largely static
features. These two bar types also correspond to the terms
swash bars and ridge and runnels (ORFORD and WRIGHT,
1978), respectively.
Despite the accessibility of the intertidal zone, the physical
processes governing intertidal bar behavior are not well understood, mainly because of the difficulties in measuring hydrodynamics and sediment transport in very shallow water
depths. In the case of slip-face ridges, it is unclear whether
DOI:10.2112/04-0214.1; received 3 May 2004; accepted in revision 20
September 2004.
these bars are in fact generated and maintained by swash
(WIJNBERG and KROON, 2002) or surf-zone processes (AAGAARD et al., 1998a; KROON and MASSELINK, 2002). They
tend to form in the mid to lower intertidal zone and migrate
onshore under low/moderate-energy conditions, whereas they
may be eroded during high-energy situations. The water level
across the bar crest would seem to be an important parameter in determining the bar behavior. KROON and MASSELINK (2002) observed landward migration associated with
mean onshore flows when water depths were less than 0.2 m
at the bar crest, while DAWSON, DAVIDSON-ARNOTT, and
OLLERHEAD (2002) observed a critical depth of 0.1 m. Similar
onshore-directed mean flows were observed at bar crests by
AAGAARD et al. (1998a, 1988b) and by KROON and DE BOER
(2001).
The origin and detailed dynamics of the ridge and runnel
type of intertidal bars may be even more obscure. This bar
type mainly occurs with relatively large tidal ranges and
small waves (KING and WILLIAMS, 1949; ORFORD and
WRIGHT, 1978), i.e., for larger values of the relative tide range
(MASSELINK and SHORT, 1993). VOULGARIS et al. (1998) mea-
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Aagaard et al.
sured hydrodynamics and sediment transport across a ridgeand-runnel system under low-energy conditions, but they
were unable to reconcile their measurements (and derived
modeling efforts) to the observed landward bar migration and
tracer movement. As their measurements were restricted to
shoaling wave conditions in water depths .0.8 m, they concluded that measurements in shallow water depths (swash
and inner surf zones) are required to quantify the sand transport and morphological development of intertidal bars. Similar problems affected the results of STEPANIAN et al. (2001),
who also measured hydrodynamics in the shoaling wave zone
and were unable to relate onshore tracer movements and
ridge migration to observed processes.
One distinguishing feature about intertidal bars is that
they are often dissected by rip channels, and onshore flows
have been recorded at bar crests in association with offshore
flows in rip channels. The currents thus form a cell circulation system with longshore rip feeder flows in the runnels,
and this type of circulation is typical for moderate-energy
conditions when waves are breaking over the landward migrating bars. While some numerical models for such threedimensional flows have started to emerge (e.g., SHORECIRC;
HAAS et al., 2003), these models have not yet been extended
to simulate sediment transport and morphological change.
Consequently, morphodynamics and bar behavior in threedimensional bathymetric settings are not well predicted by
numerical models. Indeed, most available models are two-dimensional and simulate hydrodynamics and sediment transport in a cross-shore profile; they cannot simulate the landward movement of intertidal bars under breaking waves.
More field data on hydrodynamics and sediment transport
from three-dimensional morphological settings are therefore
required to constrain future model development (SOULSBY,
1999).
The present study obtained such measurements under lowto moderate-energy conditions across an intertidal bar of the
slip-face ridge type. In the course of three tidal cycles, a large
intertidal bar migrated landward and welded to the beach.
The processes responsible for this behavior are documented
through measurements of flow velocities and sediment concentration obtained close to the bed at four measurement positions. As the tide rose and fell, the instruments were subjected to various hydrodynamic regimes and the relative effects of swash, surf, and shoaling wave processes to intertidal
bar dynamics are evaluated. Furthermore, the morphodynamic feedbacks between morphology and hydrodynamic processes are elucidated; as the bar moved onshore and closed
the landward runnel, onshore-directed mean flows were replaced by offshore-directed undertow.
MATERIALS AND METHODS
Experimental Site and Procedures
The field experiment was conducted from August 23, 2002,
to September 4, 2002, at Skallingen, which is located on the
North Sea coast of Denmark. The shoreface has a gentle
slope, b ø 0.007, the mean annual offshore significant wave
height is 1 m, and the mean tidal range is 1.5 m, increasing
Figure 1. Cross-shore profile at Skallingen, August 25, 2002. The profile
comprises an intertidal bar and two nearshore (subtidal) bars. Positions
of the four instrument stations across the intertidal bar are indicated by
the vertical lines. Mean annual sea level is at 1 0.14 m DNN (Danish
Ordnance Datum).
to 1.8 m at spring tides. The shoreface exhibits 2–3 subtidal
bars and additionally, one or two intertidal bars are common.
At the outset of the experiment, the upper shoreface had
subtidal bar crests located at x 5 250 m and x 5 150 m
relative to the survey baseline, a rather large intertidal bar
centered at x 5 90 m and an upper swash bar/berm at x 5
55 m (Figure 1). During the course of the experiment, the
intertidal bar moved landward, closed the runnel, and welded
to the beach. The behavior was consistent with that typically
displayed by such bars at Skallingen: Intertidal bars tend to
migrate landward until they weld to the beach; after welding
and runnel infilling, the bar(s) may be eroded during highenergy situations and the sediment recycled to the lower intertidal zone (AAGAARD et al., 1998a).
Initially, the survey (and instrument) transect was located
across the intertidal bar approximately midway between two
rip channels, which were spaced about 175 m apart. The difference in elevation between the bar crest and the landward
runnel was approximately 0.6 m and the bar form was
oblique to the beach, with the northern part of the bar located
closer to the shoreline, consistent with the dominant southerly longshore sediment transport at the site (Figure 2).
The sediment on this bar was well sorted with a mean
grain size of 200–240 mm. Wave-energy levels during the experiment were quite low (Figure 3). The significant offshore
wave height (recorded 18 km offshore in a water depth of ø12
m) remained below 0.5–0.6 m until August 29, when a gale
occurred and waves increased to 1.2 m and further to 2.1 m
on August 31, and subsequently wave heights decreased
again. Peak spectral wave periods increased from ø4–8 seconds during the event, which was also associated with a small
surge of ø0.2 m (Figure 3) due to the onshore winds. Tides
were recorded at the ebb delta, about 3 km away from the
field site. Unfortunately, tidal records are missing prior to
the event, which was initiated 3 days after a spring tide; the
Journal of Coastal Research, Vol. 22, No. 2, 2006
Intertidal Bars
Figure 2. Three-dimensional topographic surfaces of the beach and intertidal zone at the experimental site before (August 24, 2002) and after
(September 2, 2002) the event described in the article. The instrument
transect was located at the longshore coordinate y 5 0 m.
tidal stage was thus between spring and neap, with decreasing tidal ranges.
Four instrument stations were established in the surveyed
transect (Figure 1). These stations consisted of H-frames jetted approximately 1.5 m into the bed and all were equipped
with a Marsh-McBirney OEM 512 electromagnetic current
meter (EM) at a nominal elevation of 0.20 m above the bed
and an array of three OBS-1P optical backscatter sensors at
nominal elevations of 0.05, 0.10, and 0.20 m above the bed
for sediment-transport measurements. At the uppermost station (S4), however, the current meter was deployed at a nominal elevation of 0.12 m and the lower OBS at 0.035 m. Wave
transformations and mean water levels were measured with
pressure sensors (Viatran Model 2406A at S1 and S2 and
Druck Model PTX1830 at S3 and S4). At the upper stations
(S3 and S4), the pressure sensor elevation was kept at, or
slightly below, bed level in order to measure water depths in
the swash zone. These upper stations were also equipped
with three-dimensional sideways-looking Sontek 10 MHz
Acoustic Doppler Velocimeters (ADV) at nominal elevations
of 0.02–0.03 m above the bed and at S3, a vertical array of
five D&A Instruments UFOBS-7 fiber-optical backscatter
sensors was installed. The UFOBS-7 uses an infrared laser
to detect sediment concentration within a very small sampling volume (nominally ø10 mm3) that is centered 10–15
249
Figure 3. Mean water level (top panel) and (bottom panel) significant
offshore wave height (solid line) and peak spectral wave period (dashed
line) during the experiment period. Tidal records are missing prior to
August 29. The dashed line in the upper panel indicates the mean annual
water level.
mm away from the sensor head. Due to the small size of the
sensor head (8 mm outer diameter), the instrument is capable
of recording sediment concentrations and, in combination
with the ADV, suspended sediment transport very close to
the bed. In this experiment, sampling volumes were nominally centered at z 5 0.01, 0.02, 0.03, 0.04, and 0.05 m. All
sensors were colocated in the cross-shore and readjusted
when necessary to maintain a constant elevation relative to
the mobile bed. Sensors were hardwired to a mobile field station in the dunes where the signals were recorded on laptop
computers. When instruments were covered by water, data
bursts of 45-minute duration were recorded almost continuously at a frequency of 10 Hz. Given the relatively close spacing of the instrument stations (ø15 m), convergences and divergences of suspended sediment transport could be evaluated and compared with morphological changes.
Such changes were quantified from changes in bed elevation along a line of 62 survey rods located about 5 m south
of the instrument transect. The rods were 5 mm in diameter
and were established with 2-m individual spacing and the
line spanned the entire intertidal zone. The top of the rods
were surveyed relative to a benchmark in the dunes and the
distance from the top of the rods to the sand surface was
measured using a specially designed ruler at each low tide
throughout the field campaign. Elevations were determined
to the nearest millimeter and survey errors on the flat, wellpacked bed at low tide are estimated as being less than 5
Journal of Coastal Research, Vol. 22, No. 2, 2006
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Aagaard et al.
mm. This survey method provides an inexpensive and reasonably reliable means of estimating the net sediment (bedload and suspended load) transport across the profile. Finally, area surveys were conducted at the beginning, middle, and
end of the experiment period using a total station along seven
cross-shore transects, spaced 25 m apart, from the dune crest
to the low-tide limit of wading (Figure 2).
tration and fluid velocity. For the UFOBS array, sediment
concentrations were paired with velocities from the ADV,
whereas velocities from the EMs were used with the OBS
records. For surf-zone data, sediment fluxes were partitioned
into mean and oscillatory terms generated by mean currents
and oscillatory wave motions (at both incident and infragravity frequencies), respectively (see AAGAARD and GREENWOOD, 1994).
Date Processing and Analysis
RESULTS
Electromagnetic current meter offsets were determined in
buckets prior to the experiment as well as at times of low tide
when sensors became intermittently exposed but were still
wet; sensor gains were determined in a large tow tank prior
to the experiment. The pressure sensors were calibrated in a
stilling well at the field site. In the case of the Viatran sensors, offsets were adjusted for atmospheric pressure fluctuations during the experiment. This was not necessary for the
Druck sensors, however, because they were vented. OBS- and
UFOBS-sensors were post-calibrated in a large recirculation
tank using sand samples from the deployment locations.
Field offsets caused by minute amounts of permanently suspended organics and/or fine-grained sediment particles originating from the inlet were determined from breaks in the
cumulative frequency distribution (AAGAARD and GREENWOOD, 1994). These offsets were generally close to the second
and fifth percentile frequency output voltages for the UFOBS
and OBS sensors, respectively, and, to maintain consistency,
these percentiles were applied to all records.
Prior to analysis, the sensor outputs were screened and
checked for data quality and noisy and/or erroneous data
were discarded from further analysis. Such errors could occur
due to bed accretion resulting in (UFOBS/OBS) signal saturation or instrument emergence. Also, OBS signals sometimes become spiky in very shallow waters depths (probably
due to surface foam associated with surf/swash bores propagating past the instrument), which generally results in inverted sediment concentration profiles. This problem did not
appear to affect the output of the UFOBS sensors, which were
located closer to the bed.
Velocity measurements from the ADV tended to become
noisy in highly turbulent or aerated flows. At such times,
signal correlation values recorded by the ADV were used to
identify potentially inaccurate data. When signal correlation
for a given acoustic beam was less than 55%, the raw velocity
data was replaced by the filtered signal obtained by applying
a 1-Hz filter (cf. RAUBENHEIMER, 2002). Finally, in the swash
zone, the sensor sometimes became emerged; a signal-tonoise ratio of less than 20 was employed to identify such occasions in which the flow velocity is undefined (H UGHES and
BALDOCK, 2004).
Pressure records were detrended prior to computing wave
heights, but correction for depth attenuation was not applied
because of the small water depths in the intertidal zone.
Mean water depths and water levels were determined
through repeated surveys of instrument positions and measurements of sensor elevations relative to the bed.
Instantaneous sediment flux at a particular elevation was
calculated as the product of instantaneous sediment concen-
Morphological Change
Prior to the increased wave energy associated with the gale
occurring on August 30 to August 31 (Figure 3), the intertidal
bar was largely inactive. Only when mean water levels became sufficient to inundate the intertidal bar crest in the
afternoon of August 30 did the onshore bar migration commence. Figure 4 illustrates the morphological change occurring over the four tidal cycles between the early morning of
August 30 and the early morning of September 1.
During cycle 1 (08300015–08301300), only very limited
morphological change occurred, while cycle 2 resulted in a 5–
10-m onshore migration of the bar crest and the 10-m wide
landward runnel began to infill as sediment was scoured from
the bar crest and deposited into the trough. Large clouds of
suspended sand were driven landward with each wave stroke
and deposited on both the landward bar slope and in the runnel (Figure 5). As wave energy levels were very low in the
runnel due to wave dissipation by the shallow water depths
across the bar crest, and mean longshore (rip feeder) currents
were not sufficiently strong to remobilize the sand, infilling
progressed rapidly and was almost completed during tidal
cycle 3 (08310130–08311315; Figure 4). Tidal cycle 4 resulted
in a smoothing of the convexity marking the former intertidal
bar crest. The morphology of the intertidal zone prior to and
after intertidal bar welding is illustrated in Figure 6; the
welding process resulted in a virtually planar intertidal
beach face.
Detailed patterns of erosion and deposition across the intertidal bar during tidal cycles 2–4 are shown in Figure 7.
Initially, deposition prevailed around station S3 and in the
runnel, where up to 0.40 m of accretion occurred, while erosion occurred around station S4 at the bar crest and across
the lower seaward slope of the bar. During the two final tidal
cycles, erosion prevailed across most of the lower and upper
seaward slope of the bar and accretion was limited to the
runnel, where accretion rates systematically declined with
time as accommodation space decreased. The shifting patterns of limited erosion/accretion around station S1 was probably due to longshore migrating bedforms driven by the longshore current; visual observations indicated a prevalence of
ripples and megaripples seaward of station S2. The net
bathymetric change over the three tidal cycles is also illustrated in Figure 7. A maximum of 0.65 m of accretion occurred in the runnel, while erosion prevailed everywhere else,
with a maximum of 0.22 m at station S4. The net sediment
deposition landward of station S3 on the upper seaward slope
of the bar was 0.74 m3/m.
Alternating zones of erosion and deposition (or nonerosion)
Journal of Coastal Research, Vol. 22, No. 2, 2006
Intertidal Bars
251
AAGAARD, and NIELSEN (2004). Moreover, the height and
spacing of the present undulations (0.1–0.2 m and 20–30 m,
respectively) are consistent. Alternatively, the spatially shifting zones of erosion/accretion might indicate temporally
changing positions of sediment-transport convergence/divergence across the bar.
Waves and Currents
Figure 4. Morphological change observed in the intertidal survey transect during tidal cycles 1–4 (top to bottom panels) on August 30, 2002, to
September 1, 2002. Note the progressive landward migration of the intertidal bar and the associated closure of the runnel. The inner subtidal
bar also exhibited a net onshore migration.
occurred across the seaward slope of the bar, and there is
some evidence to suggest that there was a landward propagation of these zones, which could be analogous to the onshore migrating bed oscillations described by GREENWOOD,
Wave breaker patterns were quite persistent over the gale
event. Almost all waves broke through spilling across the inner subtidal bar and they reformed in the trough between
the subtidal and intertidal bars. Due to the filtering effects
of the subtidal bar, the secondary breakpoint on the intertidal
bar was located close to station S2; at high tide, the main
breakpoint was typically displaced some 5 m landward and
at low tide some 5 m seaward of this instrument station. The
breaker type at the intertidal bar was predominantly spilling.
Thus, shoaling waves were almost always observed at station
S1, where the relative wave height (Hs /h) remained below
ø0.4; S2 was in the shoaling zone with occasionally breaking
waves at high tide, or in the inner surf zone at low tide. S3
and S4 were in the inner surf zone with spilling bores at high
tide and in the swash zone (or dry) at low tide. The relative
wave height was almost consistently .0.6 at these stations.
Typical surface elevation spectra from a high tide are illustrated in Figure 8 at a time when the significant wave
height at the outer edge of the instrument array was 0.6 m.
The figure shows a spectral peak at the incident wave frequency (f ø 0.13 Hz) at stations S1 and S2, with suggestions
of a harmonic peak at twice that frequency, which indicates
the skewed form of these shoaling waves. As waves broke
across the intertidal bar, incident wave energy was dissipated
and infragravity waves with a peak frequency of f 5 0.01 Hz
increased progressively in amplitude.
Two instrument records have been selected for illustration
of the general hydrodynamic characteristics across the intertidal bar (Figure 9). These two examples were collected at
high tide on August 30 and August 31, respectively, with approximately similar water levels but with different bathymetries. The mean water levels at the upper instrument station
were 11.12 m DNN (Danish Ordnance Datum) and 11.03 m
DNN, respectively. In both cases, significant wave-height attenuation occurred due to breaking landward of station S2;
this dissipation caused a mean water level setup of 0.15 m
across the seaward slope of the bar (Figure 9). The limited
wave dissipation and the relative set-down between stations
S1 and S2 confirm that waves were not (or only weakly)
breaking seaward of station S2. Even though the basic hydrodynamic process regimes were thus identical in the two
situations, the mean cross-shore current characteristics at
the bar crest (station S4) were different.
The mean current velocities shown in Figure 9 were measured close to the bed by the ADVs at stations S3 and S4 and
by electromagnetic current meters at stations S1 and S2. At
stations S1–S3, the cross-shore currents were directed offshore with speeds of U ø 0.1–0.15 m/s. The smallest current
velocities were recorded around the breakpoint at station S2.
These currents were probably undertows, driven by the sea-
Journal of Coastal Research, Vol. 22, No. 2, 2006
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Aagaard et al.
Figure 5. Low surf bores propagating across the intertidal bar crest and generating a hydraulic jump at the seaward edge of the deep runnel. Waves
are reforming in the runnel. Note the large amounts of sediment trapped in the hydraulic jump; this sediment eventually settles on the landward slope
of the intertidal bar and contributes to the onshore form migration.
ward-directed setup gradient generated by waves breaking
across the intertidal bar. The relatively large mean crossshore (and longshore) current velocities observed at station
S1 were probably due to horizontal mixing and onshore surface mass transport associated with the breaking bores across
the inner subtidal bar at x 5 150 m (cf. CHURCH and THORNTON, 1993; GARCEZ-FARIA et al., 2000). At the uppermost station, S4, however, the mean cross-shore currents were directed onshore at the bar crest with a speed of 0.05 m/s in
the first example and offshore with a speed of 0.10 m/s in the
second example.
In both cases, the ADV at station S4 was permanently submerged throughout the instrument record. Unfortunately, no
mean water level measurements were obtained in the runnel,
but it is likely that the onshore current at station S4 was due
to either (a) a landward-directed pressure gradient generated
by a relative set-down in the runnel where incident waves
were reforming (Figure 5) and/or (b) the presence of the run-
nel reduced the offshore component of the oscillatory flow by
channeling this flow alongshore. Whatever the origin, the onshore-directed mean current at the bar crest (S4) represented
the onshore-directed limb of a cell circulation pattern with
the mass transport of water across the bar crest draining
along the runnel and subsequently seaward through the
downdrift rip channel (Figure 2). When the intertidal bar had
welded to the beach and the runnel had closed (hour 187.7;
Figure 9), the mean cross-shore current at station S4 clearly
became part of the undertow circulation.
These mean current characteristics were consistent
throughout the four tidal cycles for the two bathymetric configurations (Figure 10). Prior to bar welding (tidal cycle 2),
mean currents were persistently directed onshore at the bar
crest (station S4) with speeds of 0.05–0.10 m/s, at instrument
elevations of 0.02–0.07 m above the bed. When the runnel
infilled at the beginning of the tidal cycle 3 (Figure 4), the
cross-shore currents at the bar crest reversed and became
Journal of Coastal Research, Vol. 22, No. 2, 2006
Intertidal Bars
253
Figure 6. The morphology of the intertidal zone prior to and after bar welding. The upper photo shows the .10-m-wide runnel existing prior to the
event; and in the lower photo, taken during the final phase of the experiment, the beach is near planar and the former runnel position is indicated by a
slightly darker color due to increased surface moisture. The instrument stations are seen in the center of the photos.
Journal of Coastal Research, Vol. 22, No. 2, 2006
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Aagaard et al.
Figure 8. Water surface elevation spectra recorded at stations S1–S4 at
high tide, hour 187.7. The spectra have 50 degrees of freedom.
Figure 7. Detailed intertidal bathymetric change measured at the survey rods over the three tidal cycles when significant morphological changes occurred. The accumulated net change is shown by the thick line. The
cross-shore profile and instrument positions are shown in the lower panel
for reference.
offshore directed with speeds of 0.10–0.15 m/s, similar to
mean currents at the other three instrument stations. Hence,
a morphodynamic feedback existed between the morphology
and the mean current circulation across the bar crest.
Cross-Shore Suspended Sediment Transport
Visual observations and the calculated sediment fluxes indicate that considerable amounts of sediment were moved
landward across the upper seaward slope and crest of the
intertidal bar during the three tidal cycles when the bar was
active (Figure 11). Sediment-transport rates were estimated
by summing the sediment fluxes calculated for each optical
sensor bin. At S1 and S2, velocity measurements determined
by the current meters at z ø 0.2 m were paired with sediment
concentrations determined by the OBS at 0.05, 0.1, and 0.2
m; each OBS sensor output was assumed representative for
a 0.05 m (0.10 m) vertical bin. At S4, velocity measurements
from the ADV at z ø 0.03 m were paired with the OBS sensors at z 5 0.035, 0.085, and 0.135 m, and finally, at S3, ADV
velocity measurements at z ø 0.03 m were paired with sediment concentrations measured at z 5 0.01, 0.02, 0.03, 0.04,
and 0.05 m, and EM velocity measurements at z 5 0.2 m
were paired with concentrations at z 5 0.1 and 0.2 m. The
computed estimates at S3 are considered to approximate the
total suspended sediment transports occurring at this station, while transport estimates at the other stations may only
Figure 9. Cross-shore hydrodynamics recorded during two high tide
runs at hours 162.3 (tidal cycle 2) and 187.7 (tidal cycle 4). From the top
down, the panels illustrate cross-shore (U, solid lines) and longshore (V,
dashed lines) mean currents, mean water level setup relative to station
S1, and significant wave height (Hs, solid lines) and relative significant
wave height (Hs /h, dashed lines). Onshore- (U) and northward- (V) directed mean currents are positive. The beach profiles are shown in the
bottom panels for reference.
Journal of Coastal Research, Vol. 22, No. 2, 2006
Intertidal Bars
255
Figure 10. Mean cross-shore current velocities at the four instrument
stations. Positive values represent onshore currents. Times of low tide
are indicated by the vertical dashed lines and the tidal cycle number is
shown at the top of the figure.
be indicative, as sediment concentrations were not measured
very close to the bed.
During all three tidal cycles, the cross-shore sedimenttransport rate at S3 was large and directed onshore in small
water depths, with a tendency for a transport reversal at high
tide (Figure 11). There was a trend toward more seawarddirected sediment fluxes in the lower part of the water column. On balance, however, the net estimated transport was
clearly onshore directed even though mean currents were
consistently directed offshore (Figure 10). Sediment transport
at the upper station S4 is most likely underestimated because
the sediment concentrations were not measured closer than
0.03–0.04 m above the bed and visual observations indicated
that a significant fraction of the sediment transport occurred
as a thin carpet very close to the bed. Given this uncertainty,
the estimated transport at S4 was directed onshore during
cycle 2 and the beginning of cycle 3. Close to high tide during
cycle 3, however, a transport reversal occurred at this station
and offshore-directed sediment fluxes became very large.
During tidal cycle 4, the transport again became onshore directed.
Given that the direction of the cross-shore sediment transport at station S3 depended on water depth, the total transport rates at S3 were correlated against local water depth, h,
and relative wave height for surf-zone conditions (Hs /h .
0.4); see Figure 12. Apparently, there is some form of relationship between transport rate and water depth, or relative
wave height, and both regressions are significant at a 5 0.05.
The functional dependencies are not convincing, however, be-
Figure 11. Net cross-shore suspended sediment-transport rates across
the instrument array. At station S3, transports are illustrated for the
upper instrument array (EM-OBS, dashed line), the lower array (ADV/
FOBS, thin solid line), and sum of the two (heavy line). Positive transport
rates are onshore directed. The numbers of the tidal cycles are shown at
the top of the figure, and low tide occurred at hours 156, 168, 181, and
193.
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Aagaard et al.
Figure 13. Absolute values of net suspended sediment transport (the
sum of mean, incident, and infragravity fluxes) estimated at stations S1–
S3, plotted as a function of local relative wave height.
whereas maximum recorded net transports increase abruptly
at the onset of wave breaking (Hs /h . ø 0.35). Even though
net transports can still remain small under intensely breaking wave conditions due to the balancing effects of mean and
oscillatory fluxes (O SBORNE and ROOKER, 1999; see also Figure 14), there is a generally increasing trend in net sediment
transport with increasing relative wave height.
Integrated over time, there was a suspended sedimenttransport divergence between stations S2 and S3, with the
former characterized by (small) seaward-directed transports
Figure 12. Net sediment-transport rates obtained under surf-zone conditions at S3 plotted against local water depth (upper panel) and relative
wave height (lower panel). The lines of best fit are indicated by the dashed
lines. Coefficients of determination for the linear fits are r2 5 0.127 and
0.256, respectively.
cause the linear fits only explain 13 and 26% of the variance
in sediment transport, respectively.
At the two lower stations (S1 and S2) further down the
seaward slope of the bar, the suspended sediment transport
was consistently directed offshore. The only exception occurred when water levels became very low at station S2, such
that this station was located in the inner surf zone, and relatively large onshore-directed transport rates were recorded
briefly on the ebbing tide. Overall, the data (Figure 11) indicate that sediment-transport rates were about a factor of
five larger in the inner surf and swash zones (stations S3,
S4) than in the shoaling/outer surf zones (stations S1, S2).
The average estimated suspended sediment-transport rates
(absolute values) were: S1: 0.077 kgm22 s21; S2: 0.061 kgm22
s21; S3 (upper instrument array only to provide a comparison): 0.254 kgm22 s21; S4: 0.464 kgm22 s21.
The impact of relative wave height/intensity of wave breaking on cross-shore suspended sediment-transport rate is further illustrated in Figure 13, which plots absolute values of
suspended sediment transports estimated at S1–S3 as a function of relative wave height. For nonbreaking wave conditions
(Hs /h less than 0.35), absolute net transports remain small
Figure 14. Normalized cross-shore suspended sediment-transport rates
due to mean currents, incident waves, and infragravity waves recorded
during hours 162.3 and 187.7. Positive transports are directed onshore.
The beach profiles are shown in the lower panels for reference.
Journal of Coastal Research, Vol. 22, No. 2, 2006
Intertidal Bars
257
and the upper stations exhibiting a landward-directed transport (Figure 11). This sediment-transport pattern is consistent with the bar form migration and the runnel infilling. The
net calculated sediment transport (summed over the three
tidal cycles) at station S3 was 11130 kg/m (corresponding to
0.71 m3 m21), which closely corresponds to the amount of
sand deposited landward of that station.
At station S3, the largest cross-shore transport rates occurred when swash conditions prevailed (Figure 11), and at
those times, the transport was directed landward. Landwarddirected transport also persisted for a significant part of the
time when the station was subjected to surf-zone conditions
even though mean currents were directed offshore; the landward sediment transport was driven by waves at both incident and infragravity frequencies. Only at high tide did the
mean currents become sufficiently important to cause a net
seaward-directed transport. Figure 14 illustrates the relative
significance of mean currents, incident and infragravity
waves to the net cross-shore sediment transport for two example instrument records close to high tide. The normalized
transport rate due to incident waves during an instrument
record was computed as
Qinc 5
qinc
z qinc z 1 z qig z 1 z qmean z
(1)
where q inc, qig, and qmean are the sediment-transport rates accomplished by incident waves, infragravity waves, and mean
currents, respectively. Normalized transport rates due to infragravity waves and mean currents were computed accordingly.
Figure 14 indicates that sediment transport at the lower
stations in the shoaling wave and outer surf zones was dominated by the mean currents, which contributed about 80%
of the total transport rate. Note, however, that, because suspended sediment concentrations at stations S1 and S2 were
small, the net sediment-transport rates were also small. At
station S3, onshore sediment fluxes due to incident and infragravity wave action balanced, or exceeded, the offshore
sediment flux due to the undertow. At the uppermost station,
S4, all transport components were onshore directed at the
time when mean currents were due to the cell circulation.
When the undertow occurred at the upper station, the offshore sediment flux caused by this current was balanced by
an oscillatory onshore-directed flux. Interestingly, incident
waves contributed increasingly large proportions of the total
transport as the shoreline was approached, possibly because
of offshore wave-stroke attenuation due to flow diversion
along the runnel, while the infragravity contribution was
largest around station S3. In summary, the net onshore-directed sediment transport at S3 and S4 appears to have been
driven by swash processes at low tide and mainly by oscillatory wave motions at high tide.
Figure 14 provides a general impression of the relative importance of the different sediment-transport mechanisms
across the intertidal bar, but exceptions to that pattern did
exist. As mentioned earlier, the net transport at station S4
momentarily reversed from onshore to offshore and increased
dramatically around hour 176 (Figure 11). This was due to a
sudden reversal in the direction of the sediment flux due to
Figure 15. Cospectra of sediment concentration and cross-shore oscillatory velocity at stations S3 and S4 recorded during the rising tide (hour
173, dashed lines) and high tide (hour 176, solid lines) of tidal cycle 3.
Optical sensor elevations above the bed are noted in the figure. The cospectra have 50 degrees of freedom.
infragravity waves (Figure 15). The infragravity transport
rate also increased significantly and, during hour 176, it contributed about 60% of the total sediment transport at station
S4. A simultaneous switch in infragravity transport direction
also occurred at station S3 (Figure 15).
BUTT and RUSSELL (1999) suggested that infragravity
transport direction could depend on the higher order moments of the oscillatory infragravity velocity field, such as
velocity or acceleration skewness. This may not have been
the case here, however. Normalized velocity skewness can be
computed as S 5 u3/(u2)1.5 and acceleration skewness as A 5
a3/(a2)1.5, where a 5 du/dt (BUTT and RUSSELL, 1999). Infragravity velocity and acceleration skewnesses were computed
from low-passed velocity records with a high frequency cutoff of 0.067 Hz (Table 1). At both stations, velocity skewnesses were consistently negative and no convincing relationship
was apparent between the skewness magnitude and the infragravity fluxes. With respect to the acceleration skewness,
this was at least an order of magnitude smaller than the ve-
Journal of Coastal Research, Vol. 22, No. 2, 2006
258
Aagaard et al.
Table 1. Normalized infragravity velocity skewness (S) and acceleration
skewness (A) for low-tide records (hour 173) and high-tide records (hour
176).
Station
S3, hour
hour
S4, hour
hour
173
176
173
176
S
A
21.419
21.436
20.450
21.461
20.002
20.030
20.002
20.136
locity skewness. Negative acceleration skewness did increase
significantly when infragravity transport reversed offshore,
but because acceleration skewness was consistently negative,
it is difficult to convincingly attribute the observed transport
reversal to increased negative accelerations.
DISCUSSION
This field experiment demonstrated an example of onshore
migration of an intertidal bar with subsequent bar welding
to the beach and the development of a planar intertidal beach
profile (Figure 6). The intertidal bar at Skallingen was of the
slip-face ridge type (cf. WIJNBERG and KROON, 2002) and the
outcome of the bar evolution was a significant onshore sediment supply from the nearshore zone to the beach.
Previously, AAGAARD et al. (1998a) reported observations
of sediment transport and hydrodynamics across a landwardmigrating intertidal bar at Skallingen. Measurements were
then obtained at a single location on the seaward slope of the
intertidal bar under more energetic conditions than encountered in the present experiment. It was concluded that the
landward migration of that intertidal bar was mainly due to
an onshore-directed sediment transport driven by the mean
current, the direction of which depended on the presence or
absence of a runnel landward of the bar. In the study reported here, a much denser array of sensors was used, velocity and sediment transport were measured very close to the
bed, and similar conclusions on the mean current circulation
were reached: Onshore-directed currents persisted on the bar
crest until the runnel closed, subsequent to which the undertow extended landward of the bar crest. Similar onshore-directed mean currents have been observed in three-dimensional bar settings by DRøNEN et al. (1999) and KROON and
DE BOER (2001).
In this experiment, however, the onshore directed mean
currents at the bar crest did not appear critically important
to the onshore migration of the bar and the current speed
was smaller than in the example reported by AAGAARD et al.
(1998a), the reason probably being the lower wave-energy
levels. Here, onshore sediment transport did prevail across
the upper seaward slope and crest of the bar, but it was mainly caused by oscillatory wave motions under swash and inner
surf-zone conditions (Figures 11 and 13). At the upper seaward slope of the bar, onshore sediment-transport rates occurred when mean water depths were less than approximately 0.5 m or relative wave heights . ø0.7 (Figure 12).
This trend was not entirely consistent at all stations; for example, large offshore transport rates developed at station S4
when h 5 0.1–0.15 m and Hs /h . 1. The reason was that
infragravity transport momentarily became large and offshore directed. The sudden and dramatic switch in infragravity sediment-transport direction and magnitude around
hour 176 could not be confidently related to changes in infragravity velocity or acceleration skewness. Examination of the
time-series records suggests that the reversal may have had
less to do with the hydrodynamic forcing than with the processes of sediment resuspension. This is a topic of ongoing
research but falls outside the scope of the present article.
Offshore transport across the upper seaward bar slope
mainly occurred at high tide when h . 0.4 m and Hs /h # 0.7
(Figure 12). To some extent, this supports observations by
HOUSER and GREENWOOD (2003), who found landward- and
seaward-directed sediment transports being separated for
relative wave heights ø0.54. However, the functional relationship found here between transport rate and relative wave
height is certainly not convincing (r2 5 0.26) and other mechanisms were clearly important to the transport rate and direction.
Under weakly breaking or shoaling waves (stations S1 and
S2), the recorded suspended sediment transport was consistently directed offshore. The offshore transport under shoaling waves (station S1) is somewhat surprising but was due
to offshore-directed mean currents probably generated by
breaking across the subtidal bar located further seaward (e.g.,
Figure 4). The main point is that estimated sediment-transport rates under shoaling and weakly breaking waves were
generally about an order of magnitude smaller than transport
rates in the inner surf and swash zones (Figure 13), mainly
because suspended sediment concentrations in the water column are small under such conditions (AAGAARD, BLACK, and
GREENWOOD, 2002). This observation is of importance to the
question whether there is any fundamental difference between the mobile intertidal bars (slip-face ridge type) studied
here and the low-amplitude quasi-static ridge-and-runnel
type of bars, which mainly occur in meso-macrotidal settings
and with low-energy wave conditions (KING and WILLIAMS,
1949; MULRENNAN, 1992; ORFORD and WRIGHT, 1978). The
present measurements suggest that inner surf/swash zone
conditions are required in order to generate large suspended
sediment concentrations and transport rates. In settings with
a large tidal range and/or low waves, such conditions will
generally last only a small fraction of each tidal cycle at a
specific bar. This could be the reason why ridge and runnel
morphology is not very mobile and do not develop a form
asymmetry through landward migration.
If this interpretation is correct, then it is likely that many
intertidal bars or intertidal bar sequences may oscillate between one type and the other, for example, through springneap tidal cycles or as incident wave energy varies temporally on a seasonal cycle. It is therefore questionable whether
a distinction should be made between slip-face ridges and
low-amplitude ridges (ridge-and-runnels). It would seem
more prudent to use the term intertidal bar for both bar
types. The term swash bar would also appear inappropriate
as both swash and surf-zone processes are critical to the behavior of the mobile intertidal bars.
Journal of Coastal Research, Vol. 22, No. 2, 2006
Intertidal Bars
CONCLUSIONS
One of the main mechanisms for shoreline progradation is
the migration of bars across the intertidal zone and their deposition on the beach face. This study has provided hydrodynamic and suspended sediment-transport measurements
during a beach accretion event of this type. Onshore bar migration was achieved mainly by swash processes and by the
oscillatory flows of both short and long waves under surf-zone
conditions. The transport competency of the onshore stroke
of the waves was considerably larger than the competency of
the offshore stroke because water transported over the bar
crest was subsequently channeled alongshore in the runnel.
This transport asymmetry and the large sediment fluxes naturally associated with surf and swash in very shallow water
(, ø0.5 m), resulted in a relatively rapid bar-migration rate
corresponding to 10–20 m/d. Both the existence of the intertidal bar and its disappearance once it infilled the runnel
produced a strong feedback effect on the hydrodynamics and
sediment dynamics. The presence of the runnel was associated with horizontal cell circulation in the intertidal zone,
characterized by onshore-directed mean flows across the bar
crest, which augmented the sediment transport due to wave
motions. When the runnel was infilled, an offshore-directed
undertow developed which opposed the wave-induced sediment transport.
ACKNOWLEDGMENTS
We are grateful to Per Sørensen (the Danish Coastal Authority) and Erik Brenneche (Esbjerg Port Authorities) for
giving us access to offshore wave and tidal data, respectively.
Ulf Thomas and Niels Vinther helped out in the field—under
sunny conditions this time! This research was funded by the
Danish Technical Sciences Research Council (grant
99012287) and by the European Union through the CoastView Project (contract EVK3-CT-2001-0054).
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