Migration and Morphology of Asymmetric Barchans in the Central Hexi Corridor of Northwest China

Abstract

Crescent-shaped barchan dunes often display an asymmetric shape, with one limb longer than the other. As shown in previous studies, asymmetric bimodal winds constitute one major cause of barchan asymmetry, but the heterogeneous conditions of sand availability or flux, as well as topographic influences, may be also important. Understanding the morphology and dynamics of asymmetric barchans may have an impact in a broad range of areas, particularly as these dunes may serve as a proxy for planetary wind regimes and soil conditions in extraterrestrial environments. However, in addition to the existing theories and numerical models that explain barchan asymmetry, direct measurements of migration rates and morphologic changes of real asymmetric barchans over a time span of several years would be beneficial. Therefore, here we report such measurements, which we have acquired by investigating asymmetric barchans in the Hexi Corridor, northwest of China. We have found that dune interactions and asymmetric influx conditions are the most important causes of barchan asymmetry in this field. Particle size distributions in the Hexi Corridor display strong variations over different parts of the asymmetric barchans, as well as over different dunes, with gravel particles being incorporated from the substrate as the dunes migrate. Our observations have shown that upwind sediment sources are important for dune formation in the Hexi Corridor, and that interdune interactions affect dune shape in different ways, depending on their offset. The asymmetric barchans in the Hexi Corridor are active, with an average migration rate (MR) between 8 and 53 m year −1 , in spite of the different asymmetric shapes. Our data for dune migration rates can be described well by a scaling of MR = A/(W + W0), where W is the barchan cross-wind width, A ≈ 2835 m 2 s −1 , and W0 ≈ 44 m. A similar scaling fits very well the migration rate as a function of dune along-wind width L, (i.e., MR = B/(L + L0), with B ≈ 1722 m 2 s −1 and L0 ≈ 13 m). Linear relations are also found between both dune widths and the average limb and windward side lengths, thus indicating that the morphometric relations that are predicted from models for steady-state, symmetric crescent-shaped dunes can be applied to different transitional morphologies of interacting, asymmetric barchans.

Nebkha dune morphology in the gobi deserts of northern China and potential implications for dust emission

“…The similar amounts of sand and dust mean that nebkhas can both capture these materials and serve as a source of dust emission. This phenomenon has been documented for oasis and sandy desert nebkhas in north-western China (Sweeney et al, 2016;Zou et al, 2016) and in Israel's Negev Desert, where nebkhas can trap about 80% of the transported sand (Kidron & Zohar, 2016).…”

“…Nebkhas, also called coppice or shrub-coppice dunes (Hesp & Smyth, 2017), form in any region where there is a strong wind regime, available aeolian sediment capable of being entrained by these winds, and vegetation that can trap the transported sediment. Because of the vast regions they occupy worldwide, their formation and development have been extensively studied (Hasi et al, 2013;Zhang et al, 2013;Gillies et al, 2014;Hesp & Smyth, 2017;Mayaud et al, 2017;Zuo et al, 2018), along with their morphological properties (Yang et al, 2015) and environmental implications (Wang et al, 2006(Wang et al, , 2010Zou et al, 2016;Yan et al, 2019). However, there has been little research on the nebkhas that develop in areas of gobi desert (where the surface is covered with closely packed, interlocking angular or rounded rock fragments of gravel and cobble size).…”

Migration of barchan dunes in the western Quruq Desert, northwestern China

“…Based on previous results from both field observations and wind tunnel experiments, the threshold velocity in the study region is 6.0 m s À1 (Zu et al, 2008). Although this approach makes a number of interpretative simplifications and assumptions, previous studies of the wind energy environments of global sand seas have demonstrated its value (Zhang et al, 2018b). More detailed descriptions of the method can be found in previous studies (Fryberger and Dean, 1979;Zhang et al, 2015).…”

“…The main methods of studying dune migration are monitoring of changes in dune morphology (Dong et al, 2000), comparison of remote-sensing images from different years (Al-Mutiry et al, 2016;Boulghobra and Dridi, 2016;Baughman et al, 2018;Hoover et al, 2018), numerical simulation (Tsoar, 1984;Durán et al, 2010;Parteli et al, 2014;Ping et al, 2016), using multi-temporal LiDAR data and a new approach named the 'pairs of source and target points' (Dong, 2015), and dating of sediments at different depths below the dune's slipface using optically stimulated luminescence dating. Highresolution satellite images have proven to be an ideal information source for obtaining the information about dune migration (Bourke, 2010;Hereher, 2010;Dakir et al, 2016;Zhang et al, 2018b). By comparing satellite images from different dates, we can obtain key information such as the migration rate, migration direction and changes of dune morphometric parameters.…”

“…The dune migration rate and direction were measured using available Google Earth historical satellite images, which have been widely used to investigate the formation and development of barchans (Hamdan et al, 2016;Zhang et al, 2018bZhang et al, , 2018c. The acquired Google Earth images of the study sites cover the period from October 2003 to November 2014 (data from most other years were unavailable) and were extracted and analysed using the ArcGIS software (www.esri.com).…”