Erosion Control Principles

Erosion-laden Runoff

Note these materials have largely been developed by Larry Huggins for use in AGEN 521.

Goal:

To understand the erosion process and the forces involved. This is vital to the ultimate goal of quantitatively predicting field soil loss, the essential first step in planning control strategies and practices.

Scope:

Wind erosion is a serious problem in many parts of the world; within the U.S. the southwestern Great Plains have been classified as a potential "desertification" region. Nevertheless, this course's focus is on water erosion since water erosion is a much larger problem in the midwest. The erosion prediction methodologies covered in this section all are derived from the Universal Soil Loss Equation.

Introduction:

Sediment is introduced into our water resources through the erosion process. It is the number one polluter, quantity-wise at least, of our water resources. Sediment itself is detrimental to fish propagation and recreational uses of lakes and streams. Sediment also transports chemicals, especially phosphorus, which cause rapid eutrophication (aging) of those bodies of water.

Perhaps more serious than the degradation of our water resources, which often can recover reasonably quickly after erosion is abated, is the harm erosion causes to the long-term productivity to our land resources. Thus, erosion control yields dual benefits, sustainable production and improved water quality.

The Erosion Process

The terms interrill and rill erosion have gained wide acceptance because they can be quantitatively described. So-called sheet erosion,
the uniform loss of a thin layer over a large area, rarely occurs. However, it appears to occur because storm-caused rills are smoothed out by tillage tools. Visible effects of "sheet" erosion are seen in turbid waters and on the lower elevations of fields as depicted in this field.

Interrill erosion is caused mainly by raindrop splash and "film" flow between rills. Its action is the same regardless of position on the slope. Rill erosion is caused mainly by runoff. As runoff waters are progressively collected by rills they tend to become larger in the downslope direction and are also more noticeable. Thus, while rill erosion is the more visible of the two, the amount of soil moved by raindrop splash is much greater.

Erosion (also sedimentation) is a non-uniform, unsteady process with both transportation and deposition occurring simultaneously. The forces causing water erosion can be categorized as:

Attacking Forces:

Those which cause detachment and transport of soil particles. These forces are produced by rainfall and runoff. For a unit mass of water, larger raindrops and faster flowing runoff have greater energies for causing erosion.

Resisting Forces:

Those which tend to either stabilize the soil (increased cohesion or weight) or reduce the magnitude of the attacking forces (increased infiltration, better canopy, or increased surface roughness). Vegetation is especially important in that it intercepts rainfall, restrains soil movement, improves infiltration, decreases runoff velocities, and improves soil aggregation.

Erosion prediction

Effective control of soil erosion requires an ability to quantitatively to predict the amount of soil loss which would occur under alternate management strategies and practices. Without question, the model with the greatest acceptance and use, especially within the U.S., is the Universal Soil Loss Equation, USLE, developed by ARS scientists W. Wischmeier and D. Smith. While newer methods are now becoming available, most are still founded upon principles introduced by the USLE; thus, understanding those principles is quite important. USLE states that the field soil loss in tons per acre, A, is the product of six causative factors:
where:
R = rainfall and runoff erosivity index
K = soil-erodibility factor
L = length of slope factor
S = degree of slope factor
C = cropping-management factor
P = conservation practice factor
The R-term characterizes the level of attacking forces while the remaining terms characterize the level of resisting forces.

Values for the various factors (except K) were based on a statistical analysis of data gathered from plots. The K-factor was determined experimentally in tons per acre as:

for the conditions when:
S = 9 percent
L = 72.6 feet
C = fallow
P = up-and-down hill farming = 1
This model was put into its present developmental form during the 1950/60s by Mr. W.H. Wischmeier who held a joint appointment with USDA and the Department of Agricultural Engineering here at Purdue. The model is a phenomenological one (predicts the effect) and does not give a good idea of the actual mechanisms involved.

Evaluating the factors in USLE:

R, the rainfall factor:

Most appropriately called the erosivity index; it is a statistic calculated from the annual summation of rainfall energy in every storm (correlates with raindrop size) times its maximum 30-minute intensity. Empirically, this "EI-Index" was found to have the highest correlation with soil erosion from experimental plots. As expected, it varies geographically, but its seasonal distribution is also important to calculate C-values.

K, the soil erodibility factor:

This factor quantifies the cohesive character of a soil type and its resistance to dislodging and transport (particle size and density dependent) due to raindrop impact and overland flow shear forces. Some values are given in Table 6.1; however, we are now able to estimate K knowing certain soil properties.

LS, the topographic factor:

Steeper slopes produce higher overland flow velocities. Longer slopes accumulate runoff from larger areas and also result in higher flow velocities. Thus, both result in increased erosion potential, but in a non-linear manner. For convenience L and S are frequently lumped into a single term. (See Fig. 6.5).

C, the cropping-management factor:

This factor is the ratio of soil loss from land cropped under specified conditions to corresponding loss under tilled, continuous fallow conditions. The most computationally complicated of USLE factors, it incorporates effects of: tillage management (dates and types), crop, seasonal EI-index distribution, cropping history (rotation), and crop yield level (organic matter production potential).

P, the conservation practice factor:

Practices included in this term are contouring, strip cropping (alternate crops on a given slope established on the contour), and terracing. As a rule of thumb, contouring reduces to one-half the soil loss caused by up-and-down hill farming, strip cropping to one-half that of contouring, and terracing to one-half that of strip cropping. (See Table 6.3).

RUSLE

RUSLE is the Revised USLE. RUSLE has been revised in several ways, but the most important is that it is a computer program as opposed to a series of USLE tables and figures. You will have an opportunity to use RUSLE in lab to estimate soil erosion for 2 scenarios.