Aeolian particle flux profiles and transport unsteadiness

Bauer, Bernard O.; Davidson‐Arnott, Robin

doi:10.1002/2014jf003128

Cited by 52 publications

(64 citation statements)

References 78 publications

(205 reference statements)

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“…A large AP value (close to a maximum of 1) indicates essentially continuous transport in time whereas a small AP value (close to the minimum of 0) indicates only occasional transport interspersed with dominant periods of transport quiescence(Bauer and Davidson-Arnott, 2014). The spatial pattern in AP was similar for the entire 3-hour measurement period, with larger AP values (up to 0.5) at the dune crest and on the back-beach.…”

mentioning

confidence: 72%

“…Cross-multiplication of the q T and α time series was performed on the instantaneous (1 Hz) data without time lags since the instruments were closely spaced. Invoking time lag schemes of up to several seconds has minimal consequences for the gross patterns of transport because the phase relationship between peak transport events and wind direction is relatively stable during sustained gusts when transport activity is most pronounced Bauer and Davidson-Arnott, 2014). As a consequence, slight phase lags may yield slight differences in sediment flux but not in the overall cross-shore transport direction.…”

Section: Cross-shore Sediment Particle Fluxmentioning

confidence: 95%

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Sediment transport (dis)continuity across a beach–dune profile during an offshore wind event

et al. 2015

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Flow dynamics and sediment transport responses over a large, vegetated foredune at Prince Edward Island, Canada, during an offshore wind event are examined. Data were collected along an instrumented transect that extended from the dune crest, down the lee-side (seaward) slope of the dune, across a wave-cut scarp, and on to the back-beach. When the wind direction at the dune crest was approximately crest-normal (less than about 15 o deviation), mean near-surface flow directions along the dune slope and on the back beach were generally onshore, indicating reversed (onshore) flow relative to the regional (offshore) wind direction. Although flow patterns were consistent with a lee-side recirculation eddy, large excursions in flow direction were also prevalent, suggesting that the eddy was unstable and alternated with highly turbulent wake flow. As wind direction at the crest veered to greater than 20 o from crest-normal, lee-side winds shifted toward strongly alongshore flow with minimal directionally variability. On the dune slope, the wind vectors were slightly offshore whereas on the back-beach they were slightly onshore.Wind speeds and sediment transport were greatest at the foredune crest and declined rapidly downslope due to flow expansion and deceleration in the wake zone as well as to the influence of a sparse vegetation layer. Mean particle counts (averaged over a 15-minute interval) derived from laser sensors positioned at the crest were large (7.76 per second) in comparison to those measured in the immediate lee of the crest (0.52 per second) and farther down the dune slope (< 0.13 per second).In contrast, the values were as large as 25.62 per second on the middle of the back-beach, declining rapidly to a value of only 0.24 per second at the dune toe. Transport intensity was highly variable with the largest Activity Parameter (AP = 0.5) values at the dune crest and on the back-beach, and with the smallest values (AP < 0.1) on the lee-side dune slope down to the top of the scarp. Calculations of sediment (particle) flux divergence between instrument stations show that deposition was significant A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT2 immediately downwind of the dune crest but negligible across most of the lower dune slope.Deposition was also prevalent on the dune ramp below the scarp.These results demonstrate that sediment transport across the beach-dune system was spatially discontinuous during this offshore wind event. Rebuilding of the dune ramp at the toe of the scarp occurred quite independently of, and with a different sediment source than, the broadening of the dune crest, which was fed with sediment from the landward side of the foredune. Such process 'decoupling'is an example of the complexity by which foredunes evolve or are maintained, and as with previous research reinforces the importance of offshore and alongshore wind events to beach-dune morphodynamics.

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confidence: 72%

Section: Cross-shore Sediment Particle Fluxmentioning

confidence: 95%

Sediment transport (dis)continuity across a beach–dune profile during an offshore wind event

et al. 2015

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“…During each field campaign, multiple (three to nine) Wenglor optical sensors (Barchyn, Hugenholtz, et al, ) at heights from the bed surface up to ≈0.3 m counted saltating particles (at 25 Hz), which we combine into vertically integrated saltation particle count rates N (Figure a). Wenglors were mounted on one or two fixed vertical arrays (Martin et al, ), with a maximum spanwise separation among sensors of 1.3 m. Though numbers and heights of Wenglors, and thus corresponding measurements of total saltation number counts (Bauer & Davidson‐Arnott, ; Hilton et al, ), varied among field sites and deployment days (Martin et al, ), we are concerned here only with the frequency of occurrence of saltation. Based on the observed constant vertical shape of saltation profiles (Martin & Kok, ) and the fact that Wenglor sensor saturation (e.g., Hugenholtz & Barchyn, ; Sherman et al, ) appears to be absent (Martin et al, ), we assume that differences in Wenglor heights among deployments affect only the possibility of false negatives (i.e., potential under‐detection of saltation frequency), and we correct for this by treating particle arrivals as a Poisson process (see Appendix A).…”

Section: Methodsmentioning

confidence: 99%

Distinct Thresholds for the Initiation and Cessation of Aeolian Saltation From Field Measurements

2018

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Wind‐blown sand and dust models depend sensitively on the threshold wind stress. However, laboratory and numerical experiments suggest the coexistence of distinct fluid and impact thresholds for the initiation and cessation of aeolian saltation, respectively. Because aeolian transport models typically use only a single threshold, existence of separate higher fluid and lower impact thresholds complicates the prediction of wind‐driven transport. Here we extend the statistical Time Frequency Equivalence Method to derive the first field‐based estimates of distinct fluid and impact thresholds from high‐frequency wind and saltation measurements at three field sites. Our measurements show that when saltation is mostly inactive, its instantaneous occurrence is governed primarily by wind exceedance of the fluid threshold. As saltation activity increases, so too does the relative importance of the impact threshold, until it dominates under near‐continuous transport conditions. Although both thresholds are thus important for high‐frequency saltation prediction, our results suggest that the time‐averaged saltation flux is primarily governed by the impact threshold.

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“…However, such LF studies are unable to resolve the HF spatial and temporal variability in saltation flux (i.e., ≤~1 minute) resulting from wind turbulence in the atmospheric boundary layer (e.g., Baas and Sherman, 2005). Furthermore, estimates of total (vertically-integrated) saltation flux typically depend on fitting curves to vertical profiles of saltation flux, but there remains disagreement over fitting protocols (Ellis et al, 2009a) and over the proper functional form for these fits: exponential (Dong et al, 2012;Fryrear and Saleh, 1993;Namikas, 2003), power law (Zobeck and Fryrear, 1986), or some hybrid of these (Bauer and Davidson-Arnott, 2014;Dong et al, 2011). Such variability and fitting uncertainty is thought to produce much of the disagreement between measurements and models of aeolian saltation flux (Barchyn et al, 2014b).…”

Section: Introductionmentioning

confidence: 99%

“…Therefore, it is necessary to calculate vertical profile fits in order to estimate the total saltation flux. In addition to issues of deciding the best form for such profile fits (described above), turbulent variability and counting uncertainties may hinder the convergence of vertical flux profiles over short time scales (e.g., Bauer and Davidson-Arnott, 2014). Thus, existing studies of high-frequency saltation flux variability are limited to examination of relative or height-specific saltation fluxes (e.g., Baas, 2008).…”

Section: Introductionmentioning

confidence: 99%

High-frequency measurements of aeolian saltation flux: Field-based methodology and applications

et al. 2018

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Aeolian transport of sand and dust is driven by turbulent winds that fluctuate over a broad range of temporal and spatial scales. However, commonly used aeolian transport models do not explicitly account for such fluctuations, likely contributing to substantial discrepancies between models and measurements. Underlying this problem is the absence of accurate sand flux measurements at the short time scales at which wind speed fluctuates. Here, we draw on extensive field measurements of aeolian saltation to develop a methodology for generating highfrequency (25 Hz) time series of total (vertically-integrated) saltation flux, namely by calibrating high-frequency (HF) particle counts to low-frequency (LF) flux measurements. The methodology follows four steps: (1) fit exponential curves to vertical profiles of saltation flux from LF saltation traps, (2) determine empirical calibration factors through comparison of LF exponential fits to HF number counts over concurrent time intervals, (3) apply these calibration factors to subsamples of the saltation count time series to obtain HF height-specific saltation fluxes, and (4) aggregate the calibrated HF height-specific saltation fluxes into estimates of total saltation fluxes. When coupled to high-frequency measurements of wind velocity, this methodology offers new opportunities for understanding how aeolian saltation dynamics respond to variability in driving winds over time scales from tens of milliseconds to days.

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Aeolian particle flux profiles and transport unsteadiness

Cited by 52 publications

References 78 publications

Sediment transport (dis)continuity across a beach–dune profile during an offshore wind event

Sediment transport (dis)continuity across a beach–dune profile during an offshore wind event

Distinct Thresholds for the Initiation and Cessation of Aeolian Saltation From Field Measurements

High-frequency measurements of aeolian saltation flux: Field-based methodology and applications

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