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mechanism explaining convexities in the upper profile of
slopes in drylands.
As the surface becomes progressively wetter in an
event, rates of detachment will change, particularly for
finer particles as the cohesion of the surface decreases.
Al-Durrah and Bradford (1982) suggest an exponential
relationship of reduced energy required for detachment
as the water content of the surface increases. Splash can
continue after the onset of ponding and in the initial over-
land flows. Exact thresholds of cessation of splash are
debated in the literature, but relate to the changes in rain-
drop detachment as flow depth increases, which will be
considered further in the next section.
latter may be common where particles are initially lifted
up into the flow by raindrop impact. Most unconcentrated
flows are too shallow to produce conditions for transport
in suspension, even for the finest particle sizes (see the
discussion in Wainwright et al. , 2008). Although uncon-
centrated overland flows are more competent at transport-
ing sediment than splash, the overall rate of transport is a
function of the interrelationship between flow energy and
flow depth, which thus is affected by the surface rough-
ness. Distances of travel have been described as a function
that is the product of the inverse of the particle mass, the
flow energy and the raindrop energy (Parsons, Stromberg
and Greener, 1998). Deposition is thus a function of the
ability of the continued energy put into the flow to over-
come surface roughness, and particles are often deposited
in local hollows in the bed (Tatard et al. , 2008) or behind
dams formed by vegetation (usually grasses; see Parsons
et al. , 1997; Cerda, 1997; Boer and Puigdefabregas, 2005).
The effect of vegetation on unconcentrated flows is gen-
erally one of deceleration and diffusion or deposition of
transported sediment.
11.5.2
Unconcentrated overland-flow erosion
Once overland flow starts, it occurs in an unconcentrated
form. Flow in the laminar regime (Re
500) is unable to
entrain sediment in shallow flows (Ellison, 1945; Young
and Wiersma, 1973), so that initially overland flows re-
quire detachment by raindrop action to produce erosion.
As flow depths increase, there is a negative feedback
whereby the energy of the raindrop is reduced so that
less detachment and entrainment is caused. This feedback
has been described as a negative exponential function of
flow depth (Torri, Sfalanga and del Sette, 1987), so that
flow depths of more than about 12 mm effectively reduce
detachment to zero, depending on the particle size of the
surface. Although some studies have suggested that in
thin flows entrainment efficiency increased with rainfall
rate (Kilinc and Richardson, 1973; Kinnell, 1991), pre-
sumably due to acceleration of the drop through the flow,
this observation may be an artefact of the experimental
methods used (see the discussion in Wainwright et al. ,
2009). Once flows enter the transitional regime (500
<
11.5.3
Concentrated overland-flow erosion
Concentration of overland flow is caused by incision into
the surface by turbulent flows (Re > 2000). As noted
above, there is likely to be a transitional regime where
threads of flow develop due to the combination of small
amounts of local flow detachment with existing surface
irregularities. Parsons and Wainwright (2006) discussed
the conditions required for the development of permanent
concentrations as rills as a result of this combination,
which overcomes limitations with existing approaches
based on a critical shear stress (Figure 11.12). For ex-
ample, Rauws and Govers (1988) described a function
where the critical shear stress for incision is significantly
lower than the apparent shear strength of the surface, and
thus theoretically should be insufficient to produce the
erosion causing incision. Incision occurs where the total
shear stress of the flow is (a) high enough to disrupt the
root mat, (b) high enough to incise bare patches of soil be-
tween vegetation, (c) where initially unconcentrated flows
through vegetation are able to evacuate sediment loosened
(detached) by biogenic disruption or (d) where seepage
erosion triggers the initial incision through the vegetated
surface so the bare soil is exploited by saturation overland
flow. Grazing of large herbivores has apparently triggered
concentrated erosion in a number of environments be-
cause it not only contributes to the first three of these
mechanisms but also reduces the shear strength of the soil
<
Re
2000), some entrainment seems to occur through
flow detachment, although the relative amounts seem to
be of the same order of magnitude as to compensate for
the corresponding decline in raindrop detachment (see
the discussion in Wainwright et al. , 2008). The shallow
depth of flow typically means that such entrainment is by
flow shear alone, as there is insufficient development of
a pressure gradient to produce a lift force. Where exfil-
tration forms an important fraction of the overland flow,
the shear force is augmented by a small lift force associ-
ated with the emergence of water from the soil (Kochel,
Howard and MacLane, 1982). Controls on detachment are
thus very similar in unconcentrated overland flows as in
splash, with the addition of the feedback with flow depth.
Transport
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