Elsevier

Aeolian Research

Volume 53, September 2021, 100745
Aeolian Research

Sediment transport characteristics above a gobi surface in northwestern China, and implications for aeolian environments

https://doi.org/10.1016/j.aeolia.2021.100745Get rights and content

Abstract

Gobis (gravel deserts) cover large areas in northwestern China and other parts of the world, but sediment transport above gobi surfaces has not been widely investigated; thus, there is insufficient empirical data to support dust source identification. In the present study, we used the LDDSEG vertically segmented sediment sampler to collect sediment transport data above a gobi surface. The results demonstrated that the sediment transport rate above the gobi surface was larger than that above a sandy surface, with rates as high as 9.7 kg m-1h−1. The transport flux can be expressed as a Gaussian peak function, with the maximum sediment transport at 0.05 to 0.09 m above the surface. Principal-components analysis (PCA) indicated that the mean grain size of the transported sediment was controlled mainly by the content of silt and clay (<63 μm) and fine sand (125 to 250 μm); this explains the inflection height for sediment transport. PCA also indicated that dry lacustrine deposits were the main sediment source in the study region. About 90% of the cumulative sediment transport occurs at a height below 0.65 m. Our results indicate that sediment transported over a gobi surface has higher trajectories and longer distances than above a sandy surface. The larger silt and clay component (about 30%) of the sediment transported over the gobi surface means that gobi surfaces are important dust sources in northern China, although the dust likely originated from dry lacustrine sites upwind of the study site.

Introduction

Wind erosion of surface sediments is a primary cause of sediment transport, leading to dust storms and land degradation. When wind erosion occurs, erodible sediment is transported downwind from the source region, and this transport has been widely studied in the world’s arid and semiarid regions (Sweeney et al., 2011, Cheng et al., 2017, Farebrother et al., 2017, Zhang et al., 2017b, Hossein et al., 2018). The transported sediments include sand, silt, and clay particles. The sand is mainly transported below a height of 1 m and provides sediment for the formation of sand dunes or aeolian hazards such as sandstorms (Qu et al., 2005, Zhang et al., 2017b). The silt and clay particles reach heights of more than 2 m and are transported over longer distances (km), and create a significant dust hazard. The transported dust also plays a significant role in global climate change (Xuan and Sokolik, 2002), and in other Earth system processes (Prospero et al., 2002, Shao et al., 2002, Ginoux et al., 2012).

In northwestern China, gobi desert (i.e., desert with a gravel surface) covers approximately 72 × 104 km2, and represents a region where serious sand damage occurs (e.g., burial of crops and structures). The people, agricultural land, buildings, railways, and roads are frequently threatened by blowing sand. Gobis represent gravel surfaces, but the gobi deserts also include oases, areas with dry lacustrine deposits, shifting sands, and alluvial and diluvial deposits, and all of these surfaces may be the source of transported sediments. Sediment transport is a key issue in aeolian research, and has been studied over sandy surfaces (Owen, 1964, Namikas, 2003, Dong et al., 2011, Zhang and Dong, 2014, Zhang et al., 2017b, Zhang et al., 2017c), vegetation surfaces (Miri et al., 2019), agricultural land (Wang et al., 2018), and gobi surfaces (Zou et al., 1995, Qu et al., 2005, Dong et al., 2002a, Tan et al., 2013, Tan et al., 2016). However, there has been considerably less research on gobi surfaces than on other surfaces, and most of these studies were done in a wind tunnel (Qu et al., 2005, Dong et al., 2002a, Dong et al., 2004). Because wind tunnel experiments represent ideal conditions, some unavoidable limitations exist, such as a lack of the variations in surface conditions and in the wind turbulence that are observed in the field, and this causes the results to differ from what would be observed in the field. Although Tan et al. (2013) used a wind tunnel and Tan et al. (2016) measured sand transport over a gobi surface in Dunhuang, northern China, the rich available sediments upwind of the study region differed from most gobi surfaces in northern China (Zhang et al., 2016). They found that changes in the sand transport rate with height could be expressed as an exponential function, which differed from previous studies of sand transport over a gobi surface (Qu et al., 2005, Dong et al., 2002a, Dong et al., 2004). These differences show why it’s essential to study sediment transport above real gravel surfaces under field conditions.

Sediment transport processes include sediment erosion (release from the surface), transport (through creep, saltation, and suspension), and deposition. Sediment transport is controlled by the wind’s shear velocity, the surface’s aerodynamic roughness, and the sediment’s grain-size distribution. All of these factors are related to surface properties such as the vegetation cover, gravel cover, soil water content, presence or absence of a soil crust, and the content of erodible sediments (Zou et al., 1995, Martin and Kok, 2017, Hossein et al., 2018). For a gobi surface, the gravel cover, gravel particle-size distribution, and the content of erodible sediment particles all control sediment erosion and subsequent transport. Gravel requires a stronger wind than sand before sediment will be emitted, and can reduce wind velocity by increasing the surface roughness, leading to sediment deposition (Zou et al., 1995, Dong et al., 2002a, Dong et al., 2006). The surface grain size is the main factor that affects sediment transport (Martin and Kok, 2017, Zhang et al., 2017b, Hossein et al., 2018). During sand transport, winds detach sand from the surface until the wind’s maximum transport capacity is reached (i.e., until the flow becomes saturated), and this capacity is controlled by the wind’s velocity and by surface conditions (Cheng et al., 2017). The sand transport rate also differs between wind tunnel studies and field measurements (Tan et al., 2013, Tan et al., 2016).

Sediment transport characteristics above a gobi surface, such as the quantity of sediment and its grain-size distribution, must be better understood to support efforts to prevent aeolian hazards, assess air quality, and identify sediment sources. In our team's previous field research (Zhang et al., 2016), we found that the gravel cover on gobi surfaces ranged from 22 to 91%, that about 75% of field sites had a physical crust, and that the content of silt and clay particles ranged from 10 to 40%, and all of these properties differed greatly from those of other land surfaces. This causes differences in sediment erosion and transport between different types of surfaces. Gobi deserts may or may not include areas of shifting sand, leading to differences between deserts in the richness of the local sediment sources. Sediment transport over a gobi surface with a small quantity of shifting sand or a sandy surface with gravel near rich sediment sources is larger than transport above a gobi surface without shifting sand (Zhang et al., 2017a). For most gobi surfaces, the available sediments are limited (Zhang et al., 2016). This is the case in China’s Alxa Plateau, Hexi Corridor, and Heihe River basin (Zhang et al., 2016). Despite the limited sediment supply, the Alxa Plateau is one of the main dust sources for the Mongolian Plateau (Shao et al., 2002). Where local sediment sources are insufficient to generate the dust, the dust may have been transported from sources farther upwind, and temporarily deposited on the gobi surface when the wind slows (Sweeney et al., 2011). This non-local dust sources is important because land surfaces in these regions are mainly gravel, with some dry lacustrine deposits, making them a limited sediment source. Therefore, these regions are good areas to study to improve our understanding of sediment transport.

Previous studies focused mainly on sandy deserts (Namikas, 2003, Dong et al., 2011, Zhang and Dong, 2014, Zhang et al., 2017b, Zhang et al., 2017c). The differences between sandy and gobi surface characteristics create important differences in sediment entrainment and transport. Therefore, the sediment transport above sandy land is unlikely to resemble the transport that occurs above gobi surfaces. There is some debate over the relative contribution of saltation processes (Farrell et al., 2012). Namikas et al. (2009) found that sand trajectories are longer for saltating grains above a bed of coarse sand (with a grain size of 0.55 mm) because the saltating grains can rebound from coarse bed grains and retain a larger proportion of their initial impact energy, whereas finer grains absorb more of that energy. On a gobi surface, the large gravel and the presence of a soil physical crust make the gobi surfaces harder and more resistant to impacts than a sandy surface, so energy transfers between saltating particles and the surface are likely to resemble those on a coarse sandy surface. However, there is little field data to support this hypothesis.

To provide some of the missing knowledge, we designed a field experiment to observe sediment transport above a gobi surface, with the goal of quantifying the sediment transport during dust storms. A second goal was to collect data to support an explanation of the key sediment transport mechanisms. Previous research showed that on a sandy surface, sand grains larger than 500 μm in diameter are transported by creep, whereas particles between 63 μm to 500 μm are transported by saltation, and particles smaller than 63 μm are transported by suspension (Bagnold, 1941). Although different sand entrainment mechanism had been provided previously (Bagnold, 1941), recent study indicated that sand transport is mainly driven by splash, and saltation heights are approximately constant with shear velocity (Martin and Kok, 2017). Sediment saltaion height is controlled by median sand (Martin and Kok, 2017). However, the different characteristics of a gobi surface (including the presence of gravel, soil crusts, and limited sand supply compared with a sandy surface) leads to different aerodynamic roughness, shear stress, and grain ejection and falling processes above a gobi surface. Therefore, the sand transport mechanisms above a gobi are poorly understood. In the present study, we included previously published data on the differences between gobi and sandy surfaces (Zhang et al., 2017b) in our analysis to provide additional insights. The new data comprise sediment transport during seven periods with high sediment transport within a dust storm, and the previously published data comprise twelve periods with significant sediment transport during a dust storm This information will support efforts to prevent aeolian hazards using sand-control barriers, to simulate dust storms, and to perform risk assessments.

Section snippets

Surface properties and wind fetch

The study region, which is located near Ejina Qi in the Alxa Plateau of northwestern China, has frequent and severe dust storms (Han et al., 2012), and during our field experiment, a strong dust storm occurred over a large area of northwestern China (Fig. 1a). At the study site, sediments with a grain size < 63 µm account for 16.3% of the surface particle mass in the top 0.5 cm of the soil, and the mean grain size of the erodible sediment is 160 µm. The gravel cover (particles with a

Sediment transport rates

Fig. 4 show the sediment transport characteristics measured by the LDDSEG sediment sampler over the gobi surface. To determine the horizontal sediment transport flux, qz (kg m-1h−1), we fit a Gaussian peak function to the vertical transport profile (R2 = 0.93, P < 0.05, Table 1), which agrees with our previous field data for sediment transport above a coarse sand surface (Zhang et al., 2017b) and gobi surface (Qu et al., 2005, Dong et al., 2004). This data revealed inflection points at heights

Sediment transport over a gobi surface

The differences of sediment transport between sandy and gobi surfaces mainly result from differences in the surface characteristics, total sediment transport rate, cumulative sediment transport rate, and vertical distribution of the grain-size classes. The peak transport at the inflection height identified by our analysis (Fig. 4) reveals four interesting phenomena: First, the maximum transport occurred at the inflection height (0.05 to 0.09 m; Fig. 4a), and the total sediment transport was

Conclusions

Sand transport above a gobi surface is controlled by the wind’s shear velocity, the surface’s aerodynamic roughness, and the grain size distribution. However, the relationships among these variables are complex, so we focused on the grain size distribution in our study of sand transport above a gobi surface, and obtained four novel results:

  • 1.

    The sediment transport mechanism above the gobi surface appears to be complicated. Our analysis of the Ra data suggests that deposition does occur

CRediT authorship contribution statement

Zhengcai Zhang: Conceptualization, Methodology, Writing – original draft, Investigation. Lanying Han: . Kaijia Pan: Methodology.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are grateful for the financial assistance obtained from the National Natural Science Foundation of China (41930640, 41971014). We thank Doctor Chao Li and Doctor Nan Xiao (Shanxi Normal University) for their assistance during the field measurements. We thank Eric Parteli (University of Cologne) for criticisms that helped us to improve an early version of our manuscript.

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