United States Department of Part 650 Engineering Field Handbook Agriculture
Natural National Engineering Handbook Resources Conservation Service
Chapter 14 Water Management (Drainage) Chapter 14 Water Management (Drainage) Part 650 Engineering Field Handbook
Issued April 2001
Acknowledgments This chapter was prepared under the general direction of Ronald L. Marlow, national water management engineer, Natural Resources Conserva- tion Service (NRCS), Washington, DC, with assistance from Patrick H. Willey, agricultural engineer, NRCS, Portland, Oregon. Extensive drafting was previously done by Richard D. Wenberg, retired national drainage engineer, with assistance from John Rice, Walter K. Twitty, Gordon Stroup, Elwin Ross, and Virgil Backlund, retired water management engineers (drainage). Editorial and layout assistance was provided by Mary Mattinson, NRCS, Fort Worth, Texas.
The final revision were prepared by Walter J. Ochs, Richard D. Wenberg, and Bishay G. Bishay, water management engineering consultants. Peer reviews were provided by James E. Ayars, agricultural engineer, USDA ARS, Pacific West Area, Fresno, California, and Norman R. Fausey, soil scientist - research leader, USDA ARS, Midwest Area, Columbus, Ohio.
Reviewers within NRCS include: Gordon Klofstad, national construction engineer, William Hughey, national agricultural engineer, William Irwin, national design engineer, John S. Moore, national hydrogeologist, and Donald Woodward, national hydraulic engineer, Washington, D.C.
John Andrews, Denver, Colorado, Grady Adkins, Columbia, South Caro- lina, Arthur Brate, Columbus, Ohio, Dennis Carman, Little Rock, Arkan- sas, Walter Grajko, Syracuse, New York, Ronald Gronwald, Dover Dela- ware, Mark Jensen, Des Moines, Iowa, Richard Judy, Morgantown, West Virginia, Tillman Marshall, Richmond, Virginia, Perry Oakes, Auburn, Alabama, John Ourada, Salina, Kansas, Allen Stahl, Annapolis, Maryland, Jesse Wilson, Gainesville, Florida, all state conservation engineers with their respective staffs.
Michelle Burke, Brookings, South Dakota, Gene Nimmer, Juneau, Wiscon- sin, and Chris Stoner, Stillwater, Oklahoma, all agricultural engineers.
Jim Bickford, irrigation engineer, Nashville, Tennessee, William Boyd, environmental engineer, Little Rock, Arkansas, Harold Blume, water man- agement engineer, Phoenix, Arizona, Sonia Jacobsen, hydraulic engineer, St. Paul, Minnesota, Don Pitts, water quality specialist, Champaign, Illinois, and Jeffrey Wheaton, conservation engineer, Maite, Guam.
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program informa- tion (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD).
To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call (202) 720-5964 (voice and TDD). USDA is an equal opportunity provider and employer.
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Contents Subchapter A—General
650.1400 Scope 14–1 (a) Need for soil and water management ...... 14–1 (b) Natural resource and environmental setting ...... 14–3 (c) Factors affecting drainage ...... 14–3 (d) Benefits related to drainage ...... 14–4 (e) Water quality impacts ...... 14–5 (f) Agriculture drainage setting ...... 14–6 (g) Drainage setting for non-agricultural areas ...... 14–7
650.1401 Types of drainage systems 14–7 (a) Surface drainage ...... 14–7 (b) Subsurface drainage ...... 14–8 (c) Interception drainage ...... 14–8 (d) Water table management ...... 14–8 (e) Drainage pumping ...... 14–8
650.1402 Investigations and planning 14–9 (a) Reconnaissance...... 14–9 (b) Physical surveys ...... 14–10 (c) Environmental considerations ...... 14–13 (d) Decision matrix ...... 14–14
Subchapter B—Surface Drainage
650.1410 General 14–15 (a) Drainage runoff ...... 14–15 (b) Determining drainage runoff ...... 14–15 (c) Composite drainage curves ...... 14–15
650.1411 Investigation and planning of surface drainage systems 14–17 (a) Field ditches ...... 14–17 (b) Types of open drain systems ...... 14–17 (c) Landforming ...... 14–22
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650.1412 Design of open drains 14–24 (a) Drainage runoff ...... 14–24 (b) Drain alignment ...... 14–25 (c) Hydraulic gradient ...... 14–25 (d) Design methods ...... 14–27
650.1413 Construction 14–37 (a) Open drain layout ...... 14–37 (b) Structures...... 14–37 (c) Watergates ...... 14–37 (d) Surface water inlets ...... 14–37 (e) Berms, spoil banks, and seeding ...... 14–38 (f) Safety ...... 14–39
650.1414 Maintenance 14–40 (a) Mowing ...... 14–40 (b) Burning ...... 14–41 (c) Chemicals...... 14–41 (d) Biological ...... 14–42 (e) Maintenance of land forming ...... 14–42
Subchapter C—Subsurface Drainage
650.1420 General 14–45 (a) Plans ...... 14–45 (b) Soils ...... 14–45 (c) Economics ...... 14–48 (d) Use of local guides ...... 14–48
650.1421 Applications of Subsurface Drainage 14–49 (a) Field drainage ...... 14–49 (b) Interception drainage ...... 14–49 (c) Drainage by pumping ...... 14–49 (d) Mole drainage ...... 14–49 (e) Water table management ...... 14–50 (f) Salinity control ...... 14–50 (g) Residential and non-agricultural sites ...... 14–51
650.1422 Investigations and planning 14–51 (a) Topography ...... 14–51 (b) Soils ...... 14–51 (c) Biological and mineral clogging ...... 14–53
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(d) Ground water ...... 14–53 (e) Outlets ...... 14–54
650.1423 Field drainage system design 14–55 (a) Random system ...... 14–55 (b) Parallel system ...... 14–55 (c) Herringbone system...... 14–55 (d) Drainage coefficient ...... 14–55 (e) Hydraulic conductivity ...... 14–57 (f) Depth of impermeable layer ...... 14–59 (g) Depth and spacing ...... 14–59 (h) Size of drains ...... 14–64
650.1424 Drain envelopes 14–71 (a) Drain envelope materials ...... 14–71 (b) Principles of drain envelope design ...... 14–74 (c) Design of drain envelopes...... 14–74
650.1425 Materials 14–77 (a) Concrete pipe ...... 14–77 (b) Thermoplastic pipe ...... 14–78 (c) Metal pipe ...... 14–80 (d) Vitrified clay pipe (VCP) ...... 14–80 (e) Other materials and products...... 14–80
650.1426 Appurtenances 14–81 (a) Surface inlets ...... 14–81 (b) Junction boxes ...... 14–81 (c) Vents and relief wells ...... 14–81 (d) Outlet protection ...... 14–83
650.1427 Drain installation 14–84 (a) Inspection of materials...... 14–84 (b) Storage of materials ...... 14–84 (c) Staking ...... 14–84 (d) Utilities ...... 14–85 (e) Crossing waterways and roads ...... 14–85 (f) Shaping the trench bottom ...... 14–87 (g) Laying corrugated plastic pipe (CPP) ...... 14–87 (h) Drain envelope installation ...... 14–87 (i) Alignment and joints...... 14–89 (j) Safety and protection during construction ...... 14–90 (k) Blinding and backfill...... 14–90
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(l) Protection for biological and mineral clogging ...... 14–90 (m) Checking ...... 14–90 (n) As-built plans ...... 14–90
650.1428 Maintenance 14–91 (a) Jet cleaning ...... 14–91 (b) Acid solutions ...... 14–92
Subchapter D—Interception Drainage
650.1430 Interception drainage design 14–93 (a) Ground water movement ...... 14–93 (b) Location of interceptor ...... 14–94 (c) Use of surface or subsurface drains ...... 14–98 (d) Size of drains ...... 14–98 (e) Grades and velocities ...... 14–99
Subchapter E—Water Table Management
650.1440 Introduction 14–101
650.1441 Controlled drainage 14–102
650.1442 Subirrigation 14–104 (a) General requirements ...... 14–104 (b) Planning a water table management system ...... 14–104 (c) Operation of water table management system ...... 14–108
Subchapter F—Drainage Pumping
650.1450 Drainage pumping 14–109 (a) Surface drainage pumping conditions...... 14–109 (b) Subsurface drainage pumping conditions ...... 14–109 (c) Relation of pumping plant to drainage system ...... 14–109 (d) Economic justification of pumping plant ...... 14–112 (e) Pumping from subsurface aquifers ...... 14–112 (f) Basic information required for plant design...... 14–112
Glossary 14–119
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Appendixes Appendix 14A Ditch design tables using 14–143
Appendix 14B Tables for computing culvert discharge 14–153
Appendix 14C Salinity monitoring equipment 14–157
Appendix 14D Auger hole method for determining hydraulic 14–161 conductivity
Appendix 14E Transient flow method 14–171
Appendix 14F Drainage around home sites 14–187
Tables Table 14–1 Drainage curves 14–16
Table 14–2 Ditch side slopes recommended for maintenance 14–28
Table 14–3 Permissible bare earth velocities 14–28
Table 14–4 Value of Manning's n for drainage ditch design 14–29
Table 14–5 Drainage coefficients 14–55
Table 14–6 Maximum trench depths for corrugated plastic pipe 14–63 buried in loose, fine-textured soils
Table 14–7 Values of Manning's n for subsurface drains and 14–64 conduits
Table 14–8 Relationships used to obtain fabric opening size to 14–76 protect against excessive loss of fines during filtration
Table 14–9 Interception drain inflow rates 14–98
Figures Figure 14–1 Cropland in need of drainage maintenance 14–2
Figure 14–2 Field drainage system in need of maintenance 14–2
Figure 14–3 Extent of subsurface drainage for a field as shown 14–11 by infrared photo and interpretive sketch
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Figure 14–4 Areal application of drainage curves 14–16
Figure 14–5 Random drains 14–18
Figure 14–6 Parallel surface drains 14–19
Figure 14–7 Reconstructing cross slope drain system 14–20
Figure 14–8 Surface drainage bedding 14–21
Figure 14–9 Earthmoving scraper with laser grade control 14–23
Figure 14–10 Landplane 14–23
Figure 14–11 Establishing hydraulic gradeline 14–26
Figure 14–12 Surface water inlets 14–31
Figure 14–13 Watergate 14–32
Figure 14–14 Profile ditch with hydraulic gradient 14–33
Figure 14–15 Watershed sketch 14–34
Figure 14–16 Open drain design worksheet 14–35
Figure 14–17 Design profile 14–36
Figure 14–18 Main drain with spoil banks spread 14–38
Figure 14–19 Seeding newly constructed drain 14–39
Figure 14–20 Maintenance by mowing 14–40
Figure 14–21 Application of chemicals for side slope weed control 14–41
Figure 14–22 Land leveler operation for land smoothing maintenance 14–42
Figure 14–23 Land smoothing equipment 14–43
Figure 14–24 Subsurface drain plan 14–46
Figure 14–25 Subsurface drain plan and layout 14–47
Figure 14–26 Monitoring salinity level of soil-moisture 14–50
Figure 14–27 Estimating soil hydraulic conductivity using soil 14–52 interpretation record
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Figure 14–28 Soil profile with calculations of hydraulic conductivity 14–52 using soil interpretation record
Figure 14–29 Field drainage systems 14–56
Figure 14–30 Curves to determine drainage coefficient, q, for arid 14–58 areas
Figure 14–31 Nomenclature used in ellipse equation 14–60
Figure 14–32 Graphical solution of ellipse equation 14–61
Figure 14–33 Subsurface drain discharge 14–65
Figure 14–34 Determining size of corrugated plastic pipe 14–66
Figure 14–35 Determining size of clay or concrete drain tile 14–67
Figure 14–36 Subsurface drainage system 14–68
Figure 14–37 Curves to determine discharge, Qr, for main drain 14–70
Figure 14–38 Filter envelope recommendation relative to soil 14–72 texture
Figure 14–39 Typical bedding or envelope installations 14–73
Figure 14–40 Junction box and silt trap 14–81
Figure 14–41 Blind surface inlet 14–82
Figure 14–42 Vents or relief wells 14–83
Figure 14–43 Pipe outlet 14–83
Figure 14–44 Outlet pipe protection 14–83
Figure 14–45 Laser grade control 14–84
Figure 14–46 Drain crossings and outlets 14–85
Figure 14–47 Dimensions for a 90 degree V groove for corrugated 14–87 plastic pipe
Figure 14–48 High pressure jet cleaning 14–91
Figure 14–49 Ground water movement 14–93
Figure 14–50 Typical interceptor installations 14–95
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Figure 14–51 Diversion system on moderate to steep slopes 14–96
Figure 14–52 Interceptor drain on bottom land 14–97
Figure 14–53 Water table management alternatives 14–101
Figure 14–54 Water control structures 14–103
Figure 14–55 Field layout of WTM system 14–104
Figure 14–56 Field layout of hydraulic conductivity tests 14–105
Figure 14–57 Field layout of hydraulic conductivity test 14–106
Figure 14–58 Subirrigation lateral spacing and water table position 14–107
Figure 14–59 Monitoring water table depth 14–108
Figure 14–60 Pump installation 14–110
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The needed drainage improvements should be deter- 650.1400 Scope mined for each particular site. For example, drainage is needed to complement conservation tillage on naturally wet soils and on some soils that were not Water management is involved in many aspects of excessively wet under earlier tillage practices. natural resource conservation. It is a broad term used to describe application of water for irrigation, control Effective drainage is also important to facilitate good of excess ponded, and the control of excessive soil water management where environmental conditions moisture. The latter is normally called drainage. Most dictate a desire to keep soils wetter longer. With crops and vegetative material require some drainage to uncertain drainage effectiveness, landowners are more provide a favorable environment in the root zone for likely to keep water tables lower to prevent flooding of growth. crops during periods of high rainfall. Efficient drainage systems can provide a predictable response to man- An important distinction must be made between agement, providing landowners the confidence to keep improving drainage of agricultural producing land and water higher longer because of the ability to remove converting wetlands. Present agricultural trends are water in a timely fashion. toward intensive use of existing cropland, with much of the emphasis on new management technologies. Ground water drainage can be either natural or artifi- Maintaining and improving existing drainage and cial. Natural drainage takes place in soils that have a associated yields on wet agricultural soils presently in deep hydrological profile and the hydraulic conductiv- production minimizes the economic need for landown- ity values are either uniform or increase with depth. ers to convert wetlands. This encourages a new em- Consequently, impermeable soil layers are either phasis on protecting existing wetlands and establish- absent or located deep. Under such circumstances the ing new wetland areas while maintaining highly pro- ground water table cannot be detected within 6 feet of ductive agricultural land. the ground surface, and artificial drainage is not re- quired. This chapter provides guidance during the planning and implementation stages for artificial drainage A lack of natural drainage often causes a high water practices. These stages could involve investigations, table and, under some conditions, can cause surface surveys, planning, design, specifications, installation, ponding of water. These conditions are the result of a and the operation of the systems for proper water shallow hydrological profile. This indicates the pres- management. Although most of this chapter concen- ence of a shallow impermeable layer or that the soil trates on agricultural drainage, which is the manage- horizon becomes less permeable with depth to the ment of surface and subsurface water on agricultural point that percolation is restricted, creating root zone and closely related lands, the principles apply to urban water problems that deter vegetative growth. A high conditions as well. This chapter is not intended to water table and waterlogging in the soil are often provide guidance for large drainage channels. prevalent above an impermeable layer when the rate of rainfall or the rate at which water is added exceeds the permeability of this layer. Artificial drainage sys- (a) Need for soil and water tems can be installed to remove the excess water and management maintain the water table at an appropriate level for the crops or other vegetation to be grown. Visual evidences of inadequate drainage include sur- face wetness, lack of vegetation, crop stands of irregu- Drainage is a practice that can assist in the surface and lar color and growth, variations in soil color, and salt subsurface management of water. It may be designed deposits on the ground surface (figs. 14–1 and 14–2). to provide numerous benefits other than direct crop benefits. Some additional applications involve: The need for agricultural drainage varies considerably • Environmental water control for wildlife because of differences in climate, geology, topogra- habitat development or protection. phy, soil characteristics, crops, and farming methods. • Water quality management
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Figure 14–1 Cropland in need of drainage maintenance
Figure 14–2 Field drainage system in need of maintenance
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• Improve site conditions for farmsteads as well Drainage facilities and other water management sys- as urban and rural communities tems are planned, installed, and maintained consider- • Improve conditions for application of organic ing wetlands and other land use objectives. wastes • Control of excess salinity in the root zone. • Trafficability for farming operations. (c) Factors affecting drainage
Environmental considerations sometimes require The topography, geology, constructed obstructions, or careful attention to soil and water management. Water site condition can block or retard water movement quality control can be facilitated with a properly and cause poor drainage. Site factors can be placed in installed water table management system. Water several categories and may exist separately or in control for wildlife habitat enhancement, protection, various combinations. Some of the more important or development can be improved with carefully pre- site factors follow. pared drainage practices. Lack of a natural drainageway or other depres- sion to serve as an outlet. Such sites are common (b) Natural resource and in glaciated, coastal plains and in tidal areas where environmental setting natural drainage systems are still in the process of development. The natural resource and environmental setting in relation to the agricultural drainage system must Lack of sufficient land slope to cause water to receive balanced consideration. Drainage systems flow to an outlet, or natural surface barriers that need to be installed without harming the natural limit the flow of water. Such sites are: the irregular resource and environmental setting. The environmen- and pitted surface of glaciated land; depressional tal factors that must receive consideration include: areas; and land above dams, constrictions, and natural • Water quality control including both protection barriers of valley flood plains. and improvement • Wildlife habitat setting and potential changes Soil layers of low permeability that restrict the because of the drainage downward movement of water trapped in small • Wetlands and their preservation or enhance- surface depressions or held in the soil profile. ment Many soils have a slowly permeable subsoil, rock • Development of new wetlands in the context of formation, or compact (hardpan) layer below the a drainage system surface either within the normal root zone of plants or • Mitigation for wetland losses at a greater depth. Compact layers can result from human activities (equipment tillage pans) or can occur Water quality is often impacted by agricultural, indus- naturally. trial, mining, and urban development. The drainage system can be the collector and carrier for pollutants; Constructed obstructions that obstruct or limit thus water quality considerations are critical to the the flow of water. They include roads, fence rows, planning of drainage systems. Because the drainage dams, dikes, bridges, and culverts of insufficient system is a collection facility, it provides an opportu- capacity and depth. nity to treat or control the discharges. Therefore, the system often must be planned in conjunction with Subsurface drainage problems in irrigated areas other water quality control facilities. caused by deep percolation losses from irrigation and seepage losses from the system of canals and Wildlife habitat and wetlands are important natural ditches serving the area. Deep percolation losses resources. They provide ecological diversity and from irrigation may fall in the general range of 20 to 40 critical habitat for many species of plants and animals, percent of the water applied. Losses from canals and some of which are rare, threatened, or endangered. ditches vary widely and can range from near zero to 50 percent of the water conveyed.
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Many soils in arid and semiarid areas have naturally The economic benefits of drainage are typically por- high concentrations of salts. Also soils in arid and trayed by an increased yield coupled with reduced semiarid areas may have high water tables, which production costs. Uniformity and vigor of crop growth indicates restricted drainage and leads to salt accumu- leads to improved drainage and reduced unit produc- lations. High water tables can be fed by the onsite tion costs (USDA 1987). application of irrigation water, by canal seepage, or by lateral subsurface flows. A primary function of subsur- Surface drainage removes ponded water quickly, face drainage is to lower and control the water table thereby allowing the remaining gravitational water to and keep the salt concentration low in the root zone move through the soil. Subsurface drainage acceler- through leaching. Much of the subsurface drainage ates the removal of this gravitational water. Thus soils work in arid regions is for the control of saline and with production constraints caused by wetness can sodic soil conditions. become highly productive. If proper leaching opera- tions are used, subsurface drainage can improve the Intensive drainage, such as very closely spaced subsur- salt affected soils in arid and semiarid regions. face drains for most soils that have poor internal drainage, is not a cropping problem. Close spacing of Andrew Fogiel, working with Harold Belcher at Michi- drainage-ways on soils in poor condition aids in the gan State University, conducted a literature search on establishment and growth of vegetation needed for water table management impacts on water quality soil conditioning. The removal of free water in the soil (Fogiel and Belcher 1991). The study involved 43 eliminates moisture in excess of that held by soil published research reports plus 18 research articles. attraction. Drainage does not remove the capillary The conclusions were that: water used by growing plants. The depth of the • The impact of water table management prac- drainageways does control the height of the water tices is primarily on receiving surface water. table. If the water table is too low in soils that have a • Subsurface drainage systems reduce erosion. low capillary rise, sufficient moisture may not move Overland flow is reduced while the total sur- upward into the root zone. However, this condition is face and subsurface drainage increases at the normally desirable in saline and sodic soils that are edge of the field. irrigated. • Subsurface drainage results in significant reduction of sediment and pollutants, primarily Special care must be taken with some sandy soils and phosphorous and potassium not in solution. in most peat and muck areas as they can be over- • Subsurface drainage sometimes results in drained. On peat and muck soils, overdrainage also increased nitrate nitrogen concentrations increases the rate of subsidence of the soil, which can delivered to receiving water; however, control- negatively impact existing drainage systems. Maintain- ling subsurface drainage can reduce the nitrate ing a high water table in organic soils minimizes oxida- nitrogen delivered. tion and subsidence. These soils are also unique in the • Water table management provides the capabil- particular depth of water table that is best for plant ity to enhance best management practices that growth. This depth should be considered in designing deal with field surface nutrient management the drainage system. and erosion control. • Subsurface drainage intercepts downward percolating water, which allows for monitoring (d) Benefits related to drainage water quality and providing treatment if needed. Removal of free water promotes the soil bacterial • Water table management practices can contrib- action essential for manufacture of plant food by ute to reducing nonpoint pollution alone and allowing air to enter the soil and consequently improv- with other practices. ing crop production. Plant roots, as well as soil bac- • The effect of subsurface drainage on deep teria, must have oxygen. Drainage aids in soil aeration ground water quality has not been established by providing air space in the soil, which allows water because of lack of research and difficulty in passing downward through the soil to carry out car- monitoring. bon dioxide, thus allowing fresh air to be drawn in.
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• Research is limited on effects controlled drain- (e) Water quality impacts age and subirrigation have on water quality. These systems have been shown to reduce Water quality can be enhanced by installation and total flow at the edge of the field and agricul- proper management of controlled drainage systems. tural chemical losses to receiving surface The use of control structures in open drainageways to water. collect sediment benefits downstream surface water by reducing sediment loads and the nutrients and The removal of free water by drainage allows the soil pesticides normally associated with the sediment. to warm up more quickly because less heat is required Where applicable, control structures in open drain- to raise the temperature of drained soil. Soil warmth age-ways or subsurface drainage systems can help to promotes bacterial activity, which increases the re- hold a water table close to the ground surface during a lease of plant food and the growth of plants. Soil that noncropping season, which helps to develop an warms up sooner in the spring can be planted earlier anaerobic condition for denitrification. This can ben- and provide better germination conditions for seeds. efit downstream surface and ground water quality. Poor surface drainage is shown in figure 14–2. Drainage systems are not primarily installed as water The removal of ground water improves the conditions quality management tools, but they do have an impact for root growth. For example, if free water is removed on soil and chemical transport. Where it has been from only the top foot of soil, crop roots will feed in necessary to clear and drain land for agricultural this confined area. However, if free water is removed production, research has shown that drainage systems from the top 3 feet of the soil, this entire depth is can have a positive impact on some nonpoint pollution available for plant roots to obtain nutrients and mois- problems in comparison to agricultural land without ture. Removing ground water also reduces the amount drainage. For example, under certain conditions of surface runoff, thus reducing erosion and the artificial drainage acts to lower soil erosion by increas- quantity of sediment and associated nutrients and ing the movement of water through the soil profile and pesticides leaving the land. thus reducing runoff. However, some research indi- cates subsurface drainage expedites the transport of In humid areas, removing excess subsurface water nitrate-nitrogen (nitrate-N) from the soil zone to reduces runoff by allowing water intake on a continu- surface water (Fogiel and Belcher 1991) (ASCE 1995). ous basis and providing room for moisture to be stored in the soil during prolonged rain storms or Based on the published literature, research results on snowmelt. Soil tilth must also be maintained to allow water quality impacts of drainage can be summarized the continued intake to occur. Subsurface drainage by the following statements that compare agricultural can be a part of the total Resource Management Sys- land with subsurface drainage to that without subsur- tem (RMS) needed to reduce or control erosion. face drainage (Zucker and Brown 1998): • The percentage of rain that falls on a site with The removal of excess subsurface water in humid subsurface drainage and leaves the site areas can improve trafficability to facilitate the har- through the subsurface drainage system can vesting of nursery stock during the dormant period. range up to 63 percent. Generally, dormancy coincides with the prolonged • The reduction in the total runoff that leaves the winter precipitation period. During prolonged periods site as overland flow ranges from 29 to 65 of intensive rainfall, improved drainage may allow percent. agricultural wastes to be effectively spread on drained • The reduction in the peak runoff rate is 15 to pasture, hayland, or cropland, thus reducing the waste 30 percent. storage time. Care must be taken to prevent compac- • Total discharge (total of runoff and subsurface tion of the soil when spreading wastes by tractor- drainage) is similar to flows on land without drawn equipment during wet periods. Balloon or subsurface drainage, if flows are considered flotation tires on the equipment effectively minimizes over a sufficient period before, during, and compaction. after the rainfall/runoff event.
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• The reduction in sediment loss by water ero- (f) Agriculture drainage setting sion from a site ranges from 16 to 65 percent. This reduction relates to the reduction in total The agriculture setting for use of drainage systems is runoff and peak runoff rate. codependent and an integral part of the overall man- • The reduction in loss of phosphorus ranges up agement of the cropping system. The setting is critical to 45 percent and is related to the reductions in to the proper crop development as it is the key tool in total runoff, peak runoff rate, and soil loss. providing a proper root zone environment for plant • In terms of total nutrient losses, by reducing growth. The drainage system is the focal point in a runoff volume and peak flows, the reduction in well-planned water table management system and is soil bound nutrients is 30 to 50 percent. the key component in a proper salinity control system • In terms of total nitrogen (N) losses (sum of all when adequate natural drainage is not present. In N species), there is a reduction. However, humid and tropical climatic zones, it can facilitate the nitrate-N, a soluble N ion, has great potential to control of pollutants and improve effectiveness of move whenever water moves. Numerous some soil management practices. studies throughout the Midwest and Southeast United States and Canada document that the (1) Water table control presence of a subsurface drainage system Water table control is the operation and management enhances the movement of nitrate-N to surface of a ground water table to maintain proper soil mois- water. Proper management of drainage water ture for optimum plant growth, to sustain or improve along with selected best management practices water quality, and to conserve water. Controlled (BMP's) help reduce this potential loss. drainage is a method of water table management using water management structures. Supplemental water These results indicate that subsurface drainage is a may be added for subirrigation in addition to con- management tool that reduces the potential for ero- trolled drainage. Refer to subchapter E and chapter 10 sion and phosphorus enrichment of surface water of NEH part 624 for more information on water table from agricultural activities. However, nitrate-N load- control. ings exported from drainage conduits to surface water continue to be a major water quality concern. Agricul- (2) Managing salinity conditions tural drainage research results related to water quality Salinity management in coastal areas and in semiarid impacts vary by region of the country; thus the results or arid climates is paramount to the successful agricul- tend to be region specific, yet they are relevant to tural operations of farming enterprises. The provision similar circumstances. In Ohio State University Exten- of artificial drainage in these areas is related to the sion Bulletin 871 (Zucker and Brown 1998) the basic need for maintaining a proper salt balance in the crop concepts of water table management are reviewed so root zone. Water quality concerns often require the the research described for individual states can be separation of drainage water collected from larger better understood. groups of farms to provide the opportunity for proper disposal or reuse of the water. Consideration of reuse Another 10-year study in the lower Mississippi Valley options, often on more salt tolerant crops, as well as that supports the above findings showed that subsur- treatment and disposal options must be a part of face drainage effectively reduced surface runoff from planning efforts. Onfarm planning considerations, alluvial soil by an average of 35 percent. Associated which are primarily aimed at managing the salinity reductions in losses were 31 percent for soil, 31 per- levels in the crop root zone, cannot stop at this point. cent for phosphorus, 27 percent for potassium, 17 The effect of reuse, disposal, or treatment must be percent for nitrogen, and about 50 percent for pesti- considered in the framework of an entire system. cides (Bengtson, et al. 1995).
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(g) Drainage setting for non- agricultural areas 650.1401 Types of drain- Drainage for removal of excess water is a practice age systems used in settings other than agriculture under numer- ous conditions. This practice benefits many recreation areas including sport fields and parking lots. In urban Drainage is accomplished by establishing or accelerat- and suburban settings, it is closely related to storm ing water removal or water table management within water management, but is also used to facilitate land- the site, by diverting offsite surface or subsurface scaping needs for water management. Buildings and flows, or by a combination of these. Even though most structures often require artificial drainage to provide a references relate to agricultural settings, the applica- stable foundation. Roadways and airfields are other bility includes urban areas. facilities that may require water management systems for drainage. Sliding hillsides and excavated side Field drainage systems, whether surface or subsur- slopes sometimes require drainage measures to main- face, generally remove excess irrigation water, leach- tain the stability of the slope and protect facilities. ing water, storm runoff, snowmelt, excess rainfall occurring on the site, or overland flows. Surface water generally is removed by a combination of practices of open ditches, land forming, and underground outlets. Subsurface flows are removed or controlled by open ditches or buried conduits. A lateral drain or system of lateral drains are generally located to lower the water table. The alignment of the drains depends somewhat on costs, topography, design grades, and outlet conditions.
(a) Surface drainage
Surface drainage involves removal of excess surface water by developing a continuous positive slope to the free water surface or by pumping. It may be accom- plished by open ditches, land grading, underground outlets, pumping, or any combination of these that facilitates water movement to a suitable outlet. Drain- age by this method applies to nearly level topography where: • Soils are slowly permeable throughout the profile. • Soils are shallow, 8 to 20 inches deep, over an impermeable layer. • Topography consists of an uneven land surface that has pockets or ridges which prevent or retard natural runoff. • Surface drainage supplements subsurface drainage.
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(b) Subsurface drainage (d) Water table management
Subsurface drainage is the removal of excess ground Water table control systems can be an alternative to water within the soil profile. It is also used to facilitate single purpose drainage systems. The basic premise is the leaching of salts from the soil and maintenance of to install certain structural measures and to operate a salt balance. Plastic tubing, clay and concrete tile, them in a manner that controls the water table at a and mole drains are used in many cases. Open ditches predetermined elevation. The structural practices can constructed to an adequate depth and properly located range from installing a flashboard or stoplog may also be used. Subsurface drainage is applicable to structure in the outlet ditch to installing a complete wet soils having sufficient hydraulic conductivity for system consisting of land forming, subsurface drain drainage where a suitable outlet is available or an tubing, water control structures, well and pump to outlet can be obtained by pumping. provide supplemental water, and observation wells for monitoring.
(c) Interception drainage (e) Drainage pumping Interception drainage systems remove excess water originating upslope, deep percolation from irrigation Pumps or pumping systems have many applications in or rainfall, and water from old, buried stream chan- drainage systems. Pumps may be used as outlets for nels. Interception drains are open ditches or buried surface or subsurface drainage when gravity outlets conduits located perpendicular to the flow of ground are not available or the available outlet is not deep water or seepage. They are installed primarily for enough to satisfy minimum depth requirements. A intercepting subsurface flow moving downslope. more complete write-up than that in this chapter is in Although this method of drainage may intercept and the National Engineering Handbook Part 624 (Section divert both surface and subsurface flows, it generally 16) (SCS 1971). refers to the removal of subsurface water. Pumps for water supply are often needed for sub- irrigation or water table management systems. Wells and pumps for wells are also described in NEH Part 623 (Section 15) and Part 650 EFH (chapter 12).
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estimate of water table levels based on their farming 650.1402 Investigations experience. In the absence of such information, local wells can be checked and, if necessary, a few borings and planning can be made at selected locations to get some estimate of water table levels. If a high water table does exist and subsurface drainage or water table control is When drainage is considered, an investigation is neces- recommended, a more detailed investigation is re- sary to determine the feasibility of the project. The quired before planning and designing a system. investigation should provide a clear understanding of the problem, the kinds and amounts of practices The vegetative cover should be observed and associ- necessary, and an estimate of the cost and expected ated with certain ranges of water table levels or sur- benefits and impacts of the project. face ponding. Willow, cottonwood, and poplar trees often thrive in high water areas. Grasses, such as reed This information often can be obtained from a recon- grasses and sedges, are generally in these areas, as are naissance of a small problem area. More detailed many other hydrophytic plants and weeds. The pres- examinations and surveys are made where the size of ence of hydrophytic vegetation that typically occurs in the area, lack of defined drainage pattern, or such wet areas may indicate that a wetland is on the site. special situations as riparian vegetation, wetlands, or The Natural Resources Conservation Service is com- rock outcrops may require. Environmental consider- mitted publicly and by policy to help protect wetlands; ations must be a part of the planning process and therefore, technical assistance to plan and install investigations necessary for habitat enhancement or or improve drainage systems on wetlands can be mitigation and for environmental protection should be provided as outlined in the National Food Security Act an integral part. Manual (NFSA Manual).
Maps, a hand level, and a soil auger should be used in (a) Reconnaissance the reconnaissance. A standard soil survey report, aerial photographs, and county and USGS survey maps The first step in analyzing the problem is to visit the are helpful sources of information. They are useful as area proposed for drainage. Wetland determinations a guide in recording data, such as soil information, are to be made if not readily available. Determinations limits of areas to be improved, location of outlets, related to cultural resources and endangered or threat- natural and artificial channels and improvements, ened species of flora and fauna for the region should probable location of proposed improvements, general also be investigated. If a standard soil survey map of land slopes, channel slopes, wetlands to be protected, the area is available, it is a valuable source of and watershed boundaries. Pacing or scaling from a information. The investigator should walk over the map and hand level shots provide a means of estimat- area and become acquainted with the problems, topo- ing approximate channel grades, depths, and sizes. graphic conditions, and physical features. The ideal The soil auger, with extensions, provides a means of time to do this is immediately after an intensive rain- making essential subsoil examinations and determin- fall or an irrigation application. The investigator can ing water table levels at depths commonly needed for mark low areas and other important features on a map most sites. while in the field. Some field surveys may be needed to identify or locate low areas. The following items should be noted: • Location and extent of any wetlands. Special emphasis must be given to ground water • The areas in which crops show damage, as investigations for subsurface drainage and water table pointed out by the farmer, indicated by the control systems. The primary purpose of such water aerial photograph, or noted in personal management is to lower and control water table levels. observations. At the time of the reconnaissance, a determination is • Personal observations of unique landscape made of the average or usual depth to the water table, features, ecologically significant areas, land as well as the water table level during the growing use patterns, operation (land management) season. Landowners and operators may have a good aspects, and site visibility.
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• Topography and size of the watershed area. (b) Physical surveys • Size, extent, and ownership of the area being considered for drainage. The size and complexity of the area to be drained • Location of the drainage outlet and its condi- determines the kinds and number of surveys needed. tion. For the smaller, simpler jobs, the technician may only • Location, condition, and approximate size of need a few elevations at key locations and make a few existing waterways. soil borings to determine soil texture, water table • Presence of cultural resources. levels, and the need for subsurface drainage. Deter- • Potential impacts outside the area being evalu- mine the approximate drainage area, and estimate the ated. approximate cost. • General character of soil throughout the area needing drainage, including land capability, The seasonal considerations of field surveys is impor- land use, crops and yields, and salinity or tant. The dormant period versus cropping and/or sodicity. irrigation season are major periods to evaluate. • High-water marks or damaging floods and Weather patterns and significant rainfall/runoff events dates of floods. should also be evaluated. • Utilities, such as pipelines, roads, culverts, bridges, and irrigation facilities and their Concepts for physical surveys have dramatically possible effect on the drainage system (see improved in recent years. Laser levels, Geographic NEM part 503). Information Systems (GIS), and Global Positioning • Sources of excess water from upslope land or Systems (GPS) are now common tools to modernize stream channel overflow and possible disposal and simplify the physical surveys required for investi- areas and control methods. gation and planning related to maintaining drainage • Condition of areas contributing outside water systems. These tools can facilitate the proper develop- and possible treatment needed in these areas ment of field information to protect valuable wetland to reduce runoff or erosion. or other habitat areas when working on the mainte- • Condition of any existing drainage system and nance of drainage systems. Information needs for the reasons for failure or inadequacy. Old subsur- drainage system are detailed in this section. The most face drainage systems that have failed because efficient tools to use in obtaining proper and thorough of broken or collapsed sections may well be data require judgment and responsibility to assure the the cause of a wet area. information obtained presents the environmental • Estimate of surveys needed. picture in perspective with other resource and devel- • Type and availability of construction equipment. opment improvement suggestions. • Feasibility. Figure 14–3 represents an infrared photograph and For small jobs, this information may be obtained by interpretive sketch showing the extent of subsurface the technician who goes over the land with the land- drainage for a field. The infrared method appears to be owner. The technician can then obtain engineering a promising and cost effective tool as compared to and other survey data. conventional probe methods. The drain mapping procedure is based on the fact that the soil over sub- The intensity of this investigation and the makeup of surface drains dries faster than other soil. This the investigation party depend upon the size of the changed moisture condition is shown by a difference area and complexity of the problem. In all cases, as in the infrared reflectance. A sketch as shown in figure much information as possible should be obtained from 14–3 may be developed using a GIS process. local farmers and residents. The investigation must be extensive enough to provide a clear picture of the size Modern laser surveying equipment is readily available and extent of the drainage problem. and improves efficiency for obtaining the field survey information. Laser leveling has also been used with drainage trenchers for automatic grade control since the 1960's (USDA, ERS 1987). GPS facilitates the
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mapping of existing drainage facilities, which also • The capacity of the outlet should be such that contributes to the efficiency and effectiveness of the design flow can be discharged at an eleva- planning work (Farm Journal 1997). GIS data bases tion at or below the design hydraulic gradeline. are becoming more common, and the availability of If the outlet is a channel, the stage-discharge this information can greatly enhance the detail and relationship should be determined taking into scope of investigation and planning actions without consideration the runoff from the entire water- adding a great deal of extra time. GIS data and systems shed. can provide considerable environmental information • The capacity of the outlet also must be such and facilitate presentations of planned alternatives. that the discharge from the project are will not The use of these tools truly helps to make the thor- result in damaging stage increases downstream ough investigations needed for comprehensive plan- of the project. ning alternatives and guidance (FAO 1988). • The quality of drainage water and its impact on downstream areas should be considered. The objective of a survey for design purposes is to • Submerged outlets may in some cases be obtain elevations, topography, and other field informa- desirable. Special care must be taken to obtain tion necessary to design the system and prepare plans, the desirable drainage on the cropland, yet specifications, and estimates of quantities of work to keep the outlet free of undesirable vegetation, be performed. Only the field information needed for sediment, and rodent entry. This situation may this purpose should be gathered. also be a common practice in tidal areas where the daily water surface fluctuation allows free (1) Drainage outlet discharge part of the time and submergence The first major engineering survey job is to determine part of the time. Spacing and depth of a drain- the location and adequacy of the drainage outlet. age system would need to be adjusted to recog- Enough level readings and measurements should be nize this condition. made to reach a sound decision. The proper function- ing of the entire drainage system hinges upon this (2) Topographic survey point. The following requirements should be met in Topographic information of the area to be drained determining adequacy of outlets: must be obtained. This information is used in the • The depth of outlet should be such that any flatter areas for planning land forming or for locating planned subsurface drains may be discharged field ditches, drains, or other facilities. above normal low waterflow. Pumping may be considered as a last alternative.
Figure 14–3 Extent of subsurface drainage for a field as shown by infrared photo and interpretive sketch (photo: Chanpaign County Soil and Water Conservation District and the Agricultural Engineering Department, University of Illinois)
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The amount and kind of topographic surveys depend • On existing culverts and bridges along the upon the drainage problem and the topography of the ditch line, obtain the station, inlet and outlet land. The surveys vary from a detailed grid or contour invert elevations, size of culvert (or size of map to random elevations, valley cross sections, and opening if different), length, alignment angle location of important features. The type of grade with the ditch, and kind of material (concrete, control equipment used by local contractors, such as CMP). Also, obtain elevations of top of road laser, should be considered in determining the need crossing or structures and, in the case of for topographic surveys. Refer to EFH, Chapter 1, bridges, the elevation of the bottom of string- Engineering Surveys, for survey procedures. Some ers. Where bridges will be affected, note the recommended details to observe in obtaining topo- elevation of the bottom of the top footings and graphic information for drainage include: of the bridge piers and abutments along with • Obtain elevations at 100 to 300 foot their condition. horizontal intervals on flatland, depending on • Where laterals or tributaries discharge into the how nearly level the land is and whether the ditch, note the station and the bottom eleva- drainage pattern is apparent from inspection. tion of the ditch at the point of entrance and Take additional elevations in all low or de- any other pertinent data that would be useful pressed areas. The flatter the land, the more in design. important it may be to take elevations at • Locate, describe, and obtain elevations as relatively close intervals. required or any other physical features along • Where random ditches are to be used to the ditch that will affect the design, such as drain depressions or pockets, the amount of cattle ramps, fences, surface flow entering survey data should be varied according to ditch, and rock outcrops. ground conditions. Elevations at close inter- • Obtain soil information for channel stability vals will be necessary if depressed areas are considerations. numerous, whereas a few random shots may suffice for areas that have few depressions. Profiles on new ditch lines may differ from profiles of In either case the survey should be made in existing ditches. Information and elevations along the sufficient detail to locate and determine eleva- proposed ditch line should be obtained as specified for tions of depressed areas and the best outlets. existing ditches. • Physical features of adjacent land should be specified if they affect the drainage of the (4) Cross sections proposed area. The information should include A detailed procedure for surveying cross sections is the location and elevation of the bottom of described in EFH Chapter 1, Engineering Surveys. drainageways, the size of opening and flow line Some guidelines include: elevations of culverts and bridges, and any • Individual shots should be taken at all promi- other similar information needed to plan the nent breaks to accurately reflect the ground drainage system. surface. • Cross sections should be taken at intervals of (3) Profile survey 100 to 500 feet on existing ditches, depending The procedure for running profiles is described in on the irregularity of topography and the EFH, Chapter 1, Engineering Surveys. Slightly differ- variation in ditch size. Cross sections may ent procedures are required if an existing ditch is used show elevation and extent of low areas need- or if a new ditch is being considered. The following ing drainage if this information was not steps must be followed for profiles where a ditch obtained by the preliminary profiles. Other already exists: needed information is location of utilities, • Obtain elevations of the old ditch bottom fence lines, roads, land use, and existing land- natural ground at 100 to 500 foot intervals scape features (trees, vegetation) that may along the ditch. Elevations of critical points affect construction or future maintenance. between stations should also be taken. A criti- cal point may be either a high point or a low point that would affect design or system cost.
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(5) Other field information The reuse of drainage water can be noted in the salin- Other field data that should be gathered at the time of ity control section of this chapter, Applications of the design survey include: Subsurface Drainage. In-depth information on salinity • The area to be drained delineated on an aerial assessment and management is in ASCE Manual 71 photograph or other suitable map. (ASCE 1990). Other documents that describe overall • The drainage area. water quality issues include Management Guidelines • The crops to be grown and farm machinery to for Agricultural Drainage and Water Quality, pub- be used after drainage. lished by the United Nations Food and Agriculture • Soil borings to a depth of at least 1 foot below Organization and the International Commission on the proposed depth of the ditch (unless the Irrigation and Drainage (FAO 1997). These documents characteristics of the soil materials are known describe the potential for toxic materials in drainage for areas in need of surface drainage) and to water and reflect on options for controlling potential twice the depth of drain where subsurface pollution substances in drainage water. drainage is needed. • Information on frequency and depth of flood- Research has demonstrated a strong linkage between ing. Drift marks of previous flooding can be subsurface drainage and nitrate-N losses to surface seen on trees, culverts, or fence posts. Eleva- water. An obvious, but less economical method to tion and frequency of high water should be reduce nitrate-N losses is to abandon subsurface obtained if it will have a bearing on the ditch drainage systems. The practicality of this method is design. minimal, however, as crop production would be re- duced substantially on millions of acres of productive For more information, see EFH Chapter 1, Engineering poorly drained soils in areas similar to the Midwest Surveys, and EFH Chapter 5, Preparation of Engineer- Region. In addition, sediment and phosphorus concen- ing Plans. trations in surface water would increase. Research conducted in the North Central Region (Zucker and Brown 1998) suggests the following strategies would (c) Environmental considerations minimize nitrate-N loss to surface water: • Implement wetland restoration areas, denitrify- The environmental values of an area must be fully ing ponds, or managed riparian zones where considered when planning to develop a new drainage drainage water could be "treated" to remove system or improve an existing system. Alternatives excess nitrate-N before discharge into drainage and options should be evaluated from the perspective ditches or streams. of the landowner, neighbors, and the community. • Design new subsurface drainage systems or Alternatives should aim towards balanced and sustain- retrofit existing drainage systems to manage able systems that fit within the natural setting. The soil water and water table levels through action to be taken or that is proposed will be open to controlled drainage or subirrigation, lowering the scrutiny of others, and decisionmakers should be concentrations of nitrate-N in shallow ground aware of potential impacts for each alternative. Agri- water. The cost of retrofitting existing systems cultural developments, natural resource conservation, for subirrigation must be compared to the biodiversity, wildlife habitat, water quality, economics, benefit of increased yields. health, and social considerations may all play a role in • Use alternative cropping systems that contain the decisionmaking process, and appropriate evalua- perennial crops to reduce nitrate-N losses. tions should be made. Obtaining a market and a satisfactory eco- nomic return presents some barriers. (1) Water quality • Fine tune fertilizer N management. Research Installation or maintenance of drainage systems can shows that applying the correct rate of N at the cause changes to the associated ecosystem, thus care optimum time substantially affects the reduc- must be taken to consider the options and to know the tion of nitrate-N losses. adverse and beneficial impacts of actions proposed.
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• Improved management of animal manure (d) Decision matrix would contribute to lowering nitrate-N losses in livestock-producing areas. Knowing the Numerous environmental impact assessment sugges- nutrient content and application rate of the tions are available and provide guidance in developing manure, spreading it uniformly, and incorpo- a decision matrix. The decision matrix can be a critical rating it in a timely manner would all lead to tool for helping decisionmakers review options and better management and confidence in manure reach agreements on work that is to be done. In the N as a nutrient source. planning process, a number of alternative plans must be developed. The first alternative is generally to do (2) Wetlands nothing, and then more elaborate alternatives are Drainage systems impact on wetlands is an ever in- developed that may be increasingly more complex or creasing concern. Drainage equations and water more expensive. Each of these alternatives has varying movement relationships can be used to assess impacts degrees of positive or negative impact on a selected to wetland hydrology by artificial drainage. NEH, Part list of evaluations. A typical list of evaluation factors 650, Chapter 19, Hydrology Tools for Wetlands Deter- includes: mination, describes the hydrology tools or procedures • Reduce wetness impact to crops for evaluating hydrology related to wetlands and • Timeliness of field operations in wet areas appropriate distance for safe installation of subsurface • Minimize impact to downstream landowners drainage systems relative to location of wetlands. • Economics • Water quality (3) Permits • Biodiversity and wildlife habitat Required permits and regulatory issues must also • Permits and regulatory issues receive careful attention. The information for these • Soil quality and soil erosion documents or reports is frequently gathered from a • Wetland values multitude of sources, and new field data may be • Social issues needed to develop proper analytical documentation. One or more tables should be developed to display results of the impacts on the selected factors for each alternative plan. These tables or possible graphic displays benefit the decisionmaking process while working with concerned individuals or groups.
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Northern States since the 1930's. The Red River Valley 650.1410 General runoff curves, based on the SCS drainage runoff curves and adjusted to the Red River Valley of the North, have been in use for about the same period. Surface drainage is needed in flatland areas where water floods over the land. Slopes in these areas Other drainage curves have been developed for use in generally are not great enough for the flowing water to the Southeastern, Southwestern, and Western States. cause erosion; therefore, the object of drainage sys- Because of the wide variations in rainfall, soils, crops, tems in flatland areas is to remove the excess water and topography, drainage design should be based on before it damages crops. This differs from erosion drainage coefficients applicable to local conditions. control work in which the object is to control the The capacity for design given by most of the curves erosive velocity of peak flood flows. varies in accordance with the general formula (Cypress Creek equation): 0.83 When improving soils that have very low permeability Q = CM or reclaiming highly salt affected soils that need drain- age, the use of open drains has a practical installation where: advantage over pipe drainage. At a later stage when Q = flow in ft3/s for which the drain is to be the structure of these soils improves and their hydrau- designed lic conductivity values become relatively stable, cost C = the appropriate drainage curves effective subsurface pipe drains can be considered. M = area in mi2 of watershed
The drainage curves mentioned provided data to (a) Drainage runoff develop table 14–1, which facilitated the application of the Cypress Creek equation on a national basis. Ditches for drainage of common field crops generally are designed to remove runoff from the drainage area This equation provides an economical and effective within a 24-hour period following a rain event. Some design for open ditches if C is selected properly. See surface flooding of the land during this period is table 14–1 and figure 14–4, State standards and specifi- permissible. cations or local drainage guides for C values to be used. Some high-value and specialty crops require a more rapid rate of removal of runoff to prevent crop dam- The capacity for drainage ditches in arid, irrigated age. For these crops, a 6- to 12-hour removal interval areas is generally determined by the leaching require- may be necessary during the growing season. ment and amount of return flow from irrigation water. If surface runoff from precipitation is involved, the The drainage coefficient, is the rate of water removal above formula with the appropriate value of C may be per unit of area used in drainage design. For surface used. For many ditches in these areas, the depth rather drainage the coefficient generally is expressed in than the capacity required governs the size of the terms of flow rate per unit of area, which varies with drain. the size of the area.
(b) Determining drainage runoff (c) Composite drainage curves Composite curves are used where runoff from the land Curves for determining runoff for drainage design draining into an open drain must be based on different have been prepared for most of the humid areas of the drainage curves. The design capacity of ditches in United States. The curves are based on the climate, these areas can be determined by computing equiva- soils, topography, and agriculture of the particular lent drainage area information. Using this method, you area. Curves developed by John G. Sutton, known as determine the runoff from the acreage of one type of the SCS drainage runoff curves have been used in the
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Table 14–1 Drainage curves
Region Land use Drainage curves (C)
Northern humid region Pasture 25 Cultivated (grain crops) 37
Southeastern region Woodland (coastal plain) 10 Pasture (coastal plain) 30 Cultivated (coastal plain) 45 Cultivated (delta) 40 Riceland 22.5
Red River Valley (MN & ND) Cultivated 20
Southwestern region Rangeland 15 Riceland 22.5
Western region *
* Capacity for open drains is generally determined by leaching requirements and amount of return flow from irrigation water. If surface runoff from precipitation is involved, the Cypress Creek equation may be used with a value of C from the applicable state drainage guides.
Note: The drainage curves for the Cypress Creek equation (Q=CM0.83) have provided economical and effective design values for open drains in the indicated areas of the United States (Source: NRCS NEH 16).
Figure 14–4 Areal application of drainage curves
Red River Valley Washington Maine Montana North Minnesota Dakota Oregon VT Wisconsin NH Idaho South Mass New York Dakota Michigan Wyoming Rhode Island Iowa Western Region Pennsylvania Conneticut Nevada Nebraska Northern Humid Region Illinois Ind Ohio Utah W Delaware Colorado Virginia Maryland Kansas California Missouri Virginia Kentucky North Carolina Tennessee Arizona Oklahoma South New Mexico Arkansas Carolina Southeastern Region Southwestern Miss Alabama Georgia Region Texas Louisana
Florida
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land and convert it to the equivalent acreage of an- other type of land so that the acreage may be added 650.1411 Investigation together and used to find the discharge from the area. and planning of surface Example drainage systems
A watershed contains 500 acres of land requiring the runoff curve Q = 45 M0.83 and 200 acres of land requir- A surface drainage system may consist of field ditches ing the runoff curve Q = 22.5 M0.83. Either the Q = 45 and/or land forming with ditches and underground M0.83 or Q = 22.5 M0.83 curve must be converted, de- pipes to carry the drainage water to the outlet. The pending on the curve to be used on land below this drainage system should provide for an orderly removal watershed. of excess water from the surface of the land.
Assume the removal rate of this land is Q = 45 M0.83. Open drains may serve any land use. In urban uses The discharge from 200 acres on the Q = 22.5 M0.83 they are typically called open drains or surface storm curve is 8.4 ft3/s. This is about 86 acres on the Q = 45 drains, while in agricultural fields they are usually M0.83 curve. The total equivalent watershed would be referred to as ditches. 500 acres plus 86 acres, or 586 acres. The total dis- charge from 586 acres on the Q = 45 M0.83 curve is 42 Current legislation and regulations require care to ft3/s. Therefore, 586 acres and 42 cubic feet per second avoid environmental damage to existing wetlands. are used in computations below this watershed. Maintenance of drainage systems is critical to the sustainability of agriculture and for other land uses.
(a) Field ditches
Field ditches are shallow ditches for collecting and conveying water within a field. They generally are constructed with flat side slopes for ease in crossing. These ditches may drain basins or depressional areas, or they may collect or intercept flow from land sur- faces or channeled flow from natural depressions, plow furrows, crop row furrows, and bedding systems. State drainage guides and standards and specifications have criteria regarding side slopes, grades, spacing, and depth of drainage field ditches.
(b) Types of open drain systems
Drains should be located to fit the farm or other land use operations and should have capacity to handle the runoff and not cause harmful erosion. The drain sys- tem should cause excess water to flow readily from the land to the disposal drain. Five common drain systems are described in this section.
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(1) Random drain system (2) Parallel drain system This type system is adapted to drainage systems on This type system is applicable to land where the topog- undulating land where only scattered wet areas re- raphy is flat and regular and where uniform drainage is quire drainage. The ditches should be located so they needed. The ditches are established parallel but not intercept depressions and provide the least interfer- necessarily equidistant, as shown in figure 14–6. The ence with farming operations (fig. 14–5). The ditches direction of the land slope generally determines the should be shallow and have side slopes flat enough for direction of the ditches. Field ditches are generally farm equipment to cross. Precision land forming and perpendicular to the slope, and laterals run in the smoothing help to assure the removal of surface water direction of the slope. The location of diversions, cross from less permeable soil. slope ditches, and access roads for farming equipment can also influence the drain location. Spacing of the field ditches depends upon the water tolerance of crops, the soil hydraulic conductivity, and the unifor- mity of the topography. Landforming can reduce the number of ditches required by making the topography more uniform. Where possible, spacings should be adjusted to fit the number of passes of tillage and harvesting equipment.
Figure 14–5 Random drains
Depressions where runoff collects
Waste Farming operations spoil in in either direction low spots
Random field ditches (Outlet) drain
Outlet drains should be 0.5 to 1 foot deeper than (Outlet) drain the random field ditches
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Figure 14–6 Parallel surface drains
Rows continuous across field. Do not plant in the ditch bottom.
Direction of planting, cultivation, and harvesting Field slope
Outlet drain Field ditch
Drains should be about parallel but not necessarily equidistant spacing.
Outlet drain (lateral) should be about 1 foot deeper than the field ditches.
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(3) Cross slope drain system field ditches work best on slopes of less than 2 per- This system is used to drain sloping land, to prevent cent. The drain is located across the slope as straight the accumulation of water from higher land, and to as topography will permit (fig. 14–7). The spacing of prevent the concentration of water within a field. The these ditches varies with the land slope and should be
Figure 14–7 Reconstructing cross slope drain system
Cross slope drains should Land slope be constructed across the slope as straight and parallel as the topography permits with limited cutting through ridges and humps.
Main outlet drain Outlet drain After the ditches have been constructed, smooth or grade the area between the ditches.
Typical flat bottom section
Side slopes not Spread out excess excavated less than 8:1 material here
Natural ground level
Min. width 6 ft
Typical V-channel section Side slopes not less than 10:1
Natural ground level
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based on State drainage guides. The excavated mate- Beds are established to run with the land slope or in rial should be placed in low areas or on the downhill the direction of the most desirable outlet. Local infor- side of the drain. Landforming or smoothing between mation should be used to determine the width of beds, the ditches improves operation of the system by pre- the crown height, construction method, and mainte- venting the concentration of flow and the occurrence nance. of ponding. (5) Narrow raised beds (4) Bedding A narrow bed system has a raised bed wide enough for Bedding resembles a system of parallel field ditches single or double cropping rows to provide an aerated with the intervening land shaped to a raised, rounded surface profile. This system facilitates surface water surface (fig. 14–8). This drainage system generally is movement and aeration of the shallow root zone. used where slopes are flat and the soil is slowly per- When used with plastic covers for weed control, meable and where other types of drainage are not evaporation control, and nutrient management, the economically feasible. A bedding system generally is in narrow bed system can be extremely effective for small land areas and is installed using farm equipment. some cropping systems.
Figure 14–8 Surface drainage bedding
The furrows drain Land slope to field ditches Field ditch
Farming operations in either direction
300 feet to 1000 feet
Ditches100 spaced feet apart30 feet to
Grade usuallyland 1 slopepercent to 1 1/2 percent varies with Outlet drain
Bed Crown Outlet drain should be width height at least 0.5 to 1.0 foot 30 ft to 45 ft 14 in deeper than field ditch 45 ft to 65 ft 16 in 65 ft to 85 ft 20 in 85 ft to 100 ft 24 in
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(c) Landforming (2) Precision landforming Precision landforming for drainage is reshaping the Landforming refers to changing the land surface to surface of the land to planned grades so that each row ensure the orderly movement of surface water. Land or plane is graded throughout its length to field ditches smoothing and precision landforming are used in or other suitable outlets. The land is graded so that surface drainage to improve the effectiveness of the rows will carry surface runoff without overtopping. drainage system. Row or plane grades may be varied within erosive limits to provide drainage with the least amount of (1) Land smoothing grading. Local technical guides provide recommenda- Removing small irregularities on the land surface tions on precision landforming for drainage using special equipment is termed land smoothing. It does not require a grid survey and includes operations (i) Planning—Topographic surveys are needed to ordinarily classed as rough grading. plan the surface grading and the auxiliary drainage system. National Engineering Handbook, Part 623 Land smoothing is an important practice for good (Section 15), Chapter 12, Land Leveling, provides surface drainage. It eliminates minor differences in information on surveys, design methods, construction, field elevations and shallow depressions without and maintenance. Local technical guides also provide changing the general contour of the ground surface information and applicable criteria. appreciably. It results in better drainage, generally with fewer surface ditches, and enables farm equip- (ii) Construction—Precision land smoothing is ment to be operated more efficiently. Smoothing also accomplished by earth-moving scrapers (fig. 14–9), reduces ice crusting in winter. land levelers, or landplanes (fig. 14–10). The length of most land smoothing equipment varies from 20 to 50 Soils to be smoothed must have characteristics that feet, with the longer length giving a more refined job. allow small cuts. Except for isolated spots handled by The large levelers or planes that have an overall span prior rough grading, land smoothing operations sel- length of about 50 feet may require a crawler-type or 4- dom involve cuts and fills of more than 6 inches. High wheel drive tractor for power. Those that have a span and low spots generally are visible to the eye without length of 20 to 30 feet can be pulled with an ordinary use of an engineer’s level. However, sufficient spot wheel-type farm tractor. The smaller machines do a elevations should be taken and adequate planning good job on any field, but require more trips over the done to assure that water will readily move to the field field than the larger machine. ditches and laterals without ponding. To facilitate smoothing operations, the ground surface Land smoothing is accomplished best by special should be chiseled or disked before smoothing and equipment, such as the landplane or leveler, which can should be free of bulky vegetation and trash. Loosen- work efficiently to tolerances of 0.1 foot or less. This ing facilitates the movement of the dirt by the leveler degree of accuracy is necessary to remove irregulari- and mixes crop residue into the soil, thus preventing ties from flat land. Rough grading for land smoothing vegetation from collecting on the leveler blade. If the can be done with farm tractors and scrapers or with a area is in sod, it should be tilled 3 to 6 months before landplane. In making cuts, avoid removing all topsoil smoothing operations begin, or it should be farmed in from any area. It is better to take thin layers from a a cultivated crop for 1 year before smoothing. larger area. Where fills from rough grading exceed 6 inches, make an allowance for settling. The allowance The roughness of the field and the number of minor is related to the types of soil and soil conditions and depressions determine the number of passes required may be given in the local technical guides. Final to produce a smooth field. The land plane should smoothing operations may be deferred until compac- make at least three passes; one pass along each diago- tion or natural settlement of such areas has taken nal and a final pass generally in the direction of culti- place. vation.
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The first year after smoothing, settlement may occur Figure 14–10 Landplane in large depressions that were filled. On relatively flat fields, this settlement can produce pockets that collect and hold water. The field should be observed the year after smoothing. If pockets have developed, at least three more passes should be made with special em- phasis on the settled areas.
In areas where exposing the subsoil is necessary, you should remove and stockpile the topsoil, do the re- shaping, and then replace the topsoil.
Figure 14–9 Earthmoving scraper with laser grade control
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reach the junction at about the same time, and that 650.1412 Design of open therefore the drain capacity below the junction should be the sum of the two flows. This rule should be used drains in all cases for watershed areas of less than 300 acres.
Rule 2—Where the watershed area of a lateral is State drainage guides, standards, and specifications less than 20 percent of the total watershed area, give criteria for side slopes, grades, spacing, and depth the design capacity of the drain below the junc- of drainage field ditches. tion is determined from the drainage curve for the total watershed area. Mains and laterals are open ditches constructed to dispose of surface and subsurface drainage water Rule 3—Where the watershed area of a lateral is collected primarily from surface field ditches and from 20 to 40 percent of the total watershed subsurface drains. They can also intercept ground area, the discharge is proportioned from the water, help to control ground water levels, or provide smaller discharge at 20 percent to the larger for leaching of saline or sodic soils. discharge at 40 percent. In this range the discharges should be computed by both methods and the differ- Factors affecting the size and shape of ditches are ence in cubic feet per second obtained. The design drainage runoff, hydraulic gradient, depth, bottom discharge for the channel below the junction should width, side slopes, roughness of the drain bed and then be obtained by interpolation. This combination of banks, and limiting velocities. Local experience incor- rules 1 and 2 is illustrated in the following example. porated in the State drainage guide generally dictates the design factors for ditches. Example
(a) Drainage runoff A lateral drain draining 350 acres joins a drain draining 650 acres, making a total drainage area below the Runoff is determined above and below the outlet of junction of 1,000 acres. One of the watersheds is 35 contributing ditches and streams, at points of change percent of the total watershed. Using rule 3, assume in the channel slope, at culverts and bridges, and at the that the curve developed from Q = 45 M0.83 applies. outlet. Find the discharges using information in figure 14–4 and table 14–1. Runoff calculations generally begin at the upper end of the drain and proceed downstream. An empirical Rule Watershed area Runoff Q = 45 M0.83 procedure, termed the 20-40 rule, should be used in (acres) (ft3/s) computing the required capacity for a drain below a junction with a lateral. For large drainage areas, the 1 350 27 application of the procedure may have considerable 650 45 effect on the drain design. In small areas the change in 72 (total) required drain capacity may be so small that the proce- dure need not be applied. Experience in applying the 2 1,000 65 20-40 rule will guide the designer in its use. The rules 7 (difference) for computing the required capacity for a drain are: The difference between 20 and 40 percent is 20 per- Rule 1—Where the watershed area of one of the cent. Thirty-five percent is 15/20 (0.75) of the differ- ditches is 40 to 50 percent of the total watershed ence between 20 and 40 percent. Then, 0.75 times 7 area, the required capacity of the channel below ft3/s equals 5.2 ft3/s (use 5 ft3/s). Add 5 ft3/s to 65 ft3/s the junction is determined by adding the re- to arrive at 70 ft3/s for the capacity of the drain below quired design capacity of each drain above the the junction. Note that this value is an interpolation junction. This is based upon the assumption that the between the results of rule l and rule 2. flows from two watersheds of about the same size may
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Design flows are required at other points along the loss generally should not exceed 0.5 foot. drain in addition to those for ditches below junctions. • Elevations of buildings or other property within These points will be discussed later in this chapter. the area to be protected from overflow. • If the drain being designed is to outlet into an existing drain or natural stream, the elevation of (b) Drain alignment the water in the outlet drain or stream against which the designed drain must discharge should The natural topography and aesthetics should be be used as a control point. The water surface considered in determining drain alignment. Where it is elevation in the outlet ditch may be determined necessary to change direction of the drain or field from recorded data, historic observation, or high ditch, a simple curve should be used (see NEH, Part water marks. Another method of obtaining this 650, Chapter 1, Engineering Surveys). Curves that have elevation is to determine the depth of flow in the a radius greater than 600 feet are desirable, but outlet ditch by applying the same flow design sharper curves can be used if needed to follow old basis as that used for the proposed ditch. For ditches or swales, to decrease the waste area caused small outlet ditches in rather flat topography, the by the use of long radii curves, or to conform to own- water elevation may be estimated at the bankfull ership boundaries. Where the drain flows are small stage. and of low velocity, gentle curvature is not as impor- tant. Control points should be connected with a line on the profile. The hydraulic gradeline is drawn through or below the control points. The grades should be as long (c) Hydraulic gradient as possible and should be broken only where neces- sary to stay close to the control points (fig. 14–11). The hydraulic gradient is the slope of the hydraulic gradeline (water surface) and is important in deter- If the hydraulic gradeline has been well established, it mining flow velocity. Proper location of the gradeline will not be altered except at structures that have head is more important as drain flows become greater. The losses. At these control points, the head loss will be profile of the channel should be plotted showing the shown upstream from the structure as a backwater location and elevation of control points. The control curve. This will change the hydraulic gradient, al- points help to select the maximum elevation of the though generally for only a short distance. hydraulic gradeline desired for the drain. They may include, but are not limited to, the following: If the channel is in an area of flat topography and the • Natural ground elevations along the route of the hydraulic gradeline is located near field elevation, the proposed drain. hydraulic gradeline may need to be broken at culverts • Location, size, and elevation of critical low areas in amounts equal to the required head, otherwise, the to be drained. These are obtained from the backwater curve above the culvert may cause prob- topographic data. lems. The problems are compounded if several cul- • Hydraulic gradeline for side ditches or laterals verts are within a relatively short channel. established from the critical areas to the design drain. Plot the elevation where the side drain hydraulic gradeline meets the design drain as a control point. • Where laterals or natural streams enter the design drain, use the same procedure as that for hydraulic gradeline for side ditches. • Bridges across drainage ditches should not reduce the area of the design cross section. Where feasible to do so, the hydraulic gradeline should be placed 1 foot below the stringers of the bridge. The allowable head loss on culverts should be kept low. On agricultural drainage the allowable head
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Figure 14–11 Establishing hydraulic gradeline
110 110 87+50 el. 103.5 Road 80+60 Ditch from right - hydrualic gradeline el. 102.6 (control point)
NG Low area left - 300 feet 2 acres 102.0
Road 25+00 - low bridge stringer el. 101.8 S=.0004 x x Low area right - 400 feet 2 acres Hydraulic gradeline outlet ditch el. 99.50 S=.0005 x 100 x 100 HG
99.65
0+00 25+00 35+00 50+00 75+00
Note: For this design the hydraulic gradient (HG) must be 0.5 feet below natural ground (NG). The control points selected were 101.4 @ sta 35+00 and 102.90 @ sta 72+50.
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(d) Design methods • Depth to provide the capacity for removing the surface runoff plus freeboard. Design by reaches—Drain design may be done by • Depth to provide outlet for subsurface drainage. reaches (specific length of ditch). One method is to • Depth to clear bridges. design the ditch for the required capacity of the lower • Depth to allow for sufficient capacity after sub- end of the reach and use that section throughout the sidence in organic soils. reach. If this method is used, the upper end of the • Depth to trap sediment below the elevation of a reach will be overdesigned. Reaches should be se- design flow line. lected so that overdesigning is held to practical limits. To reflect the flow conditions as nearly as possible, (i) Surface discharge—Sufficient depth must be the selection of the length of reach is important. To provided for surface runoff to flow freely into the determine the beginning and the end of reaches, you ditch. Where the hydraulic gradeline has been estab- can use one of the following: lished, as shown in figure 14-11, the depth of the ditch • Tributary junctions where the required drain is measured down from the gradeline. Where the ditch capacity changes. bottom grade is established first, the depth is mea- • A break in grade of the water surface profile. sured upward to locate the water surface. Using this • Divided reach—An increasing drainage area may last method, the water surface elevation obtained must require that an otherwise long reach be divided be checked in relation to control points. The depth of into shorter reaches. A knowledge of where each reach should be determined to meet the needs of water enters the drain, as well as the amount, the specific area involved. helps to determine how the reach should be divided. (ii) Subsurface discharge—Ditches used to serve • Bridges or culverts can be used to begin or end a as outlets for subsurface drains should be deep reach. enough to provide free outfall from the underdrain with at least 1 foot of clearance between the invert of After the hydraulic gradeline has been established, the drain and the normal low water flow in the ditch. determine the drainage area and the required drain Ditches designed to intercept subsurface flows or to capacity at the upper and lower end of each reach. At serve as an outlet for subsurface drainage should be this time obtain the drainage area and the required deep enough to serve these conditions. The required capacity for any planned structure (culvert, side inlet, depth sometimes results in the ditch capacity being in grade control). This information is used in designing excess of the design flow requirements for surface the structure. runoff, and the actual hydraulic gradeline may be substantially lower. This often occurs at the upper end Point method—Drain design by the point method is of small drainage systems. sometimes preferred. In this method the required depth of the drain is determined at the control points (2) Grade of ditch bottom for the discharges at those points. This assumes that If uniform flow in the ditch is assumed, the ditch the runoff throughout the reach enters uniformly along bottom grade will have the same slope as the hydraulic the reach. In reality, the depth at the beginning and gradeline. The required depth of the ditch is deter- end of a reach differ. The depths are established at mined and measured at points below the hydraulic points below the hydraulic gradeline, and the bottom gradeline. These points are then connected to find the of the drain is drawn between the points. The slope of bottom grade of the ditch. This method of locating the the drain bottom normally is not parallel to the hy- ditch bottom is generally satisfactory in designing a draulic gradeline. At points where concentrated flow new ditch. If this method is used, you may prefer to enters, a change in depth or width, or both, may be adjust the bottom grade to eliminate the need for required. A transition may also be required at these transitions. points. In reconstruction of existing ditches, the elevation of (1) Drain depth the existing ditch bottom, the bridge footings, and the Factors that must be considered in establishing the soil strata must be considered as control points in drain depth are: establishing the bottom of the designed ditch. At
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times, it may be more convenient to locate the bottom Local information may indicate that steeper side of the ditch from these control points and measure the slopes can be used in certain soils; however, flatter design depth up from the bottom to locate the hydrau- side slopes may be desirable for more satisfactory and lic gradeline (water surface). Cross sections must be economical maintenance. Ditch side slopes that can be used in deciding upon the hydraulic grade or the ditch used with various maintenance methods are given in bottom. Ditch bottom grades that will be erosive table 14–2. during normal flows are to be avoided. (4) Velocity in ditches (3) Ditch side slopes The velocity in a ditch is ideal if neither scouring nor The side slopes of ditches are determined primarily by sedimentation occur. Because flows in most ditches the stability of the material through which the ditch is are intermittent, the velocity will fluctuate. Ditches are dug and by the methods of maintenance to be prac- generally designed for the maximum design flow and ticed. Recommended sideslopes may be found in many the allowable average velocity in the ditch section. local drainage guides. Maintenance requirements may Table 14–3 give the recommended limiting velocities at necessitate modification. The steepest side slopes design flow depth for various soils and material. If the recommended for ordinary conditions for mains or velocity is above these limits, scouring and erosion laterals are as follows: Material Side slope (Horizontal to Table 14–3 Permissible bare earth velocities vertical ratio)
Solid rock, cut section 0.25:1 Soil texture Maximum velocity Loose rock or cemented gravel, cut section 0.75:1 ft/s Heavy clay, cut section 1/ 1:1 Heavy clay, fill section 1/ 2:1 Sand or silt with clay binder, cut or fill section 1.5:1 Sand and sandy loam (noncolloidal) 2.5 Loam 2:1 Peat, muck, and sand 2/ 1:1 Silt loam (also high lime clay) 3.0 Sandy clay loam 3.5 1/ Heavy clays in CH soils often experience sliding problems caused by the structure of the clays. The recommended side Clay loam 4.0 slope for these soils is no steeper than 4:1. Stiff clay, fine gravel, graded loam to gravel 5.0 2/ Silts and sands that have a high water table in the side slopes will slough due to hydrostatic pressure gradient. The recom- Graded silt to cobbles (colloidal) 5.5 mended side slope for these saturated conditions is no steeper than 3.5:1. Shale, hardpan, and coarse gravel 6.0
Table 14–2 Ditch side slopes recommended for maintenance
Type of maintenance Recommended steepest Remarks side slopes
Mowing and grazing* 3:1 Flatter slopes desirable Dragline or backhoe 0.5:1 Generally used in ditches (more than 4 feet deep) that have steep side slopes Blade equipment 3:1 Flatter slopes desirable Chemicals Any Use caution near crops and open water. Follow manufacturer's recommendations
* Hydraulically operated booms that have a 10- to 14-foot reach may be used to mow side slopes as steep as 1:1.
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may take place. Because raw, newly dug ditches may increases. Table 14–4 gives the recommended values have a lower roughness coefficient (Manning's n) than of n. They are the values that may be expected after they will have after some aging of the ditch takes aging. The n value that occurs immediately after place, they may have higher than design velocities. construction will be lower than those given in the Where vegetation on the ditch side slopes is slow in table. Unless special site studies are available to deter- developing, the limiting velocities may need to be mine the value of n, the table 14–4 values shall be reduced. Minimum design velocities should not be less used. than 1.4 feet per second to prevent sedimentation. Short field ditches may be used to capture sediment (7) Ditch bottom width for a water quality benefit. In this case, design velocity The machinery used for construction of the ditch should be less than 1.4 foot per second with provision should be considered in the selection of ditch bottom to return sediment to the field. width. A bulldozer or blade equipment is used to construct V-shaped ditches. Flat bottom ditches fre- (5) Determination of ditch velocity quently are designed if scrapers, hydraulic hoes, or Manning's equation is used in determining the average draglines are to be used to construct the ditch. Depth velocity in a ditch section. of ditch and soil conditions affect the type of equip- ment used. Specified minimum bottom widths are 1. 486 0.. 667 0 5 V = RS often based on the available equipment. n (8) Relationship between depth and bottom where: width V = velocity (ft/s) The most economical ditch cross-section approaches n = roughness coefficient that of a semicircle. A deep, narrow ditch generally R = hydraulic radius (ft) = A/P carries more water than a wide, shallow ditch of the S = slope (ft/ft) same cross-sectional area. An excessively wide, shal- A = cross-sectional area below hydraulic gradeline low ditch tends to develop sand or silt bars, which 2 (ft ) cause ditch meandering and bank cutting, and a fairly P = wetted perimeter (ft) deep, narrow ditch tends to increase velocities and reduce siltation and meandering. Because the cross- For information on computation, refer to EFH Chapter section selected is a matter of judgment, all factors 3, Hydraulics, or use any appropriate computational involved should be considered. Ditches shall be de- procedure. signed to be stable. In some cases economy and hy- draulic efficiency must be sacrificed in the interest of (6) Value of roughness coefficient n ditch stability and maintenance. The value of n is a factor in Manning's formula for computing velocity. It indicates not only the roughness of the sides and bottom of the channel, but also other types of irregularities of the channel, such as align- ment and vegetation. The value of n is used to indicate Table 14–4 Value of Manning's n for drainage ditch design the net effect of all factors causing retardation of flow. The selection requires judgement in evaluating the material in which the channel is constructed, the Hydraulic radius n irregularity of surfaces of the ditch sides and bottom, the variations in the shape and size of cross sections, and the obstructions, vegetation, and meandering of Less than 2.5 0.040 - 0.045 the ditch. 2.5 to 4.0 0.035 - 0.040
The National Engineering Handbook, section 16, 4.0 to 5.0 0.030 - 0.035 relates n values to the hydraulic radius and indicates More than 5.0 0.025 - 0.030 that these values decrease when the hydraulic radius
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(9) Calculation of ditch capacity The structure must meet two requirements: The volume (Q) of water passing a ditch cross section • They must be of sufficient size and located so as is calculated in cubic feet per second (ft3/s) and is the to pass the design flow within the allowable head product of the flow area cross section (A) in square loss. feet (ft2) and the average velocity in the cross section • They must have adequate strength, size, and (V) expressed in feet per second (ft/s). The formula is: durability to meet the requirements of traffic. QAV= The following formula can be used to compute the minimum culvert length without headwalls. Various curves, tables, and computer software, all LW=+2 SH based on Manning's formula for velocity, have been prepared to determine ditch capacities (appendix where: 14A). L = minimum culvert length W = top width of fill over culvert, not less than levee (10) Ditch berms top width Berms should be designed to: S = side slopes of fill over culvert • provide roadways for maintenance equipment; H = height of fill measured from culvert invert • provide work areas; • facilitate spoil-bank spreading; Existing structures should be measured to determine • prevent excavated material from washing back their capacity. An existing structure may be consid- into ditches; and ered adequate if it will pass the design drainage flow • prevent sloughing of ditchbanks caused by with a head that does not cause overbank flow above placing heavy loads too near the edge of the the structure. As a safety factor, new culvert installa- ditch. tions generally are designed for 25 percent more capacity than the ditch design. A new bridge should be The recommended minimum berm widths are as designed to span the ditch, have the bottom of the follows: stringers placed at least 1 foot above the hydraulic Ditch depth Minimum berm width gradeline, and preferably have no piers placed in the ft ft center of the ditch.
2 – 68 The hydraulic gradeline at structures does not need to 6 – 810 be broken as long as the head loss required to pass the > 8 15 design flow does not cause overbank flooding above the structure. The hydraulic gradeline should be ad- (11) Spoil banks justed if such flooding will occur. The gradeline may Spoil-bank leveling or shaping is a common and desir- be broken and dropped down at the structure at an able practice. The degree of leveling, the placing of the amount equal to the head loss. spoil, and other practices related to the spoil generally are determined locally and are specified in local drain- Head loss is negligible through bridges where the age guides or State standards and specifications. channel cross section is not restricted. Head loss through culverts ordinarily should not exceed 0.5 foot. (12) Bridges and culverts It can be reduced by increasing the size of the struc- Design criteria for structures required for drainage ture. ditches, or where irrigation canals cross drainage ditches, are specified by the authority responsible for (13) Culvert flow the structure. The capacity requirement for the struc- Culverts are used for several types of flow. Detailed ture may be for flood flows that are much more in- knowledge of hydraulics is necessary under some tense than the drainage requirement. On some town- situations for the design of culverts. The following ship, private, and field roads the only requirement may situations commonly occur where culverts are used: be that the structure carry drainage flow. The follow- • Culverts flowing full with both ends submerged. ing information is limited to this type of structure. • Culverts flowing full with unsubmerged or free discharge.
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• Culvert flow limited by culvert entrance condi- (15) Surface water inlets to ditches tions. All drainage into mains and laterals should be through planned inlets, rather than at random, which can cause See EFH Chapter 3, Hydraulics, for design procedure. rills or severe bank erosion. This may be accomplished See also appendix 14B. by installing chutes, drop spillways, pipe drop inlet spillways, culverts, or other suitable structures (fig. (14) Ditch junctions 14–12). For additional information on inlets, see EFH, The bottom grades of ditches having about the same Chapter 6, Structures. depth and capacity should be designed to meet at or near the same elevation. The bottom of a shallow, (16) Swinging watergates, cattle guards, and small capacity ditch may be designed to meet a larger ramps ditch at or near the normal or low flow elevation of the Where applicable, watergates, cattle guards, and larger ditch. ramps should be used on open ditches to manage livestock and to protect the ditches. See figure 14–13a A transition is designed where a shallow ditch enters a for a typical watergate plan and figure 14–13b for much deeper ditch. Before beginning a transition, the photograph of similar watergate installation for live- grade of the shallow ditch generally is designed 10 to stock crossing. 100 feet upstream on a zero grade at the elevation of the deeper ditch. The transition should be on a non- erosive grade not to exceed 1 percent.
Where the difference in the elevation of the ditch gradelines is considerable and transition grades seem impractical, a structure should be used to control the drop from the shallow ditch to the deeper ditch. See EFH Chapter 6, Structures, for additional information.
Figure 14–12 Surface water inlets
Flared inlet
Drop Main Straight Hooded drain
Main drain
OtherOther suitable suitable types types of inlets of inlets
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Figure 14–13 Watergate
a Watergate plan
2- by 4-inch blocks for Tighten bushings 3/4-inch or 1-inch bolt deadman 14-inch posts No. 9 wire or with stick C preferably cedar L doubled strands of barbed wire Horizontal and vertical pieces are 2 feet center to center All are rough 1 inch by 6 inches except top horizontal hanger 1- by 4-inch which is 2 by 4 inches 3 ft-6 in stiffener 4 ft-6 in 2- by 4-inch 2 by 4 inches 3 ft-6 in Natural hanger ground