Geoscience Reference
In-Depth Information
6
Basic Types of Dunes
Nature has produced an amazing variety of shapes in which
sand grains have accumulated to make landforms that are
called 'dunes', a variety that accounts for the enduring
aesthetic appeal and scientific interest in aeolian landforms,
and the wide range of names in different languages (notably
Arabic) for the different forms. Yet there is order in this
variety, which somehow also promotes their beauty, as well
as the scientific tractability of their study. In fact, there are
only a handful of basic forms, which through field study,
then remote sensing, and now laboratory and computation
related to just two controlling factors: the sand supply, and
the variability in the wind direction. This latter factor is
arguably the most significant.
The advent of satellite monitoring of Earth's surface
provided a common medium with which to observe and
document sand dunes (and other features) across the planet,
and it also led to a classification of dune types based on
characteristics that can be derived from remote sensing data.
McKee (1979) presented a general dune classification sys-
tem illustrated in part using Landsat images of various sand
deposits, supplemented by ongoing field studies of various
sand dunes. The McKee system (Fig.
6.1
) has proved to be a
useful way to distinguish dune types across the Earth, as well
as now on other planetary surfaces. The classification
scheme is based on the overall shape and outline (planform)
of the sand deposit along with the number of slip faces
present on the dune, all attributes that can be determined
from field investigations, satellite images and aerial photo-
graphs. A more quantitative classification of the major types
(barchan, transverse, longitudinal and star) was made as a
function of equivalent sand thickness and directional vari-
Most planetary dunes can be considered examples of
these basic types which are a direct mapping between the
wind and sand supply regime and the resultant duneform.
This simple relationship relies on the controlling factors
being constant in time. We also show examples in this
section where varying conditions have led to horizontal
variations in dune form.
We defer to a subsequent section discussion of where
different dune forms are superposed on each other (com-
pound or complex dunes), as well as some additional types
of sand deposits (e.g., sheets) and dunes associated with
topographic obstacles and vegetation. There is extensive
and useful discussion about dune morphology in several
texts: Bagnold, as ever, is worth reading, as is (while also
somewhat dated), Cooke and Warren (1973). An excellent
more modern review is Lancaster (1995). On vegetated and
minor dune types, Pye and Tsoar is useful.
An important property in relating morphology to wind
regime is the quantitative variability in wind direction (the
ratio of resultant drift potential to total drift potential, which
gross bedform normal transport (due to Rubin and Hunter
1987),
roughly
that
the
bedform
will
arrange
itself
to
maximize the transport across it.
6.1
Dome
We begin with the simple dome (Fig.
6.2
). This is a dune
form that results from the simple accumulation of sparse
sand on a flat bed, and can be thought of as a growing
disturbance in the surface that results from the saturation
likely to accumulate on sand than on the interdune pan where
saltating sand grains may bounce more efficiently. Dome
dunes do not grow to be large, as once they reach a height of
some 10 s of cm on Earth, they develop slip faces (see
Chap. 5
) and become barchans—in essence, a dome can be
thought of as a slipfaceless barchan. Because dome dunes
ephemeral,
appearing
and
disappearing
in
response
to
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