Page 91 - Fundamentals of Geomorphology

74 INTRODUCING LANDFORMS AND LANDSCAPES

Clay Silt Sand Coarse material
10

Erosion
Erosion
1
(m/s) Entrainmen
velocity 0.1 velocity y
velocit
Entrainment t
flow
Mean Transport Deposition
ransport
T
Deposition
0.01
Fa l l
Fall
velocity
velocity
0.001
0.001 0.01 0.1 1 10 100 1,000
Particle size (mm)

Figure 3.11 The Hjulstrøm diagram showing the water velocity at which entrainment and deposition occur for
particles of a given size in well-sorted sediments.
Source: Adapted from Hjulstrøm (1935)

hollow is ground out that may eventually deepen to surface (Figure 3.12). Horton proposed that a ‘belt of no
produce a pothole. erosion’ is present on the upper part of slopes because
here the flow depth is not sufficient to cause erosion.
Channel initiation However, subsequent work has demonstrated that some
surface wash is possible even on slope crests, although
Stream channels can be created on a newly exposed sur- here it does not lead to rill development because the
face or develop by the expansion of an existing channel rate of incision is slow and incipient rills are filled by
network. Their formation depends upon water flow- rainsplash.
ing over a slope becoming sufficiently concentrated for Further studies have demonstrated that a range of
channel incision to occur. Once formed, a channel may relationships between channel network properties and
grow to form a permanent feature. topography exist, although the physical processes driv-
Robert E. Horton (1945) was the first to formalize ing these are not as well understood. In semi-arid and
the importance of topography to hillslope hydrology by arid environments, the Hortonian overland-flow model
proposing that a critical hillslope length was required provides a reasonable framework for explaining channel
to generate a channel (cf. p. 66). The critical length was initiation, but it does not for humid regions. In humid
identified as that required to generate a boundary shear regions, channel initiation is more related to the location
stress of Hortonian overland flow sufficient to overcome of surface and subsurface flow convergence, usually in
the surface resistance and result in scour. In Horton’s slope concavities and adjacent to existing drainage lines,
model, before overland flow is able to erode the soil, than to a critical distance of overland flow. Rills can
it has to reach a critical depth at which the eroding develop as a result of a sudden outburst of subsurface
stress of the flow exceeds the shear resistance of the soil flow at the surface close to the base of a slope. So channel