department of geography, university of sheffield 19th ... · the 17th windy day, october 2011 2 | p...
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A large dust storm rolling through the Phoenix area, July 5, 2011
Department of Geography, University of Sheffield
19th October 2011
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10:15 – 10:55 Arrival & Coffee/Tea
SESSION 1 Chair: Dr Robert Bryant
10:55 - 11:00 Introduction to Windy Day
11:00 - 11:30 Large roughness element effects on sand transport, Oceano Dunes, California. Gillies, J. A., and Lancaster , N
11:30 - 11:45 Modelled and measured three dimensional flow characteristics over a coastal saucer blowout. Smyth, T.A.G., Jackson D.W.T. and Cooper, J.A.G.
11:45 – 12:00 Single heavy mineral grains in loess-dust and desert sand provenance analysis in China. Stevens, T., Bird, A. F. Vermeesch, P., Parrish, R. R., Carter, A., Lu, H., Palk, C., Watson T., Sevasjanova I.
12:00 – 12:15
The vertical structure of airflow turbulence and associated shear stresses within a boundary layer as a driver of sediment transport. Lee, Z. S., and Baas A. C. W.
12:15 – 12:30
Desert sand dune dynamics and preservation: chrono-stratigraphic records of Quaternary climate change. Leighton, C.L., Bailey, R.M., and Thomas D.S.G.
12:30-13:30 Lunch
SESSION 2 Chair: Professor Ian Livingstone
13:30-13:45 New evidence for coversand deposition during MIS 3 in the Landes region, SW France. Sitzia L., Lenoble A., Mercier N., and Bertran P.
13:45- 14:00 Lee-side airflow pattern characterisation under offshore winds at a beach-dune system using high resolution three-dimensional anemometers and computational fluid dynamics modelling. Delgado-Fernandez, I., Jackson, D.W.T., Cooper, J.A.G., Baas, A.C.W., Beyers, J.H.M. and Lynch, K.
14:00-14:15 Are carbonate-rich dunefield systems a Global Climate indicator? A 300.000 years record of aeolianites along the Sardinia coasts. Andreucci, S., Sechi D., and Pascucci V.
14:15-14:30 Dust production mechanisms in the western Sahara - Observations from the Fennec IOP in June 2011: Washington, R., Engelstaedter, S., and Allen, C.
14:30-14:45 A field study of flow turbulence and sediment transport dynamics on a beach surface in the lee of a coastal foredune under offshore winds. Baas, A.C.W., Jackson, D.W.T., Cooper, J.A.G., Lynch, K, Delgado-Fernandez, I. and Beyers, M.
14:45-15:00 How old can we go? Investigating the maximum limit of optically stimulated luminescence dating. Chapot, M.S., Roberts, H.M., Duller, G.A.T., and Lai, Z.P.
15:00 – 15:30 Coffee/Tea & Nibbles
SESSION 3 Chair: Professor Mark Bateman
15:30 – 15:45 Assessing the Role of Paleo-Hydrology in Saharan Dust Production. Drake, N. A., and Schepanski, K.
15:45 – 16:00 No country for old men: the controls of geomorphology and land use on dust emission in west Texas and eastern New Mexico. Baddock, M., Lee, J., Gill, T. E., and Mbuh, J
16:00 – 16:15 Palaeodunes in the South American tropics: preliminary luminescence dating and field observations from the Llanos savannas, Colombia. Carr, A.S., Armitage, S.J., Boom A., Bilbao, B., Berrio, J-C., and Wouters H.
16:15 – 16:30 Dust Observations for Models (DO4): Observations from the initial field campaign July- October 2011. Wiggs, G, F. S., King, J., Eckardt, F. D., Thomas, D. S. G., Bryant, R. G., Nield, J, M., and Washington, R.
16:30 –16:45 The commonalities of glaciers and salt pans (or travel adventures of a terrestrial laser scanner). Nield, J. M.,
16:45 – 17:00 Reconciling the climate record in the Stari Slankamen loess with the Milankovitch climate change theory. Basarin, B., O’Hara-Dhand, K., Markovic, S., and Smalley, I.
17:00 – 17:15
Close, and announcement of future meetings (and then head to the pub)
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Large Roughness Element Effects on Sand Transport, Oceano Dunes,
California
J.A. Gillies1,*
and N. Lancaster2
1Division of Atmospheric Sciences, Desert Research Institute, Reno NV USA
2Division of Earth and Ecosystems Sciences, Desert Research Institute, Reno NV USA
*Corresponding author; Email: [email protected]; Telephone: 775-674-7035; Fax: 775-674-7016
As part of a study to evaluate the effectiveness of artificial roughness to control sand movement and dust
emissions at Oceano Dunes, California State Park, Oceano, CA, straw bales were emplaced on a portion of
the sand surface in a staggered array approximately 50 m wide and 100 m long. Sand transport through the
roughness was measured using sand traps. The bale dimensions were 1.17 m long × 0.4 m high × 0.6 m
wide, and 210 were used to create a roughness density ( ) of 0.022 ( =n b h/ S, where n = # of elements,
b=element breadth, h=element height, and S is the area of the surface that contains all the elements). The
target value was chosen based on the relationship between normalised saltation flux (NSF=outgoing sand
flux/incoming sand flux) and presented by Gillies et al. (2006), which predicts that saltation flux should be
~50% of the incoming sand flux at the trailing edge of the array for roughness of height 0.36 m. Sand flux
was measured through the array from upwind of the bales to the trailing edge of the array using two types of
sand traps: the Cox Sand Catcher (CSC, Ono et al., 2011) and the Big Spring Number Eight (BSNE, Fryrear,
1986). Twenty four CSCs were placed in six rows of four traps, beginning immediately upwind of the first
row of straw bales and then in the centre of the lane between straw bale rows four and five; eight and nine;
12 and 13; 16 and 17; and 18 and 19. Six BSNEs were installed only in the centre of each of the rows at the
CSC positions. The height of the inlet for all traps was set at 0.15 m, and adjusted daily to account for
changes in sand surface elevation. A 2.2 m high anemometer/wind direction mast located in the centre of the
array between rows 14 and 15. Measurements were conducted April 15 – May 4, 2011.
A total of 14 significant sand transport events were available for estimation of the sand flux reduction due to
the added roughness. The measurements for both types of trap are similar and closely correlated (r2 = 0.70).
There is evidence to suggest that the sand-trapping effectiveness of the array was higher in the first two days
after it was installed – giving rise to sand flux reduction factors (SFR=[sand in – sand out]/sand in) of 75-
84%, before falling to mean values of 55% and 57% for the CSC and BSNE traps, respectively. The mean
NSF at the furthest downwind distance for the two trap types were 0.44 (CSC) and 0.41 (BSNE). This is in
excellent agreement with the model prediction of Gillies et al. (2006). The Oceano Dune data corroborates
the results of Gillies et al. (2006), which demonstrated that saltation flux reduction due to roughness scales as
a power function of and that for large roughness elements there is an additional increase due to a size
effect. The reduction in saltation flux appears to be very similar for both studies even though the roughness
elements were quite different in form (bales as opposed to buckets) and horizontal porosities (Cornelis and
Gabriels, 2005) 49% and 16% for the bales and buckets, respectively. This suggests, as stated by Gillies et
al. (2006), that element height is the critical parameter enhancing the reduction in transport flux. The lower
mean NSF value at Oceano as compared to the Gillies et al. (2006) prediction could be due to the slightly
greater element height and perhaps the greater horizontal porosity of the bale roughness, but height appears
to have the greatest effect on sediment transport reduction.
References:
Cornelis, W.M. and D. Gabriels (2005). Optimal windbreak design for wind erosion control. Journal of Arid
Environments 61: 315-332.
Fryrear, D.W., 1986. A field dust sampler. Journal of Soil and Water Conservation, 41, 117-120.
Gillies, J.A., Nickling, W.G., King, J., 2006. Aeolian sediment transport through large patches of roughness in the
atmospheric inertial sublayer. Journal of Geophysical Research, 111, F02006.
Ono, D., P. Kiddoo, C. Howard, G. Davis, and K. Richmond (2011). Application of a combined measurement and
modeling method to quantify windblown dust emissions from the exposed playa at Mono Lake, California. Journal
of the Air and Waste Management Association (in press).
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Modelled and measured three dimensional flow characteristics over a
coastal saucer blowout
T.A.G. Smyth*, D.W.T. Jackson and J.A.G. Cooper
Centre for Coastal and Marine Research, School of Environmental Science, University of Ulster
*Corresponding author; Email: [email protected]; Telephone: 02870324318.
Saucer blowouts are common landforms found within interior grasslands (Hugenholtz and Wolfe, 2009) and
coastal dunes (Hesp, 2002). Enlargement of these landforms is driven primarily by surface wind stress and
aeolian transport, however the patterns of deflation within blowouts are poorly understood as near surface
wind flow is very complex. As a result previous studies have only been able to suggest that as flow enters a
blowout it separates and expands before accelerating over the rim. The effect topography has on steering
near surface airflow has yet to be quantified due to the number of three dimensional anemometers required in
an array to adequately capture such an event. This study for the first time examines near surface airflow
within and in the lee of a saucer blowout, using an array of ultrasonic, three-dimensional anemometers and a
Computational Fluid Dynamics (CFD) model.
The investigation took place on a coastal saucer blowout located in lee of the foredune crest on the Belmullet
peninsula, western Ireland. The blowout measures 13 m deep, 60 m wide and 100 m from the depositional
lobe to blowout throat, and was mapped using LiDAR and high resolution 1m x 1 m DGPS surveys (Figure
1). Fifteen anemometers were placed in areas of complex flow on the blowout‟s deflation basin and erosional
walls based on an initial CFD simulation. Two anemometers remained outside the blowout to measure
prevailing wind conditions. Anemometry data was recorded at 50Hz for a continuous 15 hour period from
which 5 minute median wind records parallel to the orientation of the blowout (252˚±2.5˚) were compared
with the CFD, a total of 33 records. Detailed flow simulations (~4.5 M cells) were conducted with the open-
source CFD modelling software OpenFOAM, using the two-equation Re-normalised Group (RNG) k-
Epsilon turbulence model.
From the selected 33 records, highest total velocities were recorded on the foredune crest immediately
upwind of the blowout, lowest velocities were recorded in lee of the relict erosional wall. Relative total flow
velocities exhibited a narrow range of velocities at each anemometer location as did the directional
components of the flow (u and v), indicating that flow acceleration, retardation and steering over the
topography is independent of wind speed.
Relative flow velocity calculated from the CFD simulation compare well with relative mean flow velocity at
the 17 anemometer locations, producing an r2 value of 0.81. Both field measurements and CFD indicate that
as flow enters the blowout it diverges and velocity reduces. However, the model also indicates that flow
reversal occurs close to the wall of the blowout in lee of the foredune (Figure 1). Flow along the axis of the
blowout remains unchanged in direction and is also less retarded, however, as flow exits the blowout it
appears to behave similarly to the windward slope of a dune, as flow decelerates at the toe of the steep slope
and increases in velocity toward the crest (Figure 1). CFD is a valuable tool in understanding blowout
morphodynamics. Their genesis and evolution can be examined by highlighting areas of potential erosion
during ranges of wind direction and strength and the potential consequences of climate change may also be
investigated by simulating the effect that changing wind climates and vegetation changes may have on
aeolian transport potential.
References: Hugenholtz, C.H., and Wolfe, S.A. (2009) Form-flow interactions of an aeolian saucer blowout, Earth Surf. Processes
and Landforms, 34, 919-928, doi:10.1002/esp.1776
Hesp, P. (2002) Foredunes and blowouts: initiation, geomorphology and dynamics, Geomorphology, 48, 245-268
Richards, P.J. and R.P. Hoxey (1993), Appropriate boundary conditions for computational wind engineering models
using k-e turbulence model, Journal of Wind Engineering and Industrial Aerodynamics, 46, 47, 145-153
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Figure 1. Map of modelled total flow velocity at 1 m above surface. Fine arrows indicate flow modelled flow
direction at 1 m above surface. Bold arrows indicate average anemometry data of 33 records selected for
comparison with CFD.
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Single heavy mineral grains in loess-dust and desert sand provenance
analysis in China
T. Stevens1,*
, A. F. Bird1, P. Vermeesch
2, R. R. Parrish
3, A. Carter
2, H. Lu
4, C. Palk
5,
T. Watson1, I. Sevasjanova
6
1Department of Geography, Royal Holloway, University of London, Egham, Surrey, UK
2School of Earth Sciences, Birkbeck College, University of London, London, UK
3Department of Geology, University of Leicester, University Road, Leicester, UK
4School of Geographical and Oceanographical Sciences, Nanjing University, Nanjing, China
5Department of Earth Science and Engineering, Imperial College London, London, UK
6Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, UK
*Thomas Stevens; Email: [email protected]; Telephone: 01784 276468; Fax: 01784 472836
Understanding the source of aeolian sediments is fundamental to their interpretation in terms of climate or
sediment transport process. Furthermore, the transport of aeolian dust in the atmosphere is a significant but
poorly understood component of climate. The past activity of dust sources can only be determined via the
provenancing of aeolian dust from sedimentary archives such as loess, notably on the Chinese Loess Plateau
in north central China. Accumulation of dust as loess is a function of wind speed, changes in sediment
deposition and capture, source location and their expansion/contraction, as well as dust production and
emission in these regions. However, the source aspects of Chinese loess are often not well understood and
hence at present it is hard to interpret accumulation rate values in terms of atmospheric flux. Furthermore,
many of the potential source regions for Chinese loess are desert basins that contain sandy sediment, also
poorly understood in terms of source. The ultimate origin of the sediments that comprise sandy deserts is
central to understanding the formation of such systems, as well as to interpretation of loess provenance data.
Here we argue that this uncertainty over source is in part a consequence of a) the use of bulk sediment
geochemical/mineralogical analyses in Chinese loess and desert sand source studies and b) the likely
multiple suite of sources that contribute to Chinese loess dust. Bulk sediment geochemical analysis of
sediment comprised of multiple source regions will average out the unique signatures that may be diagnostic
of individual source regions. We will present a heavy minerals single-grain methodology for attempting to
constrain the provenance of Chinese loess and desert sands. This utilises for example, the U-Pb and Fission-
Track age distributions and morphology of detrital zircon grains, as well as heavy mineral assemblages, in
order to isolate the multiple sand and dust source regions. Using a single-grain dataset from loess and desert
sands of Northern China we will discuss how this data can be used to determine the relative similarity of
sediments and potential source regions. We will focus on last glacial loess from the Chinese Loess Plateau
and sands of the Mu Us Desert, located in the Ordos Basin in Northern China. The latter have been
hypothesised to be a source of the former, and themselves sourced from underlying Quaternary sands and
Cretaceous aeolianite. However, there has neither been a direct quantitative test of this hypothesis, nor
characterisation of variability in geochemical signatures in sediment across the basin or through time.
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The vertical structure of airflow turbulence and associated shear
stresses within a boundary layer as a driver of sediment transport.
Z. S. Lee* and A. C. W. Baas
Department of Geography, King’s College London, Strand, London, UK
*Corresponding author; Zoë. S. Lee; Email: [email protected]
Recent and ongoing research studies have indicated that coherent turbulent flow structures, within internal
boundary layers, play an important role in driving aeolian sand transport in coastal and dryland environments
(e.g., Jackson, et al. 2011; Weaver and Wiggs, 2011). Advances in instrumentation have enabled a move
beyond the basic monitoring of shear or average velocities and bulk sediment transport over coarse time
intervals, to more detailed measurements of shear stresses and turbulence at higher spatio-temporal
resolutions. Consequently, research into best practice for calculating shear stresses and resolving turbulent
structures is necessary to gain full appreciation of how temporal changes in shear stress drive wind erosivity
and sand transport.
The results presented in this paper are from a field study of boundary layer turbulence and shear stresses
conducted under obliquely onshore winds over a beach at Magilligan Strand, Northern Ireland. High-
frequency (25 Hz) 3D wind velocity measurements were collected using sonic anemometry at five different
heights between 0.13 and 1.67 metres above the bed, and the associated sand transport response was
measured using an array of Safires and sand traps.
The wind velocity data is used to investigate the vertical structure of shear stresses and burst-sweep event
characteristics. Reynolds-derived shear stress, calculated both before and after streamline corrections are
applied, is compared to Prandtl-Von-Karman Law-of-the-Wall derived shear stress. Stresses calculated using
three different streamline correction routines (Planar fit, 2 and 3 step rotations) are compared and the
evolution of the stress tensor evaluated.
Results show how the contributions to shear stress and the average pitch of bursts and sweeps changes as a
function of height above the bed, indicating the transformation of top-down turbulent eddies as they travel
toward the surface. The study also demonstrates how different types of post processing of sonic anemometry
data significantly affect the shear stress assessment and may produce misleading results when applied to the
interpretation of sediment transport drivers.
References:
Jackson, D. W. T., Beyers, J. H. M., Lynch, K., Cooper, J. A. G., Baas, A. C. W. & Delgado-Fernandez, I. (2011)
Investigation of three-dimensional wind flow behaviour over coastal dune morphology under offshore winds using
computational fluid dynamics (CFD) and ultrasonic anemometry. Earth Surface Processes and Landforms, 36, 1113-
1124.
Weaver, C. M. & Wiggs, G. F. S. (2011) Field measurements of mean and turbulent airflow over a barchan sand dune.
Geomorphology, 128, 32-41.
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Desert sand dune dynamics and preservation:
chrono-stratigraphic records of Quaternary climate change
C.L. Leighton1*
, R.M. Bailey1 and D.S.G. Thomas
1, 2
1 School of Geography and the Environment, University of Oxford
2 Department of Environmental and Geographical Science, University of Cape Town
* Corresponding author; Email: [email protected]
Advances in laboratory and field protocols for optically stimulated luminescence (OSL) dating have
increased its chronometric utility in arid environments, but the publication of large numbers of OSL
dune ages have raised further questions regarding the interpretation of ages in terms of climate
change implications. Recent studies (e.g. Stone and Thomas, 2008; Telfer et al., 2010; Thomas and
Burrough, in press) posit that ages from dunes are not solely the result of external
palaeoenvironmental changes, but are also a product of complex dune dynamics and sampling
strategy. Extracting the relative importance of these factors when interpreting discontinuous OSL
chronologies from sand dunes has proven difficult, and is hindered in many contexts where the
internal structure of the dunes is not visible. OSL samples have been taken from three exposures of
dune internal structure in the United Arab Emirates (UAE), in order to assess how sample ages
reflect the dune profile, and how this profile is a reflection of past environmental change.
Preliminary results are presented, and their implications for reconstructing Quaternary climates
from dune chronologies are considered. Whilst visible stratigraphy may aid the identification of
under-sampling, dune internal structure reflects complexity of dune processes at a range of temporal
scales which restricts targeting OSL sampling using stratigraphy alone.
References:
Stone, A. E. C. and Thomas, D. S. G. (2008). Linear dune accumulation chronologies from the southwest Kalahari,
Namibia: challenges of reconstructing late Quaternary palaeoenvironments from aeolian landforms. Quaternary Science
Reviews 27 (17-18): 1667-1681.
Telfer, M. W. et al. (2010). Understanding linear dune chronologies: Insights from a simple accumulation model.
Geomorphology 120 (3-4): 195-208.
Thomas, D.S.G. and Burrough, S.L. (in press) Interpreting geo-proxies of late Quaternary climate change in African
drylands: implications for understanding environmental and early human behaviour. Quaternary International
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New evidence for coversand deposition during MIS 3 in the Landes
region, SW France.
SITZIA Luca1*
, LENOBLE2 Arnaud, MERCIER Norbert, and BERTRAN Pascal
1PACEA, UMR 5199, University of Bordeaux 1, avenue des facultés, 33405 Talence, France
2PACEA, UMR 5199, University of Bordeaux 1, avenue des facultés, 33405 Talence, France
*Sitzia Luca; Email: [email protected]; Telephone: 0033 6 21 32 26 65;
Fax: 33(0)5 40 00 25 45
The chronostratigraphy of aeolian deposits in the Aquitaine basin, SW France, have been recently
revised (Bertran et al. 2011). Three main aeolian phases, characterised by the formation of sand
sheets and dunes, and by distal loess accumulations have been identified and correspond to the Late
Pleniglacial (23 – 15 Ka), the Younger Dryas and the Little Ice Age. These phases fit well with the
main periods of coversand deposition in northern Europe (Kasse 2002). However, some lines of
evidence point to older (late MIS 3) aeolian phases in the Landes region. These are: 1) sand
infillings of frost wedges OSL dated between 37 and 31 ka ; 2) strongly wind-abraded Middle
Palaeolithic lithic artefacts, by contrast to fresh or weakly abraded Upper Palaeolithic (35-13 ka)
ones, 3) aeolian sand layers alternating with peat horizons and 14
C dated between 32 and 40 ka cal.
BP (Hourtin and Cestas-Pot-au-Pin sections). These sections show from top to bottom the following
sequence: 1) a Holocene podzol, 2) aeolian sands corresponding to the barkhanoid ridges visible on
satellital views, 3) cryoturbated horizons („drop‟ soils) interstratified with aeolian sand layers. Our
results show that the beginning of coversand emplacement started at ca. 35 ka, i.e. like Late
Pleniglacial loess deposition in northern France (Antoine et al. 2003). Preservation of early sand
deposits seems to be connected to depressions with a near-surface groundwater table and
subsequent low deflation potential. It is here suggested that the palaeosoils observed in the sections
are a response to Dansgaard-Oeschger type climatic events.
References:
Antoine, P., John C., Lautridou J-P & Sommé J. 2003. « The loess and coversands of northern France and southern
England ». JOURNAL OF QUATERNARY SCIENCE 18 (3-4): 309-318.
Bertran, P., Bateman M.D., Hernandez M., Mercier N., Millet D., Sitzia L. & Tastet J-P. 2011. « Inland aeolian deposits
of south‐west France: facies, stratigraphy and chronology ». Journal of Quaternary Science 26 (4) (mai 1): 374-388.
doi:10.1002/jqs.1461.
Kasse, C.. 2002. « Sandy aeolian deposits and environments and their relation to climate during the Last Glacial
Maximum and Lateglacial in northwest and central Europe ». Progress in Physical Geography 26 (4): 507-532.
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Lee-side airflow pattern characterisation under offshore winds at a
beach-dune system using high resolution three-dimensional
anemometers and computational fluid dynamics modelling
*I. Delgado-Fernandez1, D.W.T. Jackson
1, J.A.G. Cooper
1, A.C.W. Baas
2,
J.H. M. Beyers3 and K. Lynch
4
1Centre for Coastal and Marine Research, School of Environmental Sciences, University of Ulster, Cromore
Road, Coleraine, Northern Ireland, UK, BT52 1S 2Department of Geography, King's College London, Strand, London, UK, WC2R 2LS
3Klimaat Consulting & Innovation Inc., Guelph, CANADA
4Department of Geography and Archaeology, National University of Ireland, Galway,
University Road, Galway, Ireland
*Corresponding author; Email: [email protected]; Telephone: +44 (0)28 7032 4318
Studies of the role of lee-side airflow effects demonstrate the importance of offshore flows in dune growth
and maintenance. Turbulent processes at the lee side of aeolian dunes have previously been only
qualitatively described. However, appropriate numerical characterisation has been not possible to date due to
the inherent complexity of lee-side airflow behaviour. The recent use of ultrasonic anemometers (UAs),
capable of measuring the three components of the wind vector, allows quantification of flow patterns in
complex areas such as the lee side of dunes. Additionally, three-dimensional modelling using computational
fluid dynamics (CFD) tools provides valuable insight into the behaviour of turbulent zones and associated
potential sediment transport.
Using combined data from an extensive array of high-frequency, three-dimensional UAs and CFD
simulations, quantitative results from a three year NERC-funded study are presented on airflow patterns and
turbulent zones in the lee-side of a coastal foredune. Field data was collected during an experiment carried
out at Magilligan Strand, Northern Ireland, where prevailing winds from the SW (offshore) dominate its
aeolian system and previous studies have reported significant secondary airflow effects. Data were collected
using an array of twenty four UAs deployed in April-May 2010 in a 60 x 90 m grid over the beach surface.
Field locations of sensors were guided by a preliminary CFD run indicating the extent of turbulent areas.
Simulations were carried out using the open-source CFD software OpenFOAM which solves the system of
partial differential equations representing the fluid dynamics on a three-dimensional computational grid. The
one-equation eddy (k) Large Eddy Simulations (LES) were conducted because of to their ability to resolve
turbulent wind flow characteristics Simulations included a constant aerodynamic roughness length to account
for terrain roughness.
The findings confirm some of the trends observed by researchers working in wind tunnels and desert
environments, particularly with regard to the role played by different wind speeds, directions and dune
morphology. Airflow reversal at the beach surface was observed under perpendicular offshore winds as low
as 1-4 m s-1
. Once formed, the location of turbulent zones remained constant under increasing wind speed at
the dune crest, and for incident wind angles of +/-30° from offshore perpendicular. The distance to re-
attachment was approximately four times the dune crest height, and increased alongshore with increasing
aspect ratio. Observed thresholds for changes in the reversed and re-attached zones are discussed, as are the
existence of complex vortices at the beach surface as well as the potential for onshore sediment transport
under offshore winds.
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Figure 1 A) Distribution of turbulent zones during perpendicular offshore winds. Wind roses in the cross-section view show wind
slopes. Wind roses in the plan view show wind directions. Onshore (-u) directed airflow is in red; offshore (+u) directed airflow is in
blue. Sensors located at a distance (D) of 30 to 40 m showed airflow reversal, while clear re-attachment was visible at 45 m,
suggesting the existence of a narrow transition zone between 40-45 m. A new inner boundary layer (IBL) formed beyond re-
attachment, with airflow at 60 and 65 m showing relative directions similar to that at the dune crest. B) Details of a RANS k-ω SST
simulation, with blue showing areas of low wind velocity and flow reversal.
B
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Are Carbonate-rich dunefield systems a Global Climate indicator?
A 300.000 years record of aeolianites along the Sardinia coasts
S. Andreucci1,*
, D. Sechi1 and V. Pascucci
1
1Department of Botanical, Ecological and Geological Science, University of Sassari, Via Piandanna, 4,
Sassari, Italy
*Corresponding author; Email: [email protected]; Telephone: +39 079228685; Fax: +39079233600
Carbonate dunefields mostly developed under warm climate conditions during interglacial stages (Brooke,
2001). However, in the last decades several authors found evidence of “glacial-related” carbonate-rich dune
systems (e.g. Andreucci et al., 2010). The studied successions crop out along the west coast of Sardinia
Island (Mediterranean basin): (1) along a relatively flat coastal sand plain (San Giovanni di Sinis and
Alghero Bay) occasionally fragmented by Miocene marlstone substrate or Mesozoic volcanic deposits, and
(2) along a rocky coast with several small embayments bordered by a mountain range inland and sea-cliffs
composed of Mesozoic limestones at South Alghero coast and and Palaeozoic phyllites at Argentiera site.
The sedimentary successions are almost similar in all the studied sites and dominated by alternations of
shallow marine and aeolianite/colluvial units. The aeolian deposits primarily consist of metre-thick, medium
to coarse-grained sands with high-angle (15°-40°) cross stratification. The sand composition consists for
60% to 90% of marine bioclasts and for 40% to 10% of siliciclastics. This indicates that the sand largely
originates from the nearby shallow marine environment. The aeolian sand bodies, developed on the
strandplain system, built up a 5m-thick dune ridge almost parallel to the coastline. Instead, along the rocky
coasts the sandy bodies are up to 20 m-thick and interpreted as transgressive coastal dune systems produced
by bimodal winds from N-NW and NE.
Optically Stimulated Luminescence (OSL) highlighted the presence of two dune units at San Giovanni di
Sinis site: penultimate interglacial Marine Isotopic Stage (MIS 7, 250-290 ka) and last interglacial substage
MIS5e (135-75 ka). At Alghero bay two units were identified. The first developed during the last interglacial
substage MIS5e (135-125 ka) and the latter during the Late Holocene (2-1.5 ka). OSL dating revealed five
dune units along the rocky South Alghero coasts: glacial stage (MIS8, 250-300 ka), penultimate glacial stage
(MIS 6, 190-135 ka), interglacial substage MIS 5c (100-90 ka), early last glacial phase (MIS 4, 75-60 ka),
and finally during MIS 3 (60-40 ka). Finally, at Argentiera only two wind-dominated systems were
identified. The first developed during MIS 3 (60-23 ka). Here, however, humid conditions activated frequent
and powerful floods that reworked the wind-blown marine sands in water flow-dominated alluvial fans. The
latter unit, instead, was formed during the Holocene Climate Optimum (9-5 ka). Thus dunefields along the
Sardinia coasts were formed both during glacial and interglacial stages. However, the thicker and widespread
aeolianites belong to the glacial phases (MIS 4-3). Transgressive dunefields seem to be preferentially formed
along rocky shorelines during eustatic sea level drops which expos a huge amount of marine sands, which
can be blown inland by strong winds. Instead, the coastal dune ridges most likely developed along flat
coastal plains are associated to the sea level raises, which reworks deposits during the transgression and
brings material to the coast that can be wind blown inland.
In conclusion carbonate-rich dunefield systems from Sardinia seem to be formed and migrated without an
intrinsic relation to “global climate” fluctuations. Therefore, local factors seem to be more important than the
global climate in the formation of dunefield systems. In particular the sediment supply, wind regime, local
climate and accommodation space inland are the major elements that control the migration of coastal
carbonate-rich dunes.
References:
Andreucci, S., Clemmensen, LB. and Pascucci, V. (2010) Transgressive dune formation along a cliffed coast at 75 ka in
Sardinia, Western Mediterranean: a record of sea-level fall and increased windiness. TerraNova 22: 424-433.
Brooke, B. (2001) Distribution of carbonate aeolianite. Earth-Science Reviews 55: 135-164.
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Dust production mechanisms in the western Sahara - Observations
from the Fennec IOP in June 2011
R. Washington1, S. Engelstaedter
1,* and C. Allen
1
1School of Geography and the Environment, University of Oxford
*Corresponding author; Email: [email protected]; Telephone: 01865 275885
The Sahara desert is the world‟s largest source of mineral dust. Satellite observations show that dust
emissions from this source peak in boreal summer (June). Due to hyper-arid conditions and in-accessibility,
there is only a relatively small number of high quality in-situ observations in this remote region. The Fennec
project, a consortium of the universities of Oxford, Leeds, Sussex, Reading and Imperial, the UK Met Office
and European partners, aims at improving our understanding of the Saharan climate system and how it is
linked to the emission and atmospheric distribution of dust. As part of the Fennec Intensive Observation
Period (IOP) in June 2011, remote automatic weather stations, flux towers, sodars, sun-photometers and
radiosonde stations were deployed on the ground. In addition, measurements were taken during a total of 15
flights with the Facility for Airborne Atmospheric Measurement‟s (FAAM) BAE146 research aircraft,
including in-situ, dropsonde and lidar measurements.
Simultaneous high-resolution ground, aircraft and satellite measurements during the IOP allow for, for the
very first time, a detailed analysis of dust production mechanisms in the western Sahara on a variety of
temporal and spatial scales. The measurements also provide an opportunity to test model simulations of these
processes. Here, we present an overview of the dust production mechanisms observed during the Fennec IOP
with a specific focus on the role of the Saharan Heat Low (SHL), Low-Level Jets (LLJ) and Haboobs.
SHL: In boreal summer, the Saharan heat low has been observed to co-locate with the dust maximum. Data
from four flights across the heat low with the FAAM BAE146 research aircraft, including from 31
dropsondes and lidar measurements, provide valuable understanding of the interaction between dust and the
boundary layer in this region.
LLJs: The early morning breakdown of LLJs has been posed as an important mechanism for dust generation
across the Sahara. Two BAE146 flights in June 2011 sampled LLJs in northern Mauritania; the data will
provide a detailed analysis of the jets and how they led to dust mobilisation. In addition, comprehensive
observations taken throughout June 2011 in Mauritania and Algeria by regular radiosondes, sodar, lidar and
AWSs provide a description of the spatial and temporal variability of the LLJs and associated dust emission.
Haboobs: Dust-generating high-turbulent winds at the surface associated with cold pool outflow from
convective clouds have been proposed recently as one of the main contributors to atmospheric dust in the
Sahara. Ground, aircraft and satellite observations allow for a detailed analysis of the 3D structure and
evolution of these large scale events. Two BAE146 flights were able to sample Haboobs during the Fennec
IOP.
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A field study of flow turbulence and sediment transport dynamics on a
beach surface in the lee of a coastal foredune under offshore winds
Baas, A.C.W.1*
, Jackson, D.W.T. 2, Cooper, J.A.G.
2, Lynch, K
3, Delgado-Fernandez,
I. 2 Beyers, M.
4
1Department of Geography, King’s College London, Strand, London, UK; *[email protected]
2Centre for Coastal & Marine Research, School of Environmental Sciences, University of
Ulster, Northern Ireland, UK 3Department of Geography, National University of Ireland (Galway), University Road, Galway, Ireland
4Rowan Williams Davies and Irwin Inc, 650 Woodlawn Road West, Guelph, ON N1K 1B8, Canada
The past decade has seen a growing body of research on the relation between turbulence in the wind
and the resultant transport of sediment over active sand surfaces. Widespread use of sonic
anemometry and high-frequency sand transport sensors and traps have facilitated recent field
studies over dunes and beach surfaces, to move beyond monitoring of mean wind speed and bulk
transport to more detailed measurements at much higher spatio-temporal resolutions.
In this paper we present results of a field study conducted in the recirculation flow and re-
attachment zone on a beach behind a foredune at Magilligan Strand, Northern Ireland. The offshore
winds over the foredune at this site are associated with flow separation and reversal located over the
beach surface in the lee of the dune row, often strong enough to induce sand transport toward the
toe of the foredune („against‟ the overall offshore flow). The re-attachment and recirculation zone
are associated with strongly turbulent fluid flow and complex streamlines that do not follow the
underlying topography.
High frequency (25 Hz) wind and sand transport data were collected at a grid of point locations
distributed over the beach surface between 35 m to 55 m distance from the 10 m high dune crest,
using ultrasonic anemometers at 0.5 m height and co-located load cell traps and Safires at the bed
surface. The wind data are used to investigate the role of Reynolds shear stresses and quadrant
analysis techniques for identifying burst-sweep events in relation to sand transport events. This
includes an assessment of the issues involved with data rotations for yaw, pitch, and roll corrections
relative to complex flow streamlines, and the subsequently derived turbulence parameters based on
fluctuating vector components (u‟, v‟, w‟).
Results illustrate how transport may exist under threshold mean velocities because of the role
played by coherent flow structures, and the findings corroborate previous findings that shear
velocity obtained using traditional wind profile approaches does not correlate with transport as
additional stresses are generated due to turbulent structures.
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How old can we go? Investigating the maximum limit of optically
stimulated luminescence dating
M.S. Chapot,1*
H.M. Roberts,1 G.A.T. Duller,
1 and Z.P. Lai
2
1Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, SY23 3DB, U.K.
2Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, XiNing, China
*Corresponding author; Email: [email protected]; Telephone: 01970 622606; Fax: 01970 622659
Optically stimulated luminescence (OSL) dating is an absolute chronometric technique that assesses
the time since quartz or feldspar mineral grains were last exposed to sunlight (Aitken 1998). Unlike other
radiometric dating techniques, the maximum limit of OSL dating is not defined by a fixed upper age limit,
but rather by the maximum radiation dose which the sample can accurately record before the OSL signal
saturates. The maximum age limit of OSL dating is therefore variable and sample dependent. Defining this
variable maximum age limit can be problematic for older samples as some studies suggest that laboratory
induced OSL signals become unreliable at high doses. The problem of determining a reliable maximum
upper limit for OSL dating is exacerbated in such old samples due to the frequent lack of independent age
control. For this reason, a pragmatic upper limit to luminescence ages was proposed by Wintle and Murray
(2006), beyond which uncertainties become large and asymmetric and ages are said to be potentially
unreliable. However, an increasing number of studies are reporting OSL signals that do not conform with the
assumptions necessary for determining this pragmatic limit, and the physical mechanism and reliability of
high dose signal growth remains undefined for such samples. In this study we exploit the natural laboratory
offered by the loess deposits of the Chinese Loess Plateau, to address the key question regarding the
threshold of reliability for old OSL ages. Comparisons between measured results and those expected from
correlation of the loess-paleosol stratigraphy with marine isotope stages warrant caution in dating old
samples, even if the OSL signals still appear to be growing over time.
References:
Aitken, M.J., 1998. An introduction to optical dating. Oxford University Press, Oxford.
Wintle, A.G., Murray, A.S., 2006. A review of quartz optically stimulated luminescence characteristics and their
relevance in single-aliquot regeneration dating protocols. Radiat Meas 41, 369-391.
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Assessing the Role of Paleo-Hydrology in Saharan Dust Production
N. A. Drake, 1*
& K. Schepanski, 2
1. Department of Geography, King’s College, London, UK
2. School of Earth and Environment, University of Leeds, Leeds,UK
*Corresponding author: e-mail: [email protected]
The Sahara Desert is the world‟s largest source of dust yet the nature of the different dust producing regions
within it are poorly understood. We have evaluated the role of the Saharan paleo-hydrology and wind regime
in explaining dust sources by determining the wind speed and paleo-hydrology of each source. This was
achieved using the dust source map derived by Schepanski et al., (2007) from interpretation of sources in
false colour composite (FFC) images of Meteosat Second Generation (MSG) for March 2006 to February
2007. These dust sources are compared to a map of Saharan paleo-hydrology derived from interpretation of
the SRTM DEM and Landsat TM imagery (Drake et al., 2011). The SRTM3 DEM was used to identify
palaeo-river channels, closed basins that would have been likely places for lacustrine sedimentation, palaeo-
lake shorelines and spillways. Interpretation of Landsat TM FCC imagery provided further information on
palaeo-river systems. It was particularly useful for mapping lake sediment outcrops, as the resulting
limestones and gypsum-rich sediments are readily discriminated from other materials in Landsat TM FCC
imagery. The sources are also compared to wind speed calculated as a 2-year mean on a 1 degree x 1 degree
grid employing 6-hourly (00, 06,12,18 UTC) wind data from the ERA interim climatology.
The analysis showed that the paleo-hydrology can explain the vast majority of sources, though the wind also
plays an important role. Paleo-lakes are the most common and most prodigious sources (58% of sources) but
alluvial sources are also important (42%), and there is a verity of them. Some alluvial sources are found
when rivers exit the mountains and they become unconfined, depositing sediments and forming fans (e.g.
Atbara Fan, Egypt) or braid plains (e.g. Serir Tibesti, Libya) that are susceptible to deflation during arid
periods. These landforms are common in the Sahara and can be prodigious sources of dust, particularly when
they are found in areas with high wind speeds (e.g. the southern flanks of Tibesti Mountains, Chad). Some
rivers exit the mountains directly into other large rivers systems nearby. Under these conditions fans do not
form but the rivers and their flood planes are important sources of dust (e.g. western Air Mountains, Niger).
Surprisingly, regions with a high drainage density rarely form important sources. High drainage densities are
most commonly found in the mountainous headwaters of rivers. These areas are usually not sources because
Saharan mountains generally experience low wind speeds (e.g. Atlas, Hoggar and Tibesti Mountains). There
are one or two exceptions when the wind speed in the mountains is high (e.g. Jabal Umm Zeletta, Sudan) and
these regions are important dust sources, presumably because of deflation from their numerous rivers. The
poor connectivity of these arid river systems may promote dust production. Under humid conditions fine
sediments are transported by water for great distances and most is removed from the mountains. In contrast
arid river networks are not active very often, but when they are this activity is usually restricted to parts of
the networks and sediments are not transported very far before being re-deposited when the river dries up
because of high infiltration losses and deposits its load. This process may produce fines that are available for
deflation in places that have experienced recent fluvial activity.
This research shows the importance of paleo-hydrology in controlling dust production; however, the vast
majority of these paleo-hydrological sources are not depicted in maps used to parameterise dust production
in global climate models.
References:
Drake, N.A., Blench, R.M., Armitage, S.J., Bristow, C.S., White K.H. (2011) Ancient watercourses and biogeography
of the Sahara explain the peopling of the desert, Proceedings of the National Academy of Sciences of America, 108,
458-462, doi: 10.1073/pnas.1012231108
Schepanski, K; Tegen, I; Laurent, B; Heinold, B; Macke, A (2007) A new Saharan dust source activation frequency
map derived from MSG-SEVIRI IR-channels, Geophysical Research Letters, 34, doi:10.1029/2007GL030168
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No country for old men: the controls of geomorphology and land use
on dust emission in west Texas and eastern New Mexico
Matthew Baddock1*
, Jeff Lee2, Tom E. Gill
3 and Joseph Mbuh
4
1Formerly, U.S. Department of Agriculture – Agricultural Research Service, Lubbock, TX, USA
2Department of Geosciences, Texas Tech University, Lubbock, TX, USA
3Department of Geological Sciences, University of Texas at El Paso, TX, USA
4Department of Geography, Geology and Planning, Missouri State University, Springfield, MO, USA
*Corresponding author; [email protected]
West Texas and eastern New Mexico is a significant dust source region within North America.
Historically, this area was part of the 1930s Dust Bowl, and although modern land management
practices are far more sensitive to wind erosion, numerous dust days are reported in the area every
year.
This study used MODIS satellite imagery to pin-point the identifiable source points of dust plumes
in the region for 2001-2009. Dust events from 27 days were analysed, producing 625 point sources.
All dust sources were characterized according to their i) geomorphology and ii) type of land
cover/land use. Categories of geomorphology were taken from a new classification scheme based
on the emissivity of different surfaces (Bullard et al., in press).
In assessing the relative contribution of different geomorphology, 79% of sources occurred on sand
sheets which comprise 50% of the region. 3% of sources came from playas (1% area). Alluvial
surfaces contributed 9% of sources from 11% of area. Sand sheets (here, intensely farmed) produce
more sources than other geomorphic categories, with playas producing the most sources per unit
area. In terms of land cover/land use, 57% of sources were associated with cultivated cropland
(33% of area) compared to 17% in shrubland (32% area). With dust emission dominated by a
relatively uniform geomorphology throughout the region, the data highlight the strong
anthropogenic influence on actual distribution of emitting areas.
References:
Bullard, J.E., Harrison, S.P., Baddock, M.C., Drake, N., Gill, T.E., McTainsh, G.H. and Sun, Y. (In press) Preferential
dust sources: a geomorphological classification designed for use in global dust-cycle models. Journal of Geophysical
Research – Earth Surface.
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Palaeodunes in the South American tropics: preliminary luminescence
dating and field observations from the Llanos savannas, Colombia
A.S. Carr1,*
, S.J. Armitage2, A. Boom
1 B. Bilbao
3, J-C Berrio
1, H. Wouters
1
1Department of Geography, University of Leicester, UK
2Department of Geography, Royal Holloway, University of London, UK
3Dept. Estudios Ambientales, Universidad Simón Bolívar, Caracas, Venezuela
*Corresponding author; Email: [email protected] ; Telephone: 0116 2523851; Fax: 0116 2523854
Maps of active Pleistocene dune systems (e.g. Sarnthein, 1978) have often included an area of dunes in
northern South America. This area of tropical savanna, known as the Llanos (“Plains”) Orientales
(Colombia) and Orinocco Llanos (Venezuela) is covered by extensive dune systems. Despite very little study
and virtually no published chronologies these dunes have also been referred to in debates concerning aridity
in the Amazon Basin during the last glacial maximum (e.g. Colinvaux et al., 2000)
There is evidence in the local literature supporting the assumption that at least some of these tropical savanna
dunes are Pleistocene in age. Burial palaeosols in Venezuela have produced radiocarbon ages of 12,370-
16,380 cal yr BP (Roa, 1979), while TL ages published for two dunes in Apure, Venezuela produced ages of
11.6 ± 1.6 and 36 ± 5 ka (Vaz and Miragaya, 1989). Here, we report initial data from a recent field campaign
in the Llanos Orientales, Colombia. The dunes, which become rarer and generally smaller towards the
southwest, are orientated in close alignment with the trade wind system. Some dunes appear closely related
to the fluvial systems that drain from NW to SE across the landscape. Eight dunes, all completely vegetated,
were sampled for reconnaissance OSL dating across a c. 150 km transect at the western limits of the dune
system. The sampled dunes, which were all completely vegetated, ranged from subtle features barely 3 m
above the surrounding landscape, to large features as high as 25 m. Equivalent dose estimates available at
present imply Late Pleistocene and Holocene ages.
References:
Colinvaux, P.A., De Oliveira, P.E., Bush, M.B. 2000. Amazonian and neotropical plant communities on glacial time-
scales: The failure of the aridity and refuge hypotheses. Quaternary Science Reviews 19, 141-169.
Roa, P. 1979. Estudio de los medanos de los Llanos centrales de Venezuela: Evidencias de un clima desértico. Acta
Biológica Venezuelica 10, 19-49
Sarnthein, M. 1978. Sand deserts during the last glacial maximum and climatic optimum. Nature 272, 43-46.
Vaz, J.E., Miragaya, J.G. 1989. Thermoluminescence dating of fossil sand dunes in Apure, Venezuela. Acta Científica
Venezolana 40,
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Dust Observations for Models (DO4): initial field campaign results
G. F. S. Wiggs, 1*
J. King, 1 F. D. Eckardt,
2 R. G. Bryant,
3 D. S. G. Thomas,
1 J. M.
Neild,4 and R. Washington,
1
1*. School of Geography and the Environment, Oxford University Centre for the Environment (Dyson
Perrins Bldg.), University of Oxford, South Parks Road, Oxford, OX1 3QY, United Kingdom
2. Shell Environmental & Geographical Science Building, South Lane, Upper Campus, University of Cape
Town, Private Bag X3, Rondebosch 7701.
3. Sheffield Centre for International Drylands Research, Department of Geography, The University of
Sheffield, Sheffield, S10 2TN, United Kingdom
4. Geography and Environment, University of Southampton
*Corresponding Author: [email protected]
Modelling dust emissions is hampered by a lack of relevant field data. It is well recognised that there are no
observed dust source data that match the scale of climate model grid boxes, whatever their spatial resolution.
Existing data sets which have been used to develop emission scheme are typically at the microscale, are
idealised from wind tunnel experiments and/or pre-date the development (and therefore requirements) of
climate models. The complex task of building a model dust emission scheme is therefore made even more
difficult by having to scale-up data to match model resolution. Thus, significant assumptions regarding
source area characteristics and emissions have been necessary, and in addition dust model emission schemes
are often highly parameterised and tuned to dust loadings distant from source regions. The overall aim of the
Dust Observations for Models (DO4) programme is therefore to collect the first dust source-area process data
tailored to climate model grid-box resolution from targeted remote sensing and fieldwork in order to develop
a new generation of model dust emission schemes. This paper describes initial observation and results from
the on-going field campaign in Makgaikgadi pans in Botswana that aims to provide relevant erosivity,
erodibility and emissions data to improve dust models and provide information on the temporal and spatial
resolution of data required.
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The commonalities of glaciers and salt pans (or travel adventures of a
terrestrial laser scanner)
Joanna M. Nield1,*
1Geography and Environment, University of Southampton
*Corresponding author; Email: [email protected]; Telephone: 023 8059 4749
Terrestrial laser scanning is a useful technique which enables the collection of very high resolution spatial
data relatively quickly. It has great potential and many relevant applications for aeolian geomorphology.
For example, it has the ability to capture unique surface profiles which may help to elucidate relationships
between the shape and arrangement of roughness elements and the shear velocity acting over the surface.
This is particularly important for determining the turbulence flux component of energy balance models for
glacial ablation, and emissivity of potential dust sources, as well as mapping vegetation structure in semiarid
regions (e.g. Jornada, New Mexico). Here I present some preliminary results from recent trips including
those to Iceland and Botswana, investigating the relationship between surface and aerodynamic roughness
both spatially and temporally. Conveniently, the resolution of the instrument also enables small scale change
detection, which can be used for example to quantify surface change due to ice ablation or crust swelling.
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Reconciling the climate record in the Stari Slankamen loess with the
Milankovitch climate change theory
Biljana Basarin1, Ken O‟Hara-Dhand
2*, Slobodan Markovic
1, Ian Smalley
2
1Geography Department, University of Novi Sad, Novi Sad, Voyvodina, Serbia
2Geography Department, Leicester University, Leicester LE1 7RH, UK
*Corresponding author; [email protected]; [email protected]
One of the best recorders of Quaternary climate is the thick loess deposit. Some of the best loess deposits in
Europe are in Voyvodina, north Serbia. Studies are underway at Ruma brickyard, Titel Plateau, and Stari
Slankamen. The long loess section at Stari Slankamen, on the Danube, has yielded excellent results which
can be compared with the climate change determinations made by Milutin Milankovitch on the basis of
changes in the Earth‟s orbital dynamics. Comparisons using Fourier Analysis methods show interesting
relationships. There are some philosophical problems to be discussed but field observations and
mathematical analyses both offer useful data.
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Statistical Commentary on the Shape of Loess Particles
John Howarth MA Cantab, MSc London
Giotto Loess Research Group, Geography Department, Leicester University, Leicester, LE1 7RH
Email: [email protected] Telephone: 01322439075 Fax: 01322439075
Based on the 4 Zingg shape categories (disc, sphere, blade and rod) used by Rogers and Smalley (1993)1,
elementary probabilistic processes have been used to calculate the proportion of particles falling into each
category and the average dimensions for the blade category. It has been postulated that the blade (tabular)
category gives a good approximation to the observed dimensions of the loess particle. Rather than using an
underlying uniform statistical distribution from which to draw each dimension, the corresponding results will
be described from taking an unbounded distribution: viz the exponential distribution. It is argued that this
underlying distribution is more realistic than the uniform and has more physical justification in terms of
defects in the original crystal. In all cases Monte Carlo modelling has been used to support the analytical
calculations.
References:
Rogers C.D.F, Smalley I.J., The shape of loess particles, Naturwissenschaften 1993, 80, 461-462.
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A SCIDR Production – © Dr Robert Bryant2011