Desert dune processes and dynamics - Arid Zone Geomorphology - page 499

multidirectional winds, linear dunes in areas of bimodal
winds and crescentic dunes where winds are unimodal
(Figure 19.19). Such paradigms were used to explain the
distribution of dune types in the Namib Sand Sea (Lan-
caster, 1983) (Figure 19.21) and elsewhere (Breed and
Grow, 1979). Closer examination of dune patterns re-
veals sharp transitions in dune type, sediment compo-
sition, dune spacing and dune trend, which cannot be
attributed to changes in wind regime (Lancaster, 1999).
These patterns are better explained by the concept of dune
generations, in which dune fields and sand seas are com-
prised of multiple genetically distinct groups of dunes,
each formed in a different set of initial conditions of wind
regime and sediment configuration (Kocurek and Ewing,
2005; Lancaster, 1999).
Dune generations can be recognised by differences in
dune morphology, which are manifested by statistically
distinct populations of dunes with a different crest orien-
tation, spacing, length and defect density (Beveridge et al. ,
2006; Derickson et al. , 2008), by differences in sediment
composition and colour, by chronometric and relative age
of different elements of a dune pattern and by geomorphic
relations between dunes of different characteristics, e.g.
crossing patterns (Lancaster et al. , 2002) (see Box 19.2).
Because only one set of dunes can form at a time in a
given set of boundary conditions, it follows that dune
fields with simple patterns of dune morphology most
likely represent relatively young systems, whereas com-
plex patterns are the product of multiple periods of dune
construction. Such information can be used to infer the
age of a dune field as well as the degree to which it has
been subject to changes in the factors that control its de-
velopment and current dynamics.
Within a group of dunes, patterns of dune size and
spacing and the parameters of dunefield organisation vary
systematically. In areas of crescentic dunes, such as at
White Sands dune field, dunes become larger and more
widely spaced in the direction of migration away from
the linear source area (Figure 19.21). Dunes close to the
source of sand are smaller, migrate more rapidly and have
a high defect density. Downwind, these dunes interact with
each other to produce a self-organisation of the pattern in
which the dunes are larger, with longer crestlines and
fewer defects (Ewing and Kocurek, 2010b). Similar pat-
terns of dune pattern evolution have been noted in barchan
dune corridors (Elbelrhiti, Andreotti and Claudin, 2008).
In areas of linear dunes, such as the Simpson-Strzelecki
Deserts of Australia, dune spacing is apparently related
to substrate type, reflecting sediment supply from lo-
cal sources (Fitzsimmons, 2007; Mabbutt and Wooding,
1983; Wasson et al. , 1988). Within this pattern, dune spac-

creases as dunes become more widely spaced and the pat-
tern becomes more organised away from sediment source
areas (Fitzsimmons, 2007; Mabbutt and Wooding, 1983).
By contrast, in the southwestern Kalahari dune field, the
regional scale pattern of linear dunes becomes more com-
plex downwind and also from southwest to northeast
(Bullard et al. , 1995). These patterns probably reflect de-
creased aridity to the south and east of the dune field,
as well as local sources of sediment in river valleys and
pans. At a local scale, however, self-organisation of dunes
downwind of river valleys and pans is expressed by joining
of smaller dunes in Y-junctions to create a more organised
pattern (Bullard and Nash, 1998).

19.9 Conclusions

Significant advances in understanding of the dynamics
of desert dunes and the factors that determine their mor-
phology have occurred in the past three decades. This has
come about as a result of several intersecting method-
ological and conceptual advances, which include, but are
not limited to, the application of remote sensing im-
agery to understand dune patterns, field studies of key
dune-forming processes, OSL dating of periods of dune
formation and GPR investigations of dune sedimentary
structures. A rapidly evolving new development is the
numerical modelling of dunes and dune systems, facil-
itated by vastly increased computing power, as well as
better understanding of fundamental dune processes.
As a result, there is now a general understanding of the
processes that form or at least maintain most major dune
types. The importance of scale effects is recognised, so
that the processes that form simple dunes need to be con-
sidered at different temporal and spatial scales from those
that form compound and complex dunes and sand seas.
The importance of past conditions of sediment supply,
availability and mobility, determined by climatic and sea
level changes, in the formation of mega dunes and sand
seas and dune fields is now well recognised.

References

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