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Wind tunnel experiments and field measurements of aeolian dust deposition on conical hills
GHMOBPHOIOGY ELSEVIER
Geomorphology 14 (1995) 43-56
Wind tunnel experiments and field measurements of aeolian dust deposition on conical hills Zvi Y. Offer
a
Dirk Goossens b
Meteorology Unit, The Jacob Blaustein Institute for Desert Research, Sede Boqer Campus 84990, Israel b N.F.W.O., Laboratory for Experimental Geomorphology, Redingenstraat 16 bis, B-3000 Leuven, Belgium Received 6 July 1994; revised 22 January 1995; accepted 3 February 1995
Abstract
The spatial pattern of short-term aeolian dust deposition on and around cone-shaped hills is investigated via the simulation, in a wind tunnel, of dust storms over a topographic scale model of a conical hill in the Negev desert, Israel. The results are tested during a full-scale dust storm in the Negev. The wind tunnel experiments adequately predict the field pattern, although some problems may arise on steep windward slopes where the accumulation threshold is more quickly exceeded on the scale model than in the field. Conical hills create an elongated area of low deposition ( "dust shadow") in their lee. Downwind from the shadow zone, a local area of more-than-normal (compared to the undisturbed surroundings) deposition occurs. On the lateral flanks of the hill, and also on the small convex windward slope just upstream of the top (and at the top itself), dust deposition remains low to very low. It is the lowermost, concave windward slope that receives the largest amounts of dust. The pattern described above refers to short-term deposition only. Water erosion may seriously alter the final accumulation pattern since it will clean the slopes of important amounts of dust that accumulate in the wadis.
1. Introduction Topography is one of the most important factors that affect the spatial patterns of erosion, transport, deposition and accumulation of aeolian dust (Goossens, 1987a). In hilly regions, dust and dust-derived deposits (such as loess) are, therefore, characterized by an important spatial variation in occurrence and sediment depth (Rozycki, 1967; Bouten et al., 1985). The number of experimental studies in which the effect of topography on aeolian dust deposition is investigated is rather small. Wind tunnel simulations of aeolian dust deposition over topographic scale models are reported by Goossens ( 1988a, b, c, 1989, 1995a, b) and Goossens and Offer ( 1988, 1990, 1993). Field experiments are discussed by Goossens and Offer (1990, 1993) for single-storm events and Goossens 0169-555X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIOI69-555X(95)O0015-1
(1988b, 1995b) for long-term accumulation. These studies comprise both dust deposition experiments on single, isolated landforms (hills and valleys), on ranges of hills, and in complex topography. Thusfar, all wind tunnel and field studies with hilly landforms have focussed on linear hills standing perpendicular to the direction of the wind (and the dust flow). The typical sedimentation profile for such hills is shown in Fig. 1. Assuming that the area upstream of the hill is flat and horizontal, dust deposition starts to increase once the windward hillslope is reached. Deposition continuously increases up-slope until a maximum is obtained at the inflection line (where the slope changes from concave to convex). Downwind of the inflection line, dust deposition drops considerably. Typically two minima are observed in the profile: the first close to (but not necessarily exactly at) the sum-
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
44 30dust depth difference (ijmI 20-
10-
^-v~. V,.,-,,---,.~, --~ -10
-20 ¸
-30
i 60
height (cml
I t,0
210
0'
0
'6'
/'7
0
0
'
' 100
'
' 120
'
' ' 180 ' ' 200 ' ' 220 ' ' lt,' O ' 160 disfcmce to hill summit (cm)
0
Fig. 1. Typical wind tunnel dust deposition profile over a symmetrical hill standing perpendicular to the wind. Values in abscissa represent distance to the hill summit. Ordinate values indicate the difference in dust depth between the curved hill surface and a flat reference surface. Airflow is from left to right; the hill is 25 cm wide and 3 cm high. Friction velocity during the experiment was 5.8 cm. s - 5.
mit, and the second more downwind, on the leeward slope. A small zone of slightly increased sedimentation separates both minima. Dust deposition remains very low all over the leeslope. Downstream of the hill, a second zone of increased sedimentation occurs. This zone gradually damps out until "normal" deposition (corresponding to that upstream of the hill) has again been reached. No further oscillations appear in the deposition curve (Goossens, 1988c). Although linear hills occur quite frequently, they represent only one of the different types of hills found in desert environments. An even more common type is the circular (conical) hill, found abundantly in the deserts of the Middle East, and largely composed of long, concave-curved slopes with a small convex zone near the top. Also, many bornhardts, though not necessarily showing the same slope profile, have more or less conical shapes. The same holds for many domelike hills (although the " c o n e " is rather flattened for these latter). Also, many volcanoes show a cone-like shape. Since conical hills form a substantially different type of obstacle to the wind compared to linear hills, the airflow pattern over, and therefore also the dust deposition pattern on (and around), conical hills will differ from those on (and around) linear hills.
The aim of this paper is to investigate the dust deposition pattern created by conical hills. Dust storms are simulated over a topographic scale model of a conical hill located in the Negev Desert, Israel. The results of the simulation are tested by a field experiment carried out on the same hill during a dust storm.
2. Description of the test field The field experiment was carried out on a more or less isolated, conical hill in the central Negev desert. The hill is located 3 km south of the old Nabatean city of Avdat (Fig. 2). The climate in this region is typically arid, with an annual average precipitation of 87 mm. Annual average temperature is 18.3°C, with great differences between winter and summer and between day and night. The daily maximum temperature is more than 40°C in May, June and July, but the minimum sinks below freezingpoint in December and January. Atmospheric humidity is generally low, with highest values during the cold season. The predominant winds blow from the north, northwest and west. The area near Avdat consists of (mostly stony) hammadas, developed on different elevations. They include
Z E Offer, D. Goossens / Geomorphology 14 (1995) 43-56
45
Fig. 2. Situationof the experimental field. Arrowindicates locationof the test hill. both large and small plateaus and wide wadi depressions. Valleys cut many of the plateaus at their borders. The depth of incision may reach 50 m and more. Most of the hammadas are developed on limestone and dolomitic formations. The valleys are dry nearly all year. Their transverse profiles show a diversity of shapes, varying from troughs and V-forms lo small canyons. The subsoil near the test hill is composed mainly of carbonate rocks (limestone and chalk, with variable amounts of chert). The hill itself is nearly conical, with long concave slopes (Fig. 3). It has a diameter of about 450 m and a height of 66 m above the surrounding surface. The slopes are covered by a mixture of stones (diameter usually between 5 cm and 30 cm) and loess. The loess consists of some 25% fine sand, 55% silt and 20% clay (Bruins, 1986) and forms surface crusts about 2-4 mm thick. The steep convex zone near the top is largely composed of bare limestone rocks. Vegetation is scanty due to the low precipitation. Some sparse, discontinuously dispersed shrubs (20-30 cm high) locally occur on the hillslopes. The area near Avdztt is prone to a high frequency of dust storms. These occur mainly in the winter and spring, and are associated with the passage of cold fronts and low pressure systems (Goossens et al., 1993). Cases of dust haze also frequently occur in the same period. During dust storms, wind blows predominantly from the west.
3. E x p e r i m e n t a l d e s i g n in t h e w i n d t u n n e l 3.1. E x p e r i m e n t a l f a c i l i t i e s
All wind tunnel experiments were carried out in the closed return wind tunnel of the Laboratory for Experimental Geomorphology of the Katholieke Universiteit Leuven (Belgium). The test section of this tunnel is 7.6 m long, 1.2 m wide, and 0.6 m high. A complete description of the wind tunnel can be found in the work by Goossens and Offer (1988). Dust storms were generated by means of an Engelhardt laboratory dust cloud producer connected to the tunnel. This apparatus ensured a continuous feed of natural dust to the air current, and allowed the operator to adjust dust discharge (and concentration). The thickness of the dust layer on the scale model was measured with an Optocator laser instrument with a gauge probe type 2301. The accuracy of the instrument is 2/zm. 3.2. S i m i l i t u d e p a r a m e t e r s
The most important similarity criteria for the simulation of airflow over topography are (Hertig and Liska, 1983): - equal model and field densimetric Froude number: F r = u / ( g . D ) o.5 ( i f there is no temperature scaling) ; - equal model and field Reynolds number: R e = u . D/v
46
Z. Y. Offer. D. Goossens / Geomorphology 14 (1995) 43-56
Fig. 3. Topographicmap of the test hill and its surroundings. The hill is located in the centre of the map. Black dots show the location of the dust collectorsduring the field experiment of 24-25 February 1992. Stolen collectorsare not indicated. where u is the wind speed, D a characteristic length, g the gravitational acceleration and ~, the kinematic viscosity. When geometric scaling is very small, as in the conical hill experiment, it is impossible to achieve identical Froude numbers and Reynolds numbers simultaneously (Hertig and Liska, 1983). Knowing that these numbers express the relationship between kinetic energy and viscous energy dissipation, it is possible to retain the similarity of the friction energy by taking only the turbulent energy of the atmospheric flows into consideration. Consequently, both the nature and the model must have identical turbulent Reynolds numbers.
Boundary layer flow characteristics over a rough surface become independent of Reynolds number (Re) once a critical R e is exceeded (Cermak, 1984). Through selection of proper combinations of wind tunnel length, surface roughness and ambient wind speed, boundary layer flows can thus be realized that model the micrometeorological features of the high Reynolds number atmospheric boundary layer. 3.3. The s c a l e m o d e l
To investigate the sedimentation of dust on and near the conical hill near Avdat, a 1:2500 scale model, containing the hill and its surroundings was used. The
z.Y. Offer, D. Goossens/ Geomorphology 14 (1995) 43-56
model was constructed from polystyrene plates 2 mm thick, which were cut along contour lines copied from a topographical map of the study area. By adhering the different polystyrene plates to each other, a rough, terraced scale model was obtained. To simulate dust deposition over the model, the surface had to be carefully smoothed in that the small steps would generate artificial flow disturbances that could influence the sedimentation pattern on the model. The smoothing was executed by means of gypsum plaster. Only the test area itself was smoothed: the surroundings were kept terraced to increase airstream turbulence (necessary to decrease the simulation requirement of Reynolds number equality). The final model surface was smooth with a wery small micro-roughness in the order of about 100 ~m. At full scale, this corresponds to 25 cm, which is almost equal to the size of the roughness elements ( stones and shrubs) in the field. Blockage of the scale model was restricted to 3.9%. Horizontal and vertical scale were identical, which enabled us to obtain geometric similarity during the wind tunnel simulations. The maximum diameter of the modelled area (experimental field plus its surroundings) was restricted to 1.5 km. Deflection of the (near-surface) winds due to the Coriolis effect may thus be ignored (see Cermak, 1970 and Gokhan, 1!)73 for more details). 3.4. Boundary layer simulation General concepts
In hilly areas, the structure of the airflow near the ground is not only a function of local surface roughness but also depends on the general airflow pattern influenced by the topography of the surrounding region. In addition, dust is not only transported close to the ground but also at high altitudes (dust clouds of several hundreds of metres high are not an exception, especially during heavy dust storms), and the particles are so small that they largely follow the streamlines of the air. An adequate simulation of dust deposition on the conical hill scale model, therefore, requires not only a correct simulation of the local surface roughness influence, but also a correct simulation of the general airflow pattern in the region. This means that the wind tunnel experiments should .at least simulate an important part of the atmospheric boundary layer (ABL).
47
In the lowest 15% of the ABL, the wind profile (in neutral atmospheric conditions) can be described either by the logarithmic law: u / u . = ( 1 / k ) . ln(z/zo)
( 1)
or by the power law: ul/u2 = (Zl/Z2) ~
(2)
where u, Ul and u2 are the mean wind speeds at heights z, Zl and z2 respectively, u. is the friction velocity, k the Von Karman constant, Zo the roughness length and a a dimensionless exponent (usually between 0.1 and 0.4). Unlike the logarithmic profile, the power profile is not restricted only to the surface boundary layer (SBL) but can also be applied to the total boundary layer. The exponent a describes the curvature of the velocity profile and mainly depends upon surface roughness. Thus, for an adequate simulation of boundary layer flow over the topographic scale model, the following parameters must be modelled: the depth of the boundary layer, the roughness length Zo, and the power law exponent a. Boundary layer thickness
Near Avdat, the depth of the ABL is approximately 600 m (see Goossens and Offer, 1990). The surface boundary layer, in which the neutral wind profile may be approached by Eq. (1), is therefore about 80-100 m thick. As the maximum height differences in (and near) the test field are only slightly smaller (usually about 50-60 m), it is not enough to simulate only the SBL: the depth of the boundary layer to be simulated in the wind tunnel must be at least several hundreds of metres. Roughness length
The roughness length Zois determined by the roughness of the surface. Two types of roughness must be considered: micro-roughness (caused by pebbles and shrubs), and macro-roughness (caused by hills, valleys and mountains). Both the small and the large roughness elements will affect the velocity profile of the wind. In the Avdat area, micro-roughness is restricted to about 0.2-0.3 m. Moreover, the degree of micro-roughness is more or less uniform in the test field. The contribution of micro-roughness to the roughness length zo can be estimated at 6.7-10.0 mm, as it is approximately equal to 1/30 of the field roughness elements' height in
48
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
rough-turbulent flow. To calculate the contribution of the macro-roughness (relief) to the roughness length, we used the algorithm proposed by Grant and Mason (1990) reaching a Zo value of between 24 and 64 cm, depending on the size of the landforms and the orientation of the wind. Thus, the contribution of the macroroughness (to Zo) is considerably greater than that of the micro-roughness. To achieve similarity, the value of Zo in the wind tunnel must be equal to the field value multiplied by the scale of reduction (Iversen et al., 1976; Iversen, 1983). Since the field value of Zo can be estimated at 24-64 cm and the vertical scale of the model is 1:2500, the simulated value of Zo must be in the order of 96256/xm.
Shape of the velocity profile The shape (or curvature) of the wind's velocity profile can be quantified using the power law exponent a. Velocity profile curvature is largely determined by surface roughness (and atmospheric stability). Here too, it is necessary to consider the effects of both microroughness and macro-roughness. In general c~, varies from less than 0.10 (very smooth terrain) to more than 0.40 (extremely rough terrain). From meteorological data recorded at Sede Boqer, some 9 km north of the test hill, it can be calculated that, in near-neutral conditions, a is about 0.16 for winds blowing from the SW-N sector (see Goossens et al., 1993). The value of a near Avdat should be almost identical (or may be only slightly higher), since macro-roughness at Avdat is only slightly greater than at Sede Boqer. Thus, near the test hill, the simulated value of ~ should be 0.160.17.
Simulated boundary layer A 6 m upwind fetch in the wind tunnel was covered with a plastic sheet with gentle undulations. This produced a boundary layer with a depth of 24.5 cm (equivalent to 612 m in the field), a velocity profile with a power exponent of 0.17 and a Zo value (derived from a logarithmic curve fitting) of 255 /xm. Freestream velocity was 1.95 m s - ~, and friction velocity 0.11 m s - 1. The Reynolds number during the simulation was equal to 8.01 × 105. This is sufficiently high to obtain Reynolds number independency for flow over surface roughness elements higher than 0.7 cm (see Fig. 1 in Cermak, 1984). Since roughness elements were several
centimetres high, simulated flow may be considered Reynolds-independent.
3.5. The dust particles During the simulation, the aerodynamic behaviour of the airborne particles must be similar to the behaviour of the dust particles in the field. During dust storms the degree of air turbulence is high to very high, and the dust is transported in true suspension. Thus, true suspension must be simulated in the wind tunnel. Air-dry Belgian Brabantian loess, sieved at a diameter of 63 # m to exclude all sand particles, was used during the conical hill experiment. The loess had a median diameter of about 30/zm, which corresponds closely to the size of the dust particles that settle near Avdat during normal dust storm events (Yaalon and Ganor, 1979). The ratio of terminal fall velocity of the particles, ut, to the friction velocity in the wind tunnel, u., was equal to 0.64. According to Tsoar and Pye (1987), dust is transported in true suspension when u,/ u. is below 0.70. Thus, during the conical hill experiment, dust transport occurred in true suspension.
4. Airflow over, and dust deposition on, a conical hill: the general pattern Circular hills with a relatively low height-to-width ratio create an elongated wind shadow characterized by two trailing vorticies ( "horseshoe" ) --having axes parallel to the wind --that are shed from the flanks of the hill (Greeley, 1986). A general sketch is shown in Fig. 4. Circular hills with a high height-to-width ratio also create an elongated wind shadow. In the case of deposition of sand, this shadow can be characterized by a strong accumulation of sediment (with, eventu"shadow" zone
a
pointof
t
t
_~__ ~
a
c
.
h
vortex core \ .
. \~
~
.
reverse flow horseshoe vortex Fig. 4. Diagramshowinghorseshoevortexformedarounda low
topographicobstruction(modifiedfromGreeley,1986),
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
ally, the creation of impressive linear lee dunes) as well as by a general lack of sand deposition. In the latter case, the high degree of turbulence created by the conical hill causes the sands that penetrate the shadow zone to be swept away ahnost immediately. Examples of both processes are found in papers by Grolier et al. (1974) and Greeley (1986). In the case of dust deposition, the area directly downwind from a topographic obstacle always behaves as a shadow zone, as shown by several previous wind tunnel and field experiments (Goossens, 1987b, 1988a-c, 1989, 1992, 1995a; Goossens and Offer, 1990, 1993). Fig. 5 shows the dust pattern on the scale model of the conical hill near Avdat after a dust storm of 2
49
minutes (aerodynamic conditions as described in Section 3.4; dust discharge is 13.33 kg h-~ and dust concentration at 5 cm height is 1.5 g m-3; airflow from the W to the E). The picture is based on visual observations of the pattern on the model. The pattern is almost symmetric to a W - E transect passing over the summit of the hill. This could be expected since the hill was nearly perfectly symmetrical. The local deviations observed in the south and the southeast of the map are caused by the presence of other important hills in this part of the test field (see Fig. 5). The lowest part of the concave windward (W) slope of the hill is characterized by high dust accumulation. Half-way up the windward slope is a small vertical step
very much dust
no dust
Fig. 5. Dust pattern on the scale model after the simulation of a dust storm. See text for aerodynamic conditions. Airflow is from left to right. Thick black line indicates limit of smoothed (and mapped) area on the scale model.
50
Z Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
that creates a local flow separation bubble characterized by very little dust accumulation. Also, near the top, very little accumulation occurs since the steepness of the slope causes the friction velocity to increase very close to, or even beyond, the accumulation threshold. An elongated dust shadow is observed downwind of the summit, on the leeside of the hill. The shadow area extends to about 300 m downwind from the top. Within it occurs, both in the north and in the south, a small, elongated (along the wind) zone of slightly increased accumulation. These zones extend downwind to about half the dust shadow area. From about 150 m downwind of the top, the dust shadow is flanked on both of its sides by a zone of high dust accumulation. Near the end of the shadow, about 300 m downwind of the top of the hill, these zones merge. We now enter in the second accumulation maximum that can also be observed downwind of linear hills standing perpendicular to the wind (see Fig. 1 ). The most important differences between the simulated dust pattern on and around the conical hill and the pattern on and around linear hills standing perpendicular to the wind are ( 1 ) The local increase in dust accumulation immediately downwind from the top, which is always present on linear hills, does not appear on the conical hill. [ It should be noted, however, that in a check-test at a much lower wind speed (freestream velocity equal to 0.75 m . s - 1) the local increase was still present.] (2) The remarkable streaks of high accumulation that occur at each transverse side of a linear 3D hill (see Goossens, 1987b) do not occur on conical hills. This could be expected since for conical hills the wind is able to flow around the transverse hill slopes more easily than for linear hills perpendicular to the wind. The effect remains partly visible, however, both in the small, elongated zones of slightly increased accumulation inside the dust shadow and in the zones of strongly increased accumulation that envelop the downwind half of the shadow. Similar to linear hills normal to the wind, dust accumulation on the (concave) windward slope of a conical hill is considerably higher than dust accumulation on the leeslope.
5. Field experiment To evaluate the wind tunnel pattern shown in Fig. 5, a detailed field experiment was carried out on the hill
near Avdat. Since, for practical reasons, it was not possible to measure dust deposition simultaneously at more than 40 locations, two transects, perpendicular to each other and crossing at the summit, were investigated. Transect No. 1 was oriented W - E and transect No. 2 was oriented N-S. Since, during the experiment, the wind came from the west, transect No. 1 represented a longitudinal and transect No. 2 a transverse profile (Fig. 3). Originally 40 dust collectors were installed in the field. Each collector consisted of a flat, plastic tray filled with distilled water. A thin, fully wetted sponge was placed on the water surface, and a filter paper was then placed on the sponge. Dust particles that settle stick to the paper and are retained from re-deflation. Due to the water' s strong capillary forces, the dust-collecting surface remains permanently moist, even if a thick layer of dust is formed. The collector remains efficient until all the water has evaporated. The size of the filter papers used was 28 cm × 32 cm. A plastic sheet on the ground prevented erosion in the vicinity of the measuring site. After the experiment, it appeared that 6 collectors were stolen by the local population. Thirty-four data points could therefore be collected. When analysing the results, we later realized that the results of several collectors were influenced by the surrounding topography, in particular the large hill south of the conical hill (see Fig. 3). To have a clear idea of the effect of the conical hill on the pattern of dust deposition, and to avoid, as much as possible, perturbations created by the surrounding topography, we decided to take into consideration only those parts of the sedimentation transects situated on the conical hill itself, i.e., both transects start and end in the talweg of the circular depression encircling the conical hill. Twenty-four collectors were thus retained. Fig. 3 shows the location of these collectors. Transect No. 1 contains 13 collectors, and transect No. 2, 12 collectors. The collector at the top of the hill is included in both transects. The field experiment was carried out from 24 February 1992 ( 10:00 h local time) until 25 February 1992 ( 14:00 h local time). Table 1 shows the meteorological data for each of these 28 hours. Wind speed was always above 7 m s - l , with the highest values ( > 10 m s-~) between 11:00 and 17:00 h (24 February) and between 5:00 and 14:00 h (25 February). Wind direction was almost constantly west, sometimes with a (very slight) deflection to the west-southwest. Dust
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
51
Table 1 Wind speed (at 3.5 m height), wind direction (at 3.5 m height) and airborne dust concentration (at 1 m height) during the field experiment of 24-25 February 1992. No dust concentrations were measured during the night. Wind data are indicative, since they were recorded at a meteorological station 9 km north of the test site. The differences with the test site are expected to be small however. Dust measurements were carded out at the test site Date
Period (hours)
Mean wind direction
Mean wind speed (m s -j )
Mean dust concentration (/xg m-3)
24 February 1992
10:00-11:00 11:00-12:00 12:00-13:00 13:00-14:00 14:00-15:00 15:00-16:00 16:00-17:00 17:00-18:00 18:1)0-19:00 19:1)0--20:00 20:00--21:00 21:00-22:00 22:1)0-23:00 23:1)0-24:00
w w w w w w w w w w w w w w
9.6 11.1 12.9 12.4 10.6 10.9 10.9 9.9 9.4 8.4 9.1 8.3 10.0 11.1
786 803 736 702 673 507 506 451 -
25 February 1992
005)0-01:00 O1:00-02:00 02:00-03:00 03:00-04:00 04:00-05:00 05:00-06:00 06:00-07:00 07:00-08:00 08:00-09:00 09:00-10:00 10:,30-11:00
w w w w w w w w w w w
11:,oo-12:00
w
12:,~0-13:00 13:.00-14:00
w w
9.5 8.9 7.3 7.7 8.3 12.5 12.5 11.1 11.1 12.5 12.5 10.5 11.6 12.0
131 103 86 94 83 78
concentration was measured with a portable Sartorius S.M. 16711 dust c o l l e c t o r at a h e i g h t o f 1 m a b o v e the t o p o f t h e hill. C o n c e n t r a t i o n w a s b e t w e e n 4 5 0 a n d 803 /xg m - 3 b e t w e e n 10:00 a n d 18:00 h ( 2 4 F e b r u a r y ) a n d b e t w e e n 78 a n d 131 t~g.m - 3 b e t w e e n 8 h a n d 14 h ( 2 5 F e b r u a r y ) . N o dust c o n c e n t r a t i o n s w e r e m e a s u r e d during the n i g h t for p r a c t i c a l reasons. A f t e r the field e x p e r i m e n t , all filter p a p e r s w e r e collected. T h e q u a n t i t y o f the a c c u m u l a t e d dust w a s determ i n e d a f t e r the p a p e r s h a d b e e n d r i e d in a n o v e n .
6. Comparison of ~4nd tunnel data and field data T o test the r e l i a b i l i t y o f t h e w i n d t u n n e l s i m u l a t i o n , a s e c o n d w i n d t u n n e l e x p e r i m e n t , in w h i c h t h e d e p o -
sition o f dust w a s s i m u l a t e d a l o n g t h e s a m e t r a n s e c t s as in t h e field, w a s e x e c u t e d . A dust s t o r m o f 15 m i n u t e s l o n g w a s s i m u l a t e d o v e r the scale m o d e l u n d e r the s a m e c o n d i t i o n s as d e s c r i b e d in S e c t i o n 4. A f t e r the s i m u l a tion, the t h i c k n e s s o f the dust l a y e r o n t h e m o d e l surface w a s d e t e r m i n e d b y m e a n s o f the O p t o c a t o r i n s t r u m e n t . Figs. 6 a n d 7 s h o w t h e data for the w i n d t u n n e l s i m u l a t i o n a n d the field e x p e r i m e n t for the t w o transects. T h e u p p e r m o s t c u r v e in e a c h figure is the w i n d t u n n e l profile, the m i d d l e c u r v e is the field profile, a n d the l o w e s t c u r v e is the t o p o g r a p h i c profile. F o r t h e w i n d t u n n e l curves, dust t h i c k n e s s w a s m e a s u r e d at m a n y points, b u t to e n s u r e c o r r e c t c o m p a r i s o n s w i t h the field c u r v e s it is n e c e s s a r y to give the t h i c k n e s s o n l y at t h o s e spots e q u i v a l e n t to w h e r e dust d e p o s i t i o n w a s m e a s u r e d in the field.
52
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56 inflection point
320
1
:A
200 -" "~ 180 = 1600
~ 14.0120..- 100-
g
13
8060-
summit
t~020-
O-
I
I
I
I
I
I
1
2
3
t,
5 6
8
910
11
12
13 collector no.
A
inflection point 1,0-
0,9-
0,8e
0,70,6-
'-
05-
¢J
0,4-
0,30,20,10,0-
I
I
~
I
I
1
2
3
t~
5 6 7 8
I
I
I
I
l
910
I
I
11
12
I
13 collector no.
inflection summit point ,c'"
E ==
B
E
62°-I 600-1
-~ 580-J
r
I
I
i
1
2
3
/*
I
I
[
I
I
i
I
5 6 7 8 I
I
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910 I
f
I
f
I
i
11
12
13
I
collector no.
C
I
250 200 150 100 50 0 50 100 150 200 250 distance to sum~t (m} Fig. 6. Comparison of wind tunnel profile and field profile for transect 1 (along the wind). (A) Wind tunnel profile; (B) field profile; (C) topographic cross section. Airflow is from left to right for all curves. Fig. 6 ( W - E transect) shows, for the wind tunnel curve, an increase o f dust deposition along the c o n c a v e
part o f the w i n d w a r d hillslope, a m a x i m u m at the inflection point, and a sharp decrease o f deposition o n c e the
53
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56 inflection point inflection point 120 A
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Fig. 7. Comparison of wind tunnel profile and field profile for transect 2 (normal to the wind). (A) Wind tunnel profile; (B) field profile; (C)
topographic cross section. Airflowis away from the reader for all curves. slope becomes convex. The minimum is reached close to the summit of the hill. Large parts of the leeslope are characterized by a low to very low deposition. Further downwind, the amount of dust deposition starts to increase again. The field curve shows a similar pattern, with increased deposition along the concave windward slope, a maximum at the inflection point, and a sharp decrease on the convex windward slope. The minimum is reached on the leeslope, i.e., more downwind than in
the wind tunnel curve, after which deposition starts to increase again. The general picture of the wind tunnel curve and the field curve is, therefore, similar, but in the wind tunnel, the minimum is located considerably more upstream than in the field. W e will come back to this in Section 7. Fig. 7 ( N - S transect) shows, for the wind tunnel curve, an increase of dust deposition along the concave slopes (both in the north and in the south), a maximum
54
Z Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
at the inflection points, a decrease of deposition once the inflection points are crossed, and a deep minimum at the top of the hill. The maximum at the northern inflection point is higher than that at the southern inflection point, probably because of the (small) asymmetry in the hill profile and the higher topographic position of the northern inflection point. The field curve shows almost exactly the same pattern, with an increasing deposition along the concave slopes, maxima at the inflection points, a decreasing deposition closer to the top, and a minimum at the top itself. Also here, the maximum is higher at the northern inflection point than at the southern inflection point. It can be concluded that, in general, the wind tunnel profiles and the field profiles show a good agreement, so that the dust pattern shown in Fig. 5 will give at least a good indication of the real pattern in the field. One important point requires more attention, however: the upwind shift of the sedimentation minimum in the wind tunnel transect along the wind.
7. Discussion The upwind shift (compared to the field) of the deposition minimum in the wind tunnel transect along the wind can be explained as follows. First, the thickness of the dust layer on the scale model was measured on an undisturbed surface: the latter showed the same inclination as the hillslope in the field. As a corollary to the terrain method adopted (filter papers attached to a sponge floating on a (thin) water surface), deposition in the field was determined on horizontal surfaces. Secondly, the filter papers were at the same level as the top of the plastic trays containing the water. Although the height of these trays was restricted to only a few cm, there is little doubt that, due to the presence of the tray, the speed of the wind just above the filter papers was somewhat lower than above the hill surface itself. Thirdly, since there was no distortion of height, the slopes on the model were identical to those in the field. Given the strong reduction of scale, and the absence, at that scale, of important surface roughness, the accumulation threshold (i.e., the critical friction velocity where the ratio of dust deposition to dust erosion drops below unity) is exceeded more quickly on the scale model than in the field (see Goossens, 1988a or Goossens and Offer, 1988 for more details). The near
absence of dust on the convex windward slope of the scale model indicates that, during the simulation, the accumulation threshold was very closely approached (or even exceeded) on this part of the model, and the minimum in the accumulation curve was located upstream of the summit of the hill. In the field, the ratio of wind speed to hill size is much smaller, offering the wind fewer abrupt topographic changes, and the accumulation threshold was not exceeded during the dust storm of 24-25 February 1992, especially over the filter surfaces in the plastic trays. Therefore, the minimum in the dust profile was not located upstream but downstream of the summit. In a similar study executed on a linear hill (perpendicular to the wind) about 2 km north of the conical hill, the minimum in the dust profile was located on the leeslope in both the wind tunnel and the field experiment (Goossens and Offer, 1993). We repeated the conical hill experiment in the wind tunnel for a much lower wind velocity (freestream velocity of only 0.75 m s J). Now, nearly the whole of the windward slope was covered by dust, i.e., the accumulation threshold was not exceeded (similar to the field), but given the low wind speed, the airflow separation bubble downstream of the inflection point on the windward slope was much smaller than in the field, and the decrease of deposition along the convex hillslope was not significant except for a very small zone just before the top (which distracts from the field pattern). The reader should realize that both the wind tunnel patterns and the field patterns that are discussed in this paper refer to short-term geomorphic events. After a sufficiently long time, depending on the meteorological characteristics of the test site, these initial patterns may show significant changes. In the case of the conical hill described in this paper, rainfall is a (very!) important parameter since heavy floods will wash large amounts of dust from the hillslopes. This dust accumulates in the wadis (see, for example, Evenari et al., 1982). These phenomena seriously alter the final accumulation pattern of the dust in the desert. But it can be expected that, if there is any dust to be accumulated on the hillslopes, it will primarily be on the windward slopes since the leeslopes cannot benefit from an important input of dust. Moreover, for the Negev desert, the trapping efficiency is higher for the north and west facing slopes (i.e., the windward slopes for dust deposition) than for the south and east facing slopes (the leeslopes) since much dust is accumulated by microphytic crusts
z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
( D a n i n and Ganor, 1991 ) that s h o w a higher trapping efficiency on the north facing slopes than on the south facing slopes ( D e Boeck, 1994). D e t a i l e d studies o f l o n g - t e r m dust a c c u m u l a t i o n in c o l d dry e n v i r o n m e n t s (the B e l g i a n loess belt b e t w e e n 20,000 and 12,000 years B P ) clearly s h o w that, in the long term, it is nearly always the w i n d w a r d slopes that a c c u m u l a t e the largest quantities o f dust ( G o o s s e n s , 1987a, 1995b). The higher the effect of syn-and post-depositional rainwash, the lesser the difference b e t w e e n the w i n d w a r d and the leeward slopes with respect to dust accumulation.
8. Conclusions The wind tunnel experiments and the field measurements, although neither is perfect, s h o w e d at least an acceptable degree o f similarity. Both indicate that conical hills create an elongated area o f low deposition ( " d u s t s h a d o w " ) on their lee. D o w n w i n d o f the shadow zone, a local area o f e n h a n c e d ( c o m p a r e d to the undisturbed surroundings) deposition occurs. O n the lateral flanks o f the hill, and also on the small conv e x w i n d w a r d slope just upstream o f the top ( a n d at the top itself), dust deposition remains low to very low. It is the lowermost, c o n c a v e w i n d w a r d slope that r e c e i v e s the largest amounts o f dust. T h e p a t t e m described a b o v e refers to short-term deposition only. W a t e r erosion, especially during h e a v y floods, m a y seriously alter the final accumulation pattern since it will clean the slopes o f important amounts o f dust that a c c u m u l a t e in the wadis.
References Bouten, W., Van Eijsden, G., lmeson, A.M., Kwaad, F.J.P.M., Miicher, H.J. and Tiktak, A., 1985. Ontstaan en erosie van loessleemgronden in Zuid-Limburg. K.N.A.G. Geogr. Tijdschr., 19: 192-208. Bruins, H., 1986. Desert environment and agriculture in the central Negev and Kadesh-Bamea during historical times. Ph.D. Thesis, Landbouwhogeschool Wageningen, 219 pp. Cermak, J.E., 1970. Air motion in and near cities - - determination by laboratory simulation. Fluid Mechanics Program, College of Engineering, Colorado State University, Fort Collins, CO, Rep. CEP70-71JEC27, 21 pp. Cerrnak, J.E., 1984. Physical modelling of flow and dispersion over complex terrain. Boundary-Layer Meteorol., 30: 261-292.
55
Danin, A. and Ganor, E., 1991. Trapping of airborne dust by mosses in the Negev desert, Israel. Earth Surf. Process. Landforms, 16: 153-162. De Boeck, A., 1994. Relaties tussen een microfytische korst en de sedimentatie van natuurlijk atmosferisch stof. Stofneerslag-en fotosynthesemetingen. M.Sc. Thesis, K.U. Leuven, 105 pp. Evenari, M., Shanan, L. and Tadmor, N., 1982. The Negev. The Challenge of a Desert. Harvard Univ. Press, Cambridge/London, 437 pp. Gokhan, C., 1973. Air movement around buidings (an investigation study). Thesis, M.E.T.U., Ankara, 67 pp. Goossens, D., 1987a. Sedimentatiemechanismen bij natuurlijke stofdeeltjes in lucht. Ph.D. Thesis, K.U. Leuven, 265 pp. Goossens, D., 1987b. Dust deposition as a flow visualization technique. Phys. Geogr., 8: 378-389. Goossens, D., 1988a. The effect of surface curvature on the deposition of loess: a physical model. Catena, 15: 179-194. Goossens, D., 1988b. Scale model simulations of the deposition of loess in hilly terrain. Earth Surf. Process. Landforms, 13: 533544. Goossens, D., 1988c. Sedimentation characteristics of natural dust in the wake of symmetrical hills. Z. Geomorphol., 32: 499-502. Goossens, D., 1989. Height distortion and the sedimentation of dust on topographic scale models: considerations and simulations. Earth Surf. Process. Landforms, 14: 655~567. Goossens, D., 1992. Eolische processen op Venus. Heelal, 37: 172178. Goossens, D., 1995a. Wind tunnel experiments of eolian dust deposition along ranges of hills. Earth Surf. Process. Landforms, in press. Goossens, D., 1995b. Long-term aeolian loess accumulation modelled in the wind tunnel: the Molenberg case (central loess belt, Belgium). Z. Geomorphol., submitted. Goossens, D. and Offer, Z.Y., 1988. Loess erosion and loess deposition in the Negev Desert: theoretical modelling and wind tunnel simulations. The Jacob Blaustein Institute for Desert Research, Desert Meteorology Papers, Ser. A, No. 13, 65 pp. Goossens, D. and Offer, Z.Y., 1990. A wind tunnel simulation and field verification of desert dust deposition (Avdat Experimental Station, Negev Desert). Sedimentology, 37: 7-22. Goossens, D. and Offer, Z.Y., 1993. Eolian deposition of dust over symmetrical hills: an evaluation of wind tunnel data by means of terrain measurements. Z. Geomorphol., 37:103-111. Goossens, D., Offer, Z.Y. and Zangvil, A., 1993. Wind tunnel experiments and field investigations of eolian dust deposition on photovoltaic solar collectors. Solar Energy, 50: 75-84. Grant, A.L.M. and Mason, P.J., 1990. Observations of boundarylayer structure over complex terrain. Q. J. R. Meteorol. Soc., 116: 159-186. Greeley, R., 1986. Aeolian landforms: laboratory simulations and field studies. In: W.G. Nickling (Editor), Aeolian Geomorphology. Allen and Unwin, Boston, pp. 195-211. Grolier, M., Ericksen, G.E., McCauley, J.F. and Morris, E.C., 1974. The desert landforms of Peru: a preliminary photographic atlas. U.S. Geol. Survey, Interagency Report, Astrogeology, 57:146 Pp.
56
Z. E Offer, D. Goossens / Geomorphology 14 (1995) 43-56
Hertig, J.-A. and Liska, P., 1983. Simulations of regional atmospheric flows on small scale topographic models, J. Wind Eng. Ind. Aerodyn., 15: 77-89. Iversen, J.D., 1983. Saltation threshold and deposition rate modelling. In: M.E. Brookfield and T.S. Ahlbrandt (Editors), Eolian Sediments and Processes. Elsevier, Amsterdam, pp. 103-113. Iversen, J.D., Greeley, R., White, B.R. and Pollack, J.B., 1976. The effect of vertical distortion in the modelling of sedimentation phenomena: Martian crater wake streaks. J. Geophys. Res., 81: 4846-4856.
Rozycki, S.Z., 1967. Le sens des vents portant la poussibre de loess, ?~la lumi6re de l'analyse des formes d'accumulation du loess en Bulgarie et en Europe Centrale. Rev. G6omorphol. Dynam. 2~me S6rie, 9: 1-9. Tsoar, H. and Pye, K., 1987. Dust transport and the question of desert loess formation. Sedimentology, 34: 139-153. Yaalon, D. and Ganor, E., 1979. East Mediterranean trajectories of dust-carrying storms from the Sahara and Sinai. In: C. Morales (Editor), Saharan Dust. Mobilization, Transport, Deposition. Wiley, New York, pp. 187-193.
Geomorphology 14 (1995) 43-56
Wind tunnel experiments and field measurements of aeolian dust deposition on conical hills Zvi Y. Offer
a
Dirk Goossens b
Meteorology Unit, The Jacob Blaustein Institute for Desert Research, Sede Boqer Campus 84990, Israel b N.F.W.O., Laboratory for Experimental Geomorphology, Redingenstraat 16 bis, B-3000 Leuven, Belgium Received 6 July 1994; revised 22 January 1995; accepted 3 February 1995
Abstract
The spatial pattern of short-term aeolian dust deposition on and around cone-shaped hills is investigated via the simulation, in a wind tunnel, of dust storms over a topographic scale model of a conical hill in the Negev desert, Israel. The results are tested during a full-scale dust storm in the Negev. The wind tunnel experiments adequately predict the field pattern, although some problems may arise on steep windward slopes where the accumulation threshold is more quickly exceeded on the scale model than in the field. Conical hills create an elongated area of low deposition ( "dust shadow") in their lee. Downwind from the shadow zone, a local area of more-than-normal (compared to the undisturbed surroundings) deposition occurs. On the lateral flanks of the hill, and also on the small convex windward slope just upstream of the top (and at the top itself), dust deposition remains low to very low. It is the lowermost, concave windward slope that receives the largest amounts of dust. The pattern described above refers to short-term deposition only. Water erosion may seriously alter the final accumulation pattern since it will clean the slopes of important amounts of dust that accumulate in the wadis.
1. Introduction Topography is one of the most important factors that affect the spatial patterns of erosion, transport, deposition and accumulation of aeolian dust (Goossens, 1987a). In hilly regions, dust and dust-derived deposits (such as loess) are, therefore, characterized by an important spatial variation in occurrence and sediment depth (Rozycki, 1967; Bouten et al., 1985). The number of experimental studies in which the effect of topography on aeolian dust deposition is investigated is rather small. Wind tunnel simulations of aeolian dust deposition over topographic scale models are reported by Goossens ( 1988a, b, c, 1989, 1995a, b) and Goossens and Offer ( 1988, 1990, 1993). Field experiments are discussed by Goossens and Offer (1990, 1993) for single-storm events and Goossens 0169-555X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIOI69-555X(95)O0015-1
(1988b, 1995b) for long-term accumulation. These studies comprise both dust deposition experiments on single, isolated landforms (hills and valleys), on ranges of hills, and in complex topography. Thusfar, all wind tunnel and field studies with hilly landforms have focussed on linear hills standing perpendicular to the direction of the wind (and the dust flow). The typical sedimentation profile for such hills is shown in Fig. 1. Assuming that the area upstream of the hill is flat and horizontal, dust deposition starts to increase once the windward hillslope is reached. Deposition continuously increases up-slope until a maximum is obtained at the inflection line (where the slope changes from concave to convex). Downwind of the inflection line, dust deposition drops considerably. Typically two minima are observed in the profile: the first close to (but not necessarily exactly at) the sum-
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
44 30dust depth difference (ijmI 20-
10-
^-v~. V,.,-,,---,.~, --~ -10
-20 ¸
-30
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'
' 100
'
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'
' ' 180 ' ' 200 ' ' 220 ' ' lt,' O ' 160 disfcmce to hill summit (cm)
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Fig. 1. Typical wind tunnel dust deposition profile over a symmetrical hill standing perpendicular to the wind. Values in abscissa represent distance to the hill summit. Ordinate values indicate the difference in dust depth between the curved hill surface and a flat reference surface. Airflow is from left to right; the hill is 25 cm wide and 3 cm high. Friction velocity during the experiment was 5.8 cm. s - 5.
mit, and the second more downwind, on the leeward slope. A small zone of slightly increased sedimentation separates both minima. Dust deposition remains very low all over the leeslope. Downstream of the hill, a second zone of increased sedimentation occurs. This zone gradually damps out until "normal" deposition (corresponding to that upstream of the hill) has again been reached. No further oscillations appear in the deposition curve (Goossens, 1988c). Although linear hills occur quite frequently, they represent only one of the different types of hills found in desert environments. An even more common type is the circular (conical) hill, found abundantly in the deserts of the Middle East, and largely composed of long, concave-curved slopes with a small convex zone near the top. Also, many bornhardts, though not necessarily showing the same slope profile, have more or less conical shapes. The same holds for many domelike hills (although the " c o n e " is rather flattened for these latter). Also, many volcanoes show a cone-like shape. Since conical hills form a substantially different type of obstacle to the wind compared to linear hills, the airflow pattern over, and therefore also the dust deposition pattern on (and around), conical hills will differ from those on (and around) linear hills.
The aim of this paper is to investigate the dust deposition pattern created by conical hills. Dust storms are simulated over a topographic scale model of a conical hill located in the Negev Desert, Israel. The results of the simulation are tested by a field experiment carried out on the same hill during a dust storm.
2. Description of the test field The field experiment was carried out on a more or less isolated, conical hill in the central Negev desert. The hill is located 3 km south of the old Nabatean city of Avdat (Fig. 2). The climate in this region is typically arid, with an annual average precipitation of 87 mm. Annual average temperature is 18.3°C, with great differences between winter and summer and between day and night. The daily maximum temperature is more than 40°C in May, June and July, but the minimum sinks below freezingpoint in December and January. Atmospheric humidity is generally low, with highest values during the cold season. The predominant winds blow from the north, northwest and west. The area near Avdat consists of (mostly stony) hammadas, developed on different elevations. They include
Z E Offer, D. Goossens / Geomorphology 14 (1995) 43-56
45
Fig. 2. Situationof the experimental field. Arrowindicates locationof the test hill. both large and small plateaus and wide wadi depressions. Valleys cut many of the plateaus at their borders. The depth of incision may reach 50 m and more. Most of the hammadas are developed on limestone and dolomitic formations. The valleys are dry nearly all year. Their transverse profiles show a diversity of shapes, varying from troughs and V-forms lo small canyons. The subsoil near the test hill is composed mainly of carbonate rocks (limestone and chalk, with variable amounts of chert). The hill itself is nearly conical, with long concave slopes (Fig. 3). It has a diameter of about 450 m and a height of 66 m above the surrounding surface. The slopes are covered by a mixture of stones (diameter usually between 5 cm and 30 cm) and loess. The loess consists of some 25% fine sand, 55% silt and 20% clay (Bruins, 1986) and forms surface crusts about 2-4 mm thick. The steep convex zone near the top is largely composed of bare limestone rocks. Vegetation is scanty due to the low precipitation. Some sparse, discontinuously dispersed shrubs (20-30 cm high) locally occur on the hillslopes. The area near Avdztt is prone to a high frequency of dust storms. These occur mainly in the winter and spring, and are associated with the passage of cold fronts and low pressure systems (Goossens et al., 1993). Cases of dust haze also frequently occur in the same period. During dust storms, wind blows predominantly from the west.
3. E x p e r i m e n t a l d e s i g n in t h e w i n d t u n n e l 3.1. E x p e r i m e n t a l f a c i l i t i e s
All wind tunnel experiments were carried out in the closed return wind tunnel of the Laboratory for Experimental Geomorphology of the Katholieke Universiteit Leuven (Belgium). The test section of this tunnel is 7.6 m long, 1.2 m wide, and 0.6 m high. A complete description of the wind tunnel can be found in the work by Goossens and Offer (1988). Dust storms were generated by means of an Engelhardt laboratory dust cloud producer connected to the tunnel. This apparatus ensured a continuous feed of natural dust to the air current, and allowed the operator to adjust dust discharge (and concentration). The thickness of the dust layer on the scale model was measured with an Optocator laser instrument with a gauge probe type 2301. The accuracy of the instrument is 2/zm. 3.2. S i m i l i t u d e p a r a m e t e r s
The most important similarity criteria for the simulation of airflow over topography are (Hertig and Liska, 1983): - equal model and field densimetric Froude number: F r = u / ( g . D ) o.5 ( i f there is no temperature scaling) ; - equal model and field Reynolds number: R e = u . D/v
46
Z. Y. Offer. D. Goossens / Geomorphology 14 (1995) 43-56
Fig. 3. Topographicmap of the test hill and its surroundings. The hill is located in the centre of the map. Black dots show the location of the dust collectorsduring the field experiment of 24-25 February 1992. Stolen collectorsare not indicated. where u is the wind speed, D a characteristic length, g the gravitational acceleration and ~, the kinematic viscosity. When geometric scaling is very small, as in the conical hill experiment, it is impossible to achieve identical Froude numbers and Reynolds numbers simultaneously (Hertig and Liska, 1983). Knowing that these numbers express the relationship between kinetic energy and viscous energy dissipation, it is possible to retain the similarity of the friction energy by taking only the turbulent energy of the atmospheric flows into consideration. Consequently, both the nature and the model must have identical turbulent Reynolds numbers.
Boundary layer flow characteristics over a rough surface become independent of Reynolds number (Re) once a critical R e is exceeded (Cermak, 1984). Through selection of proper combinations of wind tunnel length, surface roughness and ambient wind speed, boundary layer flows can thus be realized that model the micrometeorological features of the high Reynolds number atmospheric boundary layer. 3.3. The s c a l e m o d e l
To investigate the sedimentation of dust on and near the conical hill near Avdat, a 1:2500 scale model, containing the hill and its surroundings was used. The
z.Y. Offer, D. Goossens/ Geomorphology 14 (1995) 43-56
model was constructed from polystyrene plates 2 mm thick, which were cut along contour lines copied from a topographical map of the study area. By adhering the different polystyrene plates to each other, a rough, terraced scale model was obtained. To simulate dust deposition over the model, the surface had to be carefully smoothed in that the small steps would generate artificial flow disturbances that could influence the sedimentation pattern on the model. The smoothing was executed by means of gypsum plaster. Only the test area itself was smoothed: the surroundings were kept terraced to increase airstream turbulence (necessary to decrease the simulation requirement of Reynolds number equality). The final model surface was smooth with a wery small micro-roughness in the order of about 100 ~m. At full scale, this corresponds to 25 cm, which is almost equal to the size of the roughness elements ( stones and shrubs) in the field. Blockage of the scale model was restricted to 3.9%. Horizontal and vertical scale were identical, which enabled us to obtain geometric similarity during the wind tunnel simulations. The maximum diameter of the modelled area (experimental field plus its surroundings) was restricted to 1.5 km. Deflection of the (near-surface) winds due to the Coriolis effect may thus be ignored (see Cermak, 1970 and Gokhan, 1!)73 for more details). 3.4. Boundary layer simulation General concepts
In hilly areas, the structure of the airflow near the ground is not only a function of local surface roughness but also depends on the general airflow pattern influenced by the topography of the surrounding region. In addition, dust is not only transported close to the ground but also at high altitudes (dust clouds of several hundreds of metres high are not an exception, especially during heavy dust storms), and the particles are so small that they largely follow the streamlines of the air. An adequate simulation of dust deposition on the conical hill scale model, therefore, requires not only a correct simulation of the local surface roughness influence, but also a correct simulation of the general airflow pattern in the region. This means that the wind tunnel experiments should .at least simulate an important part of the atmospheric boundary layer (ABL).
47
In the lowest 15% of the ABL, the wind profile (in neutral atmospheric conditions) can be described either by the logarithmic law: u / u . = ( 1 / k ) . ln(z/zo)
( 1)
or by the power law: ul/u2 = (Zl/Z2) ~
(2)
where u, Ul and u2 are the mean wind speeds at heights z, Zl and z2 respectively, u. is the friction velocity, k the Von Karman constant, Zo the roughness length and a a dimensionless exponent (usually between 0.1 and 0.4). Unlike the logarithmic profile, the power profile is not restricted only to the surface boundary layer (SBL) but can also be applied to the total boundary layer. The exponent a describes the curvature of the velocity profile and mainly depends upon surface roughness. Thus, for an adequate simulation of boundary layer flow over the topographic scale model, the following parameters must be modelled: the depth of the boundary layer, the roughness length Zo, and the power law exponent a. Boundary layer thickness
Near Avdat, the depth of the ABL is approximately 600 m (see Goossens and Offer, 1990). The surface boundary layer, in which the neutral wind profile may be approached by Eq. (1), is therefore about 80-100 m thick. As the maximum height differences in (and near) the test field are only slightly smaller (usually about 50-60 m), it is not enough to simulate only the SBL: the depth of the boundary layer to be simulated in the wind tunnel must be at least several hundreds of metres. Roughness length
The roughness length Zois determined by the roughness of the surface. Two types of roughness must be considered: micro-roughness (caused by pebbles and shrubs), and macro-roughness (caused by hills, valleys and mountains). Both the small and the large roughness elements will affect the velocity profile of the wind. In the Avdat area, micro-roughness is restricted to about 0.2-0.3 m. Moreover, the degree of micro-roughness is more or less uniform in the test field. The contribution of micro-roughness to the roughness length zo can be estimated at 6.7-10.0 mm, as it is approximately equal to 1/30 of the field roughness elements' height in
48
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
rough-turbulent flow. To calculate the contribution of the macro-roughness (relief) to the roughness length, we used the algorithm proposed by Grant and Mason (1990) reaching a Zo value of between 24 and 64 cm, depending on the size of the landforms and the orientation of the wind. Thus, the contribution of the macroroughness (to Zo) is considerably greater than that of the micro-roughness. To achieve similarity, the value of Zo in the wind tunnel must be equal to the field value multiplied by the scale of reduction (Iversen et al., 1976; Iversen, 1983). Since the field value of Zo can be estimated at 24-64 cm and the vertical scale of the model is 1:2500, the simulated value of Zo must be in the order of 96256/xm.
Shape of the velocity profile The shape (or curvature) of the wind's velocity profile can be quantified using the power law exponent a. Velocity profile curvature is largely determined by surface roughness (and atmospheric stability). Here too, it is necessary to consider the effects of both microroughness and macro-roughness. In general c~, varies from less than 0.10 (very smooth terrain) to more than 0.40 (extremely rough terrain). From meteorological data recorded at Sede Boqer, some 9 km north of the test hill, it can be calculated that, in near-neutral conditions, a is about 0.16 for winds blowing from the SW-N sector (see Goossens et al., 1993). The value of a near Avdat should be almost identical (or may be only slightly higher), since macro-roughness at Avdat is only slightly greater than at Sede Boqer. Thus, near the test hill, the simulated value of ~ should be 0.160.17.
Simulated boundary layer A 6 m upwind fetch in the wind tunnel was covered with a plastic sheet with gentle undulations. This produced a boundary layer with a depth of 24.5 cm (equivalent to 612 m in the field), a velocity profile with a power exponent of 0.17 and a Zo value (derived from a logarithmic curve fitting) of 255 /xm. Freestream velocity was 1.95 m s - ~, and friction velocity 0.11 m s - 1. The Reynolds number during the simulation was equal to 8.01 × 105. This is sufficiently high to obtain Reynolds number independency for flow over surface roughness elements higher than 0.7 cm (see Fig. 1 in Cermak, 1984). Since roughness elements were several
centimetres high, simulated flow may be considered Reynolds-independent.
3.5. The dust particles During the simulation, the aerodynamic behaviour of the airborne particles must be similar to the behaviour of the dust particles in the field. During dust storms the degree of air turbulence is high to very high, and the dust is transported in true suspension. Thus, true suspension must be simulated in the wind tunnel. Air-dry Belgian Brabantian loess, sieved at a diameter of 63 # m to exclude all sand particles, was used during the conical hill experiment. The loess had a median diameter of about 30/zm, which corresponds closely to the size of the dust particles that settle near Avdat during normal dust storm events (Yaalon and Ganor, 1979). The ratio of terminal fall velocity of the particles, ut, to the friction velocity in the wind tunnel, u., was equal to 0.64. According to Tsoar and Pye (1987), dust is transported in true suspension when u,/ u. is below 0.70. Thus, during the conical hill experiment, dust transport occurred in true suspension.
4. Airflow over, and dust deposition on, a conical hill: the general pattern Circular hills with a relatively low height-to-width ratio create an elongated wind shadow characterized by two trailing vorticies ( "horseshoe" ) --having axes parallel to the wind --that are shed from the flanks of the hill (Greeley, 1986). A general sketch is shown in Fig. 4. Circular hills with a high height-to-width ratio also create an elongated wind shadow. In the case of deposition of sand, this shadow can be characterized by a strong accumulation of sediment (with, eventu"shadow" zone
a
pointof
t
t
_~__ ~
a
c
.
h
vortex core \ .
. \~
~
.
reverse flow horseshoe vortex Fig. 4. Diagramshowinghorseshoevortexformedarounda low
topographicobstruction(modifiedfromGreeley,1986),
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
ally, the creation of impressive linear lee dunes) as well as by a general lack of sand deposition. In the latter case, the high degree of turbulence created by the conical hill causes the sands that penetrate the shadow zone to be swept away ahnost immediately. Examples of both processes are found in papers by Grolier et al. (1974) and Greeley (1986). In the case of dust deposition, the area directly downwind from a topographic obstacle always behaves as a shadow zone, as shown by several previous wind tunnel and field experiments (Goossens, 1987b, 1988a-c, 1989, 1992, 1995a; Goossens and Offer, 1990, 1993). Fig. 5 shows the dust pattern on the scale model of the conical hill near Avdat after a dust storm of 2
49
minutes (aerodynamic conditions as described in Section 3.4; dust discharge is 13.33 kg h-~ and dust concentration at 5 cm height is 1.5 g m-3; airflow from the W to the E). The picture is based on visual observations of the pattern on the model. The pattern is almost symmetric to a W - E transect passing over the summit of the hill. This could be expected since the hill was nearly perfectly symmetrical. The local deviations observed in the south and the southeast of the map are caused by the presence of other important hills in this part of the test field (see Fig. 5). The lowest part of the concave windward (W) slope of the hill is characterized by high dust accumulation. Half-way up the windward slope is a small vertical step
very much dust
no dust
Fig. 5. Dust pattern on the scale model after the simulation of a dust storm. See text for aerodynamic conditions. Airflow is from left to right. Thick black line indicates limit of smoothed (and mapped) area on the scale model.
50
Z Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
that creates a local flow separation bubble characterized by very little dust accumulation. Also, near the top, very little accumulation occurs since the steepness of the slope causes the friction velocity to increase very close to, or even beyond, the accumulation threshold. An elongated dust shadow is observed downwind of the summit, on the leeside of the hill. The shadow area extends to about 300 m downwind from the top. Within it occurs, both in the north and in the south, a small, elongated (along the wind) zone of slightly increased accumulation. These zones extend downwind to about half the dust shadow area. From about 150 m downwind of the top, the dust shadow is flanked on both of its sides by a zone of high dust accumulation. Near the end of the shadow, about 300 m downwind of the top of the hill, these zones merge. We now enter in the second accumulation maximum that can also be observed downwind of linear hills standing perpendicular to the wind (see Fig. 1 ). The most important differences between the simulated dust pattern on and around the conical hill and the pattern on and around linear hills standing perpendicular to the wind are ( 1 ) The local increase in dust accumulation immediately downwind from the top, which is always present on linear hills, does not appear on the conical hill. [ It should be noted, however, that in a check-test at a much lower wind speed (freestream velocity equal to 0.75 m . s - 1) the local increase was still present.] (2) The remarkable streaks of high accumulation that occur at each transverse side of a linear 3D hill (see Goossens, 1987b) do not occur on conical hills. This could be expected since for conical hills the wind is able to flow around the transverse hill slopes more easily than for linear hills perpendicular to the wind. The effect remains partly visible, however, both in the small, elongated zones of slightly increased accumulation inside the dust shadow and in the zones of strongly increased accumulation that envelop the downwind half of the shadow. Similar to linear hills normal to the wind, dust accumulation on the (concave) windward slope of a conical hill is considerably higher than dust accumulation on the leeslope.
5. Field experiment To evaluate the wind tunnel pattern shown in Fig. 5, a detailed field experiment was carried out on the hill
near Avdat. Since, for practical reasons, it was not possible to measure dust deposition simultaneously at more than 40 locations, two transects, perpendicular to each other and crossing at the summit, were investigated. Transect No. 1 was oriented W - E and transect No. 2 was oriented N-S. Since, during the experiment, the wind came from the west, transect No. 1 represented a longitudinal and transect No. 2 a transverse profile (Fig. 3). Originally 40 dust collectors were installed in the field. Each collector consisted of a flat, plastic tray filled with distilled water. A thin, fully wetted sponge was placed on the water surface, and a filter paper was then placed on the sponge. Dust particles that settle stick to the paper and are retained from re-deflation. Due to the water' s strong capillary forces, the dust-collecting surface remains permanently moist, even if a thick layer of dust is formed. The collector remains efficient until all the water has evaporated. The size of the filter papers used was 28 cm × 32 cm. A plastic sheet on the ground prevented erosion in the vicinity of the measuring site. After the experiment, it appeared that 6 collectors were stolen by the local population. Thirty-four data points could therefore be collected. When analysing the results, we later realized that the results of several collectors were influenced by the surrounding topography, in particular the large hill south of the conical hill (see Fig. 3). To have a clear idea of the effect of the conical hill on the pattern of dust deposition, and to avoid, as much as possible, perturbations created by the surrounding topography, we decided to take into consideration only those parts of the sedimentation transects situated on the conical hill itself, i.e., both transects start and end in the talweg of the circular depression encircling the conical hill. Twenty-four collectors were thus retained. Fig. 3 shows the location of these collectors. Transect No. 1 contains 13 collectors, and transect No. 2, 12 collectors. The collector at the top of the hill is included in both transects. The field experiment was carried out from 24 February 1992 ( 10:00 h local time) until 25 February 1992 ( 14:00 h local time). Table 1 shows the meteorological data for each of these 28 hours. Wind speed was always above 7 m s - l , with the highest values ( > 10 m s-~) between 11:00 and 17:00 h (24 February) and between 5:00 and 14:00 h (25 February). Wind direction was almost constantly west, sometimes with a (very slight) deflection to the west-southwest. Dust
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
51
Table 1 Wind speed (at 3.5 m height), wind direction (at 3.5 m height) and airborne dust concentration (at 1 m height) during the field experiment of 24-25 February 1992. No dust concentrations were measured during the night. Wind data are indicative, since they were recorded at a meteorological station 9 km north of the test site. The differences with the test site are expected to be small however. Dust measurements were carded out at the test site Date
Period (hours)
Mean wind direction
Mean wind speed (m s -j )
Mean dust concentration (/xg m-3)
24 February 1992
10:00-11:00 11:00-12:00 12:00-13:00 13:00-14:00 14:00-15:00 15:00-16:00 16:00-17:00 17:00-18:00 18:1)0-19:00 19:1)0--20:00 20:00--21:00 21:00-22:00 22:1)0-23:00 23:1)0-24:00
w w w w w w w w w w w w w w
9.6 11.1 12.9 12.4 10.6 10.9 10.9 9.9 9.4 8.4 9.1 8.3 10.0 11.1
786 803 736 702 673 507 506 451 -
25 February 1992
005)0-01:00 O1:00-02:00 02:00-03:00 03:00-04:00 04:00-05:00 05:00-06:00 06:00-07:00 07:00-08:00 08:00-09:00 09:00-10:00 10:,30-11:00
w w w w w w w w w w w
11:,oo-12:00
w
12:,~0-13:00 13:.00-14:00
w w
9.5 8.9 7.3 7.7 8.3 12.5 12.5 11.1 11.1 12.5 12.5 10.5 11.6 12.0
131 103 86 94 83 78
concentration was measured with a portable Sartorius S.M. 16711 dust c o l l e c t o r at a h e i g h t o f 1 m a b o v e the t o p o f t h e hill. C o n c e n t r a t i o n w a s b e t w e e n 4 5 0 a n d 803 /xg m - 3 b e t w e e n 10:00 a n d 18:00 h ( 2 4 F e b r u a r y ) a n d b e t w e e n 78 a n d 131 t~g.m - 3 b e t w e e n 8 h a n d 14 h ( 2 5 F e b r u a r y ) . N o dust c o n c e n t r a t i o n s w e r e m e a s u r e d during the n i g h t for p r a c t i c a l reasons. A f t e r the field e x p e r i m e n t , all filter p a p e r s w e r e collected. T h e q u a n t i t y o f the a c c u m u l a t e d dust w a s determ i n e d a f t e r the p a p e r s h a d b e e n d r i e d in a n o v e n .
6. Comparison of ~4nd tunnel data and field data T o test the r e l i a b i l i t y o f t h e w i n d t u n n e l s i m u l a t i o n , a s e c o n d w i n d t u n n e l e x p e r i m e n t , in w h i c h t h e d e p o -
sition o f dust w a s s i m u l a t e d a l o n g t h e s a m e t r a n s e c t s as in t h e field, w a s e x e c u t e d . A dust s t o r m o f 15 m i n u t e s l o n g w a s s i m u l a t e d o v e r the scale m o d e l u n d e r the s a m e c o n d i t i o n s as d e s c r i b e d in S e c t i o n 4. A f t e r the s i m u l a tion, the t h i c k n e s s o f the dust l a y e r o n t h e m o d e l surface w a s d e t e r m i n e d b y m e a n s o f the O p t o c a t o r i n s t r u m e n t . Figs. 6 a n d 7 s h o w t h e data for the w i n d t u n n e l s i m u l a t i o n a n d the field e x p e r i m e n t for the t w o transects. T h e u p p e r m o s t c u r v e in e a c h figure is the w i n d t u n n e l profile, the m i d d l e c u r v e is the field profile, a n d the l o w e s t c u r v e is the t o p o g r a p h i c profile. F o r t h e w i n d t u n n e l curves, dust t h i c k n e s s w a s m e a s u r e d at m a n y points, b u t to e n s u r e c o r r e c t c o m p a r i s o n s w i t h the field c u r v e s it is n e c e s s a r y to give the t h i c k n e s s o n l y at t h o s e spots e q u i v a l e n t to w h e r e dust d e p o s i t i o n w a s m e a s u r e d in the field.
52
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56 inflection point
320
1
:A
200 -" "~ 180 = 1600
~ 14.0120..- 100-
g
13
8060-
summit
t~020-
O-
I
I
I
I
I
I
1
2
3
t,
5 6
8
910
11
12
13 collector no.
A
inflection point 1,0-
0,9-
0,8e
0,70,6-
'-
05-
¢J
0,4-
0,30,20,10,0-
I
I
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1
2
3
t~
5 6 7 8
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l
910
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11
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13 collector no.
inflection summit point ,c'"
E ==
B
E
62°-I 600-1
-~ 580-J
r
I
I
i
1
2
3
/*
I
I
[
I
I
i
I
5 6 7 8 I
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910 I
f
I
f
I
i
11
12
13
I
collector no.
C
I
250 200 150 100 50 0 50 100 150 200 250 distance to sum~t (m} Fig. 6. Comparison of wind tunnel profile and field profile for transect 1 (along the wind). (A) Wind tunnel profile; (B) field profile; (C) topographic cross section. Airflow is from left to right for all curves. Fig. 6 ( W - E transect) shows, for the wind tunnel curve, an increase o f dust deposition along the c o n c a v e
part o f the w i n d w a r d hillslope, a m a x i m u m at the inflection point, and a sharp decrease o f deposition o n c e the
53
Z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56 inflection point inflection point 120 A
~. 100
t
I
I
80 "-" ?, 60 "~ 2O 0
1 I
I
I
2 3
I
I
4.
5 6
1
I
I
7
8 9
inflection point
I
I
/
10 11 12
coLLector no.
A
inflection point
0,8 t= al
I
0,7
o: 0,6
0,5 0,t~
I
1
1
2 3
I
I
1
/,
5 6
I
I
I
I
l
7 8 9
I
I
10 11 12 collector no.
summit
•-~ -. 660 6t~O E
inflection point
t
I infte~:ti°n
62o
~ 600 -£; 580
I
I
1
2 3 I
I
I
I
I
I
f
k
5 6 f
I
i
300 250 200 150 100 50
I
I
I
7 8
9
F
I
0
50
I
I
I
10 11 12 collector no. I
i
100 150
flistonce to summit (m)
Fig. 7. Comparison of wind tunnel profile and field profile for transect 2 (normal to the wind). (A) Wind tunnel profile; (B) field profile; (C)
topographic cross section. Airflowis away from the reader for all curves. slope becomes convex. The minimum is reached close to the summit of the hill. Large parts of the leeslope are characterized by a low to very low deposition. Further downwind, the amount of dust deposition starts to increase again. The field curve shows a similar pattern, with increased deposition along the concave windward slope, a maximum at the inflection point, and a sharp decrease on the convex windward slope. The minimum is reached on the leeslope, i.e., more downwind than in
the wind tunnel curve, after which deposition starts to increase again. The general picture of the wind tunnel curve and the field curve is, therefore, similar, but in the wind tunnel, the minimum is located considerably more upstream than in the field. W e will come back to this in Section 7. Fig. 7 ( N - S transect) shows, for the wind tunnel curve, an increase of dust deposition along the concave slopes (both in the north and in the south), a maximum
54
Z Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
at the inflection points, a decrease of deposition once the inflection points are crossed, and a deep minimum at the top of the hill. The maximum at the northern inflection point is higher than that at the southern inflection point, probably because of the (small) asymmetry in the hill profile and the higher topographic position of the northern inflection point. The field curve shows almost exactly the same pattern, with an increasing deposition along the concave slopes, maxima at the inflection points, a decreasing deposition closer to the top, and a minimum at the top itself. Also here, the maximum is higher at the northern inflection point than at the southern inflection point. It can be concluded that, in general, the wind tunnel profiles and the field profiles show a good agreement, so that the dust pattern shown in Fig. 5 will give at least a good indication of the real pattern in the field. One important point requires more attention, however: the upwind shift of the sedimentation minimum in the wind tunnel transect along the wind.
7. Discussion The upwind shift (compared to the field) of the deposition minimum in the wind tunnel transect along the wind can be explained as follows. First, the thickness of the dust layer on the scale model was measured on an undisturbed surface: the latter showed the same inclination as the hillslope in the field. As a corollary to the terrain method adopted (filter papers attached to a sponge floating on a (thin) water surface), deposition in the field was determined on horizontal surfaces. Secondly, the filter papers were at the same level as the top of the plastic trays containing the water. Although the height of these trays was restricted to only a few cm, there is little doubt that, due to the presence of the tray, the speed of the wind just above the filter papers was somewhat lower than above the hill surface itself. Thirdly, since there was no distortion of height, the slopes on the model were identical to those in the field. Given the strong reduction of scale, and the absence, at that scale, of important surface roughness, the accumulation threshold (i.e., the critical friction velocity where the ratio of dust deposition to dust erosion drops below unity) is exceeded more quickly on the scale model than in the field (see Goossens, 1988a or Goossens and Offer, 1988 for more details). The near
absence of dust on the convex windward slope of the scale model indicates that, during the simulation, the accumulation threshold was very closely approached (or even exceeded) on this part of the model, and the minimum in the accumulation curve was located upstream of the summit of the hill. In the field, the ratio of wind speed to hill size is much smaller, offering the wind fewer abrupt topographic changes, and the accumulation threshold was not exceeded during the dust storm of 24-25 February 1992, especially over the filter surfaces in the plastic trays. Therefore, the minimum in the dust profile was not located upstream but downstream of the summit. In a similar study executed on a linear hill (perpendicular to the wind) about 2 km north of the conical hill, the minimum in the dust profile was located on the leeslope in both the wind tunnel and the field experiment (Goossens and Offer, 1993). We repeated the conical hill experiment in the wind tunnel for a much lower wind velocity (freestream velocity of only 0.75 m s J). Now, nearly the whole of the windward slope was covered by dust, i.e., the accumulation threshold was not exceeded (similar to the field), but given the low wind speed, the airflow separation bubble downstream of the inflection point on the windward slope was much smaller than in the field, and the decrease of deposition along the convex hillslope was not significant except for a very small zone just before the top (which distracts from the field pattern). The reader should realize that both the wind tunnel patterns and the field patterns that are discussed in this paper refer to short-term geomorphic events. After a sufficiently long time, depending on the meteorological characteristics of the test site, these initial patterns may show significant changes. In the case of the conical hill described in this paper, rainfall is a (very!) important parameter since heavy floods will wash large amounts of dust from the hillslopes. This dust accumulates in the wadis (see, for example, Evenari et al., 1982). These phenomena seriously alter the final accumulation pattern of the dust in the desert. But it can be expected that, if there is any dust to be accumulated on the hillslopes, it will primarily be on the windward slopes since the leeslopes cannot benefit from an important input of dust. Moreover, for the Negev desert, the trapping efficiency is higher for the north and west facing slopes (i.e., the windward slopes for dust deposition) than for the south and east facing slopes (the leeslopes) since much dust is accumulated by microphytic crusts
z.Y. Offer, D. Goossens / Geomorphology 14 (1995) 43-56
( D a n i n and Ganor, 1991 ) that s h o w a higher trapping efficiency on the north facing slopes than on the south facing slopes ( D e Boeck, 1994). D e t a i l e d studies o f l o n g - t e r m dust a c c u m u l a t i o n in c o l d dry e n v i r o n m e n t s (the B e l g i a n loess belt b e t w e e n 20,000 and 12,000 years B P ) clearly s h o w that, in the long term, it is nearly always the w i n d w a r d slopes that a c c u m u l a t e the largest quantities o f dust ( G o o s s e n s , 1987a, 1995b). The higher the effect of syn-and post-depositional rainwash, the lesser the difference b e t w e e n the w i n d w a r d and the leeward slopes with respect to dust accumulation.
8. Conclusions The wind tunnel experiments and the field measurements, although neither is perfect, s h o w e d at least an acceptable degree o f similarity. Both indicate that conical hills create an elongated area o f low deposition ( " d u s t s h a d o w " ) on their lee. D o w n w i n d o f the shadow zone, a local area o f e n h a n c e d ( c o m p a r e d to the undisturbed surroundings) deposition occurs. O n the lateral flanks o f the hill, and also on the small conv e x w i n d w a r d slope just upstream o f the top ( a n d at the top itself), dust deposition remains low to very low. It is the lowermost, c o n c a v e w i n d w a r d slope that r e c e i v e s the largest amounts o f dust. T h e p a t t e m described a b o v e refers to short-term deposition only. W a t e r erosion, especially during h e a v y floods, m a y seriously alter the final accumulation pattern since it will clean the slopes o f important amounts o f dust that a c c u m u l a t e in the wadis.
References Bouten, W., Van Eijsden, G., lmeson, A.M., Kwaad, F.J.P.M., Miicher, H.J. and Tiktak, A., 1985. Ontstaan en erosie van loessleemgronden in Zuid-Limburg. K.N.A.G. Geogr. Tijdschr., 19: 192-208. Bruins, H., 1986. Desert environment and agriculture in the central Negev and Kadesh-Bamea during historical times. Ph.D. Thesis, Landbouwhogeschool Wageningen, 219 pp. Cermak, J.E., 1970. Air motion in and near cities - - determination by laboratory simulation. Fluid Mechanics Program, College of Engineering, Colorado State University, Fort Collins, CO, Rep. CEP70-71JEC27, 21 pp. Cerrnak, J.E., 1984. Physical modelling of flow and dispersion over complex terrain. Boundary-Layer Meteorol., 30: 261-292.
55
Danin, A. and Ganor, E., 1991. Trapping of airborne dust by mosses in the Negev desert, Israel. Earth Surf. Process. Landforms, 16: 153-162. De Boeck, A., 1994. Relaties tussen een microfytische korst en de sedimentatie van natuurlijk atmosferisch stof. Stofneerslag-en fotosynthesemetingen. M.Sc. Thesis, K.U. Leuven, 105 pp. Evenari, M., Shanan, L. and Tadmor, N., 1982. The Negev. The Challenge of a Desert. Harvard Univ. Press, Cambridge/London, 437 pp. Gokhan, C., 1973. Air movement around buidings (an investigation study). Thesis, M.E.T.U., Ankara, 67 pp. Goossens, D., 1987a. Sedimentatiemechanismen bij natuurlijke stofdeeltjes in lucht. Ph.D. Thesis, K.U. Leuven, 265 pp. Goossens, D., 1987b. Dust deposition as a flow visualization technique. Phys. Geogr., 8: 378-389. Goossens, D., 1988a. The effect of surface curvature on the deposition of loess: a physical model. Catena, 15: 179-194. Goossens, D., 1988b. Scale model simulations of the deposition of loess in hilly terrain. Earth Surf. Process. Landforms, 13: 533544. Goossens, D., 1988c. Sedimentation characteristics of natural dust in the wake of symmetrical hills. Z. Geomorphol., 32: 499-502. Goossens, D., 1989. Height distortion and the sedimentation of dust on topographic scale models: considerations and simulations. Earth Surf. Process. Landforms, 14: 655~567. Goossens, D., 1992. Eolische processen op Venus. Heelal, 37: 172178. Goossens, D., 1995a. Wind tunnel experiments of eolian dust deposition along ranges of hills. Earth Surf. Process. Landforms, in press. Goossens, D., 1995b. Long-term aeolian loess accumulation modelled in the wind tunnel: the Molenberg case (central loess belt, Belgium). Z. Geomorphol., submitted. Goossens, D. and Offer, Z.Y., 1988. Loess erosion and loess deposition in the Negev Desert: theoretical modelling and wind tunnel simulations. The Jacob Blaustein Institute for Desert Research, Desert Meteorology Papers, Ser. A, No. 13, 65 pp. Goossens, D. and Offer, Z.Y., 1990. A wind tunnel simulation and field verification of desert dust deposition (Avdat Experimental Station, Negev Desert). Sedimentology, 37: 7-22. Goossens, D. and Offer, Z.Y., 1993. Eolian deposition of dust over symmetrical hills: an evaluation of wind tunnel data by means of terrain measurements. Z. Geomorphol., 37:103-111. Goossens, D., Offer, Z.Y. and Zangvil, A., 1993. Wind tunnel experiments and field investigations of eolian dust deposition on photovoltaic solar collectors. Solar Energy, 50: 75-84. Grant, A.L.M. and Mason, P.J., 1990. Observations of boundarylayer structure over complex terrain. Q. J. R. Meteorol. Soc., 116: 159-186. Greeley, R., 1986. Aeolian landforms: laboratory simulations and field studies. In: W.G. Nickling (Editor), Aeolian Geomorphology. Allen and Unwin, Boston, pp. 195-211. Grolier, M., Ericksen, G.E., McCauley, J.F. and Morris, E.C., 1974. The desert landforms of Peru: a preliminary photographic atlas. U.S. Geol. Survey, Interagency Report, Astrogeology, 57:146 Pp.
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Hertig, J.-A. and Liska, P., 1983. Simulations of regional atmospheric flows on small scale topographic models, J. Wind Eng. Ind. Aerodyn., 15: 77-89. Iversen, J.D., 1983. Saltation threshold and deposition rate modelling. In: M.E. Brookfield and T.S. Ahlbrandt (Editors), Eolian Sediments and Processes. Elsevier, Amsterdam, pp. 103-113. Iversen, J.D., Greeley, R., White, B.R. and Pollack, J.B., 1976. The effect of vertical distortion in the modelling of sedimentation phenomena: Martian crater wake streaks. J. Geophys. Res., 81: 4846-4856.
Rozycki, S.Z., 1967. Le sens des vents portant la poussibre de loess, ?~la lumi6re de l'analyse des formes d'accumulation du loess en Bulgarie et en Europe Centrale. Rev. G6omorphol. Dynam. 2~me S6rie, 9: 1-9. Tsoar, H. and Pye, K., 1987. Dust transport and the question of desert loess formation. Sedimentology, 34: 139-153. Yaalon, D. and Ganor, E., 1979. East Mediterranean trajectories of dust-carrying storms from the Sahara and Sinai. In: C. Morales (Editor), Saharan Dust. Mobilization, Transport, Deposition. Wiley, New York, pp. 187-193.