Geoscience Reference
In-Depth Information
Box 19.1
Dune classifications
Dunes
Mega-dunes
Relative Sediment Thickness
Many different classifications of types of dunes have
been proposed: (1) those based on the external morphol-
ogy of the dunes (morphological classifications) and
(2) those that imply some relationship of dune type to
formative winds or sediment supply (dynamic classifi-
cations). Morphologic classifications of dunes facilitate
mapping of dune types from satellite image data (e.g.
Breed et al. , 1979) with no assumptions regarding for-
mative mechanisms and relations to external variables.
Dynamic classifications of dunes imply some causal re-
lationship between external variables and dune type - so
that dunes may be identified as transverse, longitudinal
or oblique to the formative winds (Hunter, Richmond
and Alpha, 1983). Even though understanding of the
conditions in which different types of dunes form has
improved significantly in recent years, this knowledge
remains imperfect, so that a largely morphological clas-
sification scheme is preferred. Figure 19.1 provides a
framework for classifying dunes based on external mor-
phology, sediment volume and other key parameters.
CRESCENTIC
Barchans
Crescentic
ridges
Compound
crescentic
dunes
Dome dunes
LINEAR
Simple
- Vegetated
- Sinuous
Compound
Linear dunes
Complex
Linear dunes
REVERSING
STAR
coarse
sand
SAND
SHEETS
ZIBARS
NEBKHAS
BLOW
OUT
DUNES
PARABOLIC
DUNES
vegetation
anchored
source
bordering
LUNETTES
topographic
influence
FALLING
DUNES
CLIMBING
DUNES
ECHO
DUNES
Figure 19.1 A scheme for the classification of desert dunes
(modified from Lancaster, 1995).
(Figure 19.3). Barchans have been described from most
desert regions and have been extensively studied in the
field, using aerial and satellite images, and most recently
via numerical models. Conditions favouring the forma-
tion of barchans are encountered on the margins of sand
seas and dune fields as well as in the transport corridors
extending from sand source areas (Figure 19.3(b)), where
a clear size selection process involves merging smaller,
faster moving dunes with slower moving, larger dunes
so that the pattern becomes more uniform. In plan view,
Dune morphology
erosion and deposition
patterns
wind regime
form - flow
interactions
slope effects
Sand transport rates
Air flow
momentum
 
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