Barchyn, Thomas

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    Aeolian dune field geomorphology modulates the stabilization rate imposed by climate
    (Blackwell Publishing Ltd., 2012-06-14) Barchyn, Thomas E.; Hugenholtz, Chris H.
    The activity of inland aeolian dune fields is typically related to the external forcing imposed by climate: active (bare) dunes are associated with windy and/or arid settings, and inactive (vegetated) dunes are associated with humid and/or calm environments. When a climate shifts the dune field reacts; however, the behavior, rate, and potential impact of diverse dune geomorphologies on these transitions are poorly understood. Here, we use a numerical model to systematically investigate the influence of dune field geomorphology (dune height, organization and collisions) on the time a dune field takes to stabilize. To generate diverse initial un-vegetated dune field geomorphologies under unidirectional winds, we varied pre-stabilization growth time and initial sediment thickness (termed equivalent sediment thickness: EST). Following dune field development from a flat bed, we introduced vegetation (simulating a climate shift) and transport-vegetation feedbacks slowly stabilized the dune fields. Qualitatively, very young and immature dune fields stabilized quickly, whereas older dune fields took longer. Dune fields with greater EST stabilized quicker than those with less EST. Larger dunes stabilized quicker because of low celerity, which facilitated higher vegetation growth rates. Extended stabilization times were associated with the extension of parabolic dunes. Dune-dune collisions resulted in premature stabilization; the frequency of collisions was related to dune spacing. Quantitatively comparing the distribution of deposition rates in a dune field to the deposition tolerance of vegetation provides a promising predictor of relative stabilization time. Dune fields with deposition rates dominantly above the deposition tolerance of vegetation advanced unimpeded and prolonged stabilization as parabolic dunes. Paleoenvironmental reconstructions or predictions of dune field activity should not assume that dune activity directly translates to climate, considerable lags to stabilizing climate shifts may exist in unidirectional dune forms.
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    Geomorphological mapping with a small unmanned aircraft system (sUAS): feature detection and accuracy assessment of a photogrammetrically-derived digital terrain model
    (Elsevier, 2013) Hugenholtz, Chris H.; Whitehead, Ken; Brown, Owen W.; Barchyn, Thomas E.; Moorman, Brian J.; LeClair, Adam; Riddell, Kevin D. A.; Hamilton, Tayler K.
    Small unmanned aircraft systems (sUAS) are a relatively new type of aerial platform for acquiring high-resolution remote sensing measurements of Earth surface processes and landforms. However, despite growing application there has been little quantitative assessment of sUAS performance. Here we present results from a field experiment designed to evaluate the accuracy of a photogrammetrically-derived digital terrain model (DTM) developed from imagery acquired with a low-cost digital camera onboard an sUAS. We also show the utility of the highresolution (0.1 m) sUAS imagery for resolving small-scale biogeomorphic features. The experiment was conducted in an area with active and stabilized aeolian landforms in the southern Canadian Prairies. Images were acquired with a Hawkeye RQ-84Z Aerohawk fixed-wing sUAS. A total of 280 images were acquired along 14 flight lines, covering an area of 1.95 km2. The survey was completed in 4.5 hours, including GPS surveying, sUAS setup and flight time. Standard image processing and photogrammetric techniques were used to produce a 1 m resolution DTM and a 0.1 m resolution orthorectified image mosaic. The latter revealed previously un-mapped bioturbation features. The vertical accuracy of the DTM was evaluated with 99 Real-Time Kinematic GPS points, while 20 of these points were used to quantify horizontal accuracy. The horizontal root mean squared error (RMSE) of the orthoimage was 0.18 m, while the vertical RMSE of the DTM was 0.29 m, which is equivalent to the RMSE of a bare earth LiDAR DTM for the same site. The combined error from both datasets was used to define a threshold of the minimum elevation difference that could be reliably attributed to erosion or deposition in the seven years separating the sUAS and LiDAR datasets. Overall, our results suggest that sUAS-acquired imagery may provide a low-cost, rapid, and flexible alternative to airborne LiDAR for geomorphological mapping.
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    Laboratory and field performance of a laser particle counter for measuring aeolian sand transport
    (Blackwell Publishing Ltd., 2011-02-23) Hugenholtz, Chris H.; Barchyn, Thomas E.
    This paper reports the results of laboratory and field tests that evaluate the performance of a new laser particle counter for measuring aeolian sand transport. The Wenglor® model YH03PCT8 (Wenglor) consists of a laser (655 nm), photo sensor, and switching circuit. When a particle passes through the 0.6 mm diameter, 30 mm long laser beam, the sensor outputs a digital signal. Laboratory tests with medium sand and a vertical gravity flume show that the Wenglor count rate scales approximately linearly with mass flux up to the saturation point of the sensor, after which the count rate decreases despite increasing mass flux. Saturation depends on the diameter and concentration of particles in the airstream and may occur during extreme events in the field. Below saturation sensor performance is relatively consistent; the mean difference between average count rate response was between 50 and 100 counts. Field tests provide a complimentary frame of reference for evaluating the performance of the Wenglor under varying environmental conditions and to gauge its performance with respect to a collocated piezoelectric impact sensor (Sensit H11-B). During 136.5 h of deployment on an active sand dune the relative proportion of time sand transport recorded by two Wenglors was 0.09% and 0.79%, compared to 4.68% by the Sensit H11-B. The weak performance of the Wenglors is attributed to persistent lens contamination from adhesion of sand grains on the sensors after rainfall. However, during dry and windy conditions the Wenglor performance improved substantially; sensors measured a concentration of sand particles in the airstream more than seven times greater than that measured by the Sensit. Between the two Wenglors, the mean absolute count rate difference was 6.16 counts per second, with a standard deviation of 8.53 counts per second. For short-term measurement campaigns in dry conditions, therefore, the Wenglor is relatively consistent and can outperform the Sensit in detecting particles in the airstream. The Sensit, however, is more reliable in detecting particle transport during longer unattended deployments. Two additional field tests show that the sensor is well-suited to the measurement of snow drifting but could be ineffective in dusty settings because of lens contamination. Overall, the main advantages of the Wenglor include (1) insensitivity to particle momentum; (2) low measurement variability; (3) low cost ($210 USD); and perhaps most important of all, (4) a consistent design that will improve comparison of results between investigations. At present, no other particle detector used in aeolian research can claim all these characteristics.
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    Predicting vegetation-stabilized dune field morphology
    (Blackwell Publishing Ltd., 2012-09-12) Barchyn, Thomas E.; Hugenholtz, Chris H.;
    The activity of inland aeolian dune fields is typically related to the external forcing imposed by climate: active (bare) dunes are associated with windy and/or arid settings, and inactive (vegetated) dunes are associated with humid and/or calm environments. When a climate shifts the dune field reacts; however, the behavior, rate, and potential impact of diverse dune geomorphologies on these transitions are poorly understood. Here, we use a numerical model to systematically investigate the influence of dune field geomorphology (dune height, organization and collisions) on the time a dune field takes to stabilize. To generate diverse initial un-vegetated dune field geomorphologies under unidirectional winds, we varied pre-stabilization growth time and initial sediment thickness (termed equivalent sediment thickness: EST). Following dune field development from a flat bed, we introduced vegetation (simulating a climate shift) and transport-vegetation feedbacks slowly stabilized the dune fields. Qualitatively, very young and immature dune fields stabilized quickly, whereas older dune fields took longer. Dune fields with greater EST stabilized quicker than those with less EST. Larger dunes stabilized quicker because of low celerity, which facilitated higher vegetation growth rates. Extended stabilization times were associated with the extension of parabolic dunes. Dune-dune collisions resulted in premature stabilization; the frequency of collisions was related to dune spacing. Quantitatively comparing the distribution of deposition rates in a dune field to the deposition tolerance of vegetation provides a promising predictor of relative stabilization time. Dune fields with deposition rates dominantly above the deposition tolerance of vegetation advanced unimpeded and prolonged stabilization as parabolic dunes. Paleoenvironmental reconstructions or predictions of dune field activity should not assume that dune activity directly translates to climate, considerable lags to stabilizing climate shifts may exist in unidirectional dune forms.
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    Real barchan dune collisions and ejections
    (Blackwell Publishing Ltd., 2012-01-26) Hugenholtz, Chris H.; Barchyn, Thomas E.
    From high-resolution satellite imagery of barchan sand dunes, we provide geomorphological evidence of collisions that result in the ejection of a barchan from the wake of another barchan dune. Previous interpretations suggest this outcome is evidence of soliton or solitary wave behaviour; however, the physical mechanisms for mass exchange are not fully understood, resulting in debate. Our evidence and interpretation indicates that mass is transferred to the upwind barchan by shadowing a portion of downwind barchan's stoss slope. Turbulent, unsaturated airflow erodes the surface between the dunes, creating a smaller dune that ejects from the wake region. Previous observations lacked the spatial resolution required to document this process; therefore, our observations clarify the collision dynamics of barchans. A broader implication of our observations is the role of collisions in maintaining an equilibrium size distribution in barchan swarms.
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    Successes of soil conservation in the Canadian Prairies highlighted by a historical decline in blowing dust
    (Institute of Physics Publishing, 2012-01-18) Fox, Thomas A.; Barchyn, Thomas E.; Hugenholtz, Chris H.
    Blowing dust from agricultural fields has serious health and economic effects, which can be mitigated by soil conservation techniques. However, it is difficult to isolate improved land management in downstream records of airborne dust. In this letter we present multi-decadal (1961–2006) records of airborne dust frequency from seven weather stations across the Canadian Prairies. We related temporal changes in dust frequency to the climatic wind erosion potential and agricultural census data. We identified a statistically significant regime shift in the region-wide dust time series at 1990, with a substantial reduction in dust frequency thereafter. The correspondence between dust frequency and the climatic wind erosion potential improved from 1961–90 (r2 = 0.154, p < 0.001) to 1991–2006 (r2 = 0.429, p < 0.001). We interpret this as indicating that the climate signal was obscured by poor soil conservation practices in 1961–90, leading to dustier conditions. Post 1990, improved land management reduced the impact of land-use practices; only the most severe climate forcings resulted in detectable dust. The dramatic reduction of dust from 1990 onward appears to represent a region-wide threshold crossing, where the effects of soil conservation efforts began to materialize. Overall, the results suggest that soil conservation initiatives have had an impact in reducing airborne dust on the Canadian Prairies.
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    Timescale dependence of aeolian sand flux observations under atmospheric turbulence
    (American Geophysical Union, 2013-07-26) Martin, Raleigh L.; Barchyn, Thomas E.; Hugenholtz, Chris H.; Jerolmack, Douglas J.
    The transport of sand in saltation is driven by the persistently unsteady stresses exerted by turbulent winds. Based on coupled high-frequency observations of wind velocity and sand flux on a desert dune during intermittent saltation, we show here how observations of saltation by natural winds depend significantly on the timescale and method used for determining shear stress and sand flux. The correlation between sand flux and excess shear stress (stress above a threshold value) systematically improves for longer averaging timescale, T, and is better for stress determined by the law-of-the-wall versus the Reynolds stress method. Fitting parameters for the stress-flux relationship do not converge with increasing T, which may be explained by the nonstationary nature of wind velocity statistics. We show how it may be possible, based on the scale-dependent statistics of stress fluctuations, to rescale saltation flux predictions for wind observations made at different timescales. However, our observations indicate hysteresis and time lags in thresholds for initiation and cessation of saltation, which complicate threshold-based approaches to predicting sediment transport at different timescales.