Environmental Engineering FOURTH EDITION

236 ENVIRONMENTAL ENGINEERING

in determining when sheetwash erosion will occur. A gentle summer shower is less
likely to result in sheetwash erosion than a sudden, torrential downpour. Slope and soil
characteristics are also important, along with antecedent precipitation and vegetation
cover. Surface runoff occurs more readily in saturated soils or very dry soils that have
developed a hard, impervious crust. Soils with a healthy vegetation cover are less
likely to experience sheetwash erosion, even when the soils are saturated or very dry,
because the cover helps slow surface runoff and hold soil particles in place.
As surface runoff flows downhill across an unconsolidated surface such as exposed
soil, the water collects into small channels (rills), and eventually forms wider, deeper
erosion gullys. When water is confined to a distinct channel, whether a rill, gully,
or stream channel, the runoff moves more rapidly and erosion becomes more intense
within the confines of the channel. The channel will develop sinuous bends and curves,
with areas of active erosion along the outer edge of the bend and depositional areas
along the inside of the bend. Only a portion of the sediment detached and transported
from upland regions in a watershed is actually carried all the way into a lake or estuary.
In many cases, significant portions of the sediments are deposited at the base of slopes,
in the floodplain, or in depositional regions along stream banks.

Estimating Sediment Erosion

Estimating the amount of sediment that will be transported out of a watershed is an

important part of measuring and abating nonpoint source pollution. Elevated sediment

concentrations are common in streams passing through agricultural regions, construc-

tion sites, freshly logged forests, and residential areas. Along with the sediments, each

nonpoint pollution source has its own characteristic array of pollutants: pathogens,

phosphorus, and pesticides from agricultural regions; phosphorus and herbicides from

cleared forests; and phosphorus, pathogens, metals, and petroleum hydrocarbons

from residential areas.

Because many urban pollutants are transported with sediment, and because sed-

iment itself is an important pollutant, mathematical models axe often used to predict

sediment erosion and transport to a body of water. Most sediment transport mod-

els include adjustments for variation in soil characteristics, slope, vegetative cover,

percent impervious surfaces, precipitation patterns, and other relevant watershed

characteristics.

One method for estimating the sediment transport from sheet and rill erosion is a

loading function based on the universal soil loss equation (USLE)

A = R x K x LS x C x P, (11.1)

where

A = soil loss (todacrs-year),

R = rainfall erosivity index,

K = soil erodibility factor,