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wards a convex form similar to that of transverse dunes
(Tsoar
et al.
, 1985), and erosion and deposition rates near
the crest decline in time as the dune comes into equilib-
rium with a new wind direction.
Erosion and deposition patterns on linear, reversing
and star dunes show that they consist of a crestal area
where erosion and deposition rates are high and a plinth
zone in which there is little surface change but consid-
erable throughput of sand (Figure 19.14). The crest lines
of Namibian complex linear dunes migrate over a lateral
distance of as much as 14 m over a 12-month period but
show little net change over decadal timescales (Living-
stone, 1989, 1993, 2003; Sharp, 1966; Wiggs
et al.
, 1995).
19.5.2
Long-term dune dynamics
Insights into dune dynamics over annual to decadal
timescales can be gained from a variety of sources. A
large data set on migration rates of barchans and crescen-
tic dunes has been developed by comparing the position
of dunes on time series of aerial photographs or satellite
images, demonstrating that there is an inverse relationship
between barchan dune height and migration rate (Figure
19.15). This relationship is important to the development
of self-organised dune patterns in that more rapidly mov-
ing smaller dunes tend to catch up and merge with larger
Figure 19.12
Windward slope erosion and deposition pat-
terns (data from Lancaster
et al.
, 1996, and McKenna Neuman,
Lancaster and Nickling, 1997).
(Lancaster, 1989b, 1989c). This is the result of winds en-
countering a dune form that is out of equilibrium with a
new wind direction. Field observations and models show
that the crestal profiles of linear and star dunes tend to-
1.00
6 m/sec (lab)
5.8 m/sec (field)
8 m/sec (lab)
8.1 m/sec (field)
0.80
0.60
0.40
0.20
0.00
0.00
0.20
0.40
0.60
0.80
1.00
x/L
