Hydrodynamics and Sediment Fluxes across an Onshore Migrating Intertidal Bar

Transcription

1 Journal of Coastal Research 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 taa@geogr.ku.dk School of Geosciences University of Sydney Sydney NSW 2006, Australia ABSTRACT AAGAARD, T.; HUGHES, M.; MØLLER-SØRENSEN, R., and ANDERSEN, S., Hydrodynamics and sediment fluxes across an onshore migrating intertidal bar. Journal of Coastal Research, 22(2), West Palm Beach (Florida), ISSN 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), WIJN- BERG 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: / ; received 3 May 2004; accepted in revision 20 September these bars are in fact generated and maintained by swash (WIJNBERG and KROON, 2002) or surf-zone processes (AA- GAARD 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 MASSE- LINK (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). VOULGARISet al. (1998) mea-

2 248 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, 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, 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 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 250 m and x 150 m relative to the survey baseline, a rather large intertidal bar centered at x 90 m and an upper swash bar/berm at x 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 m. 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 12 m) remained below 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 4 8 seconds during the event, which was also associated with a small surge of 0.2 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

3 Intertidal Bars 249 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 0m. 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 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 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 10 mm 3 ) that is centered 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 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 ( 15 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

4 250 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). Date Processing and Analysis 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 GREEN- WOOD, 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 (HUGHES 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 concentration 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 GREEN- WOOD, 1994). RESULTS 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 ( ), 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 ( ; 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 m 3 /m. Alternating zones of erosion and deposition (or nonerosion)

5 Intertidal Bars 251 AAGAARD, and NIELSEN (2004). Moreover, the height and spacing of the present undulations ( m and 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, 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 (H s /h) remained below 0.4; 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 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 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 1.12 m DNN (Danish Ordnance Datum) and 1.03 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 m/s. The smallest current velocities were recorded around the breakpoint at station S2. These currents were probably undertows, driven by the sea-

6 252 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 150 m (cf. CHURCH and THORN- TON, 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 runnel 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 m/s, at instrument elevations of 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

7 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.

8 254 Aagaard et al. Figure 8. Water surface elevation spectra recorded at stations S1 S4 at high tide, hour 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 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 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 0.03 m were paired with the OBS sensors at z 0.035, 0.085, and m, and finally, at S3, ADV velocity measurements at z 0.03 m were paired with sediment concentrations measured at z 0.01, 0.02, 0.03, 0.04, and 0.05 m, and EM velocity measurements at z 0.2 m were paired with concentrations at z 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 (tidal cycle 2) and (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 (H s, solid lines) and relative significant wave height (H s /h, dashed lines). Onshore- (U) and northward- (V) directed mean currents are positive. The beach profiles are shown in the bottom panels for reference.

9 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 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 (H s /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 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.

10 256 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 (H s /h 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 (OSBORNE 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 r 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: kgm 2 s 1 ; S2: kgm 2 s 1 ; S3 (upper instrument array only to provide a comparison): kgm 2 s 1 ; S4: kgm 2 s 1. 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 (H s /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 and Positive transports are directed onshore. The beach profiles are shown in the lower panels for reference.

11 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 1130 kg/m (corresponding to 0.71 m 3 m 1 ), 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 q inc Q inc (1) qinc qig qmean where q inc, q ig, and q mean 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 u 3 /(u 2 ) 1.5 and acceleration skewness as A a 3 /(a 2 ) 1.5, where a du/dt (BUTT and RUSSELL, 1999). Infragravity velocity and acceleration skewnesses were computed from low-passed velocity records with a high frequency cutoff of 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-

12 258 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 S A S3, hour 173 hour 176 S4, hour 173 hour 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 0.7 (Figure 12). This trend was not entirely consistent at all stations; for example, large offshore transport rates developed at station S4 when h m and H s /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 H s /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 However, the functional relationship found here between transport rate and relative wave height is certainly not convincing (r ) 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.

13 Intertidal Bars 259 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 ( 0.5 m), resulted in a relatively rapid bar-migration rate corresponding to 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 ) and by the European Union through the Coast- View Project (contract EVK3-CT ). 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