Windbreak design winderosion

Journal of
Arid
Environments
Journal of Arid Environments 61 (2005) 315–332
Optimal windbreak design for wind-erosion
control
W.M. CornelisÃ, D. Gabriels
Department of Soil Management and Soil Care, International Centre for Eremology, Ghent University,
Coupure links 653, B-9000 Ghent, Belgium
Received 28 November 2003; received in revised form 20 August 2004; accepted 21 October 2004
Abstract
In order to find the optimal windbreak design in terms of its porosity, its distribution with
height and the number of rows needed, a wind-tunnel study was conducted. Measurements of
the total wind-velocity reduction coefficient of windbreaks with a porosity ranging from 0 to
1 m2
mÀ2
showed that a Gaussian peak function fitted very well to the data. It was concluded
that a porosity of 0.20–0.35 m2
mÀ2
is optimal in terms of wind-velocity reduction. With
regards to the distribution of porosity with height, an evenly distributed porosity of stem and
canopy resulted in the longest protected area, i.e. the area where the wind-velocity reduction is
more than 50%. This was also the case for single-row barriers compared to two- and three-row
windbreaks. Experiments with dry loose dune sand were conducted to deduce zones of erosion
and deposition behind differently designed windbreaks. The observed zones could be well
explained by considering the threshold wind velocity that was computed for the dune sand
used in combination with isowind-velocity lines. The results are useful for construction of
windbreaks. However, one should bear in mind that the optimal design depends strongly on
the purpose for which it is constructed, viz. protection of a field from being eroded or
protection of infrastructure from being buried.
r 2004 Elsevier Ltd. All rights reserved.
Keywords: Wind erosion; Windbreak; Wind barrier; Sand movement; Sand dunes; Deposition
ARTICLE IN PRESS
www.elsevier.com/locate/jnlabr/yjare
0140-1963/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jaridenv.2004.10.005
ÃCorresponding author. Tel.: +32 9 264 60 40; fax: +32 9 264 62 47.
E-mail address: wim.cornelis@UGent.be (W.M. Cornelis).

1. Introduction
A windbreak is generally defined as any structure that reduces wind velocity
(Rosenberg, 1974) and is commonly associated with a natural vegetative
barrier against wind. It can be a single element or a system of elements that
through its presence in the airflow reduces the effect of wind velocity not only at the
system itself but also at a certain windward and leeward distance. The term
windscreen refers to any artificial barrier, synthetic or mechanical, obstructing wind
flow. The term wind barrier or fence can be used to indicate both windbreaks and
windscreens.
The efficiency of barriers in terms of reduction of wind velocity and turbulence
intensity, and hence on wind-erosion processes is determined by various factors.
Barrier porosity, porosity distribution, shape, height, orientation, width and spacing
all influence wind-velocity reduction and turbulence intensity in the lee of wind
barriers. Free wind velocity and the surface roughness of the surrounding area also
affect wind-barrier performance (Chepil and Woodruff, 1963; Hagen and Skidmore,
1971a; FAO, 1978; Banzhaf et al., 1992). Among these variables, barrier porosity f;
which can be defined as the ratio between the open area of the barrier and its total
area (and is hence expressed in m2
mÀ2
) (Jensen, 1954; Tillie, 1992), is generally
considered to have the highest influence on the distribution of wind velocity and
turbulence intensity (van Eimern et al., 1964; Hagen, 1976). It is widely accepted that
with increasing porosity, wake velocities increase, but the turbulence intensity
decreases accordingly. There is less shear in the flow at the fence top and thus a
weaker streamwise momentum transfer from the displacement flow back into the
sheltered region (Raine and Stevenson, 1977). However, a solid fence provides a flow
of very low turbulence in the near-wake zone (Perera, 1981), but on the other hand,
the upstream conditions are recovered faster, implying that the leeward wind velocity
tends to increase more quickly than do wind velocities leeward of more porous fences
(Skidmore and Hagen, 1970a). Also, the area of leeward sheltered ground decreases
with decreasing porosity since minimum leeward wind velocity occurs closer to the
barrier (Marshall, 1967). Furthermore, Moysey and McPherson (1966) observed that
solid windbreaks created a vortex which extended several barrier heights leeward of
the barrier. Baltaxe (1967) found a critical porosity of about 0.35 m2
mÀ2
below
which velocity-fluctuations frequency exhibited a marked peak, indicating well-
developed turbulence. When porosity exceeded 0.35 m2
mÀ2
, this peak was absent
and so presumably were separation and turbulent flow. Apart from the abrupt
elimination of the peak, the velocity-fluctuation frequency fell off evenly with
increasing porosity. These observations are supported by those of de Bray (1957).
According to Na¨ geli (1941), such medium dense barriers reduced velocity by at least
20% over a larger distance than either very dense fences or barriers with very high
porosity. In general, windbreak porosity between 0.20 and 0.50 m2
mÀ2
is considered
to give the maximum shelter over the longest leeward distance (e.g. No¨ k-
kentved,1938; Na¨ geli, 1946; Jensen, 1954; Blenk and Trienes, 1956; Tani, 1958;
Schultz and Kelly, 1960; Marshall, 1967; Skidmore and Hagen, 1970a, b; Raine and
Stevenson, 1977; Tillie, 1992). The scatter that can be observed among the
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W.M. Cornelis, D. Gabriels / Journal of Arid Environments 61 (2005) 315–332316

experimental results reported by many authors could be due to differences in
porosity distribution and shape of the tested barriers.
Although many studies have determined the optimal porosity of windbreaks, the
optimum porosity distribution with height is still much debated. Rosenberg (1974)
suggested that the porosity should decrease with height in proportion to the
logarithmic nature of the wind-velocity profile. Similarly, FAO (1969) recommends
wind barriers with closed upper parts and semi-permeable lower parts. On the other
hand, Raine (1974) and Raine and Stevenson (1977) reported that improved shelter,
in the form of lower turbulence intensity for a given mean velocity reduction, may be
obtained by reducing the concentration of a region of strong shear in the flow at the
fence top, which would quickly diffuse high wind velocities back to the surface. They
suggested a fence with a porosity increasing from 0.0 m2
mÀ2
at the base to
1.0 m2
mÀ2
at the top, with an overall porosity of 0.2–0.3 m2
mÀ2
. Hagen and
Skidmore (1971b) noted, however, that a porosity of less than 0.4 m2
mÀ2
near the
fence top causes excess shear and turbulence, while low porosity near the bottom
creates low pressure that induces a recirculation zone in the leeward area. They
concluded that either of these mechanisms could probably provide a maximum area
of shelter in the leeward area. Consequently, it is questionable that either the top or
bottom of an optimum wind barrier should be of very low porosity (Hagen, 1976).
Another windbreak property that needs some consideration is the row number.
Many of the windbreaks planted worldwide in the 1930s and 1940s were wide because
it was believed that wide belts were necessary to provide adequate reductions. In
comparing the overall wind-velocity reduction per windbreak and per row under field
conditions, Woodruff et al. (1963) concluded that two- and three-row barriers were
most effective. Bilbro and Fryrear (1997) compared the wind-velocity-reducing
efficiency of multi-row windbreaks of kenaf and slat-fences and observed that multi-
row windbarriers were most effective near the barrier. At a distance of at least 10 times
barrier height, they found the effect of additional rows already being strongly reduced.
The objective of our study was to investigate the effect of porosity distribution and
number of rows on (1) the reduction of the wind velocity and on (2) the distribution
of zones of deflation and deposition of dune sand. Five single-row and two multiple-
row windbreaks, representing vegetative barriers, each with different stem porosity
and canopy porosity were tested. In selecting the material to simulate a windbreak
canopy, screens with different but evenly distributed porosity were tested. Because of
the complexity associated with barriers with an uneven distribution of their porosity
in terms of their drag coefficient, we did not attempt to model the flow with
numerical solutions as those given by Wilson (1985) or Fang and Wang (1997).
2. Materials and methods
2.1. Equipment used
All experiments were conducted in the wind tunnel of the International Centre for
Eremology, Ghent University, Belgium. It is a 22.4-m long and 8.4-m wide closed-
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W.M. Cornelis, D. Gabriels / Journal of Arid Environments 61 (2005) 315–332 317

circuit blowing-type wind tunnel. Its working section is 12.0 m long, 1.2 m wide and
3.2 m high (Gabriels et al., 1997). The boundary layer was 0.2 m deep.
Wind-velocity measurements were conducted at a 1-Hz frequency with 16-mm
vane probes (Testo C0635.9544, Lenzkirch, Germany) connected to a digital reading
unit (Testo 454) and PC. The probe converts the rotary motion of its vane into
electric pulses, which are counted and converted into wind velocity in the reading
unit. As the measuring frequency of the reading unit was 1 Hz only, the observed
variations in wind velocity could not be used to calculate the turbulence intensity.
2.2. The different windscreens
Before studying the effect of the different simulated vegetative windbreaks, several
synthetic screens with different porosity were tested on their capability in reducing
the wind velocity. The screens used are described in Dierickx et al. (2001). The screen
width in their study was 1.2 m, which covered the total width of the wind-tunnel test
section, and the screen height was 0.1 m. In order to simulate vegetative windbreaks,
one of the screens that performed very well in terms of wind-velocity reduction was
selected from the Dierickx et al. (2001) study and combined with wooden sticks. The
selected screens (screen D in Dierickx et al., 2001), which were made of polyester,
were woven and coated to fix the openings. They had a porosity of 0.24 m2
mÀ2
. The
sticks were cylindrical and had a diameter of 9 mm. The screens were then glued to
the windward side of the sticks. Although the screens are flat and quasi two
dimensional, they can be used to represent the windbreak’s canopy. The sticks are of
course representative for (branchless) stems or stalks. Several designs of single row
windbreaks, each with a different ratio in stem or stalk and canopy area, were tested.
To determine the influence of the number of rows, a single element windbreak (one
row) was compared with several multi-row windbreaks. For the sake of clarity, we
will only present in this paper the five most representative single row windbreaks,
and a two-row and a three-row windbreak with rows that were placed successively
with alternating stems. All tested windbreaks are shown schematically in Fig. 1.
Their width was 0.5 m and their height was 0.1 m, except when otherwise mentioned.
All barriers were located at a distance from the entrance of the test section x ¼ 7:0 m
and a cross-sectional width y ¼ 0:6 m (symmetrical to the centreline of the test
section).
2.3. Wind-velocity reduction experiments
In order to study the effect of the different simulated vegetative barriers on the
reduction in wind velocity, the latter was recorded at heights z of 0.02 or 0.025, 0.05,
0.10 and 0.15 m above the tunnel surface, and at distances x of 6.60, 6.80, 6.90, 6.95,
7.00, 7.05, 7.10, 7.20, 7.40, 7.60, 8.00, 8.50, 9.00, 9.50 and 10.00 m from the test
section entrance (see Fig. 2a and b). The free-stream wind velocity, i.e. the wind
velocity above the boundary layer and not affected by the tunnel surface or the
windscreen, was 6.3 m sÀ1
during all experiments.
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From wind-velocity profiles, the roughness length z0 of the tunnel floor, which was
made of plywood during these experiments, was calculated to be 0.22 Â 10À6
m.
Using the roughness criterion of Jensen (1958):
H
z0

f
¼
H
z0

wt
; (1)
where H is the windbreak height (m), z0 the aerodynamic roughness length (m), and f
and wt denote field and wind-tunnel conditions, respectively, this corresponds, for a
windbreak height of e.g. 20 m, to a field roughness length of 50 Â 10À6
m, a typical
value for an open sand surface. This means that during the wind-velocity reduction
experiments, the windbreaks were correctly scaled with the roughness of the tunnel
floor.
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Fig. 1. Schematic view of the tested simulated vegetative windbreaks (dimensions in mm).

The efficiency in reducing wind velocity at a given height above the surface and
distance windward or leeward from the windbreak was expressed in terms of a
dimensionless reduction coefficient
RcDx;z ¼ 1 À
uDx;z
u0Dx;z
; (2)
where Dx is the distance from the windbreak (in barrier height H), z is the height
above the surface (in barrier height H), uDx,z is the time-averaged wind velocity
disturbed by the windbreak (m sÀ1
), and u0Dx,z is the time-averaged wind velocity in
the absence of a windbreak (m sÀ1
). The overall wind-velocity reduction at a given
height was expressed as the average total reduction coefficient
TRcz ¼
1
Dx1 À DxM
Z DxM
Dx1
RcDx;zdDx (3)
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wind-tunnel wall
wind
z (m)
y (m)
x (m)
6 7 8 9 10 11 12
0.4
0.8
wind
z (m)
y (m)
x (m)
6 10
0.4
0.8
(c)
vane probe
54
windbreak
sand tray
wind
y (m)
z (m)
x (m)
6 7 8 9 10 11 12
0.1
0.2
(b) windbreak
7 8 9
(a)
Fig. 2. The experimental set-up: (a) top view of the wind-velocity reduction experiments; (b) side view of
the wind-velocity reduction experiments; (c) top view of the erosion/deposition experiments.

or
TRcz ¼
1
Dx1 À DxM
XDxM
Dx1
RcDx;zDðDxÞ; (4)
where M is the number of observations. If RcDx;z ¼ 1 along the complete length
considered, then TRcz ¼ 1, which would imply a 100% effective windscreens over
the entire distance under consideration (at least in terms of wind-velocity reduction).
Note that RcDx,z and TRcz are dimensionless variables and that their value in
absolute terms depends on the chosen limits of integration (or the location of the
different vane probes).
2.4. Erosion and deposition experiments
In order to test patterns of erosion and deposition windward and leeward of
windbreaks, a uniform strip of 250-mm sized dune sand, 0.50 m wide, 0.02 m deep
and 5.80 m long, was placed 4.20 m downwind of the test section entrance (see Fig.
2c). The characteristics of the sand are given in Table 1. Runs were conducted for
30 min with windbreak 1’, which is similar to windbreak 1 except that it is 0.05 m
high only, windbreak 3 and windbreak 6 (see Fig. 1) at a constant free-stream wind
velocity of 6.3 m sÀ1
. At the end of the experiments, erosion and deposition of the
sand was determined by measuring the change in height with respect to the initial
height of the sand surface. This height difference was measured along the centreline
of the sand strip in 0.1-m intervals; close to the barrier, the interval was 0.05 m. The
zones of erosion and deposition were compared with isowind-velocity lines plotted in
the x,z plane. The threshold wind velocity u*t (m sÀ1
), that is required to initiate
particle motion and that determines the deflation zone, was calculated by combining
the threshold shear velocity equation as proposed by Cornelis and Gabriels (2004):
uÃt ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
A1 1 þ A2
1
ðrs À rf Þgd3
#v
u
u
t
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
rs À rf
rs
gd
r
; (5)
where A1 and A2 are model coefficients equal to, respectively, 0.013 and
1.695 Â 10À4
N mÀ1
, rs is the particle density (kg mÀ3
), rf is the fluid density
(kg mÀ3
), g is the gravitational acceleration (m sÀ2
), and d is the particle diameter
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Table 1
Particle-size distribution and physico-chemical properties of the used dune sand
Claya
content
(%)
Silt content
(%)
Sand content
(%)
O.M.b
content
(%)
CaCO3 content
(%)
ECe (dS mÀ1
)
1.3 0.3 98.4 0.0 3.3 0.72
a
Clay: 0-2 mm, silt: 2–50 mm, sand: 50–2000 mm.
b
O.M. is organic matter, ECe is the electrical conductivity at 25 1C measured on a saturated extract.

(m), with the well-known logarithmic law (Prandtl, 1935):
u ¼
uÃ
k
ln
z
z0
; (6)
where u is the wind velocity at height z (m sÀ1
), u* is the shear velocity (m sÀ1
), and k
is the von Ka´ rma´ n constant (E0.4), and which is assumed to be valid under the
given circumstances with 0.1 m high barriers at least up to a height of 0.02 m.
The surface roughness length during these experiments was that of sand. This
implies that, according to Eq. (1), the windbreak was not scaled to the surface
roughness for realistic field conditions, at least if the field would have a sandy
surface. The windbreak is rather to be considered as a roughness elements with its
actual size influencing the erosion and deposition process. As such, they could
represent rows of terminated forbs or terminated small grains with similar
geometries (Renard and Vandenbeldt, 1990), which are in many areas of the world
used at close spacing within the field to protect crops.
3. Results and discussion
3.1. Wind-velocity reduction experiments
In Fig. 3, the TRc0.25 values calculated at height z ¼ 0.25 H are plotted against the
porosity of the corresponding synthetic screens. It is readily apparent that the
optimal porosity ranges from about 0.20–0.35 m2
mÀ2
, which is in accordance with
values found in literature. A similar peak was also observed by Grant and Nickling
(1998) when plotting the drag coefficient vs. porosity. A Gaussian peak function was
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(m2
m-2
)
0.0 0.2 0.4 0.6 0.8 1.0
TRc0.25(-)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Observed data
Eqn. (7); R2
= 0.975
Fig. 3. The normalized total reduction coefficient TRc0.25 at z ¼ 0.25H vs. porosity f for different
synthetic screens.

fitted here to the observed data:
TRc0:25 ¼ a1 þ b1eÀð1=2ÞððfÀc1Þ=d1Þ2
; (7)
where a1, b1, c1 and d1 are regression coefficients. Non-linear least-squares regression
using the quasi-Newton algorithm (Press et al., 1992) resulted in a1 ¼ À0:185;
b1 ¼ 0.810, c1 ¼ 0.248 and d1 ¼ 0.436. The R2
value was 0.975. The scatter that can
be observed can be attributed to differences in technical characteristics of the screens,
viz. their composition, way of manufacturing (knitted or woven), ratio between warp
yarn and weft yarn, yarn width, etc. A screen with f ¼ 0:24 m2
mÀ2
and
TRc0.25 ¼ 0.59 was selected to represent the canopy of the simulated vegetative
windbreaks.
In Fig. 4, the RcDx,0.25 values measured windward and leeward of five single-row
windbreaks are plotted against the distance Dx from the windbreak. The TRc0.2
values are given in Table 2. Both illustrate that windbreak 1, where the porosity of
the stem or stalk and the canopy is evenly distributed with height, provides the
longest sheltering zone, i.e. the zone where RcDx,0.240.5, and gives the highest TRc0.2
value (which was 0.43). This is supported by the results of Hagen (1976), who
reported largest shelter when porosity is evenly distributed. Further, the wind-
velocity reduction coefficient at 10H, Rc10,0.2, was still 0.8. The other extreme is
windbreak 5, though with an evenly distributed porosity, but without a canopy. Its
effect on wind-velocity reduction was very limited. The drag force exerted by the
windbreak was too low to reduce wind velocity with more then 10%.
Windbreaks with a dense lower part and a more open upper part such as
windbreak 2, appear to result in the highest RcDx,0.2 values close to the windbreak,
up to a leeward distance of 7H. This could be attributed to the reduced shear stress in
the flow at the fence top, which quickly diffuses high wind velocities back to the
surface (Raine, 1974; Raine and Stevenson, 1977). However, its shelter length is
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∆∆x (H)
-5 0 5 10 15 20 25 30
Rc∆x,0.2
0.0
0.2
0.4
0.6
0.8
1.0
Windbreak 1
Windbreak 2
Windbreak 3
Windbreak 4
Windbreak 5
Fig. 4. Reduction coefficient RcDx,0.2 at z ¼ 0.2H for the simulated vegetative single-row windbreaks vs.
the distance from the windbreak.

shorter in comparison with windbreak 1, because the ‘effective’ height of the
windbreak, which corresponds to the canopy, is halved. The drag exerted by the
upper part is minimal as was demonstrated by windbreak 5.
With regards to windbreak 3 and windbreak 4, which both have a porous lower
part, it is apparent that they have some positive effect windward of the windbreak,
but RcDx,0.2 drops drastically just behind the leeward side of the windbreak. This
effect is most pronounced in the case of windbreak 3, which has the longest canopy,
and can be attributed to an increased pressure close to the surface. At distances
Dx415H, the behaviour of windbreak 3 in terms of wind-velocity reduction becomes
similar to that of windbreak 1 and 2. At a distance of 30H, windbreak 4 results in the
highest RcDx,0.2 value (i.e. 0.19). Owing to the lower drag exerted by this windbreak,
the difference between surface and free-stream pressure is minimal and hence the
‘Coanda’ effect, which pushes the separation streamlines towards the surface if the
above pressure difference is considerable (Plate, 1971), is limited. As a result, the
return of the displacement ﬂow to the surface at its downstream reattachment point
is gradual.
When increasing the number of rows of the windbreaks, a positive effect could
only be observed at a distance of 3–8H (see Fig. 5). Beyond this zone of highest
shelter, single-row windbreaks with evenly distributed porosity were the most
effective in reducing wind velocity. The trends observed here are in accordance with
what can be observed when reducing the windbreak porosity below an optimum
porosity, as was discussed above. As the stems of the different rows were mutually
alternating, porosity decreased with increasing number of rows. The highest overall
wind-velocity reduction was obtained with the single-row windbreak 1, though the
difference with the two multi-row windbreaks is not substantial (viz. 0.43 vs. 0.40).
3.2. Erosion and deposition experiments
In Fig. 6, the amount of eroded and deposited sand is illustrated as a function of
the distance from the windbreak. Erosion and deposition was expressed in terms of
change in height respective to the initial height of the sand surface. In the case of
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Table 2
Total reduction coefﬁcient TRc0.2 for the different windbreaks and TRc0.2 expressed as a percentage of the
highest TRc0.2 value
Windbreak TRc0.2 TRc0.2 (%)
1 0.43 100
2 0.40 94
3 0.28 65
4 0.19 45
5 0.09 20
6 0.40 92
7 0.40 92

windbreak 1’, the deposition is mainly confined to the zone just behind the leeward
side of the barrier. The wind velocity in this zone is lower than the impact threshold
wind velocity, which is 0.8ut (Bagnold, 1941; Anderson and Haff, 1988) or in the case
of our test sand 3.9 m sÀ1
at z ¼ 0.02 m (see Eqs. (5) and (6)). Consequently, the
transport capacity of the wind is too low to continue saltation in that zone and
particles that settle down will not be entrained. This is illustrated in Fig. 7, where
isowind-velocity lines are plotted in the x, z plane. The deposition that occurred
windward of windbreak 1 is due to entrapment caused by the barrier.
The effect of windbreak 3 is quite the opposite. The high porosity at its base
creates a funnel effect and results in accelerated flow near the surface compared to
the wind velocity in front of the windbreak (see Fig. 4). This leads to increased
entrainment rates and the development of score holes beneath the fences. This is
supported by field observations of Bofah and Ahmad (1990). The entrainment zone
is at least about 0.3H long as illustrated in Fig. 6. From Fig. 7, it is clear that the
surface wind velocity within this zone exceeds the impact threshold wind velocity for
deflation of 3.9 m sÀ1
at z ¼ 0.02 m. Note that the applied free-stream wind velocity
of 6.3 m sÀ1
was close to the free-stream wind velocity that initiates deflation of the
used dune sand. If the free-stream wind velocity increases, the length of the erosion
zone will increase as well. The hence entrained sediment settles down in the zone
immediately behind this zone of strong deflation. Experiments under similar
conditions but without windbreaks showed that the maximum saltation height of the
test sand is 0.09 m and that 90% of its transport occurs below a height of 0.03 m.
This implies that the hop length of the particles, which is about 12–15 times hop
height (Cooke et al., 1993), will not exceed 1.4 m, whereas most particles will be
deposited within 0.5 m. These particles hence settle down in a ‘quite’ zone where
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∆∆x (H)
-5 0 5 10 15 20 25 30
Rc∆x,0.2
0.0
0.2
0.4
0.6
0.8
1.0
Windbreak 1
Windbreak 3
Windbreak 6
Windbreak 7
Fig. 5. Reduction coefficient RcDx,0.2 at z ¼ 0.2H for the simulated vegetative single-row windbreaks and
multi-row windbreaks vs. the distance from the windbreak.

wind velocity is below the impact threshold wind velocity for deﬂation. Therefore,
they are not further transported and a rather large zone of deposition can be
observed. This ‘quite’ zone is readily apparent in Fig. 7.
By adding an additional row to windbreak 3, another pattern can be observed.
Such a windbreak corresponds to windbreak 6, which is actually a combination of
windbreak 1’ and windbreak 3. In this case, most of the sediment that is eroded from
the windward side of the barrier, is trapped by the barrier. Since this barrier is now
0.1 m high, a much smaller amount of sediment is transported over the barrier top,
compared to the 0.05-m high windbreak 1’. However, the few particles that were
transported over the fence top, settle down directly behind the barrier and are not
subsequently deﬂated as the impact threshold wind velocity is not exceeded.
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4 6 9 10
-0.02
-0.01
0.00
0.01
0.02
Windbreak 1'
4 6 9 10
-0.02
-0.01
0.00
0.01
0.02
Windbreak 3
ErorDe(m)
4 6 9 10
-0.02
-0.01
0.00
0.01
0.02
Windbreak 6
x (m)
5 7 8
5 7 8
5 7 8
Fig. 6. Longitudinal distribution of erosion Er (o0) and deposition De (40) of sand at the end of a 30-
min test run for different windbreaks vs. the distance x from the entrance of the test section. Free-stream
wind velocity was 6.3 m sÀ1
.

3.3. Practical conclusions based on both experiments
The above observations allow us to draw some practical conclusions with respect
to windbreak design. However, care should be taken when interpreting the sediment
transport and deposition data, because the particle size and hence the particle
trajectory is not scaled to dimensions of shrub and tree windbreaks. The patterns of
erosion and deposition can only represent the effect of terminated small grain and
terminated frob windbreaks when placed within the saltation layer.
The design of the barrier depends on the purpose for which it is constructed. If the
objective is to protect a ﬁeld from being eroded, single-row barriers with a
homogeneous distributed porosity are the best alternatives. They provide the longest
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7.0 7.5 8.0 8.5 9.0 9.5 10.0
0.05
0.10
0.15
windbreak 1'
7.0 7.5 8.0 8.5 9.0 9.5 10.0
0.05
0.10
0.15
windbreak 3
z(m)
7.0 7.5 8.0 8.5 9.0 9.5 10.0
0.05
0.10
0.15
windbreak 6
x (m)
Fig. 7. Isowind-velocity lines where wind velocity is in m sÀ1
in the x,z plane. Free-stream wind velocity
was 6.3 m sÀ1
.

shelter, and occupy at the same time the least amount of land area and reduce the
maintenance cost. Caution must be taken if barriers with limited height are used, i.e.
with a size smaller than the height of the saltation layer, as this will result in some
deposition of sand at its leeward side. It is generally accepted that 99% of saltating
particles do not jump higher than 1 m (Cooke et al., 1993). The most optimal are
windbreaks that are composed of three horizontal layers: a first layer close to the
surface that contains grasses, shrubs, or hedges, a second layer at medium altitude
with small and low trees, and a third layer at high altitude composed of high trees. It
is important to note that the trees, and particularly the shrubs or hedges, should be
in a foliated condition during the period in which the highest wind velocities can be
expected. Therefore, conifers seem to be the best option from a wind-erosion control
perspective.
If the objective of constructing a windbreak is to protect roads, railways, ditches,
etc. from being buried by sand, the major concern would be the deposition of
sediment in the lee of the barrier. If a barrier similar to windbreak 3 would be
planted, it should be located very close to the windward side of the infrastructure.
Such a windbreak reduces the wind velocity windward of the barrier with a value of
about 30%, whereas sediment that would settle down immediately behind the
windbreak would be blown away, but only at wind velocities exceeding the threshold
value for the particular sediment. If located at greater distances from the
infrastructure, the eroded sediment could deposit on the infrastructure and remain
there if the wind velocity would be reduced below the threshold value for the
particular sediment. Windbreaks with a homogeneously distributed porosity could
be problematic if constructed very near the infrastructure being protected. Sediment
that would settle down at the leeward side of the barrier after being transported
through the barrier or over the barrier if the height of the latter is smaller than the
saltation layer, would remain there given the reduction in wind velocity with more
than 80% compared to the upwind velocity. This is illustrated in Fig. 8 showing the
deposition behind a windbreak of about 2 m high in three consecutive years in
Southern Tunisia. Sand particles were piling up at the leeward side of the windbreak,
which was constructed for road protection. As a result, the road became gradually
buried and finally, a new road was constructed at a greater distance from the barrier.
Because the erosion and deposition data based on our small-scale windbreaks cannot
be extrapolated to large-scale windbreaks that are typical for infrastructure
protection, appropriate positions of such barriers relative to the infrastructure can
not be deduced from our wind-tunnel experiments.
However, the erosion and deposition data clearly illustrate that, when using small
grains or forbs to protect seedlings or crops, their geometry should be different from
that of windbreak 1. The latter results in sediment deposition up to a distance of
about 6H, which would result in burial of seedlings or small crops, if grown within
this zone of deposition. A better option would in this case be windbreak 3,
positioned at maximum 3H leeward of the rows of seedlings or crops. This would
prevent them from being buried. When such windbreaks are used in a network, the
overall wind velocity over the field would be reduced as well due to increased
friction, compensating the funnel effect at the immediate leeside of the barrier.
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Fig. 8. Deposition behind a windbreak for three consecutive years in Southern Tunisia. The prevailing
wind direction was from right to left.

4. Conclusions
Windbreaks have been used for many years to reduce wind velocity as a wind-
erosion control measure. However, there is yet no clear answer to what should be the
optimal design for windbreaks. In a wind tunnel, five single-row barriers, each with
different stem or stalk and canopy porosity, and a two- and a three-row barrier were
tested. Preliminary experiments were performed to determine the overall reduction
coefficient of several synthetic screens. A Gaussian peak function was fitted to the
data and it was concluded that a porosity of 0.20–0.35 m2
mÀ2
is optimal in terms of
wind-velocity reduction.
As regards porosity distribution, an evenly distributed porosity of stem and
canopy resulted in the longest protected area, where the wind-velocity reduction is
more than 50%. Dense lower parts were more efficient than more porous lower
parts. The latter resulted in an accelerated flow immediately behind the barrier. Two-
and three-row barriers were more efficient in terms of wind-velocity reduction
between 3H and 8H only (H in barrier height). At greater distances, single-row
barriers resulted in higher wind-velocity reduction. Also, the sheltering zone of
single-row barriers was higher compared to the multiple-row barriers.
Zones of erosion and deposition were determined from experiments on dune sand.
The observed zones could be well explained by considering the threshold wind
velocity that was computed for the dune sand used in combination with isowind-
velocity lines. In case of an evenly distributed porosity, erosion was rather limited.
However, a zone of deposition could be observed behind the barrier, which increased
as barrier height decreased. A barrier with a relative open lower part appeared to
induce a funnel effect and resulted in a zone of increased deflation immediately
behind the windbreak, which was followed by a zone of deposition. Such a barrier
could be useful if it is constructed to protect roads, railways, etc. from being buried,
but only if located very close to the windward side of these infrastructures. If the
purpose of the windbreak is to protect a field against wind erosion, a shrub or tree
barrier with a homogeneous porosity seems to be the best alternative. In the latter
case, the situation where the windbreak is defoliated at its lower part only should be
avoided. However, when using small grain or forb wind barriers to protect seedlings
or crops, they should be rather porous at their lower part in order to avoid burial of
the seedlings or crops.
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