NATIONAL ENGINEERING HANDBOOK SECTION 16 DRAINAGE OF AGRICULTURAL LAND CHAPTER 4. SUBSURFACE DRAINAGE Contents Page Introduction General Definition and purpose of subsurface drainage Sources of excess water Diagnosis and Improvement of Saline and Alkali Soils General Saline and alkali soils defined Saline soils Saline-alkali soils Nonsaline-alkali soils Effect of salts on crops Reclamation of saline and alkali soils Reclamation of saline soils Reclamation of nonsaline-alkali soils Reclamation of saline-alkali soils Boron Planning Subsurface Drainage Observation well hydrographs Profile flow patterns Ground-water contour maps Depth to water-table map Classification of subsurface drainage General Relief drainage Open ditches Buried drains Parallel system Herringbone system Double-main system Random system Pumping system (ground-water removal) Combination sys tern Mole drains Vertical drains Interception drains General Open ditches Buried drains Outlets for subsurface drainage Special situations Use of relief wells Salt-water intrusion in coastal areas Guidelines for prevention Planning a subsurface drainage system Design of Subsurface Drains Drainage coefficients Humid areas Arid areas Design capacity Relief drains Interception drains Combination surface and subsurface drainage systems Depth and spacing of drains General Relief drains Humid areas Irrigated areas Ellipse equation Modified ellipse equation Artesian areas Use of open ditches for relief drainage Interception drains Multiple interception drains Mole drains Drainage area Grade Outlets Length of lines Installation Design General A1 i gnmen t Connections Loads on drains General Underground conduits Classification of conduits as to rigidity Classification of conduits based on installation Bedding conditions for rigid ditch conduits Trench depth for rigid conduits Bedding conditions for flexible drainage tubing Drain grades and velocities Determining drain size Sizing of drains within the drainage system Materials for drains General Standards and specifications Clay drain tile Concrete drain tile Bituminized fiber pipe Plastic drains Metal pipes and others Page Filters and envelopes Determination of need for filters and envelopes Materials for filters and envelopes Organic filters and envelopes Fiber-glass filters Plas tic-f abric filters Sand and gravel filters and envelopes Design of filters and envelopes Design of sand and gravel filter Design of Kiber-glass filters Design of organic filters Design of sand and gravel envelopes Drain appurtenances Outlet structures Protection from animals Junction boxes Pressure relief vent Breathers Surface water inlets Blind inlet or french drain Multiple drains in depressional areas Drain crossings Construction General Drain installatioti by hand Batter boards Equipment used for trenching Excavating the trench Laying drains Blinding drains Backfilling the trench Drain installation by machine Staking the drain Trenching machines Laying the tile Installation of corrugated-plastic-drainage tubing Drain junctions and curves Placing filters and envelopes Trencher placement of granular filters and envelopes Trencher placement of fabric filters Hand placement of granular filters Blinding the drain Backfilling the trench Problems involved in drain installation Quicksands and silts Inspection of drain installation General Checking grade Mole construction Equipment Mole size Depth Spacing Construction Construction procedure Construction of open ditches for subsurface drainage Maintenance of Buried Drains General Outlets Water-surface inlets Sand traps and catch basins Blowouts Tree roots Auxiliary structures Waterways over drains Mineral deposits Miscellaneous References Figures Figure 4-la Salt tolerance of field crops Figure 4-lb Salt tolerance of vegetable crops Figure 4-lc Salt tolerance of forage crops Figure 4-2a Soil profile showing high capillarity Figure 4-2b Soil profile showing low capillarity Figure 4-3 Observation well hydrograph Figure 4-4 Observation well hydrograph Figure 4-5 Typical ground-water contour map Figure 4-6 Working drawing (canal seepage) Figure 4-7 Surface contour above ground-water contour Figure 4-8 Profile section A-A, Figure 4-7 Figure 4-9 Types of drainage collection systems Figure 4-10 Interception drain in a valley area Figure 4-11 Interception drain for barrier condition Figure 4-12 Interception drain at outcrop of aquifer Figure 4-13 Relief well installation Figure 4-14 Interception drain in a constricted aquifer Figure 4-15 Fresh water-salt water conditions Figure 4-16 Working map--topography Figure 4-17 Working map--ground-water contours Figure 4-18 Working map--depth to ground water Figure 4-19 Working map--completed Figure 4-20 Profile--Figure 4-19 Figure 4-21 Graphical solution - drainage coefficient Figure 4-22 Sketch of relief drain system showing symbols in equation 4-3 Figure 4-23 Graphical solution - drain design discharge Figure 4-24a Relief ditch Figure 4-24b Relief drain Figure 4-25a Interception ditch Figure 4-25b Interception drain Figure 4-26 Isometric profiles relief and interception drains Figure 4-27 Cross-sectional sketch showing symbols used in ellipse equation Figure 4-28 Solution of ellipse equation (Sheet 1 of 2) (Sheet 2 of 2) Figure 4-29 Graphical solution of modified (Sheet 1 of 3) ellipse equation (Sheet 2 of 3) (Sheet 3 of 3) Figure 4-30 Solution of modified ellipse equation Figure 4-31 Cross-sectional profile, interception drain and area influenced Figure 4-32 Mole drainage system on flatland Figure 4-33 Mole drainage system on sloping land Figure 4-34 Buried drain outlet for mole drain Figure 4-35 Mole drain outlets Figure 4-36 Capacity chart - n = 0.011 (Sheet 1 of 3) n = 0.013 (Sheet 2 of 3) n = 0.015 (Sheet 3 of 3) Figure 4-37 Mechanical analyses of gravel filter material Figure 4-38 Filter installation Figure 4-39 Outlet protection Figure 4-40 Drain outlets Figure 4-41 Rodent protection for outlet pipe Figure 4-42 Junction box for drains Figure 4-43 Vent Figure 4-44 Surface water inlet Figure 4-45 Manhole catch basin or sediment trap Figure 4-46 Blind inlet Figure 4-47 CLosely spaced drains in wet areas Figure 4-48 Drain crossings Tables Table 4-1 Relative salt tolerance of fruit crops 4-7 Table 4-2 Boron tolerance of crops 4-11 Table 4-3 Observation well data 4-36 Table 4-4 Drainage coefficients without surface-water inlets 4-43 Table 4-5 Maximum allowable trench depth 4-80 Table 4-6 Drain grades and velocities 4-83 Table 4-7 Classification to determine need for drain filters, or envelopes, and minimum velocities in drains 4-91 NATIONAL. ENGINEERING HANDBOOK SECTION 16 DRAINAGE OF AGRICULTURAL LAM) CHAPTER 4. SUBSURFACE DRAINAGE Introduction This chapter covers subsurface drainage in both humid and arid areas of the United States. The division between the subhumid and the semiarid areas is approximately the 100th meridian. The boundary separating the subhumid from the dry lands receives close to 18 inches of precipitation in the north and about 25 inches in Texas (1). The coastal areas of the Pacific Northwest, the Gulf Coast, and small scattered areas within the intermountain region are humid areas with average rainfall over 20 inches. Irrigated areas in the United States are largely in the arid and semiarid portions of the country. Chapter 2, Drainage Investigations, of this National Engineering Handbook discusses investigations and surveys commonly used in agricultural drainage operations. Reference will be made to Chapter 2 for information in regard to general methods and techniques for conducting these investigations. This chapter supplements the information in Chapter 2 with more detailed infor- mation on certain phases of drainage investigations. Subsurface-drainage conditions, drainage benefits, planning, design, materials, installation, and maintenance will be discussed in this chapter. In most humid areas, many years of experience with subsurface drainage instal- lation have provided the main basis for determining drainage requirements for various soil types and problem areas. Special investigations are necessary for drainage of soils where experience is lacking. There are some differences in the cause, effect, and solutions of drainage problems in the arid and semiarid regions, but in general the investigational methods, design, construction, and maintenance are similar. In areas where there have been many years of drainage experience, investigations may be standardized and routine. Where experience has been limited or special problems exist, more extensive investigations are necessary. High water tables, seepage, soil salinity and/or alkalinity are problems that usually require special investigation and consideration. General Definition and purpose of subsurface drainage Subsurface drainage is defined as the removal of excess ground water below the ground surface. In many wet areas both surface and subsurface drainage are required. Surface ditches are necessary to remove excess runoff from precipitation and to dispose of surface flow from irrigation. These surface ditches should be planned to complement the subsurface drainage system. Surface drainage reduces the amount of water to be removed by the subsurface system and permits better control of the water table. Subsurface drainage lowers the high water tables which are caused by precipitation, irrigation water, leaching water, seepage from higher lands or irrigation canals and ditches and ground water under artesian pressure. 4- 1 A high water table damages most crops to varying degrees. Soil bacterial