Geoscience Reference
In-Depth Information
channel form is strongly influenced by vertical differences
in soil strength, caused by a tougher surface or a plough
pan or other resistant layer at depth.
Transport by concentrated overland flow may also be
by bedload-type processes, especially in its early stages,
or where stones are being transported. However, as flow
energy continues to increase, transport of the finer par-
ticles becomes increasingly likely to be in the form of
suspension (Wainwright
et al.
, 2008, 2009, 2010). This
distinction is important as transport is much more effi-
cient in suspension, and as particles travel further be-
fore deposition from the flow, overall rates of trans-
port increase. Concentrated flows in rills have been ob-
served to transport stones over distances of tens of metres
(Poesen, 1987). Although the distance of movement has
been described as a function of excess flow shear stress
(Hassan, Church and Ashworth, 1992), there is some dis-
agreement between observations in unconcentrated and
concentrated flows, suggesting that investigations that
consider the problem in an integrated fashion are required
(Wainwright
et al.
, 2008). Combined with the greater ef-
ficiency of flows at detachment, the overall increase in
sediment flux can be in the region of an order of magni-
tude. The threshold for flow concentration is thus a crit-
ical one for accurate estimates of erosion from a slope
overall.
While many studies consider it useful to define a trans-
port capacity for overland flows (Govers, 1985, 1992a,
1992b; Govers and Rauws, 1986; Abrahams and Atkin-
son, 1993; Li and Abrahams, 1997), this concept can
be considered problematic on a number of levels (Wain-
wright
et al.
, 2008). First, as sediment transport increases,
the nature of the flow changes so that it is no longer
a water flow supporting sediment movement but a non-
Newtonian fluid flow of a water-sediment mix. Second,
theoretical definitions (e.g. Yalin, 1971) have been based
on entrainment by the flow shear force, but their extension
to unconcentrated overland flows (e.g. Ferro, 1998) fail
Figure 11.13
Patterns of erosion scaling as a response to storms of different intensities along a hillslope as simulated by the
Mahleran model of Wainwright
et al
. (2008). Note the thresholds along the slope for the higher-intensity storms as erosion passes
