Geoscience Reference
In-Depth Information
Figure 14.10
Drainage area to fan gradient relationships for selected Spanish, Greek and American fans, subdivided into
debris-flow and dominantly fluvial fans (data from Harvey, 1992b).
is no significant difference in gradient between fan and
nonfan channels. Only for larger drainage areas are the
fan channels steeper than the nonfan channels.
For fanhead trench channels the width (
W
, metres) rela-
tionship to drainage area can be expressed by the equation
yielding the following results:
807
A
0
.
675
F
=
0
.
(14.6)
(correlation coefficient
=
0.908 and standard error of the
estimate
=
0.208 log units) and
hA
k
W
=
(14.5)
068
A
0
.
249
G
=
0
.
(14.7)
For the three Spanish fan groups, the exponent
k
ranges
from 0.17 and 0.34 and the constant
h
between 6 and 13.
Again there are few comparable American studies, but
Denny (1965) quotes values of 0.5 for exponent
k
and
23.3 for constant
h
. It is clear that American fan channels,
at least in the Death Valley region, are much wider than
the Spanish channels and tend to increase in width down
the fanhead trench whereas the Spanish ones maintain a
near-constant width through the fanhead trenches. One
characteristic that is common on some American fans
and many Spanish fans is the presence of calcrete on
the channel floors (Van Arsdale, 1982; Harvey, 1987),
restricting the widths of these fan channels.
More recent research than much of that quoted above
has used morphometric analysis to focus on how fan mor-
phology reflects interactions between the controlling fac-
tors (e.g. Calvache, Viseras and Fernandez, 1997). For
example, from a modified sample from the Spanish fans,
using 67 fans with a single apex, Harvey (2002a) calcu-
(correlation coefficient
=−
0.693 and standard error of
the estimate
0.173 log units).
In Figure 14.11 plotting positions are identified by fan
style. Different groups of fans tend to cluster either above
or below the regression lines, suggesting the need for
an analysis of the two groups of residuals. This is the
approach first used by Silva
et al
. (1992b) and helped
to identify the influence of tectonic setting on the spatial
geometry of fans in Murcia, Spain (Figure 14.12(a)). It has
subsequently been used by Harvey
et al
. (1999a) on the
Cabo de Gata fans, identifying the effects of confinement,
and of coastal erosion of the fan toes (Figure 14.12(b)) by
Harvey (2005) in Nevada, identifying the effects of base-
level change (see below), and by Al Farraj and Harvey
(2005) in the UAE and Oman, identifying the effects of
source-area geology (Figure 14.12(c)) as well as those of
fan setting and confinement.
Another application of fan morphometry has been to
=
