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14.3.1.3
Fan morphometry
fans, except in exceptional circumstances, they can be
regarded as passive factors. The dynamic factors include
ongoing tectonic activity, climate and base-level condi-
tions (Harvey, Mather and Stokes, 2005).
A common approach to the analysis of fan morphology
has been to consider the morphometric relationships be-
tween catchment controls, especially drainage area, and
fan properties, especially fan area and fan gradient, using
regression analysis (Bull, 1962b; Denny, 1965; Hooke,
1968; Harvey, 1987, 1990, 1992b, 2002a). The results
have usually been expressed in the form below (after
Harvey, 1987):
14.3.1 Passive factors: influence
on fan morphology
14.3.1.1
Setting
pA q
F
=
(14.1)
The setting of the fan (e.g. mountain front, tributary junc-
tion, etc., see Section 14.1.1) is controlled by long-term
tectonics (see also below for a discussion of relations be-
tween passive and active tectonic control), geological and
geomorphological history, which, in turn control the avail-
able accommodation space (Viseras et al ., 2003) and the
degree of confinement offered by the surrounding bedrock
topography (Sorriso-Valvo, Antronico and Le Pera, 1998;
Al Farraj and Harvey, 2005).
aA b
G
=
(14.2)
where A is the drainage area (km 2 ), F is the fan area (km 2 )
and G is the fan gradient (dimensionless). Figure 14.9
summarises these regression relationships from a selec-
tion of earlier studies.
For the fan area relationship, from a whole series of
studies of dry-region fans there is a clear positive correla-
tion between the drainage area and fan size. The exponent
q shows very little variation between c. 0.7 and c. 1.1, but
values for constant p show a wide range of between c. 0.1
and c. 2.1, reflecting regional differences in fan age and
history as well as in catchment geology (Hooke, 1968;
Hooke and Rohrer, 1977; Lecce, 1991) and fan setting,
especially confinement (see below).
For the fan gradient relationship, again from a whole
series of studies (Blissenbach, 1952; Bull, 1961, 1962b;
Harvey, 1987, 1990, 2002a, 2002b), values for the ex-
ponent b show a limited range between
14.3.1.2 Catchment controls
The geology of the catchment area, especially rock resis-
tance to erosion, may affect the volume and nature of the
sediment delivered to the fan, which may in turn influence
the processes of sediment delivery (large amounts of fine
sediment would be conducive to debris flows) and the vol-
ume of sediment delivered. This, in turn, affects the fan
aggradation rate and the fan size (Hooke and Rohrer, 1977;
Lecce, 1991). Most important are the topographic charac-
teristics of the catchment, the drainage network and partic-
ularly the catchment size and relief characteristics. Larger
catchments produce higher flood discharges, and with the
greater potential for within-catchment sediment storage
(Harvey, 2010) are likely to have higher water-to-sediment
ratios during floods than smaller catchments. Smaller
and especially steeper catchments are likely to produce
higher sediment concentrations, coarser sediments and
more debris flows. The combined effects of catchment
size and relief on sediment delivery to the fan system are
well known (Harvey, 1984a, 1992a; Kostaschuk, Mac-
donald and Putnam, 1986; Wells and Harvey, 1987),
whereby larger and less steep catchments generate flu-
vially dominated fans and smaller steeper catchments
generate debris cones and debris-flow dominated fans.
These relationships are expressed in fan morphology,
where debris-flow fans are in general steeper than fluvially
dominated fans (Figure 14.8(a)) and there is a general
positive relationship between drainage area and fan area
0.15 and
0.35, but values for the constant a show a much
greater range from c. 0.03 to c. 0.17, probably re-
flecting different sedimentary processes, with debris-
flow fans being much steeper than fluvially dominant
fans.
The fan gradient relationship is important because dif-
ferent depositional processes have different threshold de-
positional gradients (see above, Section 14.2.1). This can
be illustrated by data from fans from a number of regions
(Figure 14.10), where the fans have been classified on the
basis of either surface morphology or deposits exposed
in fanhead trenches into debris-flow or fluvially domi-
nant (Harvey, 1992b). For these areas debris-flow fans are
generated only from catchments smaller than 5 km 2 .For
every group, fan gradients for debris-flow fans are almost
an order of magnitude steeper than those for fluvial fans
draining similar drainage areas.
This difference in depositional gradient between de-
bris flows and fluvial sediments has been used to ac-
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