364
Climatic Geomorphology
The fan shape is semicircular, or a cone segment, with concave longitudinal profiles
(Figure 15.19) and convex traverse ones. Data extracted from detailed topographical maps
allow development of the following mathematical relationships (Troeh, 1965):
Z = P + SR + LR 2
where Z is the height of a particular point on the fan surface, P the height of the fan apex, S
the fan slope in point P, R the radial distance from P to Z, and L the average slope along its
longitudinal profile.
The fan area is the more studied variable and it has been demonstrated that there is a
simple relationship between fan area and drainage basin area that may by expressed by the
equation:
Af - pA~
Where Af and A b are fan area and drainage area (measured in km2), respectively. Studies of
different groups of fans have demonstrated similar values for the exponent q in different
zones ranging from 0.7 to 1. l, but the values for p show a wider range between 0.1 and 2.1.
These variations are interpreted as a consequence of differing fan age and history between
different regions, but also to the different rock resistance within the drainage basins
(Harvey, 1997). Figure 15.21 displays the relationships among the different morphometric
fan variables for different areas. Fans of the California Coastal Ranges have larger surface
area per unit drainage area than fans in the Death Valley region. The regression line for the
fans Spain of southeast has been obtained from 68 fans (Harvey, 1987a).
The fan gradient is usually taken as the axial fan surface slope in the upper part of the
fan and the most characteristic values range from 2 to 12 ~ Its relation to the fan is:
Gf = aAbc
Where Gf is the fan gradient. Figure 15.19 illustrates that the values of the exponent b
range between -0.35 and -0.15, but values for the constant a show a greater range from
0.03 to 0.17, which is interpreted as linked to different sedimentary processes. Fans in
the Death Valley region are much steeper than the California Coastal Ranges, whereas
the SE Spain fans show an intermediate position. On the other hand, in the SE Spain fan
group of southeast Spain, those dominated by debris flows have steeper slopes than those
dominated by fluvial and/or sheet-flow processes (Figure 15.22), which reveals the
relevant impact of sedimentary processes on fan gradient (Harvey, 1984a).
2.3. Factors influencing alluvial-fan development
Fan deposition takes place if the sediment supply overcomes the transport capacity of the
feeder channel. This indicates that within the channel the critical stream power threshold
defined by Bull (1979) is surpassed. However, fan sedimentation is also affected by
different factors, the impact of which is variable depending on the different climatic and
topographic scenarios.
Topographic factors have a strong influence on sediment supply. In that way erosion
rates tend to be more important on steep slopes, favouring in addition the generation
of debris flows. These predominate in fans supplied by small steep drainage basins.
Climatic Geomorphology
The fan shape is semicircular, or a cone segment, with concave longitudinal profiles
(Figure 15.19) and convex traverse ones. Data extracted from detailed topographical maps
allow development of the following mathematical relationships (Troeh, 1965):
Z = P + SR + LR 2
where Z is the height of a particular point on the fan surface, P the height of the fan apex, S
the fan slope in point P, R the radial distance from P to Z, and L the average slope along its
longitudinal profile.
The fan area is the more studied variable and it has been demonstrated that there is a
simple relationship between fan area and drainage basin area that may by expressed by the
equation:
Af - pA~
Where Af and A b are fan area and drainage area (measured in km2), respectively. Studies of
different groups of fans have demonstrated similar values for the exponent q in different
zones ranging from 0.7 to 1. l, but the values for p show a wider range between 0.1 and 2.1.
These variations are interpreted as a consequence of differing fan age and history between
different regions, but also to the different rock resistance within the drainage basins
(Harvey, 1997). Figure 15.21 displays the relationships among the different morphometric
fan variables for different areas. Fans of the California Coastal Ranges have larger surface
area per unit drainage area than fans in the Death Valley region. The regression line for the
fans Spain of southeast has been obtained from 68 fans (Harvey, 1987a).
The fan gradient is usually taken as the axial fan surface slope in the upper part of the
fan and the most characteristic values range from 2 to 12 ~ Its relation to the fan is:
Gf = aAbc
Where Gf is the fan gradient. Figure 15.19 illustrates that the values of the exponent b
range between -0.35 and -0.15, but values for the constant a show a greater range from
0.03 to 0.17, which is interpreted as linked to different sedimentary processes. Fans in
the Death Valley region are much steeper than the California Coastal Ranges, whereas
the SE Spain fans show an intermediate position. On the other hand, in the SE Spain fan
group of southeast Spain, those dominated by debris flows have steeper slopes than those
dominated by fluvial and/or sheet-flow processes (Figure 15.22), which reveals the
relevant impact of sedimentary processes on fan gradient (Harvey, 1984a).
2.3. Factors influencing alluvial-fan development
Fan deposition takes place if the sediment supply overcomes the transport capacity of the
feeder channel. This indicates that within the channel the critical stream power threshold
defined by Bull (1979) is surpassed. However, fan sedimentation is also affected by
different factors, the impact of which is variable depending on the different climatic and
topographic scenarios.
Topographic factors have a strong influence on sediment supply. In that way erosion
rates tend to be more important on steep slopes, favouring in addition the generation
of debris flows. These predominate in fans supplied by small steep drainage basins.