Journal of Coastal Research 22 2 247–259 West Palm , 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 §School of Geosciences University of Copenhagen University of Sydney Oster Voldgade 10 Sydney NSW 2006, Australia DK-1350 Copenhagen, Denmark [email protected]

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 cycles). The data include bed-elevation changes measured at each low tide and continuous records of -surface elevation, cross- and long-shore 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 con- centration profile) and partitioned between mean and oscillatory components with the latter further partitioned be- tween 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 circu- lation when the runnel has infilled.

ADDITIONAL INDEX WORDS: bar, ridge and runnel, cell circulation, morphodynamics.

INTRODUCTION these bars are in fact generated and maintained by swash (WIJNBERG and KROON, 2002) or surf-zone processes (AA- The intertidal zone of micro/mesotidal in semien- GAARD et al., 1998a; KROON and MASSELINK, 2002). They closed are often characterized by the existence of one or tend to form in the mid to lower intertidal zone and migrate more intertidal bars. Such intertidal bars can take on various onshore under low/moderate- conditions, whereas they forms and display different types of dynamic behavior. Fol- may be eroded during high-energy situations. The water level lowing GREENWOOD and DAVIDSON-ARNOTT (1979), WIJN- across the bar crest would seem to be an important param- BERG and KROON (2002) distinguished between two main types of intertidal bars, (a) slip-face ridges that are asym- eter in determining the bar behavior. KROON and MASSE- metric forms and relatively mobile and (b) low-amplitude LINK (2002) observed landward migration associated with ridges that are more symmetric in form and largely static mean onshore flows when water depths were less than 0.2 m features. These two bar types also correspond to the terms at the bar crest, while DAWSON,DAVIDSON-ARNOTT, and swash bars and ridge and runnels (ORFORD and WRIGHT, OLLERHEAD (2002) observed a critical depth of 0.1 m. Similar 1978), respectively. onshore-directed mean flows were observed at bar crests by Despite the accessibility of the intertidal zone, the physical AAGAARD et al. (1998a, 1988b) and by KROON and DE BOER processes governing intertidal bar behavior are not well un- (2001). derstood, mainly because of the difficulties in measuring hy- The origin and detailed dynamics of the ridge and runnel drodynamics and sediment transport in very shallow water type of intertidal bars may be even more obscure. This bar depths. In the case of slip-face ridges, it is unclear whether type mainly occurs with relatively large tidal ranges and small waves (KING and WILLIAMS, 1949; ORFORD and DOI:10.2112/04-0214.1; received 3 May 2004; accepted in revision 20 WRIGHT, 1978), i.e., for larger values of the relative tide range September 2004. (MASSELINK and SHORT, 1993). VOULGARIS et al. (1998) mea- 248 Aagaard et al. sured hydrodynamics and sediment transport across a ridge- and-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 Ͼ0.8 m, they con- cluded that measurements in shallow water depths (swash and inner surf zones) are required to quantify the trans- port and morphological development of intertidal bars. Sim- ilar 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 circula- tion system with longshore rip feeder flows in the runnels, Figure 1. Cross-shore profile at Skallingen, August 25, 2002. The profile and this type of circulation is typical for moderate-energy comprises an intertidal bar and two nearshore (subtidal) bars. Positions conditions when waves are breaking over the landward mi- of the four instrument stations across the intertidal bar are indicated by grating bars. While some numerical models for such three- the vertical lines. Mean annual level is at ϩ 0.14 m DNN (Danish dimensional flows have started to emerge (e.g., SHORECIRC; Ordnance Datum). HAAS et al., 2003), these models have not yet been extended to simulate sediment transport and morphological change. Consequently, morphodynamics and bar behavior in three- to 1.8 m at spring . The shoreface exhibits 2–3 subtidal dimensional bathymetric settings are not well predicted by bars and additionally, one or two intertidal bars are common. numerical models. Indeed, most available models are two-di- At the outset of the experiment, the upper shoreface had mensional and simulate hydrodynamics and sediment trans- subtidal bar crests located at x ϭ 250 m and x ϭ 150 m port in a cross-shore profile; they cannot simulate the land- relative to the survey baseline, a rather large intertidal bar ward movement of intertidal bars under breaking waves. centered at x ϭ 90 m and an upper swash bar/berm at x ϭ More field data on hydrodynamics and sediment transport 55 m (Figure 1). During the course of the experiment, the from three-dimensional morphological settings are therefore intertidal bar moved landward, closed the runnel, and welded required to constrain future model development (SOULSBY, to the beach. The behavior was consistent with that typically 1999). displayed by such bars at Skallingen: Intertidal bars tend to The present study obtained such measurements under low- migrate landward until they weld to the beach; after welding to moderate-energy conditions across an intertidal bar of the and runnel infilling, the bar(s) may be eroded during high- slip-face ridge type. In the course of three tidal cycles, a large energy situations and the sediment recycled to the lower in- intertidal bar migrated landward and welded to the beach. tertidal zone (AAGAARD et al., 1998a). The processes responsible for this behavior are documented Initially, the survey (and instrument) transect was located through measurements of flow velocities and sediment con- across the intertidal bar approximately midway between two centration obtained close to the bed at four measurement po- rip channels, which were spaced about 175 m apart. The dif- sitions. As the tide rose and fell, the instruments were sub- ference in elevation between the bar crest and the landward jected to various hydrodynamic regimes and the relative ef- runnel was approximately 0.6 m and the bar form was fects of swash, surf, and shoaling wave processes to intertidal oblique to the beach, with the northern part of the bar located bar dynamics are evaluated. Furthermore, the morphodyn- closer to the shoreline, consistent with the dominant south- amic feedbacks between morphology and hydrodynamic pro- erly longshore sediment transport at the site (Figure 2). cesses are elucidated; as the bar moved onshore and closed The sediment on this bar was well sorted with a mean the landward runnel, onshore-directed mean flows were re- of 200–240 ␮m. Wave-energy levels during the ex- placed by offshore-directed undertow. periment were quite low (Figure 3). The significant offshore wave height (recorded 18 km offshore in a water depth of ഠ12 m) remained below 0.5–0.6 m until August 29, when a gale MATERIALS AND METHODS occurred and waves increased to 1.2 m and further to 2.1 m Experimental Site and Procedures on August 31, and subsequently wave heights decreased again. Peak spectral wave periods increased from ഠ4–8 sec- The field experiment was conducted from August 23, 2002, onds during the event, which was also associated with a small to September 4, 2002, at Skallingen, which is located on the surge of ഠ0.2 m (Figure 3) due to the onshore winds. Tides North Sea of Denmark. The shoreface has a gentle were recorded at the ebb delta, about 3 km away from the slope, ␤ ഠ 0.007, the mean annual offshore significant wave field site. Unfortunately, tidal records are missing prior to height is 1 m, and the mean is 1.5 m, increasing the event, which was initiated 3 days after a spring tide; the

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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 Figure 2. Three-dimensional topographic surfaces of the beach and in- August 29. The dashed line in the upper panel indicates the mean annual tertidal zone at the experimental site before (August 24, 2002) and after water level. (September 2, 2002) the event described in the article. The instrument transect was located at the longshore coordinate y ϭ 0m. mm away from the sensor head. Due to the small size of the sensor head (8 mm outer diameter), the instrument is capable tidal stage was thus between spring and neap, with decreas- of recording sediment concentrations and, in combination ing tidal ranges. with the ADV, suspended sediment transport very close to Four instrument stations were established in the surveyed the bed. In this experiment, sampling volumes were nomi- transect (Figure 1). These stations consisted of H-frames jet- nally centered at z ϭ 0.01, 0.02, 0.03, 0.04, and 0.05 m. All ted approximately 1.5 m into the bed and all were equipped sensors were colocated in the cross-shore and readjusted with a Marsh-McBirney OEM 512 electromagnetic current when necessary to maintain a constant elevation relative to meter (EM) at a nominal elevation of 0.20 m above the bed the mobile bed. Sensors were hardwired to a mobile field sta- and an array of three OBS-1P optical backscatter sensors at tion in the where the signals were recorded on laptop nominal elevations of 0.05, 0.10, and 0.20 m above the bed computers. When instruments were covered by water, data for sediment-transport measurements. At the uppermost sta- bursts of 45-minute duration were recorded almost continu- tion (S4), however, the current meter was deployed at a nom- ously at a frequency of 10 Hz. Given the relatively close spac- inal elevation of 0.12 m and the lower OBS at 0.035 m. Wave ing of the instrument stations (ഠ15 m), convergences and di- transformations and mean water levels were measured with vergences of suspended sediment transport could be evalu- pressure sensors (Viatran Model 2406A at S1 and S2 and ated and compared with morphological changes. Druck Model PTX1830 at S3 and S4). At the upper stations Such changes were quantified from changes in bed eleva- (S3 and S4), the pressure sensor elevation was kept at, or tion along a line of 62 survey rods located about 5 m south slightly below, bed level in order to measure water depths in of the instrument transect. The rods were 5 mm in diameter the swash zone. These upper stations were also equipped and were established with 2-m individual spacing and the with three-dimensional sideways-looking Sontek 10 MHz line spanned the entire intertidal zone. The top of the rods Acoustic Doppler Velocimeters (ADV) at nominal elevations were surveyed relative to a benchmark in the dunes and the of 0.02–0.03 m above the bed and at S3, a vertical array of distance from the top of the rods to the sand surface was five D&A Instruments UFOBS-7 fiber-optical backscatter measured using a specially designed ruler at each low tide sensors was installed. The UFOBS-7 uses an infrared laser throughout the field campaign. Elevations were determined to detect sediment concentration within a very small sam- to the nearest millimeter and survey errors on the flat, well- pling volume (nominally ഠ10 mm3) that is centered 10–15 packed bed at low tide are estimated as being less than 5

Journal of Coastal Research, Vol. 22, No. 2, 2006 250 Aagaard et al. mm. This survey method provides an inexpensive and rea- tration and fluid velocity. For the UFOBS array, sediment sonably reliable means of estimating the net sediment (bed- concentrations were paired with velocities from the ADV, load and suspended load) transport across the profile. Final- whereas velocities from the EMs were used with the OBS ly, area surveys were conducted at the beginning, middle, and records. For surf-zone data, sediment fluxes were partitioned end of the experiment period using a total station along seven into mean and oscillatory terms generated by mean currents cross-shore transects, spaced 25 m apart, from the crest and oscillatory wave motions (at both incident and infragrav- to the low-tide limit of wading (Figure 2). ity frequencies), respectively (see AAGAARD and GREEN- WOOD, 1994). Date Processing and Analysis RESULTS Electromagnetic current meter offsets were determined in Morphological Change buckets prior to the experiment as well as at times of low tide when sensors became intermittently exposed but were still Prior to the increased wave energy associated with the gale wet; sensor gains were determined in a large tow tank prior occurring on August 30 to August 31 (Figure 3), the intertidal to the experiment. The pressure sensors were calibrated in a bar was largely inactive. Only when mean water levels be- stilling well at the field site. In the case of the Viatran sen- came sufficient to inundate the intertidal bar crest in the sors, offsets were adjusted for atmospheric pressure fluctua- afternoon of August 30 did the onshore bar migration com- tions during the experiment. This was not necessary for the mence. Figure 4 illustrates the morphological change occur- Druck sensors, however, because they were vented. OBS- and ring over the four tidal cycles between the early morning of UFOBS-sensors were post-calibrated in a large recirculation August 30 and the early morning of September 1. tank using sand samples from the deployment locations. During cycle 1 (08300015–08301300), only very limited Field offsets caused by minute amounts of permanently sus- morphological change occurred, while cycle 2 resulted in a 5– pended organics and/or fine-grained sediment particles orig- 10-m onshore migration of the bar crest and the 10-m wide inating from the were determined from breaks in the landward runnel began to infill as sediment was scoured from cumulative frequency distribution (AAGAARD and GREEN- the bar crest and deposited into the trough. Large clouds of WOOD, 1994). These offsets were generally close to the second suspended sand were driven landward with each wave stroke and fifth percentile frequency output voltages for the UFOBS and deposited on both the landward bar slope and in the run- and OBS sensors, respectively, and, to maintain consistency, nel (Figure 5). As wave energy levels were very low in the these percentiles were applied to all records. runnel due to wave dissipation by the shallow water depths Prior to analysis, the sensor outputs were screened and across the bar crest, and mean longshore (rip feeder) currents checked for data quality and noisy and/or erroneous data were not sufficiently strong to remobilize the sand, infilling were discarded from further analysis. Such errors could occur progressed rapidly and was almost completed during tidal due to bed accretion resulting in (UFOBS/OBS) signal satu- cycle 3 (08310130–08311315; Figure 4). Tidal cycle 4 resulted ration or instrument emergence. Also, OBS signals some- in a smoothing of the convexity marking the former intertidal times become spiky in very shallow depths (probably bar crest. The morphology of the intertidal zone prior to and due to surface foam associated with surf/swash bores propa- after intertidal bar welding is illustrated in Figure 6; the gating past the instrument), which generally results in in- welding process resulted in a virtually planar intertidal verted sediment concentration profiles. This problem did not beach face. appear to affect the output of the UFOBS sensors, which were Detailed patterns of erosion and deposition across the in- located closer to the bed. tertidal bar during tidal cycles 2–4 are shown in Figure 7. Velocity measurements from the ADV tended to become Initially, deposition prevailed around station S3 and in the noisy in highly turbulent or aerated flows. At such times, runnel, where up to 0.40 m of accretion occurred, while ero- signal correlation values recorded by the ADV were used to sion occurred around station S4 at the bar crest and across identify potentially inaccurate data. When signal correlation the lower seaward slope of the bar. During the two final tidal for a given acoustic beam was less than 55%, the raw velocity cycles, erosion prevailed across most of the lower and upper data was replaced by the filtered signal obtained by applying seaward slope of the bar and accretion was limited to the a 1-Hz filter (cf. RAUBENHEIMER, 2002). Finally, in the swash runnel, where accretion rates systematically declined with zone, the sensor sometimes became emerged; a signal-to- time as accommodation space decreased. The shifting pat- noise ratio of less than 20 was employed to identify such oc- terns of limited erosion/accretion around station S1 was prob- casions in which the flow velocity is undefined (HUGHES and ably due to longshore migrating bedforms driven by the long- BALDOCK, 2004). shore current; visual observations indicated a prevalence of Pressure records were detrended prior to computing wave ripples and megaripples seaward of station S2. The net heights, but correction for depth attenuation was not applied bathymetric change over the three tidal cycles is also illus- because of the small water depths in the intertidal zone. trated in Figure 7. A maximum of 0.65 m of accretion oc- Mean water depths and water levels were determined curred in the runnel, while erosion prevailed everywhere else, through repeated surveys of instrument positions and mea- with a maximum of 0.22 m at station S4. The net sediment surements of sensor elevations relative to the bed. deposition landward of station S3 on the upper seaward slope Instantaneous sediment flux at a particular elevation was of the bar was 0.74 m3/m. calculated as the product of instantaneous sediment concen- Alternating zones of erosion and deposition (or nonerosion)

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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 shift- ing zones of erosion/accretion might indicate temporally changing positions of sediment-transport convergence/diver- gence across the bar.

Waves and Currents

Wave breaker patterns were quite persistent over the gale event. Almost all waves broke through spilling across the in- ner 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 ഠ0.4; S2 was in the shoaling zone with occasionally breaking waves at high tide, or in the inner 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 Ͼ0.6 at these stations. Typical surface elevation spectra from a high tide are il- lustrated 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 fre- quency (f ഠ 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 ϭ 0.01 Hz increased progressively in amplitude. Two instrument records have been selected for illustration of the general hydrodynamic characteristics across the inter- tidal bar (Figure 9). These two examples were collected at high tide on August 30 and August 31, respectively, with ap- proximately similar water levels but with different bathym- etries. The mean water levels at the upper instrument station were ϩ1.12 m DNN (Danish Ordnance Datum) and ϩ1.03 m DNN, respectively. In both cases, significant wave-height at- tenuation 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) Figure 4. Morphological change observed in the intertidal survey tran- breaking seaward of station S2. Even though the basic hy- sect during tidal cycles 1–4 (top to bottom panels) on August 30, 2002, to drodynamic process regimes were thus identical in the two September 1, 2002. Note the progressive landward migration of the in- situations, the mean cross-shore current characteristics at tertidal bar and the associated closure of the runnel. The inner subtidal bar also exhibited a net onshore migration. the bar crest (station S4) were different. The mean current velocities shown in Figure 9 were mea- sured close to the bed by the ADVs at stations S3 and S4 and by electromagnetic current meters at stations S1 and S2. At occurred across the seaward slope of the bar, and there is stations S1–S3, the cross-shore currents were directed off- some evidence to suggest that there was a landward propa- shore with speeds of U ഠ 0.1–0.15 m/s. The smallest current gation of these zones, which could be analogous to the on- velocities were recorded around the breakpoint at station S2. shore migrating bed oscillations described by GREENWOOD, These currents were probably undertows, driven by the sea-

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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 nel reduced the offshore component of the oscillatory flow by across the intertidal bar. The relatively large mean cross- channeling this flow alongshore. Whatever the origin, the on- shore (and longshore) current velocities observed at station shore-directed mean current at the bar crest (S4) represented S1 were probably due to horizontal mixing and onshore sur- the onshore-directed limb of a cell circulation pattern with face mass transport associated with the breaking bores across the mass transport of water across the bar crest draining the inner subtidal bar at x ϭ 150 m (cf. CHURCH and THORN- along the runnel and subsequently seaward through the TON, 1993; GARCEZ-FARIA et al., 2000). At the uppermost sta- downdrift rip (Figure 2). When the intertidal bar had tion, S4, however, the mean cross-shore currents were di- welded to the beach and the runnel had closed (hour 187.7; rected onshore at the bar crest with a speed of 0.05 m/s in Figure 9), the mean cross-shore current at station S4 clearly the first example and offshore with a speed of 0.10 m/s in the became part of the undertow circulation. second example. These mean current characteristics were consistent In both cases, the ADV at station S4 was permanently sub- throughout the four tidal cycles for the two bathymetric con- merged throughout the instrument record. Unfortunately, no figurations (Figure 10). Prior to bar welding (tidal cycle 2), mean water level measurements were obtained in the runnel, mean currents were persistently directed onshore at the bar but it is likely that the onshore current at station S4 was due crest (station S4) with speeds of 0.05–0.10 m/s, at instrument to either (a) a landward-directed pressure gradient generated elevations of 0.02–0.07 m above the bed. When the runnel by a relative set-down in the runnel where incident waves infilled at the beginning of the tidal cycle 3 (Figure 4), the were reforming (Figure 5) and/or (b) the presence of the run- cross-shore currents at the bar crest reversed and became

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Figure 6. The morphology of the intertidal zone prior to and after bar welding. The upper photo shows the Ͼ10-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.

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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 sur- vey rods over the three tidal cycles when significant morphological chang- es 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 in- dicate 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 ഠ 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 ഠ 0.03 m were paired with the OBS sen- sors at z ϭ 0.035, 0.085, and 0.135 m, and finally, at S3, ADV velocity measurements at z ഠ 0.03 m were paired with sed- Figure 9. Cross-shore hydrodynamics recorded during two high tide ϭ runs at hours 162.3 (tidal cycle 2) and 187.7 (tidal cycle 4). From the top iment concentrations measured at z 0.01, 0.02, 0.03, 0.04, down, the panels illustrate cross-shore (U, solid lines) and longshore (V, and 0.05 m, and EM velocity measurements at z ϭ 0.2 m dashed lines) mean currents, mean water level setup relative to station were paired with concentrations at z ϭ 0.1 and 0.2 m. The S1, and significant wave height (Hs, solid lines) and relative significant wave height (H /h, dashed lines). Onshore- (U) and northward- (V) di- computed estimates at S3 are considered to approximate the s rected mean currents are positive. The beach profiles are shown in the total suspended sediment transports occurring at this sta- bottom panels for reference. tion, while transport estimates at the other stations may only

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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 sediment- transport 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 seaward- directed sediment fluxes in the lower part of the water col- umn. 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 di- rected. Given that the direction of the cross-shore sediment trans- port at station S3 depended on water depth, the total trans- Figure 11. Net cross-shore suspended sediment-transport rates across port rates at S3 were correlated against local water depth, h, the instrument array. At station S3, transports are illustrated for the and relative wave height for surf-zone conditions (H /h Ͼ upper instrument array (EM-OBS, dashed line), the lower array (ADV/ s FOBS, thin solid line), and sum of the two (heavy line). Positive transport 0.4); see Figure 12. Apparently, there is some form of rela- rates are onshore directed. The numbers of the tidal cycles are shown at tionship between transport rate and water depth, or relative the top of the figure, and low tide occurred at hours 156, 168, 181, and wave height, and both regressions are significant at ␣ϭ0.05. 193. The functional dependencies are not convincing, however, be-

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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 Ͼ ഠ at the onset of wave breaking (Hs /h 0.35). Even though net transports can still remain small under intensely break- ing wave conditions due to the balancing effects of mean and oscillatory fluxes (OSBORNE and ROOKER, 1999; see also Fig- ure 14), there is a generally increasing trend in net sediment transport with increasing relative wave height. Integrated over time, there was a suspended sediment- transport 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 con- ditions 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 ϭ 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 oc- curred when water levels became very low at station S2, such that this station was located in the inner surf zone, and rel- atively large onshore-directed transport rates were recorded briefly on the ebbing tide. Overall, the data (Figure 11) in- dicate 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 kgmϪ2 sϪ1; S2: 0.061 kgmϪ2 sϪ1; S3 (upper instrument array only to provide a compari- son): 0.254 kgmϪ2 sϪ1; S4: 0.464 kgmϪ2 sϪ1. The impact of relative wave height/intensity of wave break- ing on cross-shore suspended sediment-transport rate is fur- ther illustrated in Figure 13, which plots absolute values of Figure 14. Normalized cross-shore suspended sediment-transport rates suspended sediment transports estimated at S1–S3 as a func- due to mean currents, incident waves, and infragravity waves recorded during hours 162.3 and 187.7. Positive transports are directed onshore. tion of relative wave height. For nonbreaking wave conditions The beach profiles are shown in the lower panels for reference. (Hs /h less than 0.35), absolute net transports remain small

Journal of Coastal Research, Vol. 22, No. 2, 2006 Intertidal Bars 257 and the upper stations exhibiting a landward-directed trans- port (Figure 11). This sediment-transport pattern is consis- tent with the bar form migration and the runnel infilling. The net calculated sediment transport (summed over the three tidal cycles) at station S3 was ϩ1130 kg/m (corresponding to 0.71 m3 mϪ1), which closely corresponds to the amount of sand deposited landward of that station. At station S3, the largest cross-shore transport rates oc- curred when swash conditions prevailed (Figure 11), and at those times, the transport was directed landward. Landward- directed 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 land- ward sediment transport was driven by waves at both inci- dent 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 ex- ample instrument records close to high tide. The normalized transport rate due to incident waves during an instrument record was computed as q Q ϭ inc (1) inc ͦ ͦ ϩ ͦ ͦ ϩ ͦ ͦ qincq igq mean where qinc, qig, and qmean are the sediment-transport rates ac- complished by incident waves, infragravity waves, and mean currents, respectively. Normalized transport rates due to in- fragravity waves and mean currents were computed accord- ingly. Figure 14 indicates that sediment transport at the lower stations in the shoaling wave and outer surf zones was dom- inated by the mean currents, which contributed about 80% Figure 15. Cospectra of sediment concentration and cross-shore oscil- of the total transport rate. Note, however, that, because sus- latory velocity at stations S3 and S4 recorded during the rising tide (hour pended sediment concentrations at stations S1 and S2 were 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 co- small, the net sediment-transport rates were also small. At spectra have 50 degrees of freedom. station S3, onshore sediment fluxes due to incident and in- fragravity 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. infragravity waves (Figure 15). The infragravity transport When the undertow occurred at the upper station, the off- rate also increased significantly and, during hour 176, it con- shore sediment flux caused by this current was balanced by tributed about 60% of the total sediment transport at station an oscillatory onshore-directed flux. Interestingly, incident S4. A simultaneous switch in infragravity transport direction waves contributed increasingly large proportions of the total also occurred at station S3 (Figure 15). transport as the shoreline was approached, possibly because BUTT and RUSSELL (1999) suggested that infragravity of offshore wave-stroke attenuation due to flow diversion transport direction could depend on the higher order mo- along the runnel, while the infragravity contribution was ments of the oscillatory infragravity velocity field, such as largest around station S3. In summary, the net onshore-di- velocity or acceleration skewness. This may not have been rected sediment transport at S3 and S4 appears to have been the case here, however. Normalized velocity skewness can be driven by swash processes at low tide and mainly by oscil- computed as S ϭ u3/(u2)1.5 and acceleration skewness as A ϭ latory wave motions at high tide. a3/(a2)1.5, where a ϭ du/dt (BUTT and RUSSELL, 1999). Infra- Figure 14 provides a general impression of the relative im- gravity velocity and acceleration skewnesses were computed portance of the different sediment-transport mechanisms from low-passed velocity records with a high frequency cut- across the intertidal bar, but exceptions to that pattern did off of 0.067 Hz (Table 1). At both stations, velocity skewness- exist. As mentioned earlier, the net transport at station S4 es were consistently negative and no convincing relationship momentarily reversed from onshore to offshore and increased was apparent between the skewness magnitude and the in- dramatically around hour 176 (Figure 11). This was due to a fragravity fluxes. With respect to the acceleration skewness, sudden reversal in the direction of the sediment flux due to 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 ϭ Ͼ when h 0.1–0.15 m and Hs /h 1. The reason was that skewness (A) for low-tide records (hour 173) and high-tide records (hour infragravity transport momentarily became large and off- 176). shore directed. The sudden and dramatic switch in infra- Station SAgravity sediment-transport direction and magnitude around hour 176 could not be confidently related to changes in infra- S3, hour 173 Ϫ1.419 Ϫ0.002 hour 176 Ϫ1.436 Ϫ0.030 gravity velocity or acceleration skewness. Examination of the S4, hour 173 Ϫ0.450 Ϫ0.002 time-series records suggests that the reversal may have had hour 176 Ϫ1.461 Ϫ0.136 less to do with the hydrodynamic forcing than with the pro- cesses 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 Ͼ Յ locity skewness. Negative acceleration skewness did increase mainly occurred at high tide when h 0.4 m and Hs /h 0.7 significantly when infragravity transport reversed offshore, (Figure 12). To some extent, this supports observations by but because acceleration skewness was consistently negative, HOUSER and GREENWOOD (2003), who found landward- and it is difficult to convincingly attribute the observed transport seaward-directed sediment transports being separated for reversal to increased negative accelerations. relative wave heights ഠ0.54. However, the functional rela- tionship found here between transport rate and relative wave DISCUSSION height is certainly not convincing (r2 ϭ 0.26) and other mech- anisms were clearly important to the transport rate and di- This field experiment demonstrated an example of onshore rection. migration of an intertidal bar with subsequent bar welding Under weakly breaking or shoaling waves (stations S1 and to the beach and the development of a planar intertidal beach S2), the recorded suspended sediment transport was consis- profile (Figure 6). The intertidal bar at Skallingen was of the tently directed offshore. The offshore transport under - slip-face ridge type (cf. WIJNBERG and KROON, 2002) and the ing waves (station S1) is somewhat surprising but was due outcome of the bar evolution was a significant onshore sedi- to offshore-directed mean currents probably generated by ment supply from the nearshore zone to the beach. breaking across the subtidal bar located further seaward (e.g., Previously, AAGAARD et al. (1998a) reported observations Figure 4). The main point is that estimated sediment-trans- of sediment transport and hydrodynamics across a landward- port rates under shoaling and weakly breaking waves were migrating intertidal bar at Skallingen. Measurements were generally about an order of magnitude smaller than transport then obtained at a single location on the seaward slope of the rates in the inner surf and swash zones (Figure 13), mainly intertidal bar under more energetic conditions than encoun- because suspended sediment concentrations in the water col- tered in the present experiment. It was concluded that the umn are small under such conditions (AAGAARD,BLACK, and landward migration of that intertidal bar was mainly due to GREENWOOD, 2002). This observation is of importance to the an onshore-directed sediment transport driven by the mean question whether there is any fundamental difference be- current, the direction of which depended on the presence or tween the mobile intertidal bars (slip-face ridge type) studied absence of a runnel landward of the bar. In the study re- ported here, a much denser array of sensors was used, veloc- here and the low-amplitude quasi-static ridge-and-runnel ity and sediment transport were measured very close to the type of bars, which mainly occur in meso-macrotidal settings bed, and similar conclusions on the mean current circulation and with low-energy wave conditions (KING and WILLIAMS, were reached: Onshore-directed currents persisted on the bar 1949; MULRENNAN, 1992; ORFORD and WRIGHT, 1978). The crest until the runnel closed, subsequent to which the under- present measurements suggest that inner surf/swash zone tow extended landward of the bar crest. Similar onshore-di- conditions are required in order to generate large suspended rected mean currents have been observed in three-dimen- sediment concentrations and transport rates. In settings with sional bar settings by DRøNEN et al. (1999) and KROON and a large tidal range and/or low waves, such conditions will DE BOER (2001). generally last only a small fraction of each tidal cycle at a In this experiment, however, the onshore directed mean specific bar. This could be the reason why ridge and runnel currents at the bar crest did not appear critically important morphology is not very mobile and do not develop a form to the onshore migration of the bar and the current speed asymmetry through landward migration. was smaller than in the example reported by AAGAARD et al. If this interpretation is correct, then it is likely that many (1998a), the reason probably being the lower wave-energy intertidal bars or intertidal bar sequences may oscillate be- levels. Here, onshore sediment transport did prevail across tween one type and the other, for example, through spring- the upper seaward slope and crest of the bar, but it was main- neap tidal cycles or as incident wave energy varies tempo- ly caused by oscillatory wave motions under swash and inner rally on a seasonal cycle. It is therefore questionable whether surf-zone conditions (Figures 11 and 13). At the upper sea- a distinction should be made between slip-face ridges and ward slope of the bar, onshore sediment-transport rates oc- low-amplitude ridges (ridge-and-runnels). It would seem curred when mean water depths were less than approxi- more prudent to use the term intertidal bar for both bar mately 0.5 m or relative wave heights Ͼ ഠ0.7 (Figure 12). types. The term swash bar would also appear inappropriate This trend was not entirely consistent at all stations; for ex- as both swash and surf-zone processes are critical to the be- ample, large offshore transport rates developed at station S4 havior of the mobile intertidal bars.

Journal of Coastal Research, Vol. 22, No. 2, 2006 Intertidal Bars 259

CONCLUSIONS DAWSON, J.C.; DAVIDSON-ARNOTT, R.G.D., and OLLERHEAD, J., 2002. Low-energy morphodynamics of a ridge and runnel system. One of the main mechanisms for shoreline progradation is Journal of Coastal Research, Special Issue No. 36, pp. 198–215. the migration of bars across the intertidal zone and their de- DRøNEN, N.; KARUNARATHNA, H.; FREDSøE, J.; SUMER, B.M., and position on the beach face. This study has provided hydro- DEIGAARD, R., 1999. The circulation over a longshore bar with rip channels. Proceedings Coastal Sediments ’99, Long , ASCE, dynamic and suspended sediment-transport measurements pp. 576–587. during a beach accretion event of this type. Onshore bar mi- GARCEZ-FARIA, A.F.; THORNTON, E.B.; LIPPMANN, T.C., and STAN- gration was achieved mainly by swash processes and by the TON, T.P., 2000. Undertow over a barred beach, Journal of - oscillatory flows of both short and long waves under surf-zone physical Research, 105, 16999–17010. conditions. The transport competency of the onshore stroke GREENWOOD, B.; AAGAARD, T., and NIELSEN, J., 2004. Swash bar morphodynamics in the Danish : sand bed oscillations of the waves was considerably larger than the competency of and suspended sediment flux during an accretionary phase of the the offshore stroke because water transported over the bar foreshore cycle. Danish Journal of Geography, 104(1), 15–30. crest was subsequently channeled alongshore in the runnel. GREENWOOD, B. and DAVIDSON-ARNOTT, R.G.D., 1979. Sedimenta- This transport asymmetry and the large sediment fluxes nat- tion and equilibrium in wave-formed bars: a review and case- study. Canadian Journal of Earth Sciences, 16, 312–332. urally associated with surf and swash in very shallow water HAAS, K.A.; SVENDSEN, I.A.; HALLER, M.C., and ZHAO, Q., 2003. (Ͻ ഠ0.5 m), resulted in a relatively rapid bar-migration rate Quasi-three-dimensional modeling of systems. Journal corresponding to 10–20 m/d. Both the existence of the inter- of Geophysical Research, 108, C7, 10.1–10.21. tidal bar and its disappearance once it infilled the runnel HOUSER, C. and GREENWOOD, B., 2003. Response of a swash bar to produced a strong feedback effect on the hydrodynamics and a sequence of storm events. Proceedings Coastal Sediments ’03, Clearwater, Florida, USA. CD-ROM published by East Meets sediment dynamics. The presence of the runnel was associ- West Productions, 13 p. ated with horizontal cell circulation in the intertidal zone, HUGHES, M.G and BALDOCK, T.E., 2004. Eulerian flow velocities in characterized by onshore-directed mean flows across the bar the swash zone: field data and model predictions. Journal of Geo- crest, which augmented the sediment transport due to wave physical Research, 109, C08009, DOI:10.1029/2003JC002213. motions. When the runnel was infilled, an offshore-directed KING, C.A.M. and WILLIAMS, W.W., 1949. The formation and move- ment of sand bars by wave action. Geographical Journal, 113, 68– undertow developed which opposed the wave-induced sedi- 85. ment transport. KROON, A. and DE BOER, A., 2001. Horizontal flow circulation on a mixed energy beach. Proceedings Coastal Dynamics ’01, Lund, ACKNOWLEDGMENTS ASCE, pp. 548–557. KROON, A. and MASSELINK, G., 2002. Morphodynamics of intertidal We are grateful to Per Sørensen (the Danish Coastal Au- bar morphology on a macrotidal beach under low-energy wave con- thority) and Erik Brenneche (Esbjerg Port Authorities) for ditions, North Lincolnshire, England. Marine Geology, 190, 591– giving us access to offshore wave and tidal data, respectively. 608. Ulf Thomas and Niels Vinther helped out in the field—under MASSELINK, G. and SHORT, A.D., 1993. The effect of tide range on beach morphodynamics: a conceptual model. Journal of Coastal sunny conditions this time! This research was funded by the Research, 9, 785–800. Danish Technical Sciences Research Council (grant MULRENNAN, M.E., 1992. Ridge and runnel beach morphodynamics: 99012287) and by the European Union through the Coast- an example from the central east coast of Ireland. Journal of View Project (contract EVK3-CT-2001-0054). Coastal Research, 8, 906–918. ORFORD, J.D. and WRIGHT, P., 1978. What’s in a name—descriptive or genetic implications of ‘‘ridge and runnel’’ topography. Marine LITERATURE CITED Geology, 28, M1-M8. OSBORNE, P.D. and ROOKER, G.A., 1999. Sand re-suspension events AAGAARD, T.; BLACK, K.P., and GREENWOOD, B., 2002. Cross-shore suspended sediment transport in the surf zone: a field-based pa- in a high energy infragravity swash zone. Journal of Coastal Re- rameterization. Marine Geology, 185, 283–302. search, 15, 74–86. AUBENHEIMER AAGAARD, T. and GREENWOOD, B., 1994. Suspended sediment trans- R , B., 2002. Observations and predictions of fluid ve- port and the role of infragravity waves in a barred surf zone. Ma- locities in the surf and swash zones. Journal of Geophysical Re- rine Geology, 118, 23–48. search, 107, C11, 11.1–11.7 AAGAARD, T.; NIELSEN, J., and GREENWOOD, B., 1998b. Suspended SOULSBY, R.L., 1999. Coastal sediment transport: the COAST3D sediment transport and nearshore bar formation on a shallow in- project. Proceedings ICCE98, Copenhagen, ASCE, pp. 2548–2558. termediate-state beach. Marine Geology, 148, 203–225. STEPANIAN, A.; VLASWINKEL, B.; LEVOY, F., and LARSONNEUR, C., AAGAARD, T.; NIELSEN, J.; NIELSEN, N., and GREENWOOD, B., 2001. Sediment transport on a macrotidal ridge and runnel beach 1998a. Suspended sediment transport and morphological evolu- during accretionary conditions. Proceedings Coastal Dynamics ’01, tion on an intertidal beach. Proceedings Coastal Dynamics ’97, Lund, ASCE, pp. 1017–1027. Plymouth, ASCE, pp. 824–833. VOULGARIS, G.; SIMMONDS, D.; MICHEL, D.; HOWA, H.; COLLINS, BUTT, T. and RUSSELL, P., 1999. Suspended sediment transport M.B., and HUNTLEY, D.A., 1998. Measuring and modelling sedi- mechanisms in high-energy swash. Marine Geology, 161, 361–375. ment transport on a macrotidal ridge and runnel beach: an inter- CHURCH, J.C. and THORNTON, E.B., 1993. Effects of comparison. Journal of Coastal Research, 14, 315–330. induced turbulence within a longshore current model. Coastal En- WIJNBERG, K.M. and KROON, A., 2002. Barred beaches. Geomor- gineering, 20, 1–28. phology, 48, 103–120.

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