Page 196 - Fundamentals of Geomorphology

HILLSLOPES 179

Slope-dependent (diffusive) processes events, as well as predicting total runoff and soil loss
(Table 7.2). Many physically based soil-erosion models
have benefited from GIS technology.
q s = KS

Hillslope erosion along trails

The trampling of humans (walking or riding) and other
Steady-state convex form Perturbations damp out animals along trails may lead to soil erosion. Any-
one who has walked along footpaths, especially those
in hilly terrain, is bound to have firsthand experience
Water flow and slope-dependent (advective) processes
of the problem. The problem has become acute over
the last twenty or thirty years as the number of people
using mountain trails, either on foot or in some form
q s =( f q S)
w´
of off-road transport, has risen sharply. A study in Costa
Rican forest confirmed that trails generate runoff more
quickly, and erode sooner, than is the case in off-trail set-
tings (Wallin and Harden 1996). This finding, which is
typical of trail erosion studies in all environments, under-
Steady-state concave form Perturbations grow
scores the need for careful management of ecotourism in
Figure 7.9 An explanation for the development of trail-dependent activities. Strategies for combating trail
ridge-and-valley topography in soil-mantled terrain. erosion can work. Smedley Park lies in the Crum Creek
Slope-dependent (diffusive) transport leads to convex watershed, Delaware County, near Media, Pennsylvania,
hillslopes, and when the topography is laterally perturbed USA.Thetrailsintheparkpassthroughseveralareaswith
the transport direction (black lines) causes the topographic fragile environments (Lewandowski and McLaughlin
highs to lower and topographic lows to fill in, resulting in 1995). A strategy was devised using network analysis,
smooth topography, as suggested by the dashed line. In which altered the efficiency of the trail system by more
contrast, advective transport, which depends on water flow fully connecting sites with robust environments and
and slope gradient, carries sediment downslope and reducing the potential for visitors to use environmentally
produces concave hillslopes. Flow concentrations (black fragile sites. Some of the severest erosion is associated
flowpaths) resulting from lateral topographic perturbation
lead to incision, as suggested by the dashed lines. The with logging trails. In the Paragominas region of east-
competition of these two processes leads to ern Amazonia, tree damage in unplanned and planned
diffusion-dominated ridges and advection-dominated logging operations was associated with each of five log-
valleys. ging phases: tree felling, machine manoeuvring to attach
Source: Adapted from Dietrich and Perron (2006) felled boles to chokers, skidding boles to log landings,

Box 7.2

THE UNIVERSAL SOIL LOSS EQUATION (USLE)

The USLE (Wischmeier and Smith 1978) predicts soil The equation is usually written as:
loss from information about (1) the potential erosivity
of rainfall and (2) the erodibility of the soil surface. E = R × K × L × S × C × P