Turbulence-driven saltation in the atmospheric surface layer

Abstract

The relations between the turbulent structures that contribute to the Reynolds shear stress and the transport of saltating grains and between the wind speed and the saltation are studied. The saltation of sand particles and the turbulent flow field over a beach surface in the Western Baltic Sea are measured with a saltiphone, a sonic anemometer and cup anemometers. Continuous, synchronous measurements of horizontal sand transport at 0.1 m above ground and turbulent velocity fluctuations at 0.55 m were made with a sampling frequency of 1 Hz and 10 Hz, respectively. The stress is considered to be resulting from various turbulent structures including sweeps, ejections, inward interactions and outward interactions using the quadrant technique. The instantaneous horizontal drag force on grains, which initiates saltation, seems to be well represented by the instantaneous horizontal wind measured well above the surface. Results indicate an almost instantaneous response of transport to instantaneous horizontal wind velocity. A new method to establish the threshold friction velocity for intermittent aeolian sand transport is introduced. The observed threshold friction velocity varied between 0.28 ms −1 to 0.36 ms −1. Zusammenfassung Der Zusammenhang zwischen den turbulenten Strukturen, die zur Reynolds-Schubspannung beitragen und dem Transport von durch Saltation bewegten Sandkörnern und der Zusammenhang zwischen der Windgeschwindigkeit und der Saltation wurden untersucht. Die Saltation von Sandkörnern und das turbulente Strömungsfeldüber einer Strandfläche der Westlichen Ostsee wurden mit einem Saltifon, einem akustischen Anemometer und mit Schalensternanemometern untersucht. Es wurden kontinuierliche, synchrone Messungen des horizontalen Sandtransports 0.,1 müber Grund und turbulente Windgeschwindigkeitsfluktuationen 0,55 müber Grund mit einer Abtastrate von 1 Hz bzw. 10 Hz durchgeführt. Die Schubspannung wurde der Quadranten-Technik folgend in die Klassen verzögerte Aufwärtsbewegung (ejection), beschleunigte Aufwärtsbewegung (outward interaction), beschleunigte Abwärtsbewegung (sweep) und verzögerte Abwärtsbewegung (inward interaction) eingeteilt. Die unmittelbaren an den Sandkörnern wirkenden horizontalen Widerstandskräfte, die zur Saltation führen, werden offenbar gut durch die augenblickliche horizontale Windgeschwindigkeit etwas weiterüber den Sandkörnern beschrieben. Die Ergebnisse zeigen fast eine augenblickliche Reaktion des Sandtransports auf die augenblickliche Horizontalwindgeschwindigkeit. Eine neue Methode zur Bestimmung der Schwellenschubspannungsgeschwindigkeit wird vorgestellt. Die beobachteten Schwellenschubspannungsgeschwindigkeiten variierten zwischen 0,28 ms −1 und 0,36 ms −1 .

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

“…This is particularly problematic in the field, where the dynamic complexity of the flow structures cannot be controlled in the same way as in modelled or wind tunnel environments [104]. This issue is pertinent given the evidence that turbulence is an important driving force behind sediment entrainment and transport in aeolian environments [105][106][107][108][109][110][111][112][113][114].…”

Sediment Mobilisation by the Wind

“…A variety of designs exist but all operate on a similar basis of responding to the individual impacts of saltating grains on a sensitive surface. In the case of the Saltiphone, this surface is a highly responsive microphone (Arens, 1996;Schönfeldt and von Löwis, 2003;Sterk, Jacobs and van Boxel, 1998;Zobeck et al, 2003), while in the case of the Sensit and Safire, the surface is a piezoelectric transducer (Gillette and Stockton, 1989;Stout and Zobeck, 1997;Baas, 2004). The advantage of such instruments over traditional sand traps is that they are small, offer little resistance to the flow and are omnidirectional in response.…”

“…While variability in surface conditions can account for some irregularity in entrainment, it is now clear that, in a similar manner to river flows (Bennett and Best, 1996;Kostaschuk and Villard, 1996;Venditti and Bauer, 2005), turbulence in the boundary layer is also a major driving force behind aeolian sediment entrainment and transport (Livingstone, Wiggs and Weaver, 2007;Weaver, 2008;Sterk, Jacobs and van Boxel, 1998;Schönfeldt and von Löwis, 2003). Indeed, it has also been shown that the influence of peak instantaneous turbulent stresses on sand transport dynamics is required to explain the observed development of mobile sand dunes (Castro and Wiggs, 1994;Wiggs, Livingstone and Warren, 1996;Walker andNickling, 2002, 2003;Weaver and Wiggs, 2010) and sand streamers (Baas, 2008).…”

“…These findings led Butterfield (1993) to conclude that with naturally fluctuating winds over a sand bed, the grain-laden boundary layer is in constant adjustment and may rarely achieve an equilibrium state. With the increasing application of sonic anemometers in the field (Walker, 2005;van Boxel, Sterk and Arens, 2004;Weaver, 2008) and the use of high-frequency grain impact sensors (Schönfeldt and von Löwis, 2003;Baas, 2004), investigations have now begun to observe the relationship between saltation dynamics and instantaneous turbulent peaks in wind velocities at much higher frequencies (1 to > 10 Hz). While the relationship between turbulent wind flow and sand transport is complex at such frequencies ( Figure 18.25) the association between the parameters has led many to question the efficacy of 'mean' measures of erosivity, such as mean wind velocity or shear velocity (u * ) to predict sediment fluxes (Stout, 1998;Baas and Sherman, 2005;Weaver, 2008).…”

Based on the earth's surface layer dust flux field observation and study on collection efficiency

“…Saltation, a mode of airborne particle motion in the atmosphere, plays a key role in aeolian sand transport studies (Schönfeldt and von Löwis, 2003). The transportation of most aeolian sediments due to wind erosion is by means of saltation Chepil, 1945).Research level of dust flux is very important to clear dust transport mechanism, on the one hand, strengthen the quantitative study on dust transport, on the other hand provide important parameters for numerical simulation of dust transport.…”