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The effect of unsteady winds on sediment transport on the stoss slope of a transverse dune, Silver Peak, NV, USA (2000)

Neuman, C. Mckenna ; Lancaster, N. ; Nickling, W. G.

Oxford UK : Blackwell Publishing Ltd

Sedimentology 47 (2000), S. 0

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ISSN: 1365-3091

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Rapid (10 s) measurements of sediment transport and wind speed on the stoss slope of a transverse dune indicate that the majority of sand transported is associated with fluctuations in wind speed with a periodicity of 5–20 min duration. Increases in the sediment transport rate towards the dune crest are associated with a small degree of flow acceleration. The increase in wind speed is sufficient, however, to greatly increase values of the intermittency index (γ), so that the duration of saltation is extended in crestal regions of the dune. The pattern of sediment transport on the stoss slope and, therefore, the locus of areas of erosion and deposition change with the regional wind speed. Erosion of the crest occurs during wind speed events just above transport threshold, whereas periods of higher magnitude winds result in deposition of sand upwind of the crest, thereby increasing dune height. Although short-term temporal and spatial relations between sand transport and wind speed on the stoss slope are well understood, it is not clear how these relations affect dune morphology over longer periods of time.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-3091.2000.00289.x

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2

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Grainfall processes in the lee of transverse dunes, Silver Peak, Nevada (2002)

Nickling, W. G. ; Neuman, C. Mckenna ; Lancaster, N.

Oxford UK : Blackwell Science Ltd

Sedimentology 49 (2002), S. 0

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ISSN: 1365-3091

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Grainfall deposition and associated grainflows in the lee of aeolian dunes are important in that they are preserved as cross-beds in the geological record and provide a key to the interpretation of the aeolian rock record. Despite their recognized importance, there have been very few field, laboratory or numerical simulation studies of leeside depositional processes on aeolian dunes. As part of an ongoing study, the relationships among grainfall, wind (speed and direction), stoss sand transport rates and dune morphometry (height and aspect ratio) were investigated on four relatively small, straight-crested transverse dunes at Silver Peak, Nevada. Between 55% and 95% of the total grainfall was found to be deposited within 1 m of the crest, and 84–99% within 2 m, depending primarily on dune size and shape. Grainfall decay rates on high dunes of large aspect ratio were observed to be very consistent, with a weak positive dependence on wind speed. For small dunes with low aspect ratios, grainfall deposition was more varied and decreased rapidly within 1 m of the dune crest, whereas at increased distance from the dune crest, it eventually approached the smaller decay rates observed on the large dunes. No dependence of grainfall on wind speed was observed for these small dunes. Comparison of field data with predictions from Anderson's (1988) saltation model of grainfall, based on the computation of saltation path lengths, indicates lack of agreement in the following areas: (1) deposition rate magnitude; (2) variation in decay rate with wind speed; and (3) the magnitude and location of the localized lee-slope depositional maxima. The Silver Peak field results demonstrate the importance of dune aspect ratio and related wake effects in determining the rate and pattern of grainfall. This work confirms earlier speculation by McDonald & Anderson (1995) that temporary, turbulent suspension (or `modified saltation') of relatively large grains does occur within the dune wake, so that transport distances generally are larger than predicted by numerical simulations of `true' saltation.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-3091.2002.00443.x

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3

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Relations between dune morphology, air flow, and sediment flux on reversing dunes, Silver Peak, Nevada (1997)

NEUMAN, C. McKENNA ; LANCASTER, N. ; NICKLING, W. G.

Oxford, UK : Blackwell Publishing Ltd

Sedimentology 44 (1997), S. 0

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ISSN: 1365-3091

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Relations between wind speed, sediment flux and dune morphology were measured for two reversing dunes situated in the south-western part of the Silver Peak dunefield in Clayton Valley, west-central Nevada. The larger dune was 120 m in length with a height of 12.5 m and the smaller dune 80 m long and 6 m high. Both dunes were sharp crested, aligned approximately E-W perpendicular to the dominant wind direction, and had slightly concave stoss profiles. Twenty-seven rotating cup anemometers were placed (0.3 m elevation) along N-S transects on each of the dunes. At each anemometer site a passive wedge-shaped sediment trap was used to measure sediment flux. Amplification of wind speed was observed towards the crest on the stoss side of both dunes with speed-up factors (ucrest/Ubase) ranging from 1·50–3·19, with a corresponding increase in sediment flux by 1–2 orders of magnitude. In general, the ratio of crest flux to base flux (qc/qt,) increased with increasing incident basal wind speed on both dunes. Direct measurements of the stoss slope variation in sediment flux relative to the dune crest are in good agreement with Owen's transport model. Friction speed (u) was approximated from near surface (0·3 m) point wind speed. Although not all assumptions of the Owen model are upheld, the modified model performance is sufficiently robust to predict short-term variation in stoss sediment flux on the study dunes.Improved models that adequately account for variation in sediment flux under changing air flow and transport conditions are necessary for the prediction of longterm evolution of dunes. In this regard, further progress in model development will require increased understanding of the spatial and temporal variability of airflow and the short term response of sediment flux to these flow conditions.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-3091.1997.d01-61.x

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4

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Development of deflation lag surfaces (1995)

NICKLING, W. G. ; NEUMAN, C. McKENNA

Oxford, UK : Blackwell Publishing Ltd

Sedimentology 42 (1995), S. 0

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ISSN: 1365-3091

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: A series of wind tunnel tests were carried out to investigate the development of deflation lags in relation to the non-erodible roughness element concentration. Glass spheres (18 mm in diameter) were placed along the complete length of the wind tunnel working section in regular staggered arrays using three different spacings (d=18, 30 and 60 mm) and completely covered with a 0.27-mm erodible sand. A pre-selected free stream velocity above threshold (8m s−1) was established above the surface and the sediment transport measured at 2-s intervals using a wedge-shaped trap in which an electronic balance is incorporated. Throughout each test, the emerging lag surface was periodically photographed from above at two locations upwind of the trap. The photographs were electronically scanned and analysed to calculate the lag element coverage and location, as well as mean height and frontal area for each time period. Test results indicate that lag development has a profound effect on both the sediment flux and wind profile characteristics. Initially, there is an increase in sediment flux above that for a rippled sand bed because of increased erosion around and reduced kinetic energy loss in highly elastic collisions with the emerging roughness elements. With further emergence, a dynamic threshold is reached whereupon the sediment flux decreases rapidly, tending towards zero. At this point, the supply of grains to the air stream through fluid drag follows a reduction in aerodynamic roughness and, therefore, surface shearing stress. At least as important is the lesser potential for grain ejection at impact because of reduced momentum imparted from the air stream during saltation. Although recent shear stress partitioning models indicate when particle movement may commence on varying surfaces, our experimental results demonstrate that this partitioning has a further direct bearing upon the saltation flux ratio.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-3091.1995.tb00381.x

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5

Electronic Resource

Momentum extraction with saltation: Implications for experimental evaluation of wind profile parameters (1994)

Neuman, C. McKenna ; Nickling, W. G.

Springer

Boundary layer meteorology 68 (1994), S. 35-50

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract In wind tunnel studies of aeolian transport, the number and position of pitot tubes are decided by the researcher, so that there are important variations in the computation ofU * between studies. Velocity measurements seldom are made very close to mobile surfaces because the tubes become blocked by drifting sand grains. This practice is fortuitous as demonstrated by recent selfregulatory models of saltation which indicate that fluid and grain-borne shear stress vary substantially within the lowest 0.01 m and application of the logarithmic law is therefore unsound. This study reports detailed velocity measurements which further suggest that no single logarithmic expression, based on fixed values of κ and τ, adequately represents the full wind profile which includes the inner saltation cloud above 0.01 m and the outer grain-free region of the boundary layer. A much improved fit over the logarithmic wind profile model is achieved with a square root relation, although there is no known physical basis for this specific form of power model. Relatively shallow boundary-layer development in wind tunnels forces the velocity gradient above the region of momentum extraction to attain exceptionally large values, uncommon in natural settings.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00712663

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