Wind tunnel measurements of adobe abrasion by blown sand: profile characteristics in relation to wind velocity and sand flux

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1 Journal of Arid Environments (2003) 53: doi: /jare Wind tunnel measurements of adobe abrasion by blown sand: profile characteristics in relation to wind velocity and sand flux Liu Lian-You %,w,z, *, Gao Shang-Yu %, w,z, Shi Pei-Jun %, w,z, & Li Xiao- Yan}, Dong Zhi-Bao} % The Key Lab of Environment Change and Natural Disaster, Ministry of Education, Beijing, , China wchina Center of Desert Research at Beijing Normal University, Beijing , China z Institute of Resources Sciences, Beijing Normal University, Beijing , China } Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou , Peoples Republic of China (Received 30 January 2002, accepted 1 June 2002) Blown sand causes various damages, such as extensive abrasion to crops, structural wear of facilities and buildings, and abrasion of soil clods and clayey materials generating fine particulate matter. In this study, experiments conducted in a straight-line blowing wind tunnel confirmed field observations and provided useful information for understanding abrasion profiles created by sand drift. Abrasion rates of 20 adobe blocks in a pile 50 cm high were determined at wind velocities of 10, 12, 14, 16, 18, 20 and 22 m s 1. Wind velocities at eight different heights were measured with a wind profiler, and the transport rate of sand flux was determined by a step-like sand trap. The rate of abrasion and sand transport increased exponentially with increasing wind velocities. Abrasion profiles obtained in the wind tunnel showed good consistency with previous observations in the field. The height of maximum abrasion shifted upward from 5 to 12?5 cm as wind velocity increased. Also, as wind velocity increased, the relative amount of saltating sand decreased under the height of 5?0 cm, but increased above 7?5 cm. Comparison of the rate of abrasion with the mass transport rate yielded a strong linear relationship (r 2 =0?99). The abrasion capacity of the saltating sand (defined as the ratio of abrasion rate to aeolian sand transport rate) increased logarithmically with wind velocity, and its peak value occurred at a height of 20 22?5 cm. The experiment demonstrated that adobe abrasion by aeolian sand was affected by both abrasion capacity and the sand transport rate. Shifting of height of maximum abrasion with wind velocity was mainly due to a corresponding mass change in the structure of the aeolian sand flow. # 2002 Elsevier Science Ltd. *Corresponding author. Current address: Institute of Resources Sciences, Beijing Normal University, Beijing , China Fax: lianyou@public.lz.yc.cn /02/ $35.00/0 # 2002 Elsevier Science Ltd.

2 352 L. LIAN-YOU ET AL. Keywords: abrasion by blown sand; adobe; wind tunnel experiment; aeolian sand flux Introduction Sand blown by wind is an important near-surface transport phenomenon in sandy lands and sand deserts (Bagnold, 1941; Pye & Tsoar, 1990; Cooke et al., 1993). Blown sand particles with high saltating and spinning speeds possess potent abrasive capacity (Zou et al., 1994; Livingstone & Warren, 1996). As a mode of wind erosion, abrasion by sand drift has been investigated in the field and tested in the laboratory. Erosion by wind-driven particles is often responsible for the development of unusual aeolian landforms ranging from centimeter-sized ventifacts to kilometer-long yardangs (Blackwelder, 1930, 1934; Whitney & Dietrich, 1973; McCauley et al., 1977; Ward & Greeley, 1984; Lancaster, 1984; Greeley & Iversen, 1985). Soil erosion and dust entrainment by wind is significantly intensified by bombardment of sand particles (Hagen, 1984; Dong et al., 1987; Zobeck, 1991; Shao et al., 1993; Gillette & Chen, 2001; Houser & Nickling, 2001). And shifting sand causes destructive impacts on buildings, abrasion of crops and many other damages in arid regions (Woodruff, 1956; Armbrust, 1972; Fryrear et al., 1975; Zhu et al., 1980; Wu, 1987). For example, in North-western China, there are many well-known cultural relics under special protection by local governments, the State Council, and the World Cultural Heritage Committee (Qu et al., 2001). But due to frequent aeolian activity, many of them are facing more and more damages by both sand burying and severe abrasion. Adobe, a kind of sun-dried, unburned brick of clay material, has been widely used in arid regions as construction material both in the past and today. Therefore, studying abrasion of adobe has a practical bearing on protection of historical sites and residential houses in arid regions (Fig. 1). Figure 1. Historical and cultural sites threatened by aeolian sand at (a) the entrance of the Black City (AD. 1200); (b) the ancient walls in Inner Mongolia (AD. 1200); and (c) Dunhuang Mogao Grottoes threatened by sand burying and abrasion (AD ).

3 WIND TUNNEL MEASUREMENTS 353 There are a number of studies on profiles of aeolian sand abrasion in the saltation boundary layer. Field observations on lucite rods exposed to sand drift displayed a maximum abrasion at cm above the ground (Sharp, 1964, 1980), and the maximum abrasion in fence posts during a dust storm event occurred at 24 cm high (Sakomoto-Arnold, 1981). The rate of abrasion is closely related to the kinetic energy of the sand particles and is affected by impact angle (Suzuki & Takahashi, 1981; Scattergood & Routbort, 1983). To explain the vertical pattern of abrasion, energy distribution of saltating sand particles was calculated by theoretical analysis and observation (Anderson, 1986; Zou et al., 2001). Due to temporal and spatial variability of erosive winds, understanding the vertical distributions of abrasion at various wind conditions is important in protecting facilities and buildings from degradation by sand in different aeolian environments (Skidmore, 1965). In this study, a shifting sandy surface was simulated, and adobe was tested as target material for abrasion in a wind tunnel. Through measurement of wind velocities, abrasion rates and corresponding sand transport rates, this experiment was intended to compare abrasion profiles under different wind velocities, to examine the relationship of abrasion to wind velocities and sand transport rates, and to evaluate the abrasion capacity of sand particles on adobe. Materials and methods Target material and abrader sand The adobe material used for targets was composed of very fine sand (38%), silt (58%) and a little clay (4%). Blocks (10?0-cm long, 5?0-cm wide and 2?5-cm thick) were prepared and sun-dried prior to testing. The abrader sand was collected from the checkerboard dune field of Tengger Sand Desert in Ningxia Hui Autonomous Region, China. The mechanical composition of the sand was 19% fine sand, 68% medium sand and 13% coarse sand (Fig. 2). Grain size distributions were analysed using a pipet method for the target material and dry sieving for the abrader sand (Xiong & Wen, 1953; Gee & Bauder, 1986). Wind tunnel tests The tests were carried out in the straight-line, blow-type wind tunnel at the Laboratory of Blown Sand Physics and Environment, Lanzhou Institute of Desert Research, Chinese Academy of Sciences. The wind tunnel has a total length of Cumulative%finer (by weight) (a) Grain diameter (µm) (b) Grain diameter (µm) Figure 2. Distributions of particle size for (a) the target material, and (b) the abrader sand

4 354 L. LIAN-YOU ET AL. 37?8 m; the working section is 16.2 m long, 1.0 m wide and 0.6 m high (Fig. 3(a)). Wind velocity can be changed continuously from 2 to 40 m s 1. Detailed descriptions of this facility and related instrumentation were given by Buckley (1987), Liu (1995) and Dong et al. (2001). For the experiments, a sand bed was placed on the tunnel floor upwind the adobe blocks in the working section. The sand bed was 3-cm thick and 15-m long, and served as the source for the development of aeolian sand flow. Twenty adobe blocks were stacked flat upon one another on the floor of the wind tunnel to form a target 50 cm tall. The frontal impact area of each block was 25 cm 2. In the wind tunnel, the adobe blocks were set up on the left-hand side at the back and tested in comparison with other materials (Fig. 3(b)). In designing the experiment, possible flow interference from the surrounding stacks was considered. The total width of the tunnel working section is 100 cm; the width of the three stacks was 10 cm each. The diameter of metal rods and bars in the front varied from 0?5 to 2 cm each. According to the wind tunnel s capability, the effect of flow interference from the surrounding stacks was minimum and under acceptable range. Abrasion tests were conducted under seven wind speed levels (10, 12, 14, 16, 18, 20, and 22 m s 1 measured by a standard Pitot tube at 20 cm above the floor). During the test, one stack of adobe blocks was used for all wind speeds. With increasing wind speed, testing time decreased from 60 to 10 min for each run. To examine development of the boundary layer, wind velocities at eight heights (0?3, 0?6, 1?0, 1?5, 3, 6, 12, and 20 cm above the sand surface) were measured by a wind profiler (Dong et al., 2001). Wind gradients indicated that the thickness of the boundary layer in the working section was 22 cm (Fig. 5). In order to keep good consistency with a natural setting, discussion about abrasion was made mainly in the section from the mobile sand bed to 22?5 cm high. The quantity of material abraded (q w ) at different heights was calculated from changes in weight of the adobe blocks (with a 0?01 g electronic balance) before and after each experiment. To avoid the influence of air moisture on the weight of the blocks, each weighing procedure was made after 24-h of oven drying at 1051C. The rate of abrasion (q a ) was defined as q a ¼ q w =ðstþ ðeqn 1Þ where S is the frontal impact surface area on the target, and t is the test time in minutes. The transport rate of sand (q s ) was measured by a 50-cm tall step-like sand trap. The rate of sand transport was measured at height increments of 1 cm. The Figure 3. The working section of the wind tunnel: (a) The exterior framework with top glass dormers and removable windows; (b) the adobe stack (50 cm high) on the left side at the back was tested in comparison with other materials.

5 WIND TUNNEL MEASUREMENTS 355 efficiency of the sand trap is 70 90% at wind velocities from 6 to 18 m s 1 (Liu, 1999a). An average efficiency of 80% was used to adjust data presented in this report. Results and discussion Characteristics of the abrasion profiles Abrasion occurred mainly on the front surface. The original surface was abraded progressively and unevenly. During the test, there was no obvious structural weakening or failure as could be observed in the field. The reason for this may have something to do with the relatively fine mechanical component and smaller size of the blocks. At each test velocity, there was an abrasion rate maximum at a certain height (H max ) above the ground (Fig. 4). However, the patterns of the abrasion profiles were similar to one another. The abrasion profiles in wind tunnel showed very good consistency with Sharp s original model established in the field (Sharp, 1964, 1980; Sakomoto- Arnold, 1981). The knowledge of abrasion profiles is important for it may be used in controlling aeolian sand abrasion. Because the test was conducted under controlled conditions in a wind tunnel, it was possible to analyse the response of the abrasion profiles to varying wind velocities. It could be readily observed that the H max shifted upward from 5 to 12?5 cm when wind velocity increased from 10 to 22 m s 1 (Fig. 4). The increase of abrasion rate with Figure 4. Profiles of the rate of abrasion at different wind velocities measured 20 cm above the tunnel floor (curves were obtained by polynomial fitting):, 10ms 1 ;, 12ms 1 ;, 14 m s 1 ;,16ms 1 ;,18ms 1 ;,20ms 1 ;,22ms 1.

6 356 L. LIAN-YOU ET AL. height is consistent with the kinetic energy distribution of saltating particles in the boundary layer (Gillette & Stockton, 1986; Anderson, 1986; Zou, 2001). At heights above H max, abrasion rate decreased with height above the sand bed. Aeolian sand abrasion in relation to erosive winds Based on wind velocity measurements, the velocity distribution in the boundary layer of the wind tunnel can be expressed as U z ¼ U =k lnðz=z 0 Þ; r 2 ¼ 098 ðeqn 2Þ where z is the height and z 0 is the roughness length (Fig. 5). In the wind tunnel, the threshold wind velocity was about 6 m s 1 at a height of 20 cm above the sand bed. At each height, the rate of abrasion increased exponentially with wind velocity. The correlation could be described as q a ¼ Ae BU ; r 2 ¼ ðeqn 3Þ where q a is the abrasion rate, U is the wind velocity, and A and B are regression coefficients. Figure 6 showed the relationship of average abrasion rate in the profile to wind velocities. Further regression analysis revealed that A and B changed with the height above the sand bed (Table 1). A decreased exponentially with the increase of height, and B increased logarithmically with height: A ¼ 00101e ; r 2 ¼ 098 ðeqn 4aÞ B ¼ ln ðhþþ03115; r 2 ¼ 095 ðeqn 4bÞ where H is the height above the sand bed in centimeter. The relationship between rate of abrasion and wind velocity was consistent with many other aeolian sand phenomena, such as that between the rate of sand transport and wind velocity, and that between soil loss with wind velocities (Wu & Ling, 1965; Zhu et al., 1980; Liu, 1999b). At high wind speeds such as 22 m s 1, it was very likely that the smallest grains in the abrader sand were travelling in modified saltation or true suspension. Also, a small fraction of grains ricocheted off the tunnel walls and roof prior to striking the target. 100 Height, H (cm) Wind velocity, U (ms 1 ) Figure 5. Wind velocity profiles in the wind tunnel boundary layer at free-stream wind velocities (20 cm high above the tunnel floor):,15?0ms 1 ; 18?5ms 1 ; and,24?5ms 1.

7 WIND TUNNEL MEASUREMENTS 357 Rate of abrasion, q a (g cm 2 min 1 ) Wind velocity U (ms 1 ) Figure 6. Relation curve between average rate of abrasion and wind velocity at 20 cm above the tunnel floor. Table 1. Correlation between rate of abrasion at different heights and wind velocities at 20 cm above the tunnel floor * Height (cm) A B r 2 0?0 2?5 0? ?3395 0?98 2?5 5?5 0? ?3654 0?98 5?0 7?5 0? ?3973 0?97 7?5 10?0 0? ?4326 0?98 10?0 12?5 0? ?4450 0?99 12?5 15?0 0? ?4592 0?98 15?0 17?5 0? ?4729 0?97 17?5 20?0 0? ?4847 0?98 20?0 22?5 0? ?4730 0?99 * Regression function q a = Ae BU, q a in g cm 2 min 1, and U the erosive wind velocity (20 cm high) in m s 1. Aeolian sand abrasion in relation to sand flux The rate of sand transport (q s ) is defined as the amount of sand passing through a unit area in unit time (Bagnold, 1941; Pye & Tsoar, 1990). Based on sand trap measurements, the vertical distributions of sand transport rates are shown in Fig. 7. At all heights, the sand transport rate, q s, increased with wind speed. The relationship between q s and wind velocity U can be expressed as q s ¼ Ce DU ; r 2 ¼ ðeqn 5Þ where C and D are regression coefficients closely related to height. At any given height, C and D are constants, and the rate of sand transport is dependent only on wind velocity. With increasing height, C decreased exponentially and D increased

8 358 L. LIAN-YOU ET AL. Figure 7. Vertical percentage of the sand transport rate under different wind velocities at 20 cm high:,10ms 1 ;,12ms 1 ;,14ms 1 ;,16ms 1 ;,18ms 1 ;,20ms 1 ; 22 m s 1. logarithmically: C ¼ 05292e 06464H ; r 2 ¼ 098 ðeqn 6aÞ D ¼ lnðhþþ02244; r 2 ¼ 099 ðeqn 6bÞ At any wind velocity, the rate of sand transport decreased with increasing height. The relationship between q s and H could be written as q s ¼ Ee FH ; r 2 ¼ ðeqn 7Þ where E and F are regression coefficients closely relevant to wind velocity. At a constant wind velocity, both E and F are constants, and q s is only a function of height. When wind velocity increased, E increased exponentially, while F decreased linearly: E ¼ 05410e 02343U ; r 2 ¼ 098 ðeqn 8aÞ F ¼ 00213U þ 06399; r 2 ¼ 095 ðeqn 8bÞ Figure 8 illustrates changes in the vertical structure of the aeolian sand flux with wind velocity. From the sand bed to a height of 22?5 cm, more than 70% of sand was transported below a height of 10 cm above the sand bed. However, the percentage of the sand flux at different heights depended on wind velocity. In the heights below 5 cm, the percentage of transported sand decreased with the increase of wind speed, from 62% at 10 m s 1 to 33% at 22 m s 1. At a height of 5?0 7?5 cm, the percentage of sand was relatively stable and remained 13 14% at varying wind velocities. At heights above 7?5 cm, the percentage of transported sand increased with wind velocities, from 24% at 10 m s 1 to 52% at 22 m s 1. Overall, the profiles of aeolian sand flux indicated that, on average, sand particles saltate to higher positions with increasing wind velocity (Zinamenski, 1960; Wu, 1987).

9 WIND TUNNEL MEASUREMENTS 359 Figure 8. Vertical percentage of the rate of sand transport at different wind velocities at 20 cm high:,10ms 1 ;,12ms 1 ;,14ms 1 ;,16ms 1 ;,18ms 1 ;,20ms 1 ; 22 m s 1 Abrasion rate, q a (g cm 2 min 1 ) Average sand transport rate, q s (g cm 2 min 1 ) Figure 9. Relationship between average adobe abrasion rate and average sand transport rate of all the heights, the dots indicated the seven testing wind speed levels. For all the nine different heights, comparing the average rate of abrasion (q a ) with the average sand transport rate (q s ) yielded a strong linear relationship (Fig. 9), which can be written as q a ¼ 00053q s 00109; r 2 ¼ 099 ðeqn 9Þ Distinct positive linear relationships existed between the rate of abrasion and the rate of sand transport at each height in the profile (Table 2). Hence, the rate of abrasion is directly affected by the rate of aeolian sand transport. It is similar to the relationship between the rate of dust emission and the rate of sand transport from a disturbed claycrusted surface (Houser & Nickling 2001). Previous work has suggested a linear

10 360 L. LIAN-YOU ET AL. Table 2. Correlation between rate of abrasion and sand transport rate at different heights Heights (cm) Formula r 2 0 2?5 q a =0?0017q s ?98 2?5 5 q a =0?0031q s 0?0102 0?97 5 7?5 q a =0?0050q s 0?0067 0?99 7?5 10 q a =0?0056q s 0?0075 0? ?5 q a =0?0062q s 0?0054 0?99 12?5 15 q a =0?0069q s 0?0021 0? ?5 q a =0?0081q s 0?0026 0?99 17?5 20 q a =0?0093q s 0?0033 0? ?5 q a =0?0098q s 0?0024 0?99 relation between the kinetic energy of the impacting grains and the abrasion rate (Suzuki & Takahashi, 1981). The existence of linear relationship between q a and q s,is perhaps, due to saltating sand was entrained by the kinetic energy of erosive winds. Abrasion capacity of sand particles on adobe Aeolian abrasion of the target material was caused by impacts of the saltating sand particles. In this experiment, the observations of abrasion rate and the sand flux at different heights provide a possibility for evaluating the abrasion capacity of the sand particles on adobe at different saltation heights. The abrasion capacity of sand particles can be expressed by the amount of abrasion per unit impacting sand (i.e. the ratio of the rate of abrasion to the rate of sand transport) expressed as Z ¼ q a =q s ðeqn 10Þ where Z is a dimensionless indicator of abrasion capacity. Using Eqn. (10) we calculated profiles of abrasion capacity of aeolian sand at each wind velocity (Fig. 10). The vertical distribution of abrasion capacity of aeolian sand on adobe had similar features at varied wind speeds. The peak value of abrasion capacity occurred on top of the profile at height from 20 to 22?5 cm. The value of abrasion capacity at heights below 22?5 cm, was 10 90% of the peak and increased with height. The heights in which the sand particles had relatively higher abrasion capacity on adobe occurred at the upper part of the boundary layer, so abrasion capacity may approximately reflect the average kinetic energy of the sand particles. The lower abrasion capacity at 0 5 cm heights, where high sand transport rate occurred, confirmed the preponderance of low-energy trajectories in the saltation population (Anderson & Willetts, 1991). At each height in the profile, the abrasion capacity increased with wind velocity. The relationship between abrasion capacity and wind velocity can be written as Z ¼ 0005 lnðuþ 00091; r 2 ¼ 096 ðeqn 11Þ The relationships in Eqns. (3), (5) and (7) revealed that, as wind velocity increased in the saltating boundary layer, the rate of abrasion, the sand transport rate (i.e. the number of sand particles) and the abrasion capacity of sand particles increased. From 10 to 22 m s 1, the rate of abrasion increased more than 100 times, the rate of sand transport increased about 40 times but the abrasion capacity of the sand particles increased by only about 3 times (Fig. 11). This implied that the increase of abrasion with wind velocity was mainly affected by the increase of sand transport rate, and partially by the increase of abrasion capacity of the sand particles.

11 WIND TUNNEL MEASUREMENTS Height, H (cm) (a) (b) (c) (d) (e) (f) (g) Abrasion capacity of aeolian sand on adobe, η Figure 10. Abrasion capacity (the ratio of the rate of abrasion to the rate of sand transport) at different wind velocities at 20 cm high: (a) 22 m s 1 ; (b) 20 m s 1 ; (c) 18 m s 1 ; (d) 16 m s 1 ; (e) 14 m s 1 ;(f)12ms 1 ; (g) 10 m s Average abrasion capacity, η Wind speed, U (m s 1 ) Figure 11. Relationship between average abrasion capacity and wind speed at 20 cm high above the tunnel floor. Conclusions Tests of aeolian sand abrasion on adobe confirmed that its vertical distribution in the wind tunnel was similar to that observed in the field. The position of maximum abrasion shifted upward as wind velocity increased. The characteristics of the abrasion profiles should be taken into consideration in designing controls on abrasion damage to engineering facilities. The rate of abrasion and the rate of sand transport both increased exponentially with increasing wind velocity. There was a strong linear relationship between the rate

12 362 L. LIAN-YOU ET AL. of abrasion and the sand transport rate. The abrasion capacity of the saltating sand increased logarithmically with wind velocity. The heights in which the sand particles had relatively higher abrasion capacity on adobe occurred at the upper part of the boundary layer, so abrasion capacity may approximately reflect the average kinetic energy of the sand particles. The increase of abrasion with wind velocity was mainly affected by the increase of sand transport rate, only partially by the increase of abrasion capacity. Because the vertical pattern of abrasion capacity remained almost identical at various wind velocities, the shifting of the position of peak abrasion was due chiefly to change in the vertical mass structure of the sand flow. This research only presented some measurement results of adobe abrasion on a sand bed in a wind tunnel. More field observation data are needed to verify the wind tunnel results. Further work such as study on bed properties, characteristics of engineering materials, the kinetic energy of the sand particles and their interaction would be necessary in understanding the process of aeolian sand abrasion. This work was jointly carried out under the auspices of the National Key Project for Research The Process of Desertification and its Control in Northern China (G ), the Hundred Talents Project of the Chinese Academy of Sciences and the National Natural Science Foundation of China ( ). We sincerely thank Liu Xianwan, Li Changzhi, Wang Guochang, Liu Yuzhang and other colleagues for helping with the wind tunnel tests. Sincere thanks are due to Dr. L. Hagen, Dr. E. Skidmore and Dr. Simon van Donk for their scientific reviews of the manuscript. The authors highly appreciate the comments of two anonymous reviewers, which greatly improved the manuscript. References Anderson, R.S. (1986). Erosion profiles due to particle entrained by wind: application of an aeolian sediment-transport model. Geological Society of America Bulletin, 97: Anderson, R.S. & Willetts, B.B. (1991). A review of recent progress in our understanding of aeolian sediment transport. Acta Mechanica (Suppl) 1: Armbrust, D.V. (1972). Recovery and nutrient content of sandblasted soybean seedlings. Agronomics Journal, 64: Bagnold, R.A. (1941). The Physics of Blown Sand and Desert Dunes. London: Methuen. 241 pp. Blackwelder, E. (1930). Yardang and Zastruga. Science, 72: Blackwelder, E. (1934). Yardangs. Geological Society of America Bulletin, 45: Buckley, R.C. (1987). The effect of sparse vegetation on the transport of dune sand by wind. Nature, 325: Cooke, R.U., Warren, A. & Goudie, A.S. (1993). Desert Geomorphology. London: UCL Press. 526 pp. Dong, G.R., Li, C.Z., Jin, J. et al. (1987). Some experimental results of wind tunnel simulation on soil wind erosion. Chinese Science Bulletin, 32: Dong, Z.B., Wang, X.M., Zhao, A.G. et al. (2001). Aerodynamic roughness of fixed sandy beds. Journal of Geophysical Research, 106: Fryrear, D.W., Armbrust, D.V. & Downes, J.D. (1975). Plant response to wind-erosion damage. Proceedings of the 30th Annual Meeting of Soil Conservation Society of America, San Antonio, TA, pp Gee, G.W. & Bauder, J.W.(1986). In: Klute. A. (ed) Methods of Soil Analysis. Part 1FPhysical and Mineralogical Methods (2nd edn). Madison: WI pp. Gillette, D.A. & Chen, W.N. (2001). Particle production and aeolian transport from a supplylimited source area in the Chihuahuan Desert, New Mexico, United States. Journal of Geophysical Research, 106: Gillette, D.A. & Stockton, P.H.(1986). Mass momentum and kinetic energy fluxes of saltating particles. In: Nickling, W.G. (Ed.), Aeolian Geomorphology, London: Allen and Unwin, pp pp. Greeley, R. & Iversen, J.D.(1985). Wind as a Geological Process on Earth, Mars, Venus, and Titan. Cambridge: Cambridge University Press. 333 pp.

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