ANR-790Water Quality 4.3.5

not usually vegetated. Essentially, they reduce erosionby shortening the length of the slope. The shorter theslope length, the less chance there is for runoff tobuild enough momentum to cause excessive erosion.

Terracing is more effective for erosion controlthan strip cropping and contouring because it dividesthe slope into discrete segments. Although there maybe soil movement within terrace intervals, most ofthis material accumulates in the terrace channel or inbasin areas. This measure is one of the most widelyused practices to reduce erosion on cropland. Withsome types of terraces, more than 95 percent of thedetached soil particles remain on the land.

Terraces also channel runoff safely away throughoutlets—either underground pipes, grassed water-ways, or other stabilized outlets. Because they controlthe flow of water over long slopes, they prevent theformation of gullies and often allow more intensiverow crop production on land that otherwise woulderode excessively. High yields can be sustained formany years on fields where terraces have been con-structed and maintained.

Tile Outlet Water Disposal SystemsThese consist of terrace systems augmented with

underground plastic pipe or tile water outlets. Thepipe or tile is installed in natural drainageways on agradient to allow for proper drainage. Small basinsaround outlets allow for most sediment to settle out.

Terrace systems, especially tile outlet systems,have the highest initial cost of all row crop conserva-tion practices, and they require periodic maintenance.In addition, nutrient leaching to groundwater may beincreased when this practice is used.

Grassed Waterway Or OutletA grassed waterway is a natural or constructed

watercourse kept in grass and designed to carry waterdown slope without causing erosion. The sod slows

Structural measures for soil management and ero-sion or sediment control are designed primarily to

control runoff water. They remove the excess waterwithout allowing it to carry the soil away with it. Thiscontrol of runoff water is accomplished by effectivelyreducing slopes and slope lengths, by increasing infil-tration or settling of sediment, and by stabilizing wa-terways or other outlets that transport excess water.

Structural measures for soil management and ero-sion or sediment control include diversions, terraces,tile outlet water disposal systems, grassed waterwaysor outlets, lined waterways or outlets, improved sur-face drains, subsurface drains, grade stabilizationstructures, land smoothing or leveling, and debris orsediment basins.

DiversionsDiversions are ridges of soil or channels with a

supporting ridge on the lower side. They are builtacross the land slope to allow interception and dispos-al of runoff at a selected location. They are used tobreak up long slopes, to move water away from activeerosion sites, to direct water around barnyards orother sites, and to channel surface runoff to suitableoutlet locations. Their primary purpose is to protectland or water below the structure. They often are usedin combination with contour strip-cropping, grassedwaterways, sediment filters, or other sod filters thatcan trap nutrients and fecal bacteria in the runoffwater.

TerracesTerraces are earthen embankments or ridges with

a channel constructed across the slope. Their spacingdepends on slope. Terraces have gentle grades, whichallow them to intercept surface runoff and conduct itat low velocity to a stable outlet. These structuresfunction much the same as diversions, except that ter-races are usually constructed closer together, supportless drainage area, are smaller than diversions, and are

ANR-790-4.3.5

A L A B A M A A & M A N D A U B U R N U N I V E R S I T I E S

Agriculture and Natural Resources WATER QUALITY:Controlling Nonpoint Source(NPS) Pollution

Soil Management To Protect Water QualityStructural Measures For Soil Management

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4.3.5-2

runoff and traps soil and nutrients carried in runoffwaters from cropland or animal production areas.Grassed waterways serve as outlets for terraces, di-versions, or other concentrations of water, but theydon’t take the place of erosion-control measures onadjoining slopes.

If waterways are used primarily as filter strips toremove sediment, they can quickly silt in and losetheir effectiveness. To prevent this, crop residue man-agement, contour buffer strips, or other methodsshould be used to control erosion in the fields thatcontribute the runoff water.

A stable outlet from the waterway is needed toprevent a gully from forming and growing in sizewith each storm. This may involve the construction ofa water control structure, perhaps concrete-blockspillways.

Lined Waterway Or OutletIn some cases, because of land slope or other

conditions, a waterway or outlet must be lined withan erosion-resistant material such as stones or evenconcrete. This structure is expensive but sometimesnecessary to stabilize critically eroding areas, and it isgenerally used in combination with vegetative prac-tices such as critical area planting.

Surface DrainsDitches or channels may be constructed in rela-

tively low-lying areas or flat fields next to streams totransport surface flow through a filter strip prior to itsentering a stream. This practice has limited applica-bility because it has no erosion control benefit. Infact, these channels or ditches are not even vegetated.Their sole purpose is to facilitate the trapping of erod-ed sediment before it can enter a watercourse.

Subsurface DrainsPipe or tile, usually plastic, may be installed un-

derground to lower shallow groundwater tables or tohandle seepage on side slopes or in drainageways.This practice can reduce surface runoff, allow formore efficient use of machinery around wet spots,and serve as a water outlet for other conservationmeasures in some cases. However, subsurfacedrainage has limited applicability because it maycause additional nitrate leaching and may be definedas illegal drainage of wetlands in the future.

Grade Stabilization StructuresThese structures substantially reduce erosion and

off-site sediment damage associated with channel andgully erosion. They are designed to reduce the ero-sion effects of sharp changes in elevation or to pro-vide a stable area where the change in elevation takesplace. They are commonly used in natural as well asartificial channels and may be used in conjunctionwith other surface water outlets.

Land Smoothing And LevelingDesigned to remove surface irregularities, this

practice tends to reduce rill and gully erosion byspreading the flow of water over a larger drainagearea. This allows for better control of surface runoffand is more effective when used in combination withother practices such as terraces.

Debris Or Sediment Control BasinsThese basins consist of drainageways and em-

bankments, dams, ridges, or channels constructedacross the slope. Debris basins reduce the velocity ofrunoff water, allowing much of the sediment and as-sociated nutrients to settle to the basin bottom. Sedi-ment basins are effective in reducing sediment deliv-ery from severe storms and in trapping small soilparticles, but they do not control erosion at thesource.

These structures are generally used where ter-races cannot be installed and farmed with reasonableeffort and where slope length and runoff rate or vol-ume must be reduced. These shallow basins need agrassed overflow section and may require periodic re-moval of trapped sediment.

ConclusionStructural measures usually have higher initial

costs but lower annual costs than cropping and tillagepractices. They require proper design and construc-tion and have a degree of permanency. They may re-quire some limited land use changes, but they aremost likely to require changes in tillage or manage-ment techniques (like a row direction change, for ex-ample). Structural measures can be a very effectivemeans of water quality improvement, and they mayallow continuation of a more intense cropping prac-tice than would be possible without them.

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ReferencesAlabama Department of Environmental Manage-

ment. 1989. Alabama Nonpoint Source ManagementProgram. Montgomery, AL.

Amemiya, Min, Stewart Melvin, and J. ClaytonHerman. 1980. Soil Management To Control Erosion.Pm-901e. Iowa Cooperative Extension Service. IowaState University. Ames, IA.

Grass Waterways And Buffer Strips: Soil-SavingTools. 1991. Fact Sheet. Alliance For A Clean RuralEnvironment. Washington, DC.

Humenik, Frank J., DeAnne D. Johnson,Jonathan M. Greglow, Steven A. Dressing, Richard P.Maas, Fred A. Koehler, Lee Christensen, WilliamSnyder, James W. Meek, and Fred N. Swader. 1982.Best Management Practices For Agricultural Non-point Source Control: III Sediment. North CarolinaCooperative Extension Service. Biological and Agri-cultural Engineering Department. North CarolinaState University. Raleigh, NC.

Walker, Robert, Steve Probst, and Doug Peterson.1985. A Plan For the Land: Erosion-Control Alterna-tives. Land And Water Number Four. Illinois Cooper-ative Extension Service. University of Illinois. Ur-bana-Champaign, IL.

Weinberg, Anne, Steve Berkowitz, and FredMadison. 1979. Nonpoint Source Pollution: Land UseAnd Water Quality. G3025. Wisconsin CooperativeExtension Service. University of Wisconsin. Madi-son, WI.

Wheaton, Rolland Z., and Edward B. Hale. 1980.Best Management Practices For Row Crop Agricul-ture. Virginia Cooperative Extension Service. Vir-ginia Polytechnic Institute and State University.Blacksburg, VA.

ANR-790-4.3.5

This publication, supported in part by a grant from the Alabama Department ofEnvironmental Management and the Tennessee Valley Authority, was preparedby James E. Hairston, Extension Water Quality Scientist, assisted by Leigh Strib-ling, Technical Writer.

For more information, call your county Extension office. Look in your telephonedirectory under your county’s name to find the number.

Issued in furtherance of Cooperative Extension work in agriculture and home eco-nomics, Acts of May 8 and June 30, 1914, and other related acts, in cooperation withthe U.S. Department of Agriculture. The Alabama Cooperative Extension System (Al-abama A&M University and Auburn University) offers educational programs, materials,and equal opportunity employment to all people without regard to race, color, nationalorigin, religion, sex, age, veteran status, or disability.

UPS, New June 1995, Water Quality 4.3.5