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Barchans of Minqin: Sediment transport
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Geomorphology 96 (2008) 233 – 238 www.elsevier.com/locate/geomorph
Barchans of Minqin: Sediment transport Zhen-Ting Wang a,b,⁎, Jia-Wu Zhang a , Qian-Hua Zhang b , Ming-Rui Qiang a , Fa-Hu Chen a , Yu-Quan Ling b a
CAEP, Key Laboratory of Western China's Environmental Systems (Ministry of Education), Lanzhou University, Lanzhou 730000, PR China b Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, PR China Received 20 April 2007; received in revised form 13 July 2007; accepted 19 July 2007 Available online 10 August 2007
Abstract Spatial changes in the rates of sand transport are a fundamental control of dune morphology. A detailed field measurement of sand flux along the brink of a barchan was performed to explore the spanwise property of sand flux on the barchan surface. Setting the arc length of the brink at the tip of the longer horn to zero, it was found that the value of sand flux increases with the arc length of the brink at first and then decreases at both low and high wind speeds. The maximum occurs at the crest. Our result suggests that the spanwise distribution of sand flux cannot be neglected in the study of transverse dunes. © 2007 Published by Elsevier B.V. Keywords: Barchan; Sand flux; Spanwise; Brink
1. Introduction Dune dynamics involve wind flow, dune morphology, sediment transport and their interactions. Although physical and numerical models offer more detailed information of these complex processes (Stam, 1997; Momiji and Warren, 2000; Shao, 2000; Parsons et al., 2004; Schwämmle and Herrmann, 2005), field investigation is still the dominant approach in the studies of transverse dunes (Livingstone et al., 2007).
⁎ Corresponding author. CAEP, Key Laboratory of Western China's Environmental Systems (Ministry of Education), Lanzhou University, Lanzhou 730000, PR China. E-mail address: [email protected] (Z.-T. Wang). 0169-555X/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.geomorph.2007.07.018
Spatial changes in the rates of sand transport are a fundamental control of dune morphology (Lancaster, 1995). The streamwise characteristics of sand flux vary remarkably under the influence of barchan dunes as obstacles. It has been shown that the sandflux increases to a maximum at the crest or brink along the barchan centre-line (Lancaster et al., 1996; Wiggs, 2001). As barchans occur while sand supply is limited, the streamwise change of the sand flux from undersaturated to saturated has also been observed (Lancaster et al., 1996). If the sand flux is uniformly spanwise or transverse, the streamwise measurements associated with field observations on the wind velocity and surface shear stress will suffice in understanding barchan dynamics. However, the sediment transport by wind is not isotropic even on the flat sandy bed because of the existence of sand streamers (Baas, 2003). Here we present a detailed field measurement of sand flux along
234
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
the brink of a barchan for the purpose of exploring the spanwise properties of sand flux on the barchan surface. 2. Field methods A dust storm broke out on the afternoon of 27 March 2007 at Minqin oasis. One observation (run A) was taken at the beginning of the storm, the other (run B) when the storm was strongest. To reduce the influence of sand traps on the local wind velocity, we selected a large bare barchan, No. A5 in our previous study (Wang et al., 2007). The test site (38° 47′ 22″ N, 102° 29′ 25″ E) lies in the southeastern margin of Badain Jaran Desert, a large mobile desert in Northwest China, see Fig. 1 in Wang et al. (2007). This isolated barchan is 93 m high with a horizontal distance of 94 m along the axis of symmetry from the trailing edge to the upper edge of the slipface. The crest coincides with the top of the slipface. One horn is 90 m wide and 92 m long. The other is 76 m wide and 79 m long. There are some small fixed dunes less than 2.0 m high downwind of the barchan, see Fig. 1. The surface is hard and flat within 50 m upwind of the toe of windward slope. The movement of the barchan takes place by means of erosion on the windward slope and simultaneous deposition at the lee side. The brink, the boundary between the slipface and the windward slope, is an ideal curve to perform the test. Setting the arc length of the brink at the tip of the longer horn to zero or s = 0 m, the crest location is s = 135 m. The distribution of the mean grain size d along the brink (Fig. 2) shows that the grain size profile is disordered when s b 80 m and the grains at s = 130–180 m are slightly larger in size than those at s = 80–130 m and s = 180–260 m. The
average grain diameter of 16 samples taken from the surface of centre-line is d = 414.33 μm. It is wellknown that the threshold friction velocity u⁎t for medium and coarse sand can be computed by (Wang, 2006) rffiffiffiffiffiffiffiffiffiffiffi qs u⁎t ¼ A gd ð1Þ qa where the value of empirical coefficient A is approximately 0.1, ρs, ρa and g are sand density, air density and the acceleration of gravity, respectively. Substituting parameter values into Eq. (1), we obtain u⁎t = 0.30 m/s. The sand traps were placed at: 9:6ði 1Þ iV3 s¼ ð2Þ 6:0ði 4Þ þ 28:8 iz4 where s is in meters, and i is an integer. In the test, we adopted the MWAC type of aeolian sand trap recommended by Goossens et al. (2000). Five plastic bottles (9.0 cm in diameter, 25 cm in height) with an inlet/outlet diameter of 2.0 cm/3.6 cm, were installed horizontally at heights of 6 cm, 16 cm, 26 cm, 36 cm and 46 cm above the surface at each site to measure the vertical profile of sand flux density (of dimensions M L− 2 T− 1). The sand flux density at the surface was obtained by linear extrapolation of the data given by two lower catchers. Based on the results of 6 points, we can easily calculate the sand flux through the numerical integration of a trapezoidal rule. The incoming flow speeds were recorded using anemometers placed at the centre-line upwind of the barchan. The distance between the anemometer mast and the barchan was 3 times of the dune height. We expect that the
Fig. 1. Photograph of the study site, Minqin, China.
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
effect of the studied dune on the wind velocity can be neglected at this distance and the recorded data can then represent the free incoming flow. 3. Results and discussions The instantaneous velocity u can be decomposed into an average part u¯ and a fluctuating part u′, P
u ¼ u þ uV
ð3Þ
For convenience, we use the average velocity u¯ to represent the wind property although the vertical and streamwise velocity fluctuations play an important role in sediment transport. u¯ can be calculated by P
u¼
1 T
Z
T
uðt Þdt
ð4Þ
0
where u at different height h and time t is shown in Figs. 3 and 4. The profile of time–average wind velocity follows the form: P
u¼
u⁎ z ln z0 j
ð5Þ
in which the von Kármán constant κ = 0.4, z is height above surface, u⁎ is the friction velocity, and z0 is the roughness length. The roughness length z0 is essentially a property of surface. We measured wind speeds at 4 different heights to accurately determine this parameter and the friction velocity u⁎ simultaneously in run A. At the beginning stage of the storm, there were no apparent saltation motions near the anemometer mast because sand supply was very limited and the wind speed was low. Substituting the wind speed records (Fig. 3) into Eqs. (4) and (5), we find u⁎ = 0.29 m/s and z0 = 0.0748 cm, see Fig. 5. When the wind became stronger (run B), we only recorded the wind speeds 1.0 m above the surface so as to avoid the influence of undersaturated aeolian sand flow. The friction velocity is 0.61 m/s in Fig. 5(b). The changes of sand flux q with the arc length of the brink s are plotted in Fig. 6. It is found that q increases with s at first and then decreases at both low (run A) and high (run B) wind speeds. This tendency was not apparent in Lancaster et al.'s (1996) observations since they had only 6 sand traps placed along the brink. It should be pointed out that the abnormal value of sand flux at s = 0 in
235
Fig. 5 is the result of abundant sand supplied by three proximal small fixed dunes when wind speeds become sufficiently high. The streamline pattern for a barchan given by the k–ɛ model of turbulence indicates that the surface shear stress is higher near the crest compared to the horns (see Fig. 3 in Herrmann et al., 2005). The spanwise variation of shear stress or friction velocity leads to the change in sand flux along the brink in Fig. 6. Considering that the lateral or spanwise component of wind velocity on the windward side of the barchan is very small compared to the component in the direction of the incoming flow (Herrmann et al., 2005), this result can also be interpreted by the 2-dimensional speed-up flow theory (Jackson and Hunt, 1975; Hunt et al., 1988a,b). A very recent report on a 1-year field test of barchans found that the sand flux at the crest is about 3 times greater than the interdune mass transport rate (Ahmedou et al., 2007). However, our study suggests that the ratio of sand flux at the crest to the horns should be a variable mainly determined by the wind speed. Fig. 6a and b clearly illustrate that this ratio is much higher for low wind speeds than for high wind speeds because the wind speed-up effect is prominent in weak wind conditions (Momiji and Warren, 2000). The comparison of dune morphology before and after the storm, which lasted about 5 h, was conducted. The barchan maintained its shape during the storm. Further measurements show that the brink decreased 15 ± 5 cm in height and increased 7.00 ± 2.00 m in length, the barchan moved 21 ± 5 cm and its volume reduced 5.1 ± 2.0 × 103 m3 . Although the understanding of dune dynamic relies on the long-term field investigations (Long and Sharp, 1964; Hastenrath, 1967,1987; Gay, 1999] or GPR surveys [Bristow et al., 2000, 2005], the short-term data we obtained provides
Fig. 2. Distribution of the mean grain size along the brink.
236
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
Fig. 3. Instantaneous wind speed recorded using anemometers, run A.
Fig. 4. Instantaneous wind speed recorded using anemometers, run B.
Fig. 5. Profile of time–average wind velocity.
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
237
project No. 40601053, NSFC Innovation Team Project No. 40421101 and “111” Project No. B06026. References
Fig. 6. Sand flux along the brink.
a good example for examining the validity of theoretical or numerical models of dune dynamic, such as those of Stam (1997) and Parsons et al. (2004). The semi-empirical models such as Howard et al. (1978) and Momiji and Warren (2000) will also benefit from the present study. For instance, the model of Momiji and Warren (2000) could be further extended to the three dimensional condition. 4. Summary We performed a detailed field measurement of sand flux along the brink of a barchan. It was found that the sand flux increases with the arc length of the brink firstly and then decreases at both low and high wind speeds. This investigation indicates that the spanwise properties of sand flux on the dune surface cannot be neglected in the study of barchan dynamic. Acknowledgements We are grateful to Prof. Xun-Ming Wang for his helpful discussions, to Mr. Shuo Chen, Mr. Ge-Chao Zhang and Dr. Guang-Hui Dong for their help in the field. Thank Ms. Qiong Li for her assistance in grain size analysis. This research was supported by NSFC
Ahmedou, D.O., Mahfoudh, A.O., Dupont, P., Moctar, A.O.E., Valance, A., Rasmussen, K.R., 2007. Barchan dune mobility in Mauritania related to dune and interdune sand fluxes. Journal of Geophysical Research 112, F02016. Baas, A.C.W., 2003. The Formation and Behavior of Aeolian Streamers. PhD Thesis, University of Southern California. Bristow, C.S., Bailey, S.D., Lancaster, N., 2000. The sedimentary structure of linear sand dunes. Nature 406, 56–59. Bristow, C.S., Lancaster, N., Duller, G.A., 2005. Combining ground penetrating radar surveys and optical dating to determine dune migration in Namibia. Journal of the Geological Society of London 162, 315–321. Gay, S.P., 1999. Observations regarding the movement of barchan sand dunes in the Nazca to Tanaca area of southern Peru. Geomorphology 27, 279–293. Goossens, D., Offer, Z., London, G., 2000. Wind tunnel and field calibration of five aeolian sand traps. Geomorphology 35, 233–252. Hastenrath, R.L., 1967. The barchans of the Arequipa region, southern Peru. Zeitschrift für Geomorphology N.F. 11, 300–331. Hastenrath, R.L., 1987. The barchan dunes of southern Peru revisited. Zeitschrift für Geomorphology N.F. 31, 167–178. Herrmann, H.J., Andrade, J.S., Schatz, V., Sauermann, G., Parteli, E.J.R., 2005. Calculation of the separation streamlines of barchans and transverse dunes. Physica A 357, 44–49. Howard, A.D., Morton, J.B., Gad-el-Hak, M., Pierce, D.B., 1978. Sand transport model of barchan dune equilibrium. Sedimentology 25, 307–338. Hunt, J.C.R., Leibovich, S., Richards, K.J., 1988a. Turbulent shear flow over low hills. Quarterly Journal of the Royal Meteorological Society 114, 1435–1470. Hunt, J.C.R., Richards, K.J., Brighton, P.W.M., 1988b. Stably stratified shear flow over low hills. Quarterly Journal of the Royal Meteorological Society 114, 859–886. Jackson, P.S., Hunt, J.C.R., 1975. Turbulent wind flow over low hills. Quarterly Journal of the Royal Meteorological Society 101, 929–955. Lancaster, N., 1995. Geomorphology of Desert Dunes. Routledge, London, p. 122. Lancaster, N., Nickling, W.G., McKenna Neuman, C.K., Wyatt, V.E., 1996. Sediment flux and airflow on the stoss slope of a barchan dune. Geomorphology 17, 55–62. Livingstone, I., Wiggs, G.F.S., Weaver, C.M., 2007. Geomorphology of desert sand dunes: a review of recent progress. Earth-Science Reviews 80, 239–257. Long, J.T., Sharp, R.P., 1964. Barchan-dune movement in the Imperial Valley, California. Geological Society of America Bulletin 75, 149–156. Momiji, H., Warren, A., 2000. Relations of sand trapping efficiency and migration speed of transverse dunes to wind velocity. Earth Surface Processes and Landforms 25, 1069–1084. Parsons, D.R., Walker, I.J., Wiggs, G.F.S., 2004. Numerical modelling of flow structures over idealized transverse aeolian dunes of varying geometry. Geomorphology 59, 149–164. Schwämmle, V., Herrmann, H.J., 2005. A model of barchan dunes including lateral shear stress. The European Physical Journal E 16, 57–65.
238
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
Shao, Y.P., 2000. Physics and Modelling of Wind Erosion. Kluwer Academic, Dordrecht, pp. 341–350. Stam, J.M.T., 1997. On the modelling of two-dimensional aeolian dunes. Sedimentology 44, 127–141. Wang, Z.-T., 2006. Influence of moisture on the entrainment of sand by wind. Powder Technology 164, 89–93.
Wang, Z.-T., Tao, S.-C., Xie, Y.-W., Dong, G.-H., 2007. Barchans of Minqin: morphometry. Geomorphology 89, 405–411. Wiggs, G.F.S., 2001. Desert dune processes and dynamics. Progress in Physical Geography 25, 53–79.
Geomorphology 96 (2008) 233 – 238 www.elsevier.com/locate/geomorph
Barchans of Minqin: Sediment transport Zhen-Ting Wang a,b,⁎, Jia-Wu Zhang a , Qian-Hua Zhang b , Ming-Rui Qiang a , Fa-Hu Chen a , Yu-Quan Ling b a
CAEP, Key Laboratory of Western China's Environmental Systems (Ministry of Education), Lanzhou University, Lanzhou 730000, PR China b Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, PR China Received 20 April 2007; received in revised form 13 July 2007; accepted 19 July 2007 Available online 10 August 2007
Abstract Spatial changes in the rates of sand transport are a fundamental control of dune morphology. A detailed field measurement of sand flux along the brink of a barchan was performed to explore the spanwise property of sand flux on the barchan surface. Setting the arc length of the brink at the tip of the longer horn to zero, it was found that the value of sand flux increases with the arc length of the brink at first and then decreases at both low and high wind speeds. The maximum occurs at the crest. Our result suggests that the spanwise distribution of sand flux cannot be neglected in the study of transverse dunes. © 2007 Published by Elsevier B.V. Keywords: Barchan; Sand flux; Spanwise; Brink
1. Introduction Dune dynamics involve wind flow, dune morphology, sediment transport and their interactions. Although physical and numerical models offer more detailed information of these complex processes (Stam, 1997; Momiji and Warren, 2000; Shao, 2000; Parsons et al., 2004; Schwämmle and Herrmann, 2005), field investigation is still the dominant approach in the studies of transverse dunes (Livingstone et al., 2007).
⁎ Corresponding author. CAEP, Key Laboratory of Western China's Environmental Systems (Ministry of Education), Lanzhou University, Lanzhou 730000, PR China. E-mail address: [email protected] (Z.-T. Wang). 0169-555X/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.geomorph.2007.07.018
Spatial changes in the rates of sand transport are a fundamental control of dune morphology (Lancaster, 1995). The streamwise characteristics of sand flux vary remarkably under the influence of barchan dunes as obstacles. It has been shown that the sandflux increases to a maximum at the crest or brink along the barchan centre-line (Lancaster et al., 1996; Wiggs, 2001). As barchans occur while sand supply is limited, the streamwise change of the sand flux from undersaturated to saturated has also been observed (Lancaster et al., 1996). If the sand flux is uniformly spanwise or transverse, the streamwise measurements associated with field observations on the wind velocity and surface shear stress will suffice in understanding barchan dynamics. However, the sediment transport by wind is not isotropic even on the flat sandy bed because of the existence of sand streamers (Baas, 2003). Here we present a detailed field measurement of sand flux along
234
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
the brink of a barchan for the purpose of exploring the spanwise properties of sand flux on the barchan surface. 2. Field methods A dust storm broke out on the afternoon of 27 March 2007 at Minqin oasis. One observation (run A) was taken at the beginning of the storm, the other (run B) when the storm was strongest. To reduce the influence of sand traps on the local wind velocity, we selected a large bare barchan, No. A5 in our previous study (Wang et al., 2007). The test site (38° 47′ 22″ N, 102° 29′ 25″ E) lies in the southeastern margin of Badain Jaran Desert, a large mobile desert in Northwest China, see Fig. 1 in Wang et al. (2007). This isolated barchan is 93 m high with a horizontal distance of 94 m along the axis of symmetry from the trailing edge to the upper edge of the slipface. The crest coincides with the top of the slipface. One horn is 90 m wide and 92 m long. The other is 76 m wide and 79 m long. There are some small fixed dunes less than 2.0 m high downwind of the barchan, see Fig. 1. The surface is hard and flat within 50 m upwind of the toe of windward slope. The movement of the barchan takes place by means of erosion on the windward slope and simultaneous deposition at the lee side. The brink, the boundary between the slipface and the windward slope, is an ideal curve to perform the test. Setting the arc length of the brink at the tip of the longer horn to zero or s = 0 m, the crest location is s = 135 m. The distribution of the mean grain size d along the brink (Fig. 2) shows that the grain size profile is disordered when s b 80 m and the grains at s = 130–180 m are slightly larger in size than those at s = 80–130 m and s = 180–260 m. The
average grain diameter of 16 samples taken from the surface of centre-line is d = 414.33 μm. It is wellknown that the threshold friction velocity u⁎t for medium and coarse sand can be computed by (Wang, 2006) rffiffiffiffiffiffiffiffiffiffiffi qs u⁎t ¼ A gd ð1Þ qa where the value of empirical coefficient A is approximately 0.1, ρs, ρa and g are sand density, air density and the acceleration of gravity, respectively. Substituting parameter values into Eq. (1), we obtain u⁎t = 0.30 m/s. The sand traps were placed at: 9:6ði 1Þ iV3 s¼ ð2Þ 6:0ði 4Þ þ 28:8 iz4 where s is in meters, and i is an integer. In the test, we adopted the MWAC type of aeolian sand trap recommended by Goossens et al. (2000). Five plastic bottles (9.0 cm in diameter, 25 cm in height) with an inlet/outlet diameter of 2.0 cm/3.6 cm, were installed horizontally at heights of 6 cm, 16 cm, 26 cm, 36 cm and 46 cm above the surface at each site to measure the vertical profile of sand flux density (of dimensions M L− 2 T− 1). The sand flux density at the surface was obtained by linear extrapolation of the data given by two lower catchers. Based on the results of 6 points, we can easily calculate the sand flux through the numerical integration of a trapezoidal rule. The incoming flow speeds were recorded using anemometers placed at the centre-line upwind of the barchan. The distance between the anemometer mast and the barchan was 3 times of the dune height. We expect that the
Fig. 1. Photograph of the study site, Minqin, China.
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
effect of the studied dune on the wind velocity can be neglected at this distance and the recorded data can then represent the free incoming flow. 3. Results and discussions The instantaneous velocity u can be decomposed into an average part u¯ and a fluctuating part u′, P
u ¼ u þ uV
ð3Þ
For convenience, we use the average velocity u¯ to represent the wind property although the vertical and streamwise velocity fluctuations play an important role in sediment transport. u¯ can be calculated by P
u¼
1 T
Z
T
uðt Þdt
ð4Þ
0
where u at different height h and time t is shown in Figs. 3 and 4. The profile of time–average wind velocity follows the form: P
u¼
u⁎ z ln z0 j
ð5Þ
in which the von Kármán constant κ = 0.4, z is height above surface, u⁎ is the friction velocity, and z0 is the roughness length. The roughness length z0 is essentially a property of surface. We measured wind speeds at 4 different heights to accurately determine this parameter and the friction velocity u⁎ simultaneously in run A. At the beginning stage of the storm, there were no apparent saltation motions near the anemometer mast because sand supply was very limited and the wind speed was low. Substituting the wind speed records (Fig. 3) into Eqs. (4) and (5), we find u⁎ = 0.29 m/s and z0 = 0.0748 cm, see Fig. 5. When the wind became stronger (run B), we only recorded the wind speeds 1.0 m above the surface so as to avoid the influence of undersaturated aeolian sand flow. The friction velocity is 0.61 m/s in Fig. 5(b). The changes of sand flux q with the arc length of the brink s are plotted in Fig. 6. It is found that q increases with s at first and then decreases at both low (run A) and high (run B) wind speeds. This tendency was not apparent in Lancaster et al.'s (1996) observations since they had only 6 sand traps placed along the brink. It should be pointed out that the abnormal value of sand flux at s = 0 in
235
Fig. 5 is the result of abundant sand supplied by three proximal small fixed dunes when wind speeds become sufficiently high. The streamline pattern for a barchan given by the k–ɛ model of turbulence indicates that the surface shear stress is higher near the crest compared to the horns (see Fig. 3 in Herrmann et al., 2005). The spanwise variation of shear stress or friction velocity leads to the change in sand flux along the brink in Fig. 6. Considering that the lateral or spanwise component of wind velocity on the windward side of the barchan is very small compared to the component in the direction of the incoming flow (Herrmann et al., 2005), this result can also be interpreted by the 2-dimensional speed-up flow theory (Jackson and Hunt, 1975; Hunt et al., 1988a,b). A very recent report on a 1-year field test of barchans found that the sand flux at the crest is about 3 times greater than the interdune mass transport rate (Ahmedou et al., 2007). However, our study suggests that the ratio of sand flux at the crest to the horns should be a variable mainly determined by the wind speed. Fig. 6a and b clearly illustrate that this ratio is much higher for low wind speeds than for high wind speeds because the wind speed-up effect is prominent in weak wind conditions (Momiji and Warren, 2000). The comparison of dune morphology before and after the storm, which lasted about 5 h, was conducted. The barchan maintained its shape during the storm. Further measurements show that the brink decreased 15 ± 5 cm in height and increased 7.00 ± 2.00 m in length, the barchan moved 21 ± 5 cm and its volume reduced 5.1 ± 2.0 × 103 m3 . Although the understanding of dune dynamic relies on the long-term field investigations (Long and Sharp, 1964; Hastenrath, 1967,1987; Gay, 1999] or GPR surveys [Bristow et al., 2000, 2005], the short-term data we obtained provides
Fig. 2. Distribution of the mean grain size along the brink.
236
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
Fig. 3. Instantaneous wind speed recorded using anemometers, run A.
Fig. 4. Instantaneous wind speed recorded using anemometers, run B.
Fig. 5. Profile of time–average wind velocity.
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
237
project No. 40601053, NSFC Innovation Team Project No. 40421101 and “111” Project No. B06026. References
Fig. 6. Sand flux along the brink.
a good example for examining the validity of theoretical or numerical models of dune dynamic, such as those of Stam (1997) and Parsons et al. (2004). The semi-empirical models such as Howard et al. (1978) and Momiji and Warren (2000) will also benefit from the present study. For instance, the model of Momiji and Warren (2000) could be further extended to the three dimensional condition. 4. Summary We performed a detailed field measurement of sand flux along the brink of a barchan. It was found that the sand flux increases with the arc length of the brink firstly and then decreases at both low and high wind speeds. This investigation indicates that the spanwise properties of sand flux on the dune surface cannot be neglected in the study of barchan dynamic. Acknowledgements We are grateful to Prof. Xun-Ming Wang for his helpful discussions, to Mr. Shuo Chen, Mr. Ge-Chao Zhang and Dr. Guang-Hui Dong for their help in the field. Thank Ms. Qiong Li for her assistance in grain size analysis. This research was supported by NSFC
Ahmedou, D.O., Mahfoudh, A.O., Dupont, P., Moctar, A.O.E., Valance, A., Rasmussen, K.R., 2007. Barchan dune mobility in Mauritania related to dune and interdune sand fluxes. Journal of Geophysical Research 112, F02016. Baas, A.C.W., 2003. The Formation and Behavior of Aeolian Streamers. PhD Thesis, University of Southern California. Bristow, C.S., Bailey, S.D., Lancaster, N., 2000. The sedimentary structure of linear sand dunes. Nature 406, 56–59. Bristow, C.S., Lancaster, N., Duller, G.A., 2005. Combining ground penetrating radar surveys and optical dating to determine dune migration in Namibia. Journal of the Geological Society of London 162, 315–321. Gay, S.P., 1999. Observations regarding the movement of barchan sand dunes in the Nazca to Tanaca area of southern Peru. Geomorphology 27, 279–293. Goossens, D., Offer, Z., London, G., 2000. Wind tunnel and field calibration of five aeolian sand traps. Geomorphology 35, 233–252. Hastenrath, R.L., 1967. The barchans of the Arequipa region, southern Peru. Zeitschrift für Geomorphology N.F. 11, 300–331. Hastenrath, R.L., 1987. The barchan dunes of southern Peru revisited. Zeitschrift für Geomorphology N.F. 31, 167–178. Herrmann, H.J., Andrade, J.S., Schatz, V., Sauermann, G., Parteli, E.J.R., 2005. Calculation of the separation streamlines of barchans and transverse dunes. Physica A 357, 44–49. Howard, A.D., Morton, J.B., Gad-el-Hak, M., Pierce, D.B., 1978. Sand transport model of barchan dune equilibrium. Sedimentology 25, 307–338. Hunt, J.C.R., Leibovich, S., Richards, K.J., 1988a. Turbulent shear flow over low hills. Quarterly Journal of the Royal Meteorological Society 114, 1435–1470. Hunt, J.C.R., Richards, K.J., Brighton, P.W.M., 1988b. Stably stratified shear flow over low hills. Quarterly Journal of the Royal Meteorological Society 114, 859–886. Jackson, P.S., Hunt, J.C.R., 1975. Turbulent wind flow over low hills. Quarterly Journal of the Royal Meteorological Society 101, 929–955. Lancaster, N., 1995. Geomorphology of Desert Dunes. Routledge, London, p. 122. Lancaster, N., Nickling, W.G., McKenna Neuman, C.K., Wyatt, V.E., 1996. Sediment flux and airflow on the stoss slope of a barchan dune. Geomorphology 17, 55–62. Livingstone, I., Wiggs, G.F.S., Weaver, C.M., 2007. Geomorphology of desert sand dunes: a review of recent progress. Earth-Science Reviews 80, 239–257. Long, J.T., Sharp, R.P., 1964. Barchan-dune movement in the Imperial Valley, California. Geological Society of America Bulletin 75, 149–156. Momiji, H., Warren, A., 2000. Relations of sand trapping efficiency and migration speed of transverse dunes to wind velocity. Earth Surface Processes and Landforms 25, 1069–1084. Parsons, D.R., Walker, I.J., Wiggs, G.F.S., 2004. Numerical modelling of flow structures over idealized transverse aeolian dunes of varying geometry. Geomorphology 59, 149–164. Schwämmle, V., Herrmann, H.J., 2005. A model of barchan dunes including lateral shear stress. The European Physical Journal E 16, 57–65.
238
Z.-T. Wang et al. / Geomorphology 96 (2008) 233–238
Shao, Y.P., 2000. Physics and Modelling of Wind Erosion. Kluwer Academic, Dordrecht, pp. 341–350. Stam, J.M.T., 1997. On the modelling of two-dimensional aeolian dunes. Sedimentology 44, 127–141. Wang, Z.-T., 2006. Influence of moisture on the entrainment of sand by wind. Powder Technology 164, 89–93.
Wang, Z.-T., Tao, S.-C., Xie, Y.-W., Dong, G.-H., 2007. Barchans of Minqin: morphometry. Geomorphology 89, 405–411. Wiggs, G.F.S., 2001. Desert dune processes and dynamics. Progress in Physical Geography 25, 53–79.