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Wind erosion of residue waste. Part I. Using the wind profile to characterise wind erosion
¢ATENA ELSEVIER
Catena 21 (1994) 291-303
Wind erosion of residue waste. Part I. Using the wind profile to characterise wind erosion W.D. Scott Environmental Science Murdoch University Perth Western Australia
Abstract
A portable wind tunnel is used to assess the erodibility of prepared surfaces of residue waste. The data are extensive and include wind profiles, collections of saltation and suspension material as well as particle sizing by elutriation. Still, the data show no definitive information on the effects of treatment. The experiments, however, can be characterized by the structure of the measured wind profile; a special plot of the friction velocity and dynamic roughness suggests that only one of the six plots is "all erodible". Surfaces may have a fixed roughness, eroding roughness or exhibit an anomalous "constant friction velocity". There are difficulties in the use of the gradient of the log profile and the intercept as measures of the shear stress and the scale.
I. Introduction
Dust blown off the dykes and dry red m u d surfaces o f A L C O A ' s * residue waste storage lakes has been a problem to nearby residents. As a result, a different strategy o f waste storage has evolved, termed " d r y stacking". The waste residue m u d and sand are p u m p e d at a high solids content to the sites where the fluid/solid material is dried to form a " m o u n d " . The shape o f the m o u n d s that are evolving is designed using simplified models o f flow over hills (Coffey et al., 1986). The wind erosion or dust formation potential is assessed t h r o u g h the wind speed-up factor. The research effort included a detailed study o f the "soil" mixes to find out the "erosivity" of the different combinations o f residue mud, residue sand, chemical treatments and roughness. The m e t h o d o f assessment used the standard procedures developed by Marsh and Carter o f the Western Australian D e p a r t m e n t o f Agriculture, W A D A . This includes the use o f an artificial wind produced by the W A D A wind tunnel. The tunnel features and test results are explained by Carter • ALCOA of Australia Limited. 0341-8162/94/$07.00 ~c) 1994 Elsevier Science B.V. All rights reserved SSDI 0341-8162(93)E0009-0
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W.D. Scott et al. / Catena 21 (1994) 291-303
and Moore (1990) and Raupach and Leys (1990). The tunnel was manufactured with a tent-shaped cross-section to allow maximum ground coverage and minimise the power requirements while producing maximum winds to 25 m/sec. The tunnel is 7 m long with a working section of about 5 m. The design follows the earlier design of Zingg, Chepil and Fryrear (see Zingg, 1953; Armburst and Box, 1967). A special feature is the manufacture of turbulence with a frequency of about 1 Hz with a series of turning vanes. The turbulence intensity is around 30%. Vanes of this sort are used to develop an appropriate turbulent "equilibrium" and can produce some desired wind flow characteristics in short tunnels (Counihan, 1969; McDonald and Camarata, 1969). In this case the unique design, including an asymmetric rotation, is of questionable value (Raupach and Leys, 1990; Scott and Tubb, 1990). An approach to the assessment of erosion and surfaces has evolved that not only considers the classic "all erodible" surfaces but any variation of surface conditions, roughness, or drag partitioning. Linked with a general theory of suspension with saltation (Scott et al., 1993), the data are viewed as scaled with the "roughness length" and the Rouse number. This paper presents a first view of this particular usage of the tunnel together with a specific technique of assessment of the type of erosion using the wind profile.
2. Measurements The studies were conducted over prepared plots of 5 m by 1.5 m (See Bell, 1984). The experimental runs follow the standard procedure which includes 1 minute "blows" at increasing speeds (see Carter and Marsh, 1980). Sample collections, manometer zeroing and equipment checks are completed during intervals when the fan is off. Recorded during the study of each plot were: 1. Mean horizontal wind at 0.05, 0.1, and 0.35 m averaged over approximately 30 sec. 2. Fractions of material collected with a vacuum saltation gauge in height intervals 0-0.05 m, 0.05-0.25 m, and 0.25 to 0.5 m. This is a three level saltation collector designed after the standard Bagnold collector with a width of 1.5 cm (Bagnold, 1941). 3. Size sorting of the 0.05-0.25 m fraction as well as the original surface material. This used the elutriation technique of the W A D A (Tommerup and Carter, 1982). 4. Fine (suspended) particle concentrations in sizes above 0.5 #m at 0.33 m height. An attempt was also made to direct collect samples of suspended material with small vacuum impactors. These samplers suffered from the clogging effects of mm particles; this was overcome with a 1 mm screen filter. The sample analysis, however, was not completed because of time commitments. Fig. 1 is a picture of the portable wind tunnel at the site in Coolup, W.A. Shown is the lattice of instruments used to monitor the wind profile and airborne dust. Note that the instruments, generally, are stationed downwind of the blowing wind and
W.D. Scott et al. / Catena 21 (1994) 291-303
293
Fig. 1. Wind tunnel study underway.
placed so that little obstruction to the airflow occurs. The operation required the attendance of approximately 5 members of the staff of the WADA, ALCOA and University students. The information was acquired by manual reading of manometers or direct weighing of the sample bags from the vacuum cleaners. These bags were weighed before the experiment; humidity changes and adjustments during weighing produced a drift of about 0.05 g. Accounting for this error allows the reporting of results to about 0.02 g. A complete listing of the digested data is presented in Table 1. Eight experimental runs (blows) were conducted on each plot, each experiment at a higher wind speed as represented by an increase in the engine rpm (EREVS). Since wind erosion occurs roughly as the cube of the wind speed, this minimises the interdependence of experiments. After a 1 minute experimental run, samples collected in the three-level saltation collector were immediately weighed. Background particle counts were collected between runs. These counts and the counts at the lowest wind speeds were used as background corrections. The prepared surfaces of each plot were manually constructed of materials from the sand separation plant at Alcoa's Pinjarra Alumina Refinery and allowed to weather for a seven week period from mid-October through 8 September 1984. The surfaces were slightly larger than the working sections of the tunnel, 5 m x 1.5 m, set in 100 mm high forms and screeded in a wet condition, approximately 50% solids. Surfaces were then amended with mulch or variously treated. The final plots are described as: Plot 1: Rain-washed residue sand covered with an evenly spread layer of rock mulch (about 13 kg/m2).
4.1 4.8 5.8 7.1 7.9 9.1 10.4 11.4
4.0 5.5 6.7 8.2 9.6 11.4 12.5 13.7
Plot 2 800 1000 1200 1400 1600 1800 2000 2200
0.05
5.4 6.5 8.3 9.5 11.2 13.2 14.4 15.8
5.1 5.7 7.2 9.0 10.3 11.7 13.1 14.3
0.10
5.4 7.0 8.6 10.2 11.8 13.7 14.6 16.7
5.4 7.0 8.8 10.5 11.8 13.6 15.4 16.8
0.35
Wind a metres/sec Pitot height (m)
Plot 1 800 1000 1200 1400 1600 1800 2000 2200
Ereus
0.03 0.06 0.04 0.02 0.08 I).19 0.18 0.20
0.07 0.09 0.10 0.08 0.21 0.88 1.35 2.64
Soil elutriation 0.03 0.03 0.02 0.06 0.08 0.02 0.17 0.14 0.17 0.18 0.34 0.43 0.48 0.66 0.46 0.83
Soil elutriation 0.20 O.15 O. 11 0.08 0.09 0.09 0.10 0.12 4.08 1.72 18.39 14.62 12.77 13.91 16.85 12.69
0.06
0.00
0.0173
0.0046
0.0011
0.0038
0.0282
0.0037
0.0039
0.0019
1.24
0.0044
0.0017
4.34
0.0011
7.78
0.0023
1.26
0.4762
0.0116
0.0007
37.69
0.1934
0.0159
-0.0008
31.73
9.1778
1.0597
0.0018
23.44
0.6435
0.0556
0.0030
15.86
whole
50-100
< 10
I0 50
300 500
1050 100 50 100 300 Mid-level sample (gms)
< 10
250500
0 50
50 250
Elutriation data c Soil sample (%) in/~m range
Saltation b grams
Table 1 Interpreted raw data set. Compiled data from Bell (1984)
33.23
27.69
5002000
0
15.75
36.0 39.5
0.5 2
19.4 26.2
2-10
0.2 2.0
> 10
Fine particle data d particles/cm 3 of air 0.33 M corrected for > 2000 background in #m range
~"
-~
~"
r'--.-
2
.~ ce
4~
Plot 5 800 1000 1200 1400 1600 1800 2000 2200
Plot 4 800 1000 1200 1400 1600 1800 2000 2200
Plot 3 800 1000 1200 1400 1600 1800 2000 2200
2.9 4.4 4.8 7.7 8.8 8.3 9.0 13.1
-
2.9 4.2 -
9.2
4.1 4.8 5.2 6.4 6.3 6.7
4.8 6.0 7.0 8.3 9.9 9.0 9.7 14.5
4.8 6.0 7.2 8.3 9.4 10.8 12.0 14.9
4.8 6.4 7.6 9.0 10.1 11.7 12.7 13.8
6.2 7.6 8.6 10.3 11.8 10.7 11.8 16.8
6.0 7.4 9.0 10.7 12.3 13.9 15.6 20.9
6.5 8.1 9.5 11.2 12.7 14.6 16.0 17.9
Soil elutriation 0.27 0.00 0.00 0.01 0.18 0.08 0.14 0.02 0.35 0.28 0.10 0.04 1.07 1.83 3.53 5.57
Soil elutriation 0.03 0.00 1.64 1.00 42.15 15.85 87.75 32.46 116.97 58.47 182.68 93.43 200.11 132.62 363.0 444.7
Soil elutriation 1.20 0.06 22.41 6.05 53.75 11.12 94.62 32.05 117.86 51.38 170.33 88.14 210.02 134.95 304.87 173.96
0.02 0.00 0.02 0.01 0.06 0.02 0.61 1.62
0.04 -0.02 0.60 1.51 3.48 6.40 10.54 37.67
0.05 0.17 0.45 1.56 3.10 6.62 9.98 14.67
0.03
0.00
0.00
-0.0001
0.0162
0.6914
0.0026
0.0101
10.85
0.0006
5.34
0.1626
0.1496
0.0039
0.0046
0.0021
8.00
0.0030
1.26
0.0757
0.1981
0.0036
0.0035
0.0170
4.54
0.0044
0.84
1.2341
0.0540
0.0015
19.71
1.4411
0.8725
0.0492
39.85
0.5127
0.9273
0.3068
42.48
2.3778
0.1803
0.0021
9.95
5.8936
6.0425
2.2153
24.46
7.7230
7.7151
5.0623
24.93
54.10
26.42
27.22
0
0
0
58.9 88.9
10.5 7.8
90.1
7.9 7.1
0.0 0.4 1.9 11.1 12.5 20.4 30.1 13.7
46.9 14.6
14.6 15.9
0.5
(continued on p. 296)
1.3
10.2 7.3 33.0 61.0 58.9 72.5 74.2 87.4
63.5 78.3
34.9 21.5
27.1 2.2 34.6 30.1 35.2 16.3 10.2 24.2
7.7
25.4
i
c~
.~
3.3 3.8 5.3 6.4 7.4 8.3 9.8 I0.6
0.05
4.0 5.1 6.7 8.3 9.2 10.8 12.5 13.4
0.10
5.7 7.0 9.0 11.0 11.8 13.7 15.5 16.6
0.35
Wind ~ metres/sec Pitot height (m)
Soil elutriation 0.03 0.18 0.03 0.02 0.00 0.06 0.05 0.13 0.05 0.04 0.02 0.01 0.05 0.09 0.06 0.05 0.05 0.01 0.01 0.02 0.02 0.02 0.08 0.05
0.02
0.0016
0.0021
0.0008
0.00
6.46
0.0024
0.0026
3.28
0.0020
0.0013
-0.0011
17.09
0.0010
0.0054
0.0016
13.12
whole
50 100
< 10
10-50
300 500
10 50 10050 100 300 Mid-level sample (gms)
< 10
250 500
0 50
50250
Elutriation data c Soil sample (%) in #m range
Saltation b grams
60.03
500 2000
0
0.5-2
2 10
> 10
Fine particle data d particles/cm 3 of air 0.33 M corrected for > 2000 background in #m range
Calculated from pitot tube pressures (Bell, page 218). b Corrected for water loss, 0.06 gms added. Grams of sample collected over a 1 minute run (page 177). c The soil elutriation is the original soil sample in 7 size classes. Otherwise particulates collected in the mid-level Bagnold trap were separated in 4 size classes. The mid-level sample is in grams (0.05 to 0.25 m); the soil sample is in % (pp. 159, 335). d Fine particles collected with the optical particle counter placed directly in the airstream, 0.33 m above ground.
1600 1800 2000 2200
Plot 6 800 1000 1200 1400
Ereus
Table 1 (continued)
~
2 ''"
'~ ¢,a ~"
,~ a,
W.D. Scott et al. / Catena 21 (1994) 291 303
297
Plot 2: Manually-washed residue sand whose surface was treated with a 1% solution of Napcoseal M 130 one week before the experiments. The application rate was 1 litre/m 2. Plot 3: Untreated residue sand which was covered with black plastic until the experiments. Plot 4: Rain-washed residue sand which was treated with Napcoseal CE40 just before the experiments. Plot 5: Crushed gypsum - - amended mud from Alcoa's Kwinana Refinery. Plot 6: Untreated residue mud. These plot tests concentrate on the residue sand because it has the appropriate saltation components and is the main component in the dykes that surround the mud lakes. The chemical treatments follow the recommended amounts and are viable alternatives, should they be effective.
3. Profile characterization
The work of Scott et al. (1993) and Hopwood and Scott (1990) make it clear that, at least for suspension and saltation, the airborne concentrations are scaled with the height measured in numbers of roughness lengthsand the velocity measured with the dimensionless Rouse number, which is, effectively, the ratio of mass-weighted fall velocity to the friction velocity. Hence any experiment to measure erosion must measure both the roughness length, z 0 and the friction velocity u,. Classically the wind profile, in both the atmosphere and in the inner boundary layer of the wind tunnel, is fit by the form (Monin and Yaglom, 1973): U,
u = ~- In Z/Zo where k is the Von Karman constant, 0.4 (H6gstr6m, 1985; Wieringa, 1980). This form holds in the case of wind blowing below the threshold for erosion to occur. Bagnold (1941) makes it abundantly clear that in eroding conditions the same profile is observed and this has been corroborated by other classic studies since Bagnold (Zingg, 1953; Williams, 1964). Consider the case when the above formula applies and the wind speed is just at the threshold that allows erosion to occur. Then, with a disturbance, erosion proceeds and continues in an all-eroding situation. The logarithmic profile that obtains has the same slope but the effective roughness is increased by, perhaps, 100 fold: /g*
/
u = -~- In z/z o The surface has adjusted to a new roughness level z~ which varies with the overriding wind strength, as indicated by the wind stress or friction velocity u,. The wind speeds are lower. However, with an increase in the overriding wind, the wind speed increases with a concurrent increase in z~ such that all the semilogarithmic lines pass through the same point, the "focus". The threshold velocity ut and the threshold friction
298
W.D. Scott et al. / Catena 21 (1994) 291-303
length zt can be defined as that focus where** U,
Ut = ~-In z/zlo for all values o f u, and z~ (Fig. 2 ) . Dividing up the logarithm, this equation can be written: In z~ = - u t k ( 1 / u , ) + In z, This is a standard linear form with y = lnz~ and x = 1 / u , . We expect that a plot of In z~ versus l/u, should yield a straight line with slope = - u t k intercept = In z t provided the surface is effectively "all-erodible". T h a t is, there is a continuing source of all particles at the surface and the winnowing o f particles in the air does not leave heavy particles behind. O f course, if the surface is not erodible or the roughness elements are p e r m a n e n t features, Zo is zo which is constant anyway. I f the surface roughnesses blow away during the experimental runs with increasing wind speeds, then zd actually decreases with time and the surface is not all-erodible. If the plot is non-linear, there is an alteration o f the surface structure with each different, overriding wind. This later condition normally obtains in nature. With depletion of the surface and armoring, vegetation growth, wetting or whatever, it is expected that the surface will exhibit a hystersis and history but, importantly, this m e t h o d of analysis gives a characterisation of the erosion process.
,oc,,s
I I
, -fi
!
N
I
/
! U[
u
Fig. 2. Schematic plot of logarithmic profile and the effects of erosion. ** These terms are not precisely the same as used by Bagnold.
W.D. Scott et al. / Catena 21 (1994) 291 303
299
Table 2 u, and z~ values. Data interpreted from wind profile prepared plots Friction velocity (m/s)
Roughness length (mm)
Plot 1
800 1000 1200 1400 1600 1800 2000 2200
0.25 0.45 0.60 0.67 0.76 0.89 0.99 1.07
0.045 0.657 0.979 0.617 0.637 0.683 0.648 0.612
Plot 2
800 1000 1200 1400 1600 1800 2000 2200
0.25 0.29 0.36 0.39 0.42 0.44 0.39 0.58
0.050 0.020 0.017 0.008 0.004 0.001 0.000 0.003
Plot 3
800 1000 1200 1400 1600 1800 2200
0.50 0.66 0.85 0.95 1.26 1.54 1.73
1.972 2.472 3.655 2.911 5.505 6.922 5.160
Plot 4
800 1000
0.61 0.63
5.936 2.949
Plot 5
800 1000 1200 1400 1600 1800 2000 2200
0.65 0.64 0.75 0.55 0.62 0.50 0.59 0.76
6.979 2.836 3.185 0.198 0.163 0.068 0.118 0.049
Plot 6
800 1000 1200 1400 1600 1800 2000 2200
0.50 0.65 0.76 0.94 0.90 1.09 1.15 1.21
3.744 4.657 2.989 3.092 1.757 2.164 1.483 1.364
300
W.D. Scott et al. / Catena 21 (1994) 291-303
4. Experimental profiles The experimental velocity profiles were easily fitted to the logarithmic profile equation; data are presented in Table 2. These profiles are corrected for the air density effect as outlined in Scott and Carter (1986) using the 3-level saltation collection (see follow-up paper - - Part II). Though this correction has little effect on this particular data, it allows a logical definition of u, and z~ that are unaffected by considerations of any "kink" in the profile (Gerety, 1985). As presented above, there is no single roughness of the eroding surface but the roughness varies with u,. An overall assessment of these data was completed following the above procedure, using l/u, and lnz~; this is shown in Fig. 3. As presented, the data for a given experimental plot are connected with a broken line with arrows that represent the temporal course of the experiment. Following the above generalities regarding this method of displaying an experiment, we see that at least 4 different trends: 1. Constant Roughness. Large anchored roughness elements disallow penetration of the air into the surface. Plot 1 with the blue metal mulch exhibits this characteristic with an aerodynamic roughness of about 0.7 mm, independent of the value of u*. This
10'1~i~"'
~/'-o
1.0
0.1'
~
0.01'
~
PLOT
SYMBOL
1 2
-O-0--
0.001'
3 4 5
--~
6
0
i
~ 1
J
/U, Fig. 3. The changesin the windprofileparametersshowthe erosionpattern.
W.D. Scott et al. / Catena 21 (1994) 291-303
301
does not say that no erosion is occurring though there is little saltation (Table 1). The erosion process is not influencing the roughness. The normal feedback of an alleroding surface is absent. Saltation is also unlikely to be effective though there may be direct aerodynamic entrainment. 2. All Erodible. This is the "normal" type of erosion treated and observed by Bagnold on sand dunes and treated above. Roughness is truly affected by saltation. Plot 3 shows an all-erodible situation with a threshold velocity of 2.2 m/sec as obtained from the slope. Note that Plot 3 exhibited substantial saltation and a large amount of suspension (see Table 1). 3. Eroding Roughness. Roughness elements are composed oferodible fractions that become depleted with time. Plot 2 and Plot 6 show this effect. Plot 2 definitely shows substantial saltation as might accommodate this effect. However, Plot 6 shows almost no saltation. This is difficult to explain as this plot was a heavy clay bed with a smooth surface and cracks throughout. There are two possibilities: (a) Surface creep has allowed movement of the material into cracks. This is unlikely to have involved surface features of the centimetre sizes required. (b) There is a gradation of roughness down the tunnel. The scaling and size of the inner boundary layer is determined by the surface roughness. Equilibration to these changes is not instantaneous (Jensen, 1985) so that with increased wind speed, the smoother, upwind surfaces have more influence on the wind profile. The latter suggestion is most likely. Corroboration is required; this condition would not allow that the slope of the log profile be used as a measure of the shear, u,. An alternate explanation of this effect is that the profile becomes more displaced as the experiment proceeds; the displacement distance simply increases and the profile shifts downward in an absolute sense. Displacement distances are unknown in these experiments but are unlikely to exceed 1 cm. The variation ofz 0 in Plot 6 is larger than this; hence, this is unlikely to be the cause of the trend in the data. 4. Constant Friction Velocity. This is unexpected but exhibited by Plot 5 and perhaps Plot 4. This could be the case of a strong, eroding roughness but this is not confirmed by the saltation data, particularly in Plot 5. Three suggestions about how this could happen are: (a) The surface is compliant, like a wheat field or grass surface that exhibits the Honami, in which the energy of the wind is not absorbed by the surface but reflected by the waving grain stalks. (b) The surface has no influence on the wind profile. Perhaps the Reynold's Stress or the correlation form of it is not predicted by the lower, soil surface, but by the walls and upper surface of the tunnel. This might be expected when the roughness of the lower surface is small and the inner layer, accordingly, would be small. In fact, in a very smooth surface, the law of the wall (Schlichting, 1979, p. 640) predicts scaling accordingly as u/u, ~ 0.02 ram, where u is the kinematic viscosity. If the surface were smooth enough, there would then no longer be any dependence on the roughness. This might be the case with Plot 2 but is unlikely with Plot 5; z~ is greater than 0.05 mm. (c) The profile simply is not a measure of u,. The velocity measurements may simply be made above the inner layer where r is not constant; they are
302
W.D. Scott et al. / Catena 21 (1994) 291-303
representative of the " m a i n s t r e a m " turbulence. This influence should be related to the size of the boundary layer and might be expected if the inner layer was poorly developed or very small. Perhaps a short distance upwind of where the profile was measured the roughness was much greater. Unfortunately, this cannot be confirmed. It should be noted that logarithmic profiles are common and occur without a constant shear stress (Scott and Tubb, 1990). Both (b) and (c) are the likely causes of the effect. O f course, these considerations are inherently linked with the "equilibrium" or lack thereof in the tunnel's wind flow characteristics. It may be simply a case where the profile is not a measure of the shear stress, Without erosion, a logarithmic profile is nearly always present through most of the tunnel (Carter and Moore, 1990; Carter and Marsh, 1980). This does not mean that the inner layer is this deep and in fact the shear stress drops in a nearly linear way to zero a few tens of centimetres off the surface (Raupach and Leys, 1990). During erosion the inner layer should be greatly extended since it scales with the roughness so a deep inner layer should form. It is a primary purpose of this paper to show that most surfaces are not "all erodible" and experiments conducted without a broad perspective of erosion processes, especially in natural and agricultural environments, must fail or give misleading information. The approach presented allows at least some of the additional complications to be assessed. The data do show the value of a wind flow analysis. Specifically, we see that the largest amount of erosion occurred with a surface treatment intended as a prevention (Table 1, Plot 2); rock mulch and surface texture changes (Plots 1, 5, 6) did not allow much erosion; and that an "all erodible" surface is by no means obtained in ordinary experiments. Note that the full data set (Table 1) is presented here for review and criticism; further work, characterising suspension and saltation, will appear in a follow-up paper.
Acknowledgements Peter Bell acquired the present data during the development of his honour's thesis. Without his efforts and attendance to detail, this paper would not have been possible. Also, A L C O A of Australia funded the research; John Summers of A L C O A arranged the prepared plots and guided the project. Dr. Bill Robertson and several students at Murdoch helped collect the data in a difficult, noisy and hot environment.
References Armburst, D.V. and Box, J.E., 1967. Design and operation of a portable soil-blowingwind tunnel, ARS41131. Agricultural Research Service, U.S. Department of Agriculture, Big Spring, TX. Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Reprinted 1984, Chapman and Hall, London, 265 pp. Bell, P., 1984. Micrometeorologicalmeasurementsof dust from residue waste. Honours thesis in Environmental Science, Murdoch University.
W.D. Scott et al. / Catena 21 (1994) 291 303
303
Carter, D.J. and Marsh, B. a'B., 1980. A portable wind tunnel for erosion research. In: Proceedings National Soils Conference (Soil Science Society Australia), Sydney. Australian Society of Soil Science; available from Department of Soil Science, Waite Agricultural Institute, Glen Osmond, S.A., 105 pp. Carter, D.J. and Moore, G.H., 1990. Flow characteristics of a portable wind tunnel. In: D.J. Carter et al. (Editors), Wind Erosion Study Group, a Compilation of Papers. Murdoch University Library, Perth, W.A., 172 pp. Coffey, P.S., Scott, W.D. and Summers, K.J., 1986. The effects of tailing dam profiles on relative erosion rates. J. Environ. Qual., 15:168 172. Counihan, J., 1969. An improved method of simulating an atmospheric boundary layer in a wind tunnel. Atmos. Environ., 3:197 214. Gerety, K.M., 1985. Problems with determination of u, from wind velocity profiles measured in experiments with saltation. In: O.E. Barndorff-Neilsen, J.T. Moller, K. Romer Rasmussen and B.B. Willetts (Editors), Proceedings of the International Workshop on the Physics of Blown Sand. Dept. of Theoretical Statistics, Inst. of Mathematics, Univ. of Aarhus, Mem. 8, pp. 271 300. H6gstr6m, U., 1985. Von Karman's constant in atmospheric boundary layer flow: Reevaluated. J. Atmos. Sci., 42: 263-270. Hopwood, J.M. and Scott, W.D., 1990. The sizing of particles in terms of fall velocity. Catena, 17:327 332. Jensen, N.O., 1985. Atmospheric boundary modelling, Report RISO-M-2493 from Riso Bibliotek, Forsogsanl~eg, DK-4000 Roskilde, Denmark. See also Jensen, N.O. (1978), Change in surface roughness and the planetary boundary layer. Q. J. Roy. Met. Soc. 104: 351--356. McDonald, H. and Camarata, F.J., 1969. An extended mixing length approach for computing the turbulent boundary layer development. In: S.J. Kline et al. (Editors), Computation of Turbulent Boundary Layers, AFOSR-IFP-Stanford Conference, 1968, Vol. 1. Thermosciences Division, Department of Mechanical Engineering, Stanford University, Stanford, CA, p. xxii. Monin, A.S. and Yaglom, A.M., 1973. Statistical Fluid Mechanics. M.I.T. Press, Cambridge, MA, 769 pp. Raupach, M.R. and Leys, J.F., 1990. Aerodynamics of a portable wind erosion tunnel for measuring soil erodibility by wind. Aust. J. Soil Res., 28:177 191. Scott, W.D. and Carter, D.J., 1986. The logarithmic profile in wind erosion: an algebraic solution. Boundary-Layer Meteorol., 34:303 310. Scott, W.D. and Tubb, J., 1990. An assessment of agricultural wind tunnels. In: Proceedings of the 5th Conference on Agricultural Engineering. Joint publication of the Institute of Engineers, Australia, the American Society of Agricultural Engineering and the University college of Southern Queensland, Toowoomba, Qld. Scott, W.D., Hopwood, J.M. and Summers, K.J., 1993. A mathematical model of suspension with saltation. Acta Mechanica, in press. Schlicting, H., 1979. Boundary Layer Theory. McGraw Hill, New York, 817 pp. Tommerup, I.C. and Carter, D.J., 1982. Dry separation of microorganisms from soil. Soil Biol. Biochem., 14:69 71. Wieringa, J., 1980. A revaluation of the Kansas mast influence on measurements of stress and cupanemometer overspeeding. Boundary-Layer Meteorol., 18:411 430. Williams, G.P., 1964. Some aspects of the Eolian saltation load. Sedimentology, 3:257 287. Zingg, A.W., 1953. Wind-tunnel studies of the movement of sedimentary material. In: Proc. of the Fifth Hydraulics Conference, Bulletin 34. State University of Iowa Studies in Engineering, pp. 111 135.
Catena 21 (1994) 291-303
Wind erosion of residue waste. Part I. Using the wind profile to characterise wind erosion W.D. Scott Environmental Science Murdoch University Perth Western Australia
Abstract
A portable wind tunnel is used to assess the erodibility of prepared surfaces of residue waste. The data are extensive and include wind profiles, collections of saltation and suspension material as well as particle sizing by elutriation. Still, the data show no definitive information on the effects of treatment. The experiments, however, can be characterized by the structure of the measured wind profile; a special plot of the friction velocity and dynamic roughness suggests that only one of the six plots is "all erodible". Surfaces may have a fixed roughness, eroding roughness or exhibit an anomalous "constant friction velocity". There are difficulties in the use of the gradient of the log profile and the intercept as measures of the shear stress and the scale.
I. Introduction
Dust blown off the dykes and dry red m u d surfaces o f A L C O A ' s * residue waste storage lakes has been a problem to nearby residents. As a result, a different strategy o f waste storage has evolved, termed " d r y stacking". The waste residue m u d and sand are p u m p e d at a high solids content to the sites where the fluid/solid material is dried to form a " m o u n d " . The shape o f the m o u n d s that are evolving is designed using simplified models o f flow over hills (Coffey et al., 1986). The wind erosion or dust formation potential is assessed t h r o u g h the wind speed-up factor. The research effort included a detailed study o f the "soil" mixes to find out the "erosivity" of the different combinations o f residue mud, residue sand, chemical treatments and roughness. The m e t h o d o f assessment used the standard procedures developed by Marsh and Carter o f the Western Australian D e p a r t m e n t o f Agriculture, W A D A . This includes the use o f an artificial wind produced by the W A D A wind tunnel. The tunnel features and test results are explained by Carter • ALCOA of Australia Limited. 0341-8162/94/$07.00 ~c) 1994 Elsevier Science B.V. All rights reserved SSDI 0341-8162(93)E0009-0
292
W.D. Scott et al. / Catena 21 (1994) 291-303
and Moore (1990) and Raupach and Leys (1990). The tunnel was manufactured with a tent-shaped cross-section to allow maximum ground coverage and minimise the power requirements while producing maximum winds to 25 m/sec. The tunnel is 7 m long with a working section of about 5 m. The design follows the earlier design of Zingg, Chepil and Fryrear (see Zingg, 1953; Armburst and Box, 1967). A special feature is the manufacture of turbulence with a frequency of about 1 Hz with a series of turning vanes. The turbulence intensity is around 30%. Vanes of this sort are used to develop an appropriate turbulent "equilibrium" and can produce some desired wind flow characteristics in short tunnels (Counihan, 1969; McDonald and Camarata, 1969). In this case the unique design, including an asymmetric rotation, is of questionable value (Raupach and Leys, 1990; Scott and Tubb, 1990). An approach to the assessment of erosion and surfaces has evolved that not only considers the classic "all erodible" surfaces but any variation of surface conditions, roughness, or drag partitioning. Linked with a general theory of suspension with saltation (Scott et al., 1993), the data are viewed as scaled with the "roughness length" and the Rouse number. This paper presents a first view of this particular usage of the tunnel together with a specific technique of assessment of the type of erosion using the wind profile.
2. Measurements The studies were conducted over prepared plots of 5 m by 1.5 m (See Bell, 1984). The experimental runs follow the standard procedure which includes 1 minute "blows" at increasing speeds (see Carter and Marsh, 1980). Sample collections, manometer zeroing and equipment checks are completed during intervals when the fan is off. Recorded during the study of each plot were: 1. Mean horizontal wind at 0.05, 0.1, and 0.35 m averaged over approximately 30 sec. 2. Fractions of material collected with a vacuum saltation gauge in height intervals 0-0.05 m, 0.05-0.25 m, and 0.25 to 0.5 m. This is a three level saltation collector designed after the standard Bagnold collector with a width of 1.5 cm (Bagnold, 1941). 3. Size sorting of the 0.05-0.25 m fraction as well as the original surface material. This used the elutriation technique of the W A D A (Tommerup and Carter, 1982). 4. Fine (suspended) particle concentrations in sizes above 0.5 #m at 0.33 m height. An attempt was also made to direct collect samples of suspended material with small vacuum impactors. These samplers suffered from the clogging effects of mm particles; this was overcome with a 1 mm screen filter. The sample analysis, however, was not completed because of time commitments. Fig. 1 is a picture of the portable wind tunnel at the site in Coolup, W.A. Shown is the lattice of instruments used to monitor the wind profile and airborne dust. Note that the instruments, generally, are stationed downwind of the blowing wind and
W.D. Scott et al. / Catena 21 (1994) 291-303
293
Fig. 1. Wind tunnel study underway.
placed so that little obstruction to the airflow occurs. The operation required the attendance of approximately 5 members of the staff of the WADA, ALCOA and University students. The information was acquired by manual reading of manometers or direct weighing of the sample bags from the vacuum cleaners. These bags were weighed before the experiment; humidity changes and adjustments during weighing produced a drift of about 0.05 g. Accounting for this error allows the reporting of results to about 0.02 g. A complete listing of the digested data is presented in Table 1. Eight experimental runs (blows) were conducted on each plot, each experiment at a higher wind speed as represented by an increase in the engine rpm (EREVS). Since wind erosion occurs roughly as the cube of the wind speed, this minimises the interdependence of experiments. After a 1 minute experimental run, samples collected in the three-level saltation collector were immediately weighed. Background particle counts were collected between runs. These counts and the counts at the lowest wind speeds were used as background corrections. The prepared surfaces of each plot were manually constructed of materials from the sand separation plant at Alcoa's Pinjarra Alumina Refinery and allowed to weather for a seven week period from mid-October through 8 September 1984. The surfaces were slightly larger than the working sections of the tunnel, 5 m x 1.5 m, set in 100 mm high forms and screeded in a wet condition, approximately 50% solids. Surfaces were then amended with mulch or variously treated. The final plots are described as: Plot 1: Rain-washed residue sand covered with an evenly spread layer of rock mulch (about 13 kg/m2).
4.1 4.8 5.8 7.1 7.9 9.1 10.4 11.4
4.0 5.5 6.7 8.2 9.6 11.4 12.5 13.7
Plot 2 800 1000 1200 1400 1600 1800 2000 2200
0.05
5.4 6.5 8.3 9.5 11.2 13.2 14.4 15.8
5.1 5.7 7.2 9.0 10.3 11.7 13.1 14.3
0.10
5.4 7.0 8.6 10.2 11.8 13.7 14.6 16.7
5.4 7.0 8.8 10.5 11.8 13.6 15.4 16.8
0.35
Wind a metres/sec Pitot height (m)
Plot 1 800 1000 1200 1400 1600 1800 2000 2200
Ereus
0.03 0.06 0.04 0.02 0.08 I).19 0.18 0.20
0.07 0.09 0.10 0.08 0.21 0.88 1.35 2.64
Soil elutriation 0.03 0.03 0.02 0.06 0.08 0.02 0.17 0.14 0.17 0.18 0.34 0.43 0.48 0.66 0.46 0.83
Soil elutriation 0.20 O.15 O. 11 0.08 0.09 0.09 0.10 0.12 4.08 1.72 18.39 14.62 12.77 13.91 16.85 12.69
0.06
0.00
0.0173
0.0046
0.0011
0.0038
0.0282
0.0037
0.0039
0.0019
1.24
0.0044
0.0017
4.34
0.0011
7.78
0.0023
1.26
0.4762
0.0116
0.0007
37.69
0.1934
0.0159
-0.0008
31.73
9.1778
1.0597
0.0018
23.44
0.6435
0.0556
0.0030
15.86
whole
50-100
< 10
I0 50
300 500
1050 100 50 100 300 Mid-level sample (gms)
< 10
250500
0 50
50 250
Elutriation data c Soil sample (%) in/~m range
Saltation b grams
Table 1 Interpreted raw data set. Compiled data from Bell (1984)
33.23
27.69
5002000
0
15.75
36.0 39.5
0.5 2
19.4 26.2
2-10
0.2 2.0
> 10
Fine particle data d particles/cm 3 of air 0.33 M corrected for > 2000 background in #m range
~"
-~
~"
r'--.-
2
.~ ce
4~
Plot 5 800 1000 1200 1400 1600 1800 2000 2200
Plot 4 800 1000 1200 1400 1600 1800 2000 2200
Plot 3 800 1000 1200 1400 1600 1800 2000 2200
2.9 4.4 4.8 7.7 8.8 8.3 9.0 13.1
-
2.9 4.2 -
9.2
4.1 4.8 5.2 6.4 6.3 6.7
4.8 6.0 7.0 8.3 9.9 9.0 9.7 14.5
4.8 6.0 7.2 8.3 9.4 10.8 12.0 14.9
4.8 6.4 7.6 9.0 10.1 11.7 12.7 13.8
6.2 7.6 8.6 10.3 11.8 10.7 11.8 16.8
6.0 7.4 9.0 10.7 12.3 13.9 15.6 20.9
6.5 8.1 9.5 11.2 12.7 14.6 16.0 17.9
Soil elutriation 0.27 0.00 0.00 0.01 0.18 0.08 0.14 0.02 0.35 0.28 0.10 0.04 1.07 1.83 3.53 5.57
Soil elutriation 0.03 0.00 1.64 1.00 42.15 15.85 87.75 32.46 116.97 58.47 182.68 93.43 200.11 132.62 363.0 444.7
Soil elutriation 1.20 0.06 22.41 6.05 53.75 11.12 94.62 32.05 117.86 51.38 170.33 88.14 210.02 134.95 304.87 173.96
0.02 0.00 0.02 0.01 0.06 0.02 0.61 1.62
0.04 -0.02 0.60 1.51 3.48 6.40 10.54 37.67
0.05 0.17 0.45 1.56 3.10 6.62 9.98 14.67
0.03
0.00
0.00
-0.0001
0.0162
0.6914
0.0026
0.0101
10.85
0.0006
5.34
0.1626
0.1496
0.0039
0.0046
0.0021
8.00
0.0030
1.26
0.0757
0.1981
0.0036
0.0035
0.0170
4.54
0.0044
0.84
1.2341
0.0540
0.0015
19.71
1.4411
0.8725
0.0492
39.85
0.5127
0.9273
0.3068
42.48
2.3778
0.1803
0.0021
9.95
5.8936
6.0425
2.2153
24.46
7.7230
7.7151
5.0623
24.93
54.10
26.42
27.22
0
0
0
58.9 88.9
10.5 7.8
90.1
7.9 7.1
0.0 0.4 1.9 11.1 12.5 20.4 30.1 13.7
46.9 14.6
14.6 15.9
0.5
(continued on p. 296)
1.3
10.2 7.3 33.0 61.0 58.9 72.5 74.2 87.4
63.5 78.3
34.9 21.5
27.1 2.2 34.6 30.1 35.2 16.3 10.2 24.2
7.7
25.4
i
c~
.~
3.3 3.8 5.3 6.4 7.4 8.3 9.8 I0.6
0.05
4.0 5.1 6.7 8.3 9.2 10.8 12.5 13.4
0.10
5.7 7.0 9.0 11.0 11.8 13.7 15.5 16.6
0.35
Wind ~ metres/sec Pitot height (m)
Soil elutriation 0.03 0.18 0.03 0.02 0.00 0.06 0.05 0.13 0.05 0.04 0.02 0.01 0.05 0.09 0.06 0.05 0.05 0.01 0.01 0.02 0.02 0.02 0.08 0.05
0.02
0.0016
0.0021
0.0008
0.00
6.46
0.0024
0.0026
3.28
0.0020
0.0013
-0.0011
17.09
0.0010
0.0054
0.0016
13.12
whole
50 100
< 10
10-50
300 500
10 50 10050 100 300 Mid-level sample (gms)
< 10
250 500
0 50
50250
Elutriation data c Soil sample (%) in #m range
Saltation b grams
60.03
500 2000
0
0.5-2
2 10
> 10
Fine particle data d particles/cm 3 of air 0.33 M corrected for > 2000 background in #m range
Calculated from pitot tube pressures (Bell, page 218). b Corrected for water loss, 0.06 gms added. Grams of sample collected over a 1 minute run (page 177). c The soil elutriation is the original soil sample in 7 size classes. Otherwise particulates collected in the mid-level Bagnold trap were separated in 4 size classes. The mid-level sample is in grams (0.05 to 0.25 m); the soil sample is in % (pp. 159, 335). d Fine particles collected with the optical particle counter placed directly in the airstream, 0.33 m above ground.
1600 1800 2000 2200
Plot 6 800 1000 1200 1400
Ereus
Table 1 (continued)
~
2 ''"
'~ ¢,a ~"
,~ a,
W.D. Scott et al. / Catena 21 (1994) 291 303
297
Plot 2: Manually-washed residue sand whose surface was treated with a 1% solution of Napcoseal M 130 one week before the experiments. The application rate was 1 litre/m 2. Plot 3: Untreated residue sand which was covered with black plastic until the experiments. Plot 4: Rain-washed residue sand which was treated with Napcoseal CE40 just before the experiments. Plot 5: Crushed gypsum - - amended mud from Alcoa's Kwinana Refinery. Plot 6: Untreated residue mud. These plot tests concentrate on the residue sand because it has the appropriate saltation components and is the main component in the dykes that surround the mud lakes. The chemical treatments follow the recommended amounts and are viable alternatives, should they be effective.
3. Profile characterization
The work of Scott et al. (1993) and Hopwood and Scott (1990) make it clear that, at least for suspension and saltation, the airborne concentrations are scaled with the height measured in numbers of roughness lengthsand the velocity measured with the dimensionless Rouse number, which is, effectively, the ratio of mass-weighted fall velocity to the friction velocity. Hence any experiment to measure erosion must measure both the roughness length, z 0 and the friction velocity u,. Classically the wind profile, in both the atmosphere and in the inner boundary layer of the wind tunnel, is fit by the form (Monin and Yaglom, 1973): U,
u = ~- In Z/Zo where k is the Von Karman constant, 0.4 (H6gstr6m, 1985; Wieringa, 1980). This form holds in the case of wind blowing below the threshold for erosion to occur. Bagnold (1941) makes it abundantly clear that in eroding conditions the same profile is observed and this has been corroborated by other classic studies since Bagnold (Zingg, 1953; Williams, 1964). Consider the case when the above formula applies and the wind speed is just at the threshold that allows erosion to occur. Then, with a disturbance, erosion proceeds and continues in an all-eroding situation. The logarithmic profile that obtains has the same slope but the effective roughness is increased by, perhaps, 100 fold: /g*
/
u = -~- In z/z o The surface has adjusted to a new roughness level z~ which varies with the overriding wind strength, as indicated by the wind stress or friction velocity u,. The wind speeds are lower. However, with an increase in the overriding wind, the wind speed increases with a concurrent increase in z~ such that all the semilogarithmic lines pass through the same point, the "focus". The threshold velocity ut and the threshold friction
298
W.D. Scott et al. / Catena 21 (1994) 291-303
length zt can be defined as that focus where** U,
Ut = ~-In z/zlo for all values o f u, and z~ (Fig. 2 ) . Dividing up the logarithm, this equation can be written: In z~ = - u t k ( 1 / u , ) + In z, This is a standard linear form with y = lnz~ and x = 1 / u , . We expect that a plot of In z~ versus l/u, should yield a straight line with slope = - u t k intercept = In z t provided the surface is effectively "all-erodible". T h a t is, there is a continuing source of all particles at the surface and the winnowing o f particles in the air does not leave heavy particles behind. O f course, if the surface is not erodible or the roughness elements are p e r m a n e n t features, Zo is zo which is constant anyway. I f the surface roughnesses blow away during the experimental runs with increasing wind speeds, then zd actually decreases with time and the surface is not all-erodible. If the plot is non-linear, there is an alteration o f the surface structure with each different, overriding wind. This later condition normally obtains in nature. With depletion of the surface and armoring, vegetation growth, wetting or whatever, it is expected that the surface will exhibit a hystersis and history but, importantly, this m e t h o d of analysis gives a characterisation of the erosion process.
,oc,,s
I I
, -fi
!
N
I
/
! U[
u
Fig. 2. Schematic plot of logarithmic profile and the effects of erosion. ** These terms are not precisely the same as used by Bagnold.
W.D. Scott et al. / Catena 21 (1994) 291 303
299
Table 2 u, and z~ values. Data interpreted from wind profile prepared plots Friction velocity (m/s)
Roughness length (mm)
Plot 1
800 1000 1200 1400 1600 1800 2000 2200
0.25 0.45 0.60 0.67 0.76 0.89 0.99 1.07
0.045 0.657 0.979 0.617 0.637 0.683 0.648 0.612
Plot 2
800 1000 1200 1400 1600 1800 2000 2200
0.25 0.29 0.36 0.39 0.42 0.44 0.39 0.58
0.050 0.020 0.017 0.008 0.004 0.001 0.000 0.003
Plot 3
800 1000 1200 1400 1600 1800 2200
0.50 0.66 0.85 0.95 1.26 1.54 1.73
1.972 2.472 3.655 2.911 5.505 6.922 5.160
Plot 4
800 1000
0.61 0.63
5.936 2.949
Plot 5
800 1000 1200 1400 1600 1800 2000 2200
0.65 0.64 0.75 0.55 0.62 0.50 0.59 0.76
6.979 2.836 3.185 0.198 0.163 0.068 0.118 0.049
Plot 6
800 1000 1200 1400 1600 1800 2000 2200
0.50 0.65 0.76 0.94 0.90 1.09 1.15 1.21
3.744 4.657 2.989 3.092 1.757 2.164 1.483 1.364
300
W.D. Scott et al. / Catena 21 (1994) 291-303
4. Experimental profiles The experimental velocity profiles were easily fitted to the logarithmic profile equation; data are presented in Table 2. These profiles are corrected for the air density effect as outlined in Scott and Carter (1986) using the 3-level saltation collection (see follow-up paper - - Part II). Though this correction has little effect on this particular data, it allows a logical definition of u, and z~ that are unaffected by considerations of any "kink" in the profile (Gerety, 1985). As presented above, there is no single roughness of the eroding surface but the roughness varies with u,. An overall assessment of these data was completed following the above procedure, using l/u, and lnz~; this is shown in Fig. 3. As presented, the data for a given experimental plot are connected with a broken line with arrows that represent the temporal course of the experiment. Following the above generalities regarding this method of displaying an experiment, we see that at least 4 different trends: 1. Constant Roughness. Large anchored roughness elements disallow penetration of the air into the surface. Plot 1 with the blue metal mulch exhibits this characteristic with an aerodynamic roughness of about 0.7 mm, independent of the value of u*. This
10'1~i~"'
~/'-o
1.0
0.1'
~
0.01'
~
PLOT
SYMBOL
1 2
-O-0--
0.001'
3 4 5
--~
6
0
i
~ 1
J
/U, Fig. 3. The changesin the windprofileparametersshowthe erosionpattern.
W.D. Scott et al. / Catena 21 (1994) 291-303
301
does not say that no erosion is occurring though there is little saltation (Table 1). The erosion process is not influencing the roughness. The normal feedback of an alleroding surface is absent. Saltation is also unlikely to be effective though there may be direct aerodynamic entrainment. 2. All Erodible. This is the "normal" type of erosion treated and observed by Bagnold on sand dunes and treated above. Roughness is truly affected by saltation. Plot 3 shows an all-erodible situation with a threshold velocity of 2.2 m/sec as obtained from the slope. Note that Plot 3 exhibited substantial saltation and a large amount of suspension (see Table 1). 3. Eroding Roughness. Roughness elements are composed oferodible fractions that become depleted with time. Plot 2 and Plot 6 show this effect. Plot 2 definitely shows substantial saltation as might accommodate this effect. However, Plot 6 shows almost no saltation. This is difficult to explain as this plot was a heavy clay bed with a smooth surface and cracks throughout. There are two possibilities: (a) Surface creep has allowed movement of the material into cracks. This is unlikely to have involved surface features of the centimetre sizes required. (b) There is a gradation of roughness down the tunnel. The scaling and size of the inner boundary layer is determined by the surface roughness. Equilibration to these changes is not instantaneous (Jensen, 1985) so that with increased wind speed, the smoother, upwind surfaces have more influence on the wind profile. The latter suggestion is most likely. Corroboration is required; this condition would not allow that the slope of the log profile be used as a measure of the shear, u,. An alternate explanation of this effect is that the profile becomes more displaced as the experiment proceeds; the displacement distance simply increases and the profile shifts downward in an absolute sense. Displacement distances are unknown in these experiments but are unlikely to exceed 1 cm. The variation ofz 0 in Plot 6 is larger than this; hence, this is unlikely to be the cause of the trend in the data. 4. Constant Friction Velocity. This is unexpected but exhibited by Plot 5 and perhaps Plot 4. This could be the case of a strong, eroding roughness but this is not confirmed by the saltation data, particularly in Plot 5. Three suggestions about how this could happen are: (a) The surface is compliant, like a wheat field or grass surface that exhibits the Honami, in which the energy of the wind is not absorbed by the surface but reflected by the waving grain stalks. (b) The surface has no influence on the wind profile. Perhaps the Reynold's Stress or the correlation form of it is not predicted by the lower, soil surface, but by the walls and upper surface of the tunnel. This might be expected when the roughness of the lower surface is small and the inner layer, accordingly, would be small. In fact, in a very smooth surface, the law of the wall (Schlichting, 1979, p. 640) predicts scaling accordingly as u/u, ~ 0.02 ram, where u is the kinematic viscosity. If the surface were smooth enough, there would then no longer be any dependence on the roughness. This might be the case with Plot 2 but is unlikely with Plot 5; z~ is greater than 0.05 mm. (c) The profile simply is not a measure of u,. The velocity measurements may simply be made above the inner layer where r is not constant; they are
302
W.D. Scott et al. / Catena 21 (1994) 291-303
representative of the " m a i n s t r e a m " turbulence. This influence should be related to the size of the boundary layer and might be expected if the inner layer was poorly developed or very small. Perhaps a short distance upwind of where the profile was measured the roughness was much greater. Unfortunately, this cannot be confirmed. It should be noted that logarithmic profiles are common and occur without a constant shear stress (Scott and Tubb, 1990). Both (b) and (c) are the likely causes of the effect. O f course, these considerations are inherently linked with the "equilibrium" or lack thereof in the tunnel's wind flow characteristics. It may be simply a case where the profile is not a measure of the shear stress, Without erosion, a logarithmic profile is nearly always present through most of the tunnel (Carter and Moore, 1990; Carter and Marsh, 1980). This does not mean that the inner layer is this deep and in fact the shear stress drops in a nearly linear way to zero a few tens of centimetres off the surface (Raupach and Leys, 1990). During erosion the inner layer should be greatly extended since it scales with the roughness so a deep inner layer should form. It is a primary purpose of this paper to show that most surfaces are not "all erodible" and experiments conducted without a broad perspective of erosion processes, especially in natural and agricultural environments, must fail or give misleading information. The approach presented allows at least some of the additional complications to be assessed. The data do show the value of a wind flow analysis. Specifically, we see that the largest amount of erosion occurred with a surface treatment intended as a prevention (Table 1, Plot 2); rock mulch and surface texture changes (Plots 1, 5, 6) did not allow much erosion; and that an "all erodible" surface is by no means obtained in ordinary experiments. Note that the full data set (Table 1) is presented here for review and criticism; further work, characterising suspension and saltation, will appear in a follow-up paper.
Acknowledgements Peter Bell acquired the present data during the development of his honour's thesis. Without his efforts and attendance to detail, this paper would not have been possible. Also, A L C O A of Australia funded the research; John Summers of A L C O A arranged the prepared plots and guided the project. Dr. Bill Robertson and several students at Murdoch helped collect the data in a difficult, noisy and hot environment.
References Armburst, D.V. and Box, J.E., 1967. Design and operation of a portable soil-blowingwind tunnel, ARS41131. Agricultural Research Service, U.S. Department of Agriculture, Big Spring, TX. Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Reprinted 1984, Chapman and Hall, London, 265 pp. Bell, P., 1984. Micrometeorologicalmeasurementsof dust from residue waste. Honours thesis in Environmental Science, Murdoch University.
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