Geoscience Reference
In-Depth Information
19
Desert dune processes and dynamics
Nick Lancaster
19.1
Introduction
range of height, width and spacing. Aerial photographs
and satellite images show the general regularity of dune
patterns (manifested by close correlations between dune
height, width and spacing) and also indicate that essen-
tially similar dune forms occur in widely separated local-
ities. Even dunes on other planetry bodies (e.g. Mars,
Titan) have a simliar morphology to terrestrial dunes.
These commonalities indicate that there are general phys-
ical principles that govern dune form and dynamics.
Dunes are created and modified by interactions be-
tween sand transport rates, dune topography and airflow
(Figure 19.2). As the dune projects into the atmospheric
boundary layer, the primary air flow is modified by in-
teractions between the dune form and the airflow, which
give rise to modifications of the local wind speed, shear
stress and turbulence intensity, and therefore sand trans-
port rates. Such interactions also create secondary flow
circulations, especially in the lee of the dune, as a result of
flow separation and flow diversion. In addition, superim-
posed dunes on megadunes or draa respond to changes in
airflow and sediment transport on the megadune itself. The
nature of these interactions is discussed in detail below.
Desert sand dunes form part of a self-organised hierar-
chical system of aeolian bedforms, which comprises: (a)
wind ripples, (b) individual simple dunes or superimposed
dunes on mega dunes and (c) mega dunes (also called
draa or compound and complex dunes) - characterised by
superimposition of simple or elemental dunes on larger
forms. The majority of dunes are composed of quartz and
feldspar grains of sand size, although dunes composed of
gypsum, carbonate and volcanic sand also occur. Most
sand dunes occur in contiguous areas of dunes known as
sand seas or ergs (with an area of
100 km 2 ). Smaller
areas of dunes are called dune fields (see Chapter 17).
Dunes occur in self-organised patterns that develop over
time as the response of sand surfaces to the wind regime
(especially its directional variability) and the supply of
sand (Werner, 1995). The dune types discussed below
represent the steady state attractors of the aeolian sand
transport system and can evolve from a wide range of
initial conditions. Key variables that determine dune mor-
phology and dynamics are the spatial and temporal charac-
teristics of: (1) the wind regime (especially its directional
variability), (2) sand supply (the amount of sand available
for dune building) and (3) vegetation cover. Sand particle
size does not appear to be as important as once thought.
As dunes and dune patterns evolve over time, the legacy
of past climates and wind regimes influences present-day
dune morphology and dune patterns in many areas.
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19.3
Dune types and environments
19.3.1
Crescentic dunes
In wind regimes characterised by a narrow range of wind
directions (unidirectional wind regimes), crescentic dunes
form with their crests aligned normal to the direction of
sand transport.
Where sand supply is low relative to the capacity of
the wind to transport sand (undersaturated sand transport
conditions), isolated crescentic dunes or barchans form
19.2
Desert dune morphology
Desert dunes occur in a variety of morphologic types (see
Box 19.1 and Figure 19.1), each of which displays a
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