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

Abstract

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.

Sorting in grain flows at the lee side of dunes

“…The larger the delta height, the larger the sediment wedge and therefore the thicker the laminae. This was confirmed with field data of wind dunes by Nickling et al (2002). A hypothetical explanation is that the flow velocity over higher dunes is larger, leading to higher velocities of the grains downstream of the brink point and therefore a longer and larger sediment wedge on the upper foreset slope, leading to larger grain flows.…”

“…Termes (1986), Love et al (1987), and Shaw and Gorrell (1991). Turbulence in the wake of wind dunes was also found to affect the paths of falling grains (Nickling et al, 2002).…”

“…4) is much larger in water than in air. So in subaqueous dunes, the cross-stratification generally consists of grain flow deposits only, whereas in air the cross-stratification may consist of couplets of grain fall and grain flow deposits (Hunter, 1985b;Nickling et al, 2002). Allen (1963) forced a certain combination of dune height and flow velocity in his experiments, and it could be that his conditions were such that both grain fall and flow deposits were preserved in his deposit, leading to the disappearance of the net fining upward sorting.…”

“…Thus fine-grained sediments, which are much easier suspended, will have a much stronger grain fall deposition all over the lee slope (Fig. 4a), especially in wind-blown deposits (Nickling et al, 2002), whereas coarse-grained sediments have little or no grain fall deposition on most of their lee slope, resulting in a grain flow dominated deposit (Fig. 4b), or, for the coarsest sediments, rolling grain deposits.…”

Desert Dune Processes and Dynamics

“…The rate of deposition by grainfall decreases exponentially with distance from the brink of the lee face (Figure 19.13), so that most of the sediment is deposited within 1 m of the brink (Nickling, McKenna Neuman and Lancaster, 2002). Grain flows occur when grainfall deposits build up so that the lee slope is steepened above the angle of repose and fails, initiating a grain flow or avalanche.…”