Erosion Control Practices

Classic Bench Terraces

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

Goal:

To understand the role of "structural" means of erosion control.

Scope:

Sizing, building and maintaining: vegetated waterways; drop structures; and terraces, including parallel-tile-outlet (pto) ones.

Perspective:

Cropping and tillage management are among the most effective means, both economically and conservationally, of controlling soil erosion. They are not covered in detail (except as they influence tabular values and/or computation of C-values) because there are numerous agronomic considerations outside the purview of this course. Rather, the emphasis is on planning and construction of those structural measures often essential to the full effectiveness and long-term stability of environmentally sensitive agronomic management.

Vegetated Waterways:

The purpose of vegetated waterways is to protect the soil against erosive actions of concentrated flow. They have many uses in a comprehensive conservation plans, but primarily they collect and concentrate flows and then safely transport the water to major drainage systems. Dense vegetation is used to minimize the area required, i.e. the protective action of the vegetation permits higher flow velocities and thus smaller waterway cross-sections. Most vegetated waterways run directly down a slope; however, they can also be constructed somewhat across the slope as diversions or sometimes to just reduce channel slope.

Vegetated channels should not be used where continuous flow occurs because the vegetation will die out. Base flow, even for short periods, can usually be controlled by one or more tile drains laid either directly under or alongside a watercourse. Tillage near a waterway should be accomplished in a direction essentially across the waterway. Grass cover should be cut periodically, fertilized as needed, and not subjected to prolonged traffic by either livestock or vehicles.

Channel Shapes:

There are basically three shapes to be considered for vegetated waterways: triangular, parabolic and trapezoidal shapes. The triangular shape concentrates low flows if the "V" is not smoothed out during the construction process with a tillage tool. On the other hand, the flow tends to meander on the flat bottom of a trapezoidal waterway. The parabolic shape is the most stable shape. Often, however, the shape is decided based on available equipment. A triangular shape is usually made using a motorized grader, a trapezoidal shape by a motorized scraper or pan, and a parabolic shape with a bulldozer going back and forth across the waterway.

Channel lining and permissible velocity:

Grass linings should be hardy, dense-growth perennials adapted to the geographical region and soil. Waterways are usually constructed in this area during August and September because that coincides with the time of the year when most perennial grasses are best established. A permissible velocity for the chosen channel lining, the existing soil based on its resistance to erosion, and the established slope is given in Table 7.3.

Waterway dimensions:

The waterway dimensions can be determined by simultaneously solving two equations for the waterway depth, d, and the channel top width, t, or bottom width, b. One equation is the continuity equation, an equation for the discharge or capacity of a channel.
The capacity of the channel, q, is usually sized to handle runoff from the watershed for a 10-year return period storm.

The other equation is a velocity equation called Manning's formula. This is an empirical formula, i.e. not dimensionally consistent.

Determining channel dimensions to handle a specified flow rate requires an iterative solution of the continuity and Manning's equations. Nomographs provide a quick solution.

Waterway failures:

Failure generally occurs when a waterway is overtopped. Alternate flow paths become established, usually just alongside the vegetated waterway. Common causes of failure are sedimentation (due to excessive watershed erosion), improper edge cultivation (due to plowing, etc. parallel to the waterway), over-grazing and trampling by livestock, and low, longtime flows, i.e. base flows. Proper edge cultivation is accomplished by raising or lowering tillage tools in an irregular manner as the waterway is crossed. Low, longtime flows can be prevented by placing subsurface drains alongside the waterway.

Contour Practices

We will use this term to refer only to those mechanical practices of contouring, strip cropping and terracing. These three practices require the adjustment of tillage operations to on or near the contour and give more-or-less permanent protection against erosion. They influence values of the cropping and management factor, C, and the conservation practice factor, P, in the USLE.

Reduced or conservation tillage will be used to refer to a collection of tillage practices carried out without regard to topographic conditions. With conservation tillage some or all of the crop residue remains on the surface and it is usually left in a rough condition. While influencing only the C-factor of the USLE, conservation tillage nearly always results in a significant reduction in soil erosion.

Various forms of conservation tillage are now fairly widespread, especially in the intense mid-agricultural sections of the U.S. Most farmers are aware of the serious nature of soil erosion, but must balance real conflicts with labor, machinery, pest and disease control, and the speculative nature of today's agriculture when implementing erosion control measures.

Contouring:

Contouring farming entails performing all tillage operation on or near the same elevation or "contour". It is accomplished by putting one or more guide lines in a field and then starting tillage operations from these guide lines. Contouring is applicable on relatively short slopes up to about 8 percent steepness with fairly stable soils. Contouring's impact on annual soil loss rates vary with slope steepness, but typically it's reduced on the order of one-half from up-and-down hill farming when the slope is between 4 and 7 percent.

The disadvantages of contouring are point rows and break-overs caused by reduced water storage capabilities behind the contour ridges as the slope increases. Once a break-over occurs it is likely to progress all the way down a slope as the runoff volume increases. Break-overs are a definite hazard on slopes greater than 10 percent. On steeper slopes erosion control may have to be accomplished through vegetative means, i.e., grass or timber.

The advantages of contouring are: the velocity of runoff is checked; the adsorption of rainfall is increased; erosion is reduced by slowing the rate of runoff; and, temporarily storing runoff behind contour ridges increases infiltration and collects sediment.

Strip Cropping:

Strip cropping is a combination of contouring and crop rotation in which alternate strips of row crops and closely grown crops such as meadow or a small grain are grown on the same slope. C-C-W-M is a good rotation for strip cropping since it has the same number of years for corn as for wheat and meadow. Strips generally vary from 50 to 130 feet in the Midwest. In general, strip cropping is applicable to a steeper but mainly longer slope than contouring alone. Values of A are reduced on the order of one-half that for contouring.

The advantages and disadvantages of strip cropping are similar to those for contouring. Strip cropping also tends to filter out the soil in the runoff through the strip with the closely grown crop. On the negative side, one crop may tend to harbor (host) plant diseases and pests which are detrimental to the other crop.

Terracing:

Terracing is a combination of contouring and land shaping in which the slope length is reduced by the construction of ridges or channels across the slope. The two types in common use in the U.S. (in contrast to the very steep areas with bench terraces in older countries as shown in the title graphic of this lesson) are broad-based, graded terraces and pto terraces. Both are named after their means of outleting water. Graded terraces are constructed as channels with a slight cross-slope grade to outlet water into surface waterways. PTO terraces pond water which is then slowly released through underground tile lines.

Terraces are suitable on slopes similar to contouring but preferably with long slope lengths. Long terrace lengths are also desirable because of the high initial costs associated with land forming. Values of P are reduced on the order of one-half that of strip cropping. However, annual loss is further reduced because the L-factor is the terrace spacing interval rather than the entire slope length.

Advantages of terracing are similar to those for contouring, but since erosion is better controlled the land can be farmed more intensively. Most modern terraces are also parallel and then spaced to fit exactly with farm machinery. The primary disadvantage of terracing is the expense, ranging from $300 to $500 per acre.

PTO terrace:

A PTO terrace system incorporates the parallel concept so terrace ridges are high when crossing natural drainage-ways and low or even non-existent (may be channels) when crossing natural ridge lines. The parallel feature eliminates points rows between terraces. The PTO terrace also reduces peak runoff rates substantially and eliminates the need for waterways.

The ridge top of a PTO terrace is at a constant elevation. Then as the terrace crosses a natural drainage-way it acts as a dam ponding runoff water behind it. This water can be released slowly through a tile system. The recommended temporary storage for PTO terraces in this area of the United States is 2-inch of runoff. The tile system is designed to release it in 24 hours. Peak runoff rates are therefore reduced on the order of 10 to 20 times over that which would have normally occurred. One can easily visualize that gully production below a PTO terraced field could be virtually eliminated.

One of two terrace cross-sections are generally used: the broad-based terrace over which machinery may operate or the steep back-sloped terrace with the back-slope usually grassed. If constructed from the downhill side, the steep back-sloped terrace also tends to form a bench similar to the benched terraces in the Far East used for rice culture.

Conservation Tillage:

The most rapidly growing conservation practice is a variety of techniques classified as "reduced tillage practices." These range from substitution of a chisel plow (which leaves crop residue on the surface) for traditional moldboard plowing to "no-till" systems. The major reasons such practices are gaining acceptance are increased knowledge about what soils and topographies are most responsive \f2and\f1 the availability of special equipment and chemicals to support these tillage methods. The combination of those forces has made these practices more profitable that conventional tillage. Thus, it's the opportunity for increased profitability that has caused accelerated adoption, probably more so than increased environmental concern.

Conservation Structures

The purpose of erosion control structures is to transfer runoff from a higher to a lower elevation over a short distance without allowing erosion to occur. In addition to dissipating flow energy, some structures also act to retain earth, i.e., they act like a retention wall. They are especially effective in arresting gully development, a situation which might need both a structure(s) and some vegetated channel.

Types of Structures:

Basically there are three types of structures:

drop spillway with box- or straight-inlets.

chute

pipe spillway

The drop spillway can handle a high discharge rate but is limited with regard to overfall height. Because the vertical headwall also acts as an earth retention structure, cost becomes prohibitive when the headwall width exceeds 10 feet. On the other hand, a pipe spillway can transfer runoff over wide ranges in elevation, but is limited with regard to discharge. Flow is totally enclosed in a pipe which acts as a control. The chute can handle high discharge because its entrance is similar to that of the drop spillway and it can transfer runoff over a large change in elevation. However, the inclined section rests on the soil and the variations of the soil over which a chute may be installed will decrease the operational life of the chute significantly.

Advantages and Disadvantages:

drop spillway:
  • suitable for a high q and low overfall height, especially if it has a box inlet
  • economical for a low overall height; a box-style inlet may be used to increase flow capacity when the upstream channel is shallow
  • can be used as a soil retention and tile outlet structure
  • can be built with various materials: concrete blocks, section of an aluminum arch, etc.
  • skilled labor is required except with the concrete block construction method
  • uses less space than the other structures
  • subject to undercutting if a stable slope is not maintained below the structure
chute:
  • economical for both a high q and a high overfall height, but if only one feature is necessary other structures are more appropriate
  • usually built as a monolithic reinforced concrete structure
  • skilled labor required
  • need good soil conditions for foundation; still, the life expectancy is less than for the other structures
pipe spillway:
  • suitable for a high overfall height and a low q
  • economical for a high overfall height
  • constructed with jointed pipe or corrugated metal pipe (CMP) with a variety of inlet and outlet choices
  • requires less material with a high overfall height than for the drop spillway
  • relative simple to construct but still needs skilled labor in most cases
  • versatile; can be used with small reservoirs, as a culvert, or as an outlet through the spoil banks of ditches
  • some space is needed above the structure for temporary storage of runoff
  • good backfill material is needed and it should be properly placed and compacted
  • the inlet opening is subject to blockage by debris

Criteria for a Reliable Erosion Control Structure:

  • adequate capacity, usually designed for 50-year recurrence interval or higher peak runoff rate
  • underseepage is minimized
  • constructed of erosion resistant materials
  • must safely dissipate the energy of flowing water
  • must have a stable channel below the structure