COOPERATIVE EXTENSION SERVICE, PURDUE UNIVERSITY, WEST LAFAYETTE, INDIANA
Terracing as a ‘Best Management Practice’
for Controlling Erosion and Protecting Water Quality
Rolland Z. Wheaton and Edwin J. Monke, Agricultural Engineering Department, Purdue University
Indiana farmers have long recognized the importance of controlling soil erosion and the benefits of applying conservation measures. For years, the basic motivation behind their conservation efforts was "to maintain the soils resource in a highly productive state."
Although that's reason enough to use sound land management practices, today farmers have an additional 'motive'-compliance with the national Clean Water Act (Public Law 92-500), which seeks to eliminate the pollution of streams, rivers and lakes by any source, including runoff from agricultural land.
The law specifies that control of no point source pollution is to be accomplished primarily by application of 'best management practices.' BMPs are those conservation measures and/ or land management techniques deemed most effective in preventing pollution by runoff or seepage from a given field or land area into watercourses.
One BMP applicable to some of the cropland of Indiana is terracing. This publication seeks to answer the commonly-asked questions about terraces, including: what are the different types?, how do they work?, where are they most effective (or ineffective)?, what does it take to construct and maintain them?, and where does one get help with design, installation and financing? From the information presented, hopefully farmers can better assess the potential value of this BMP to their particular situations.
What is terracing?
Terracing is a soil conservation practice applied to prevent rainfall runoff on sloping land from accumulating and causing serious erosion. Terraces consist of ridges and channels constructed across-the-slope.
What types of terraces are there, and how do they differ?
To some people, the word 'terrace' brings to mind bench terraces that are used in various mountainous regions of the world. More often, one pictures contour terraces, with their point rows and grassed waterway outlets, that follow the lay-of- the-land (Figure 1).
Today, however, the type most common on agricultural land in the U .S. are parallel terraces, so named because they are constructed parallel to each other and, where possible, to the direction of field operations (Figure 2). Parallel terraces eliminate the production losses associated with point rows and minimize interference to farming operations when spaced at multiple widths of planting and harvesting equipment.
Some terraces are constructed with steep back slopes that are kept in grass. Most, however, are broad-based having gently sloped, ridges that are cultivated as a part of the field.
In semi-arid regions (e.g., western Kansas and Nebraska), contour terraces may have level channels to collect and hold the water until it seeps into the soil. In humid regions like Indiana, most terraces will have graded channels that convey the water to an outlet.
Terraces that are constructed in parallel and discharging runoff through subsurface drains are known as parallel tile outlet terraces. With PTO terraces, water that accumulates behind a terrace ridge is discharged through a surface inlet into a subsurface drain. The surface inlet, called a riser, has a restricted section to control the discharge rate, causing some of the runoff to be temporarily stored (Figure 3). This storage period is long enough for sediment to settle out of the water, but not so long as to damage the crop.
What are the benefits of a terrace system?
The major benefit, of course, is the conservation of soil and water. Terraces reduce both the amount and velocity of water moving across the soil surface, which greatly reduces soil erosion. Terracing thus permits more intensive cropping than would otherwise be possible.
PTO terraces provide these added benefits: (1) the total area can be farmed, since grassed waterways are not needed; (2) elimination of grassed 'waterways also eliminates the inconvenience they cause when tilling or applying herbicides; (3) peak discharges are reduced because runoff is temporarily stored; and (4) sediment and other contaminants settle out behind the terrace ridge before polluting water in a receiving stream.
To what field conditions are terraces best adapted?
Fields with long, fairly-uniform slopes that are not too steep (generally less than 8 percent) are best adapted to broad-based terraces. If the slopes are very irregular or if the soil is shallow (less than 6 inches), alternative BMPs should probably be used.
The overall slope of a field being terraced can be improved by taking fill material from the 'right' locations in that field. Topsoil should be removed from both the cut and the fill areas and stockpiled, especially with shallow soil. It can later be spread back over the terrace and borrow areas.
In some cases, on land where a total terrace system is not practical, terraces are sometimes placed across minor watercourses. This type of terrace uses subsurface outlets similar to PTO terraces. It controls sediment discharge but does not check soil erosion as effectively, since steep slopes are likely to exist above the terrace.
Figure 1. A contour terrace system with many point rows and grassed waterways.
How effective are terraces in controlling erosion?
Generally, very effective! They intercept rainfall runoff as it starts down a slope, thus preventing a large accumulation of flow on the surface. This reduces the potential for sheet and rill erosion.
In effect, terraces break up one long slope into a number of short ones. As already mentioned, PTO terraces also give soil particles that do erode a chance to settle out in the basin behi1d the terrace ridge, while the excess water is slowly discharged through a subsurface drain.
Figure 2. A parallel terrace system.
Figure 3. Water temporarily stored behind a newly constructed PTO terrace.
To what extent do terraces interfere with field operations?
Any permanently-installed practice will, to some degree, obstruct field operations and/or take some land out of production. However, these obstructions are greatly minimized under present methods of design. For instance, with a PTO terrace system, elimination of the grassed waterways more than offsets the interference caused by cross-slope ridges and channels.
Does terracing require changes in tillage practices?
Since terraces are constructed across the slope, tillage will also be done in that direction. Cross-slope tillage may be the only additional change needed. However, use of conservation tillage practices also might permit greater distances between terraces.
Can (or should) I install terraces myself?
Some terrace systems can be built using conventional farm equipment, such as a mold- board plow. More often, however, bulldozers, motor patrols (graders) and towed or self- propelled scrapers are needed. If earth must be moved a considerable distance, a scraper is probably best.
Aside from equipment availability, another consideration in the decision to do-it-yourself is the farmer's time commitment and know-how. For many, it's wisest to hire an experienced land contractor to do the job.
Regardless of who installs it, a terrace system must be well designed. Terraces must be spaced correctly, have adequate ridge height and channel cuts to provide the necessary water storage, and have properly-sized outlets. Considerable field surveying work may be needed to design and stake out a system. Usually this will require the services of an engineer or someone specially trained in terrace design.
Must the job be done at one time?
It's best if it is but doesn't have to be. By starting at the top and working downhill, construction of a terrace system can be spread over more than one season.
A diversion may be needed at the upper edge of the field, if an uncontrolled runoff area exists above it. The water outlet should be installed the year before terrace construction begins.
What type of maintenance is required?
The main thing is annual preventive maintenance, such as removing debris from outlets and smoothing out silt bars in the channels. However, yearly tillage activity will eventually wear down the terrace ridges, requiring that they be rebuilt occasionally. This is done by plowing toward the ridge top from both sides with a mold-board plow. The use of conservation tillage methods is a good means of assuring long life for a terrace system.
Where can one go for further information or assistance?
For more detailed information on terracing and for help in assessing its potential as a BMP on a given farm, contact the Cooperative Extension Service agricultural agent or U.S. Soil conservation Service district conservationist at your local county seat.
Technical assistance in the design and layout of terrace systems is available from the Soil Conservation Service through the local Soil and Water Conservation District.
System installation may qualify for cost-sharing assistance from the Agricultural Stabilization and Conservation Service, also located in the county seat. Additional financing by intermediate and long-term agricultural loans can be sought from the Production Credit Association, Federal Land Bank, Farmers Home Administration, commercial banks or other agricultural lending institutions.
Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U, S. Department of Agriculture Cooperating. H. G. Diesslin, Director, West Lafayette, Ind. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to its programs and facilities without regard to race, religion, color, sex or national origin.