Geology Reference
In-Depth Information
place in the body of the regolith and along subsurface
lines of concentrated water flow, including throughflow
in percolines and pipes.
stems; and, in forests with a thick litter layer, overland
flow occurs under decaying leaves and twig. The efficacy
of sheet wash in transporting material is evident in the
accumulation of fine sediment upslope of hedges at the
bottom of cultivated fields.
Rainsplash
Rainsplash
and
sheet wash
are common in arid environ-
ments and associated with the generation of Hortonian
overland flow (p. 66). There is a continuum from rain-
splash, through rainflow, to sheet wash. Falling raindrops
dislodge sediment to form 'splash', which moves in all
directions through the air resulting in a net downslope
transport of material. Experimental studies using a sand
trough and simulated rainfall showed that on a 5
◦
slope
about 60 per cent of the sediment moved by raindrop
impact moves downslope and 40 per cent upslope; on a
25
◦
slope 95 per cent of the sediment moved downslope
(Mosley 1973). Smaller particles are more susceptible
to rainsplash than larger ones. The amount of splash
depends upon many factors, including rainfall proper-
ties (e.g. drop size and velocity, drop circumference, drop
momentum, kinetic energy, and rainfall intensity) and
such landscape characteristics as slope angle and vege-
tation cover (see Salles
et al
. 2000).
Rain power
is a
mathematical expression that unites rainfall, hillslope,
and vegetation characteristics, and that allows for the
modulation by flow depth (Gabet and Dunne 2003). It is
a good predictor of the detachment rate of fine-grained
particles.
Through-wash (suffossion)
In well-vegetated regions, the bulk of falling rain passes
into the soil and moves to the water table or moves under-
neath the hillslope surface as throughflow. Throughflow
carries sediment in solution and in suspension. This
process is variously called
through-wash
,
internal ero-
sion
, and
suffossion
, which means a digging under or
undermining (Chapuis 1992). Suspended particles and
colloids transported this way will be about ten times
smaller than the grains they pass through, and through-
wash is important only in washing silt and clay out of
clean sands, and in washing clays through cracks and
roots holes. For instance, in the Northaw Great Wood,
Hertfordshire, England, field evidence suggests that silt
and clay have moved downslope through Pebble Gravel,
owing to through-wash (Huggett 1976). Where through-
flow returns to the surface at seeps, positive pore pressures
may develop that grow large enough to cause material
to become detached and removed. Throughflow may
occur along percolines. It may also form pipes in the
soil, which form gullies if they should collapse, perhaps
during a heavy rainstorm.
Rainflow
Creep and dry ravel
Rainflow
is transport caused by the traction of overland
flow combined with detachment by raindrop impact,
which carries them further than rainsplash alone.
Sheet
wash
carries sediment in a thin layer of water running
over the soil surface (p. 66). This is not normally a
uniformly thick layer of water moving downslope; rather,
the sheet subdivides and follows many flow paths dictated
by the microtopography of the surface. Sheet wash results
from overland flow. On smooth rock and soil surfaces,
a continuous sheet of water carries sediment downslope.
On slightly rougher terrain, a set of small rivulets link
water-filled depressions and bear sediment. On grassed
slopes, sediment-bearing threads of water pass around
Soil creep
(p. 66) is common under humid and tem-
perate climates. It occurs mainly in environments with
seasonal changes in moisture and soil temperature. It
mainly depends upon heaving and settling movements in
the soils occasioned by biogenic mechanisms (burrowing
animals, tree throw, and so on), solution, freeze-thaw
cycles, warming-cooling cycles, wetting-drying cycles,
and, in some hillslopes, the shrinking and swelling of
clays and the filling of desiccation cracks from up-
slope.
Dry ravel
is the rolling, bouncing, and sliding
of individual particles down a slope (Gabet 2003). It
is a dominant hillslope sediment-transport process in
steep arid and semiarid landscapes, and includes the
