Page 189 - Fundamentals of Geomorphology

172 PROCESS AND FORM

place in the body of the regolith and along subsurface stems; and, in forests with a thick litter layer, overland
lines of concentrated water flow, including throughflow flow occurs under decaying leaves and twig. The efficacy
in percolines and pipes. of sheet wash in transporting material is evident in the
accumulation of fine sediment upslope of hedges at the
Rainsplash bottom of cultivated fields.

Rainsplash and sheet wash are common in arid environ- Through-wash (suffossion)
ments and associated with the generation of Hortonian
overland flow (p. 66). There is a continuum from rain- In well-vegetated regions, the bulk of falling rain passes
splash, through rainflow, to sheet wash. Falling raindrops into the soil and moves to the water table or moves under-
dislodge sediment to form ‘splash’, which moves in all neath the hillslope surface as throughflow. Throughflow
directions through the air resulting in a net downslope carries sediment in solution and in suspension. This
transport of material. Experimental studies using a sand process is variously called through-wash, internal ero-
trough and simulated rainfall showed that on a 5 slope sion, and suffossion, which means a digging under or
◦
about 60 per cent of the sediment moved by raindrop undermining (Chapuis 1992). Suspended particles and
impact moves downslope and 40 per cent upslope; on a colloids transported this way will be about ten times
25 slope 95 per cent of the sediment moved downslope smaller than the grains they pass through, and through-
◦
(Mosley 1973). Smaller particles are more susceptible wash is important only in washing silt and clay out of
to rainsplash than larger ones. The amount of splash clean sands, and in washing clays through cracks and
depends upon many factors, including rainfall proper- roots holes. For instance, in the Northaw Great Wood,
ties (e.g. drop size and velocity, drop circumference, drop Hertfordshire, England, field evidence suggests that silt
momentum, kinetic energy, and rainfall intensity) and and clay have moved downslope through Pebble Gravel,
such landscape characteristics as slope angle and vege- owing to through-wash (Huggett 1976).Where through-
tation cover (see Salles et al. 2000). Rain power is a flow returns to the surface at seeps, positive pore pressures
mathematical expression that unites rainfall, hillslope, may develop that grow large enough to cause material
and vegetation characteristics, and that allows for the to become detached and removed. Throughflow may
modulation by flow depth (Gabet and Dunne 2003). It is occur along percolines. It may also form pipes in the
a good predictor of the detachment rate of fine-grained soil, which form gullies if they should collapse, perhaps
particles. during a heavy rainstorm.

Rainflow Creep and dry ravel

Rainflow is transport caused by the traction of overland Soil creep (p. 66) is common under humid and tem-
flow combined with detachment by raindrop impact, perate climates. It occurs mainly in environments with
which carries them further than rainsplash alone. Sheet seasonal changes in moisture and soil temperature. It
wash carries sediment in a thin layer of water running mainly depends upon heaving and settling movements in
over the soil surface (p. 66). This is not normally a the soils occasioned by biogenic mechanisms (burrowing
uniformly thick layer of water moving downslope; rather, animals, tree throw, and so on), solution, freeze–thaw
thesheetsubdividesandfollowsmanyflowpathsdictated cycles, warming–cooling cycles, wetting–drying cycles,
by the microtopography of the surface. Sheet wash results and, in some hillslopes, the shrinking and swelling of
from overland flow. On smooth rock and soil surfaces, clays and the filling of desiccation cracks from up-
a continuous sheet of water carries sediment downslope. slope. Dry ravel is the rolling, bouncing, and sliding
On slightly rougher terrain, a set of small rivulets link of individual particles down a slope (Gabet 2003). It
water-filled depressions and bear sediment. On grassed is a dominant hillslope sediment-transport process in
slopes, sediment-bearing threads of water pass around steep arid and semiarid landscapes, and includes the