Geoscience Reference
In-Depth Information
Figure 14.8
Size and gradient of alluvial fans. (a) Musandan mountain front, Wadi Al-Bih, Oman. Note the steep angle debris-flow
cone behind the lower-angle fluvially dominant fan in the centre of the photo. The main wadi cuts the toe of the fluvially dominant
fan in the foreground. (b) Mountain-front fans, Panamint Valley, California. Note the triangular slope facets indicating a faulted
mountain front. Note also the general positive relationship beween catchment size and fan area and the inverse relationship with
the fan gradient.
rivers (Blair and McPherson, 1994b), defined as the
'slope gap'. While there is undoubtedly a difference in
threshold depositional gradient, the concept of a specific
slope gap is flawed. Blair and McPherson compared the
moderate-sized Death Valley fans with larger rivers. Saito
and Oguchi (2005) have demonstrated that fan gradients
do, in fact, span the slope gap and Harvey (2002a) has
demonstrated that gradients on small rivers also span
the gap.
In an attempt to improve on the general drainage area
to fan gradient regression, Harvey (1987), on a sample of
77 Spanish fans, used multiple regression, also taking into
account drainage basin relief characteristics. The overall
simple regression was
(correlation coefficient
=−
0.64 and standard error of the
estimate
0.170 log units).
Subdividing the data into three groups (shown as three
separate lines in Figure 14.9: lower), in two of the three
cases correlations improve and standard errors reduce
marginally. When multiple regression is used the corre-
lations improve markedly to between 0.74 and 0.89, and
standard errors are reduced further (see Harvey, 1987,
1990, for details).
Morphometric analysis has also be used to exam-
ine other fan properties, particularly those of fan chan-
nels (Harvey, 1987, 2002a). For channels within fanhead
trenches, the channel gradient relationship to drainage
area can be expressed by the equation
=
