WATERSHEDS - Water Quality Engineering in Natural Systems - page 225

and observations indicate that almost all sediment
reaching waterways from forestlands originate from the
construction of logging roads and from clearcuts.

sediments-primarily, clay and organic particulates-to
adsorb and make the pollutants biologically unavailable
is considered by some as a partial water-quality benefit
of sediment discharges.
The term denudation refers to the process of weath-
ering or breakdown of parent rock materials, entrain-
ment of the weathered debris, and transportation and
deposition of the debris. The term erosion is often used
synonymously with denudation, although erosion
applies to entrainment and transportation of debris by
water and wind but not to weathering. Geomorphology
is the science dealing with the shaping of Earth surfaces,
including erosion, tectonic processes, weathering, and
other stresses, including those caused by humans.
Erosion can be divided into the sequential processes of
sheet erosion, rill erosion, gully erosion, stream bank
erosion, floodplain scour, and shoreline or bluff erosion.
Sheet erosion occurs when sediment is entrained and
transported by sheet flow, typically over very flat areas,
such as roadways, rill erosion occurs when sediment is
entrained and transported in small eroded channels
called rills , and gully erosion occurs when sediment is
entrained and transported by runoff in large eroded
channels called gullies . Sheet erosion, rill erosion, and
gully erosion are sequential, as illustrated in Figure
6.17a, and these evolving erosion features are shown on
a steep slope in Figure 6.17b. Typically, rills are only
several centimeters in depth and occur mainly in culti-
vated soils, while gullies are formed when rills grow to
0.5-30 m (1.5-100 ft) in depth. Gully erosion is often
defined for agricultural land in terms of channels too
deep to be easily remedied with ordinary farm tillage
equipment. Examples of a rill and gully are shown in

6.3.2 Fate and Transport Processes

The two main pathways by which agricultural pollutants
travel from their sources to receiving water bodies are
via overland flow into a surface water body or via infil-
tration and percolation through the soil into the ground-
water. In the case of overland flow, a significant portion
of contaminants are usually adsorbed onto eroded sedi-
ment particles and transported with these particles. In
the case of percolation through the soil into the ground-
water, contaminants exist in both the aqueous (dis-
solved) phase and the sorbed (solid) phase attached to
soil particles, and only the dissolved phase of the pollut-
ant affects the saturated zone. Fate and transport pro-
cesses for both overland flow and subsurface flow are
described in the following sections.

6.3.2.1 Erosion. Each year, millions of tons of soil
and weathered geological material are washed from the
land surface into receiving water bodies, and human
activities and land use can increase the rate of erosion
dramatically. For example, deforestation in areas of high
precipitation and high slope can be devastating to both
water quality and flooding.
Land uses or watershed modifications that produce
elevated sediment yields are considered polluting activ-
ities. Eroded soil particles carry pollutants that can
be harmful to the ecology of receiving water bodies
and to humans. however, the strong affinity of fine

(b)

(a)

Figure 6.17. Sequential relationship between sheet, rill, and gully erosion. (a) Schematic diagram; (b) rills on steep slope.
Source : (a) Natural Resources Conservation Service (2005b).