FLOOD09931

• County: Statewide
• Community: Urbanizing Areas of Colorado
• Basin: Statewide
• Title: A Guide for Erosion & Sediment Control in Urbanizing Areas of Colorado
• Date: 1/1/1973
• Prepared For: State of Colorado
• Prepared By: USDA Soil Conservation Service
• Floodplain - Doc Type: Educational/Technical/Reference Information

FACTORS INFLUENCING WATER
EROSION

The inherent erosion potential of any area is determined
by four principal factors: the characteristics of its soil;
its vegetative cover; its topography; and its climate.
Each of these factors is discussed in the following
pages. Although they are discussed separately, it
should be noted that they are interrelated in determin-
ing erosion potential.

Soil Characteristics

Soil properties which influence erosion by rainfall and
runoff are those factors which affect the infiltration
(water passing through the surface of the ground)
capacity of a soil and those which affect the resistance
of the soil to detachment and being carried away by
falling or flowing water. The vulnerability of a soil to
erosion is known as its erodibility. Some important or
key factors which control erodibility are:

1. Average particle size and gradation
2. Percentage of organic content
3. Soil structure
4. Soil permeability (the ease by which water passes
through the soil)

Soils which contain high proportions of silt and very
fine sand are generally the most erodible. Particles in
this size range are easily detached and carried away.
The erodibility of these soils is decreased as the per-
centage of clay or organic matter content increases.
Clay acts as a binder between particles and tends to
limit erodibility. Most soils with a high clay content are
relatively resistant to detachment by rainfall and
runoff. Once eroded, however, clays are easily trans-
ported and settle out very slowly.

An increase in organic matter reduces erodibility in a
different manner by maintaining a favorable structure
which improves its stability and permeability. This in-
creases infiltration capacity and delays the start and
reduces the amount of runoff.

Well-drained and well-graded gravels and gravel sand
mixtures with little or no silt are the least erodible soils.
(Well-graded soils are those which contain a wide range
of particle sizes.) Coarse, granular soils also have high
permeabilities and a good infiltration capacity which
either prevents or delays runoff.

Clayey soils have a very high water holding capacity
relative to sands and gravels, but poor infiltration
characteristics. In this respect, they are more vulnera-
ble to erosion because they tend to increase the amount
of runoff,

The study of the soil characteristics which relate to soil
erodibility is a complex, technical field. The K factor
index of soil erodibility is currently considered to be the
best index of soil erodibility.

It is difficult tojudge the relative erodibility of different
soils from field observations. Even a soil with a rela-
tively low erodibility rating (K factor) may show signs
of serious erosion when it appears on long or steep
slopes. On the other hand, a soil with a high erodibility
factor may show little evidence of erosion when it ap-
pears on short, gentle slopes or when good soil man-
agement is practiced.

Vegetative Cover

Vegetative cover plays an extremely important role in
controlling erosion in the following four ways. Vegeta-
tion:

I. Shields the soil surface from the impact of falling
rain
2. Slows the velocity of runoff
3. Maintains the soil's capacity to absorb water
4. Holds soil particles in place

By limiting and staging the removal of existing vegeta-
tion, and by decreasing the area and duration of expo-
sure, soil erosion and sedimentation can be signifi-
cantly reduced. Special consideration should be given
to the maintenance of existing vegetative cover on
areas of high erosion potential such as erodible soils,
steep slopes, drainageways, and the banks of streams.

Topography

The size and shape of a watershed influences the
amount and rate of runoff. Diversions as well as a
number of other control measures, intercept runoff
from higher watershed areas, store or divert it away
from vulnerable areas, and direct it toward stabilized
outlets.

Slope length and gradient are key elements in determin-
ing the volume and velocity of runoff and erosion risks.
As both slope length and gradient increase, the rate of
runoff increases and the potential for erosion is mag-
nified.

Slope orientation can also be a factor in determining
erosion potential. For example, a slope that faces south
and contains droughty soils may have such poor grow-
ing conditions that vegetative cover will be very dif-
ficult to reestablish.

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