Internationa l Journal of Geosciences, 2014, 5, 20-26
Published Online January 2014 (
The Effect of a Single Shrub on Wind Speed and Nabkhas
Dune Development: A Case Study in Kuw ait
Jasem M. Al-Awadhi
Department of Earth and Environmental Sciences, Faculty of Science, Kuwait University, Kuwait City, Kuwait
Received October 26, 2013; revised November 28, 2013; accep ted December 25, 2013
Copyright © 2014 Jasem M. Al-Awadhi. Th is i s an open access article distributed under the Creative Commons Attribution License,
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Thirty coastal nabkhas were selected for morphometrical measurements. The studied nabkhas were mostly
elongated, with an average total length of about 12.9 m, an average width of 3.4 m, and an average height of 1.2
m. Optical porosity of nabkha shrub c rown w as measure d and no appare nt relatio nship with t he horizonta l size
of trapped w ind laden sand was found. A simple wind tunnel experi ment was carried out to investigate the hori-
zonta l w ind-flow distri butio n a cro ss a pr o-typed shrub. The results o f the experiment revealed that the degree of
wind shelt e r i ng might extend up t o a downwind dista nce approximately e qual to 4.5 times the height of the shrub,
where an effective velocity recovery started.
Nabkhas; Sediment; Poro sity; Wind Tu nne l
1. Introduction
Nabkha is a type of Aeolian landforms, which is com-
monly developed as a result of sand accumulation around
coastal and desert shrubs [1]. The morphology of nabk-
has is controlled by growth patterns of shrub [2]. The
height of nabkha, to some extent, is related to the height
of the shrub crown, while its length is related to the
overall height of the shrub [3], width of the basal shrub
and wind velocity [4]. Other important factors control-
ling nabkhas morphology include type of sediment
supply and climate [5] as well as the porosity of the
shrub crown (e.g., [2]). The wind regime is a major cli-
matic factor that influences Aeolian processes including
nabkha development [7]. The air-flow patterns over
nabkhas were studied in a series of wind-tunnel experi-
ments by Zhizhong et al. [8]. Nabkhas form a surface
roughness, where they interact with airflow and, there-
fore, cause significant variations in wind velocity and
direction over the surface.
Grant and Nickling [9] carried out a field study to in-
vestigate the effect of vegetation porosity (leaf density)
on the drag coefficient of small conifer trees (1.4 m in
height ). T hey fou nd that the po rous e lement had a hi gher
drag coefficient than a solid element, and the drag coef-
ficie nt cha nged o n a cont inuu m with porosity, reaching a
peak at an intermediate porosity value (0.32), and even-
tually falling to zero when the element was removed.
Grant and Nickling [9] came to a conclusion that this
peak in the drag coefficient (momentum extraction of
vegetation) versus porosity curve corresponded to shel-
terbelt efficienc y that peaked at medium-poro sities.
Aeolian processes including deflation, transportation
and deposition are very active in Kuwait, where they are
related to the dry, hot, windy climate, the detrital nature
of bed rock and its location downwind from the high-
deflation area of Mesopotamian floodplain [10,11]. As a
result, more than 50% of the Kuwait surface is covered
by 13 mobile sandy bodies. These are continually drifted
along the surface by wind, mainly during the summer
season (May to September), to form different land forms
including coastal nabkhas [12]. To provide a better un-
derstanding of the mechanism of nabkha development,
this paper discusses the results of the horizontal wind-
flow distribution across a pro-typed shrub, obtained from
a wind-tunnel experiment and supported by field mea-
surements of horizontal size of trapped wind laden sand
around shrubs.
2. Materials and Methods
An attempt was made to carry out field measurements of
morphome trical characteristics of thirty coastal nabkhas
to support the results of the wind-tunnel. The morphme-
tric parameters are defined in Figure 1. The field study
area is located within the northern coastal plain of Ku-
wait Bay and downwind of the major mobile sand corri-
dor extending continuously from the northwestern border
of Kuwait in a SE direction. The study area is well de-
scribed by Al-Awadh i an d Al -Do usari [13 ]. I n this stud y,
another attempt was also made to investigate the effect of
shrub crowns porosity (p) of nabkhas on the horizontal
dimension of sand trapped using image processing tech-
nique built in PAX-it software. Digital images of the
shrubs, with placing a red board behind the shrub as a
distinguish background, were captured and analyzed for
determining the porosity percentage. The angle of inci-
dence of the image was selected to be consistent with the
dominant wind direction (NW).The porosity of the shrub
was then calculated as the percentage of red spots to the
total selected area (F igure 2). Fo r ea ch s hrub t wo i mages
were captured, one upwind side and other downwind side,
the calculated values were then averaged for the porosity
Variation in up and downwind velocities over pro-
typed shrub due to its interaction with airflow was de-
termined by conducting a simple experiment using the
Kuwait Institute for Scientific Research (KISR) wind
tunnel (20 m long wind tunnel with a test-section di-
mension of 1.2 m wide and 0.95 m height). The wind
tunnel is of the blower type driven by a centrifugal fan.
The components of the wind tunnel are shown in Fig-
ure 3.
A profile for wind flow distribution across a single
shrub model (10 cm height) was obtained by using
Dwyer Pito t-static t ubes fixe d a t a hei ght o f 1 5 cm; i.e., 5
cm above the height of the model. As a reference point,
the mean velocity was also measured at 140 cm upstream
from the upwind edge of the model, on continuous bases
at the same height; i. e. , 15 cm. For a better simulation
with the case in the field: 1) the tested shrub model was
erected among other similar shrub models in the test-
section of the wind tunnel along a line perpendicular to
the flow direction and uniformly spaced at a center-to-
center distance of 10 cm (Figure 4); 2) the whole up-
stream length of the wind tunnel up to upwind edge of
the model was laden by a sand o f median grain size equal
to 0.25 mm, obtained locally from the slip face of a bar-
chan dune; and 3) based on the field measurements, the
shrub model crown (7.5 cm) was l ifted up fro m the wind
tunnel testing floor to a height equal to 50% of its total
height ; this he ight re presents the aver age hei ght of nab k-
ha dune crest, measured in the field, with respect to the
its shrub height; and 4) a preselected velocity (6.3 m/s;
fan-settings = 11 Hz) representing an average wind speed
during summer period where Aeolian processes are ac-
Figure 1 . Schematic diagram illustrating the morphometric parameters of nabkhas.
Figure 2. Change detection input and output s howing the size and distribution of air void, in shades of dark color, within a
nab kha shrub.
Figure 3 . Components of Kuwait Institute for Scientific Research (KISR) wind tunnel.
Figure 4. Ex perimental setup s howi ng the erection of pro-t y pe d shr u b model s a cr os s the test -section of the wind tunnel.
3. Result
3.1. Morphom etr ica l Parameters o f the Studied
The studied nab khas ha ve an oval shape elongated paral-
lel to the NW prevailing wind direction. Measurements
of the morphometrical parameters of the studied nabkha
are presented in Table 1. The total length (L) of nabkha
dunes va ri es b e t wee n 8 a nd 1 8 m wit h an aver a ge of 1 2 .8
m, while their width (W) across their crest ranges be-
tween 2.1 and 4.9 m. T he length of the base (LB ), where
the shrub grows, varies between 1.7 and 4.3 m with an
average of 2.9 m. The length of the tail (LT) varies be-
tween 4.1 and 10.8 m with an average of 7.1 m, whil e t he
length of the nose (LN) varies between 1.2 and 4.3 m
with an average of 2.8 m.
The crest height of the nabkhas (h) ranged between 0.5
to 2.6 m above their base level with an average of 0.9 m,
while the height of shrub (H) ranged between 1.1 to 3.7
m above the ground with an average of 1.8 m. The opti-
cal porosity of shrubs crown (p), the percentage of voids
to total area of the shrub, of the studied nabkhas ranged
between 7.1% to 18% with an average of 11.7%. Using
the relationship obtained by Grant and Nickling [9]:
op vp=
where op is the optical porosity and vp is the volumetric
porosity, the volumetric porosity (3-diemtional porosity
the percentage of voids to total volume of the shrub) of
the studied nabkhas ranged between 38.6% to 54% with
an average of 46%.
Figure 5 illustrates the effect of shrub porosity (p) on
the le ngt h d i me nsions o f sa nd tr a pp e d. T he fi gur e wea kl y
reveals that as the porosity increases the horizontal
length of the nabkha decreases; i.e., it reveals weak cor-
relations of such effect either on total length (L) or par-
tial length ( LN or LT ). T he possible exp lanatio n for s uch
weakness in relationship is t hat the air flow, carr ying the
sedi ments, p assi ng the shrub ma y var y due to high va ria-
tion in the porosity over the upwind shrub coverage area
(leaf density), thus restricting the maximum growth of
the nabkha in the ho rizo ntal as well as vertica l dir ectio ns.
The other possible explanation could be related to the
difficulty with using optical porosity; i.e., two elements
of similar optical porosities may have significantly dif-
ferent amounts of pore space within them, altering the
way in which the wind would interact with these ele-
ments [9]. For this reason, the crown porosity of the
tested shrub model, used in the wind tunnel experiment,
was not ta ken into consider ation.
3.2. Wind Flow Pattern across a Shrub
Nabkha dunes are formed as a result of significant varia-
tions in wind velocity caused by the interaction of the
shrub with airflo w carrying Aeolian sand. Such variations
Table 1. Morphometrical parameters of the studied nabk-
has (all value s in m).
No. H LN LB LT h W P (%)
1 2.1 3.3 2 .5 6.3 1.2 4 .9 11.4
2 1.8 2.5 2 .5 4.9 0.8 2 .6 7.6
3 1.7 1.2 2 .4 4.4 0.8 2 .7 13.6
4 1.4 2.4 2 .6 7.6 0.7 2 .7 9.5
5 1.8 2.9 3 .9 4.9 1.4 3 .7 13.6
6 1.5 2.4 2 .2 8.1 0.6 2 .2 11.9
7 2.3 3.3 2 .6 7.1 0.9 4 .3 11.0
8 2.7 2.4 1 .9 5.5 1.0 3 .7 10.7
9 1.1 3.9 2 .9 7.9 1.1 3 .7 7.1
10 1.6 3.1 3.8 9.1 1.0 3.8 10.6
11 1.9 2.3 1.7 5.9 0.7 2.1 8.4
12 1.8 2.9 2.6 7.4 0.9 2.9 17.7
13 1.2 3.6 3.3 10.1 0.7 3 .6 17.2
14 1.5 3.4 3.1 7.1 0.7 3.1 8.7
15 1.8 4.3 2.9 10.8 0.9 3 .2 9.3
16 2.1 2.1 2.9 7.6 0.9 4.1 13.2
17 1.9 2.6 2.3 10.5 1.1 3 .8 11.3
18 3.7 2.5 2.2 9.5 2.6 3.6 14.1
19 2.1 4.1 3.5 8.1 0.9 3.8 12.7
20 1.4 3.4 3.5 7.1 1.0 3.6 15.0
21 2.0 3.9 4.3 8.1 0.9 4.3 7.2
22 2.3 2.3 3.4 7.1 1.3 3.8 10.5
23 1.3 1.9 3.4 5.1 0.5 2.1 11.1
24 1.1 1.9 2.4 4.8 0.6 2.3 18.0
25 1.3 2.7 3.3 5.9 0.7 3.5 13.0
26 2.2 2.8 3.0 7.5 1.0 3.4 9.6
27 2.8 3.7 3.4 6.3 1.0 4.0 12.4
28 2.0 2.4 3.2 7.3 1.0 2.9 8.1
29 1.8 2.8 3.9 7.3 0.8 3.4 10.1
30 1.3 2.2 2.1 4.1 1.0 3.1 15.9
Ave 1 .8 2.8 2.9 7.1 0.9 3 .4 11.7
Max 3.7 4.3 4.3 10.8 2.6 4.9 18.0
Min 1 .1 1.2 1.7 4.1 0.5 2 .1 7.1
in wind velocity were determined by conducting a simple
wind tunnel experiment, and Figure 6 shows the non-
dimensional mean flow distribution measured across a
single shrub model.
The figure reveals that the mean wind velocity imme-
diately over the shrub; i. e. , the direct interaction with
airflow, is increased by a factor of 39%, and greater
mean velocity reduction (83%) occurred downstream
from the shrub at a distance 3.5 times its height (H),
while in upstream the greater reduction (37%) occurred
Figure 5. Corre latio n betw een lengt hs of na bkha d une with
the shrub porosity (p).
Figure 6 . Velocity distri bution across a model shrub; H and
x are the height of shrub and distance from it, respe ctively;
U0 is far upstream velocity an d U is velocity at dis tance x.
at a distance 1 time H. The effective velocity recovery in
downstream starts at distance approximately 4.5 times
the shrub height, while the effective recovery in upwind
stream seems to start at a distance 2 time the s hrub he i g ht.
Greater mean velocity reduction occurred immediately
downstream from the shrub with a maximum value at a
distance 2 times H.
Relating the wind tunnel findings, shown in Figure 6,
with the horizontal dimensions of the studied nabkha
dunes, it is obvious that the reduction or the degree of
sheltering (tail length of nabkha dune) of studied shrubs
extends in average to a downwind distance equal to 3.9
times the height of the shrub, where an effective wind
velocity recovery starts (x/H = 4). Similarly, the upwind
deposition forming the nabkha nose extends to an aver-
age distance equal to 1.5 times the height of shrub from
the base, where an effective upwind velocity reduction
starts at (x/H = 2). The up wi nd p eak r educ tion i n vel oc-
ity occurs at x/H = 1, while the peak reduction in veloc-
ity occurs at a downwind distance equal to 2 time the
height of shrub (x/H = 2), where the crest of nabkha is
formed. The length of the base (LB), where the crest oc-
curs and the shrub grows, has an average value equal to
68 10 12 14 16 18 20
Total Length of Nabkha (m)
Optical Porsity of Shrub , p(%)
Tail Length (TL(
Total Length (L(
-2 02468 10 12 14 16
1.6 times the height of shrub where the peak reduction in
velocity occurs. It must be pointed out that these mea-
surements are rather preliminary, but they are encourag-
The extent of reduction in erosion, or the degree of
sheltering sand can be observed from Figure 6. It ex-
tends up to an average downwind distance equal to 4.5
times the height o f the shrub mode l (x/H = 4.5), where an
effective velocity recovery starts. This finding agrees
with the field measurements stated in the Section 3.1
(morpho metric of nabkha); th e tail length, which is asso-
ciated with the degree of sheltering effect, may extend in
average 4.7 times (0.5LB + LT) that of shrub height.
From the same figure, it can be figured out that the up-
wind deposition forming the nabkha nose may extend to
a distance equal to two times the height of the shrub
model. Reviewing the field measurements, it was found
that the nose may extend in average 2.3 times (0.5LB +
LN) that of shr ub hei ght.
4. Discussions
Nabkha is a special depositio nal Aeolian land for m where
accumulation of wind-laden particles is facilitated by
vegetation as micro-wind breakers. Therefore, the mor-
phometrical characteristics of nabkha sediments are
closely related to vegetation morphology [1,14-16]. The
nabkha thus grows along with plant, both vertically and
horizontally [13].
Despite the importance of the continuous supply of
sand, the size variability of nabkhas in the study area is
controlled chiefly by the total shrub hei ght [13]. I t is also
believed that, like sand fence porosity, the optical poros-
ity of the shrub crown may affect the percentage of the
trapped sand. The effect of porosity of sand fences on
trapping wind laden sands has been studied by many a u-
thors [17-21]. Most of related studies concluded in gen-
eral that there is a relationship between drag coefficient
(Cd) (wind force) and the optical porosity (op), and the
most effective porosity in reducing the mean, near-
ground wind speeds for the longest distances (10 to 15
times the height of the porous fence) ranges between
30% to 50%. Taylor [22] presented a curve showing the
relationship between Cd and op; the porous elements
have higher Cd than solid elements and it may increase
up to 50%. In this study, however, the investigation of
the effect of shrub o ptical porosit y on the horizontal size
of trapped wind laden sand does not show apparent rela-
tionship in this regard (Figure 5). In addition to the ex-
planations given in the result sectio n, for such weak rela-
tionship, the effect of shrub porosity on downwind re-
gime may not be comparable to similar effect of porous
fences, simply because a three dimensional porous shrub
consists of a number of individual solid leaves, which
interact with one another in a flo w similar to wake inter-
ference. Thus, some momentum would be extracted by
an upwind leaf, more by the next downstream leaf, until
either all of the momentum is extracted or the other side
of the shrub is reached [9]. This will certainly affect the
Cd magnitude and subsequently affect in reducing the
mean, near-ground wind speeds for the shortest distances
(about 4 times the height of the shrub; (Figure 6)).
It is known that the total forces imparted by the wind
to composite surfaces will be portioned between the
erodible (nabkha dune) and non-erodible (shrub) surface
elements [23]. The main influence of non-erodible ele-
ment on erosion is sheltering effect. It captures some of
the disturbing forces of wind and incomi ng saltations that
would otherwise move the erodible surface sediments
[24]; i.e. , the shrub dec rea ses the wind gr ain st re ss on t he
nabkha dune surface by absorbing a significant fraction
of the downwind momentum flux [24-26]. Thus, the re-
duction in the flow strength near a shrub bed reduces the
sand flux, forming the crest part of nabkha, then part of
this flux is deposited upwind and further downwind,
forming the nose and tail, respectively, until an equili-
brium condition exists between the height of the shrub
(including leaf density) and sediment deposition. Accor-
dingly, (Figure 7), based on field and wind-tunnel mea-
surements, illustrates schemat ically the role o f a shrub in
interfering wind flow and sediment deposition patterns.
5. Conclusions
Wind tunnel experiment on a shrub reveals that greater
mean velocity reduction (wake zone) occurs immediately
down and upstream from the shrub, where most of the
sand flux deposits, forming the crest part of nabkha dune.
Then part of this flux is deposited further up and down-
wind, forming the ends of nose and tail of nabkha at up-
wind and downwind distances of approximately 2.3 and
4.7 times the height of the shrub, respectively, until an
equilibrium condition exists between the height of shrub
and wind sediment deposition. Field observations indi-
cate that nabkha dune volume is closely related to shrub
height rather than its porosity (leaf density), the higher
the shrub the larger the nabkha, while no apparent rela-
tionship with respect to the horizontal component of the
nabkha is found as the crown porosity of the shrub
The study shows that the pattern of horizontal wind
flo w red uctio n in front and be hind a shr ub is gene rall y in
agreement with the horizontal sediment deposition pat-
tern around a shrub.
Ackno wledgements
The author would like to express his appreciation and
gratitude to Mr. Nabil Basili for helping in field works.
This work was supported by Kuwait University Research
Figure 7 . Schematic diagram fo r wi nd interference with the shrub t o form nabkh a dune.
Grant No. [SE01/11].
[1] K. Ardon, H. Tsoar and D. G. Blumberg, “Dynamics of
Nebkhas Superimposed on a Parabolic Dune and Their
Effect on the Dune Dynamics,” Journal of Arid Environ-
ments, Vo l . 73, No. 11, 2009, pp. 1014-1022.
[2] A. Tengber g and D. L. Chen, “A Comparative Analysis
of Nebkhas in Central Tunisia and Northern Burkina Fa-
so,” Geomorphology, Vol. 22, 1998, pp. 181-192.
[3] F. I. Khalaf, R. Misak and A. Al-Dousari,” Sedimento-
logical and Morphological Characteristics of Some Na-
bkha Deposits in the Northern Coastal Plain of Kuwait,
Arabia,” Journal of Arid Environments, Vol. 29, No. 3,
1995, pp. 267-292.
[4] P. A. Hesp , “Ecological Processes and Plant Adaptations
on Coastal Dunes,” Journal of Arid Environments, Vol.
21, No. 2, 1991, pp. 165-191.
[5] R. Cooke, A. Warren and A. Goudie, “Desert Geomor-
phology,” UCL Press Limited, University College Lon-
don, London, 1993.
[6] S. J. Lee, K. C. Park and C. W. Park, “Wind Tunnel Ob-
servations about the Shelter Effect of Porous Fences on
Sand Particle Movements,” Atmospheric Environment,
Vol. 36, No. 9, 2002, pp. 1453-1463.
[7] J. M. Al-Awadhi, A. Al-Hellal and A. Al-Enezi, “Sand
Drift Potential in the Desert of Kuwait,” Journal of Arid
Environments, Vol. 63, No. 2, 2005, pp. 425-438.
[8] L. I. Zhizhong, W. U. Shengli, D. L. Jani s, G. E. Lin, H.
E. Mudan, W. Xiaofeng, J. Jianhui, L. Jinwei, L. I. Wan-
juan and M. A. Rong, “Wind Tunnel Experiments of Air
Flow Patterns over Nabkhas Modeled after Those From
the Hotan River Basin, X injiang, China (II): Vegetated,”
Earth Science China, Vol. 2, No. 3, 2008, pp. 340-345.
[9] P. F. Grant and W. G. Nickling, “Direct Field Measure-
ment of Wind Drag on Egetation for Application to
Windbreak Design and Modelling,” Land Degradation
and Development, Vol. 9, No. 1, 1998, pp. 57-66.
[10] F. I. Khal af and D. Al-Aj mi, “Aeolian P rocesses and Sand
Encroachment Problems in Kuwait,” Geomorphology,
Vol. 5, No. 2, 1993, pp. 111-134.
[11] A. Al-Enezi, K. Pye, R. Misak and S. Al-Hajraf, “Mor-
phologic Characteristics and Development of Falling
Dunes, Northeast Kuwait,” Journal of Arid Environments,
Vol. 72, No. 4, 2008, pp. 423-439.
[12] J. M. Al-Awadh i and R. M. Mis ak, “Field Assessment of
Aeolian Sand Processes and Sand Control Measures in
Kuwait,” Kuwait Journal for Science and Engineering,
Vol. 27, No. 1, 2000, pp. 156-176.
[13] J. M. Al-Awadhi and A. M. Al-Dousari, “Morphological
Characteristics and Development of Coastal Nabkhas,
North-East Kuwait,” International Journal of Earth Sci-
ences (Geologische Rundschau), Vol. 102, No. 3, 2013,
pp. 949-958.
[14] X. Wang, T. Wang , Z. Dong, X. Liu and G. Qian,
Nebkha Development and Its Significance to Wind Ero-
sion and Land Degradation in Semi-Arid Northern China,”
Journal of Arid Environment, Vol. 65, No. 1, 2006, pp.
[15] P. Hesp and A. McLachlan, “Morphology Dynamics,
Ecology and Fauna of Arctotheca Populifolia and Gaza-
nia Rigens Nabkha Dunes,” Journal of Arid Environ-
ments, Vo l . 44, No. 2, 2000, pp. 155-172.
[16] A. J. Dougill and A. D. Thomas, “Nebkha Dunes in the
Molopo Basin, South Africa and Botswana: Formation
Controls and Their Validity as Indicators of Soil Degra-
dation,” Journal of Arid Environments, Vol. 50, No. 3,
2002, pp. 413-428.
[17] K. K. Bofah and K. G. Al-hinai, “Field Tests of Porous
Fences in The Regime of Sand-Laden Wind,” Journal of
Wind Engineering and Industrial Aerodynamics, Vol. 23,
1986, pp. 309-319.
[18] J. E. C ermak, “Advances in Physical Modeling for Wind
Engineering,” Journal of Engineering Mechanics, ASCE ,
Vol. 113, No. 5, 1987, pp. 737-756.
[19] M. D. Perera, “Shelter Behind Two-Dimensional Solid
and Porous Fences,” Journal of Wind Engineering and
Industrial Aerodynamic, Vol. 8, No. 1-2, 1981, pp. 93-
[20] G. M. Heisl er and D. R. DeWalle, “International Sympo-
sium on Windbreak Technology,” Reprinted from Agri-
culture, Ecosystems and Environment, Vol. 22-23, pp.
41-69, Elsevier Science Publishers BV, Amsterdam,
[21] S. J. Lee, K. C. Park and C. W. Park, “Wind Tunnel Ob-
servations about the Shelter Effect of Porous Fences on
Sand Particle Movements,” Atmospheric Environment,
Vol. 36, No. 9, 2002, pp. 1453-1463.
[22] P. A. Taylo r, “Turbulent Wakes in the Boundary Layer,”
In: W. L. Steffen and O. T. Denmead , Eds, Flow and
Transport in the Natural Environment: Advances and Ap-
plications, Springer-Verlag, Berlin, 1988, pp. 270-292.
[23] J. M. Al-Awadhi (Abdullah), “Sand Transport and Depo-
sition over No nerod ible Elements,” Ph.D. Thesis, Univer-
sity of Aber deen, Aberdeen, 1996.
[24] D. A. Gillette and P. H. Stockton, “The Effect of Non-
Erodible Particles on Wind Erosion of Erodible Surfaces,”
Journal of Geophysical Research, Vol. 94, No. D10, 1989,
pp. 12885-12893.
[25] L. K. Marshall, “Drag Measurements in Roughness Ar-
rays of Varying Density and Distribution,” Agricultural
Meteorology, Vol. 8, 1971, pp. 269-292.
[26] L. Lyles, R. L. Schrandt and N. F. Schneidler, “How
Aerodynamic Roughness Elements Control Sand Move-
ment,” Transactions, American Society Agricultural En-
gineer s , Vol. 17, No. 1, 1974, pp. 134-139.