United States Department of Agriculture 606-CPS-1

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NRCS reviews and periodically updates conservation practice standards. To obtain the current 

version of this standard, contact your Natural Resources Conservation Service State office or 

visit the Field Office Technical Guide online by going to the NRCS website at 

https://www.nrcs.usda.gov/ and type FOTG in the search field. 

USDA is an equal opportunity provider, employer, and lender.

Natural Resources Conservation Service 

CONSERVATION PRACTICE STANDARD 

SUBSURFACE DRAIN 

CODE 606 

(ft)

DEFINITION 

A conduit, or system of conduits, installed beneath the ground surface to manage soil water conditions. 

PURPOSE 

Use this practice to accomplish one or more of the following purposes: 

• Remove or distribute soil water 

• Remove salts and other contaminants from the soil profile 

• Mitigate degraded plant health and vigor and undesirable plant productivity due to saturated soil,

ponding, and flooding 

• Mitigate degraded animal health productivity due to saturated soil, ponding, and flooding 

CONDITIONS WHERE PRACTICE APPLIES 

This practice applies to all land uses where a shallow water table exists or where a subsurface drainage

system can mitigate one or more of the following adverse conditions caused by excessive soil moisture: 

• Poor health, vigor, and productivity of plants 

• Poor field trafficability 

• Accumulation of salts in the root zone 

• Health risk and livestock stress due to pests 

• Adverse soil conditions around farmsteads, structures, and roadways 

This practice also applies to water distribution through subsurface drain pipe for utilization or treatment. 

CRITERIA 

General Criteria Applicable to All Purposes 

Plan, design, and construct this practice to comply with all applicable Federal, State, Tribal, and local laws

and regulations. The landowner must obtain all necessary permissions from regulatory agencies or

document that no permits are required. The landowner or contractor is responsible for locating all buried

utilities in the project area, including drainage tile and other structural measures. 

If wetlands are present, complete an appropriate wetland determination per established procedures. Avoid

adverse effects of the drainage system on the ecology and hydrology of the site and on adjacent lands,

especially potential or delineated wetlands, existing easements, and wildlife habitat. 

Capacity 

Base design capacity on the following, as applicable: 
• Recommendations from the State drainage guide 

• Application of a locally proven drainage coefficient for the acreage drained 

• The yield of ground water based on the expected deep percolation of irrigation water from the

overlying fields, including leaching requirement 

• Comparison of the site to other similar sites with known subsurface drain yields 

• Measurement of the rate of subsurface flow at the site during a period of adverse weather and

ground water conditions 

• Application of Darcy’s law to lateral or artesian subsurface flow 

• Contributions from surface inlets based on hydrologic analysis or flow measurements 

• Drain inlet opening size versus drain intake rate 

Size 

Compute the drain size by applying Manning’s formula, using roughness coefficients recommended by the

manufacturer of the conduit or coefficients adopted by the regulating authority. Base the size on the

maximum design flow rate and compute using one of the following: 

• The hydraulic grade line parallel to the bottom grade of the subsurface drain with the conduit

flowing full at design flow (normal condition, no internal pressure) 

• Conduit flowing partly full where a steep grade or other conditions require excess capacity 

• Conduit flowing under internal pressure with hydraulic grade line set by site conditions, which differs

from the bottom grade of the subsurface drain 

All subsurface drains must have a nominal diameter that equals or exceeds 3 inches. 

Internal hydraulic pressure 

Drains are normally designed to flow with no internal pressure, and the flow is normally classified as open

channel. Maintain the design internal pressure of drains at or below the limits recommended by the

manufacturer of the conduit. 

Horizontal alignment 

Accomplish a change in horizontal direction of the subsurface drain by one of the following methods: 

• The use of manufactured fittings 

• The use of junction boxes or manholes 

• A gradual curve of the drain trench on a radius in accordance with the limitations of the installation

equipment and the recommendations of the pipe manufacturer while maintaining grade 

Location, depth, and spacing 

Base the location, depth, and spacing of the subsurface drain on site conditions including soils, plants,

topography, ground water conditions, crops, land use, outlets, saline or sodic conditions, and proximity to

wetlands. 

The minimum depth of cover over subsurface drains may exclude sections of conduit near the outlet or

through minor depressions, providing these sections of conduit are not subject to damage by frost action

or equipment travel. 

In mineral soils, provide a minimum depth of cover over subsurface drains of 2 feet. 

In organic soils, provide a minimum depth of cover after initial subsidence of 3 feet. If water control

structures are installed and managed to limit oxidation and subsidence of the soil, the minimum depth of

cover may be reduced to 2.5 feet. 

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For flexible conduits, base maximum burial depth on the manufacturer’s recommendations for the site

conditions or on a site-specific engineering design consistent with methods in NRCS National Engineering

Handbook (NEH) (Title 210), Part 636, Chapter 52, “Structural Design of Flexible Conduits.” 

For computation of maximum allowable loads on subsurface drains of all materials, use the trench and

bedding conditions specified and the compressive strength of the conduit. Base the design load on the

conduit from a combination of embedment, backfill, and live loads. 

Base live loads on the maximum equipment or vehicle wheel loads. Equipment loads on the conduit may

be neglected when the depth of cover exceeds 6 feet. 

Minimum velocity and grade 

In areas where sedimentation of fine sands and silts is not a hazard, design the minimum grade based on

site conditions and velocity of not less than 0.5 feet per second. If a sedimentation potential exists, use a

velocity of not less than 1.4 feet per second to establish the minimum grade. Otherwise, include provisions

for preventing sedimentation by use of filters or by collecting and periodically removing sediment from

installed traps, or by periodically cleaning the lines with high-pressure jetting systems or cleaning

solutions. Prior to using high-pressure jetting systems, verify that the jetting system will not damage the

pipe or the pipe embedment. 

Maximum velocity 

Limit the maximum velocity in a perforated corrugated plastic drainage pipe under open channel flow to 12

feet per second unless further restricted by the manufacturer’s recommendation. 

Limit the maximum design velocities in open-joint pipe (clay or concrete) to those given in table 1 if

protective measures are not installed. Refer to NRCS 210-NEH, Section 16, Chapter 4, “Subsurface

Drainage.” 

Table 1. Maximum Flow Velocities by Soil Texture 

Protective measures for high velocities in open-joint pipe may include the following, as appropriate: 

• Bed the conduit in a sand and gravel envelope that is filter-compatible with the joint openings and

surrounding soil 

• Wrap the joints with nonwoven geotextile 

Releases from water control structures must not cause flow velocities in perforated or open-joint drains to

exceed allowable velocities in table 1, unless protective measures are installed. 

Thrust control 

Follow pipe manufacturer’s recommendations for thrust control or anchoring where the following

conditions exist: 

• Axial forces that tend to move the pipe down steep slopes 

• Thrust forces from abrupt changes in pipeline grade or horizontal alignment that exceed soilbearing strength 

• Reductions in pipe size 

In the absence of manufacturer’s data, design thrust blocks in accordance with NRCS 210-NEH-636-52. 

Soil Texture Velocity (ft/s)

Sand and sandy loam 3.5

Silt and silt loam 5.0

Silty clay loam 6.0

Clay and clay loam 7.0

Coarse sand or gravel 9.0

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Outlets 

Provide drainage outlets adequate for the quantity and quality of water to be discharged. 

Avoid submerged outlets unless intermittent submerged outlets are designed for protection from root

clogging. For discharge to streams or channels, locate the outlet invert above the elevation of normal flow

and at least 1 foot above the channel bottom. 

For outlets into sumps, locate the discharge elevation above the elevation at which pumping is initiated. 

Protect outlets against erosion and undermining of the conduit, entry of tree roots, damaging periods of

submergence, and entry of rodents or other animals into the subsurface drain. 

Use a continuous section of rigid pipe, without open joints or perforations and with stiffness necessary to

withstand expected loads at the outlet end of the drain line. Table 2 shows minimum lengths for the outlet

section of the conduit. Unsupported single-wall corrugated plastic pipe is not suitable for the section that

outlets into a ditch or channel. 

Table 2. Minimum Length of Outlet Pipe Sections 

The use and installation of outlet pipe must conform to the following requirements: 

• If burning vegetation on the outlet ditch bank is likely to create a fire hazard, select fireproof

material for the pipe 

• Bury at least two-thirds of the rigid outlet pipe section in the ditch bank and project the cantilever

section past the toe of the ditch side slope; or protect the side slope from erosion below the outlet

pipe 

• If ice or floating debris may damage the outlet pipe, protect the pipe by recessing the cantilevered

part of the pipe to protect it from the current of flow in the ditch or channel 

• Headwalls used for subsurface drain outlets must be adequate in strength and design to avoid

washouts and other failures 

For outlets into sumps, locate the discharge elevation above the elevation at which pumping is initiated. 

Protection from biological and mineral clogging 

Drains in certain soils are subject to clogging of perforations by bacterial action in association with ferrous

iron, manganese, or sulfides. Iron ochre can clog drain openings and seal manufactured (fabric) filters.

Manganese deposits and sulfides can clog drain openings. 

Where bacterial activity is expected to lead to clogging of drains, provide access points for cleaning the

drain lines. 

Where possible, outlet individual drains to an open ditch to isolate localized areas of contamination and to

limit the translocation of contamination throughout the system. 

Protection from root clogging 

Problems may occur where drains are near perennial vegetation. Drain clogging may result from root

penetration by water-loving trees, such as willow, cottonwood, elm, and soft maple, and some shrubs,

grasses, and deep-rooted perennial crops growing near subsurface drains. Refer to the USDA PLANTS

Database website as a preliminary planning tool for obtaining vegetation properties and qualities

information. 

Pipe Diameter (in) Minimum Section Length (ft)

8 and smaller 10

10 to 12 12

15 to 18 16

Larger than 18 20

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Use one or more of the following steps to reduce the incidence of root intrusion: 

• Install a continuous section of nonperforated pipe or tubing with sealed joints through the root zone 

• Remove water-loving trees for a distance of at least 100 feet on each side of the drain and locate

drains a distance of 50 feet or more from noncrop tree species 

• Provide for intermittent submergence of the drain to limit rooting depth by installing a structure for

water control that allows for raising the elevation of the drain outlet (e.g., an inline weir with

adjustable crest) 

Utilize the intermittent submergence option only when the raised elevation of the drain outlet will not

adversely impact the performance of the land use supported by the subsurface drain, and where the

raised elevation will not adversely impact flooding on neighboring properties. 

Water quality 

Do not connect septic systems to the subsurface drainage system, nor allow animal waste to be directly

introduced into the subsurface drainage system. 

Materials 

Subsurface drains include flexible conduits of plastic, bituminized fiber, or metal; rigid conduits of vitrified

clay or concrete; or other materials of acceptable quality. 

The conduit must meet strength and durability requirements for the site. All conduits must meet or exceed

the minimum requirements of the appropriate and current specifications published by the American

Society for Testing and Materials (ASTM), American Association of State Highway Transportation Officials

(AASHTO), or the American Water Works Association (AWWA). 

Conduit foundation 

If soft or yielding foundation conditions are encountered, stabilize the conduit foundation and protect the

area from settlement. The following methods are acceptable foundation treatments: 

• Remove the unstable material and provide stable bedding of granular envelope or filter material. 

• Provide continuous cradle support for the conduit through the unstable section. 

• Bridge unstable areas using long sections of a conduit having adequate strength and stiffness to

ensure satisfactory subsurface drain performance. 

• Place conduit on a flat, treated plank. This method must not be used for flexible conduits (e.g.,

plastic pipe) without proper bedding between the plank and conduit. 

• Use bedding to avoid laying the pipe on rock, rocky soil, or extremely hard soil. 

Conduit Placement and bedding 

Placement and bedding requirements apply to both trenching and plow-type installations. 

Place the conduit on a firm foundation to ensure proper alignment. 

The conduits must not be placed on exposed rock, stones greater than 1.5 inches for conduits 6 inches or

larger in diameter, or stones greater than three-fourths of an inch for conduits less than 6 inches in

diameter. Where site conditions do not meet this requirement, the trench must be over-excavated a

minimum of 6 inches and refilled to grade with suitable bedding material. For trench installations where a

sand-gravel envelope or compacted bedding is not specified, the conduit embedment must be suitable

backfill. Soil excavated from the trench may be used for backfill as long as it contains no hard objects

larger than the specified stone sizes above. Place initial backfill to a minimum of 3 inches above the

conduit. Compact backfill to a density similar to the surrounding soil material. Mound the backfill over the

trench to provide material for settling. 

If installation will be below a water table or where unstable soils are present, special equipment,

installation procedures, or bedding materials may be needed. These special requirements may also be

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necessary to prevent soil movement into the drain or plugging of the envelope if installation will be made

in materials such as soil slurries. 

For the installation of corrugated plastic pipe with diameter of 8 inches or less, specify one of the following

bedding methods: 

• Provide a shaped groove with an angle of support of 90 degrees or greater in the bottom of the

trench for tubing support and alignment 

• Provide a sand-gravel envelope, at least 3 inches thick, for support 

• Provide compacted embedment material beside and to 3 inches above the conduit 

For the installation of corrugated plastic pipe with diameter larger than 8 inches, the same bedding

requirements apply except that a semicircular or trapezoidal groove shaped to fit the conduit with a

support angle of 120 degrees will be used rather than a V-shaped groove. 

For rigid conduits installed in a trench, the same requirements apply except that a groove or notch is not

required. 

Filter envelopes and materials 

Design filters around conduits, as needed, to enhance water entry and stabilize the structure of the

surrounding soil material. Determine the need for a filter by the characteristics of the surrounding soil

material, site conditions, and the velocity of flow in the conduit. Use a suitable filter if any of the following

conditions exist: 

• Local experience with soil site conditions indicates a need 

• Soil materials surrounding the conduit are dispersed clays, silts with a plasticity index less than 7,

or fine sands with a plasticity index less than 7 

• The soil is subject to cracking by desiccation 

• The method of installation may result in inadequate consolidation between the conduit and backfill

material 

Design the sand or sand-gravel filter gradation in accordance with NRCS 210-NEH, Part 633, Chapter 26,

“Gradation Design of Sand and Gravel Filters.” 

Specified filter material must completely encase the conduit such that all openings are covered with at

least 3 inches of filter material, except where the top of the conduit and side filter material will be covered

by a sheet of plastic or similar impervious material to reduce the quantity of filter material required. In all

cases, the resulting flow path through the filter material must be a minimum of 3 inches in length. 

Use geotextile filter materials with the effective opening size, strength, durability, and permeability to

prevent soil movement into the drain throughout the expected life of the system. Where the silt content in

the soil exceeds 40 percent, ensure the geotextile filter material will not clog during its design life. 

Hydraulic envelopes and materials 

Use an envelope around subsurface drains, as needed, to improve flow conditions in the area immediately

adjacent to the drain. 

Materials used for envelopes must not contain materials that will cause an accumulation of sediment in

the conduit or render the envelope unsuitable for bedding of the conduit. 

Envelope materials must consist of sand-gravel, organic, or similar material. Use an envelope gradation

such that 100 percent of sand-gravel passes a 1.5-inch sieve, not more than 30 percent passes a number

60 sieve, and not more than 5 percent passes a number 200 sieve. 

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Organic or other compressible envelope materials must not be used below the centerline of flexible

conduits. If organic or other compressible materials are used they must be of a type that will not readily

decompose within the expected lifespan of the practice. 

Refer to NRCS 210-NEH, Part 650, Chapter 14, “Water Management (Drainage),” for more complete

definitions of envelopes (e.g., hydraulic envelope, filter envelope, and bedding). 

Auxiliary structures and protection 

The capacity of any structure installed in the drain line must be no less than that of the line or lines feeding

into or through the structure. 

Structures for water table management, with provisions to elevate the outlet and allow submergence of

the upstream drain, must meet applicable design criteria in NRCS Conservation Practice Standard (CPS)

Structure for Water Control (Code 587). Mark buried boxes with surface evidence or referenced to fixed

aboveground markings or structures. 

Underground outlets connected to the subsurface drainage system must meet the applicable provision of

NRCS CPS Underground Outlet (Code 620). Design the capacity of the surface water inlet to be no

greater than the maximum design flow in the downstream drain line or lines. Install sediment traps if

sediment might pose a problem. 

Specify pressure relief wells as needed to allow excess flow to escape the conduit and flow over the

ground surface. Use pressure relief wells where there is a stable outlet for the flow from the relief well.

Cover the relief well with a grate or other appropriate means to prevent accidental entry of machines,

animals, humans, and debris. Design the subsurface drain system to have a positive hydraulic grade to

the relief well flow line. Base the relief well system capacity on the flow from the drainage system and

other site conditions. Capacity must be adequate to lower the water head to the desired level. Relief wells

must not be less than 4 inches in diameter. 

Junction boxes, manholes, catch basins, and sediment traps must be accessible for maintenance. Provide

a clear opening of not less than 2 feet in diameter or 2 feet by 2 feet square. 

Protect the drain system against turbulence created near outlets, surface inlets, or similar structures. Use

continuous nonperforated or closed-joint pipe in drain lines adjoining the structure where excessive

velocities will occur. 

As an alternative to manufactured fittings, install a junction box where three or more lines join or if two

lines join at different elevations. Use a solid cover if the junction box is buried. The junction box should

have a minimum of 1.5 feet of soil cover. Protect buried boxes from traffic. 

If not connected to a structure, close the upper end of each subsurface drain line with a tight-fitting cap or

plug of the same material as the conduit, or other durable materials. 

Use watertight conduits designed to withstand the expected loads where subsurface drains cross under

irrigation canals, ditches, or other structures. 

CONSIDERATIONS 

When planning, designing, and installing this practice, consider— 

• Protection of shallow drains, auxiliary structures, and outlets from damage due to freezing and

thawing 

• Proper surface drainage to reduce the required capacity of the subsurface drainage system 

• Designs that can incorporate drainage water management practices (or facilitate future

incorporation of drainage water management) to reduce nutrient loading of receiving waters,

including downstream drinking water sources. 

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• Drainage laterals oriented along elevation contours to improve the effectiveness of drainage water

management structures 

• The effects of drainage systems on runoff volume, seepage, and the availability of soil water

needed for plant growth 

• Confirmation of soil survey information with site investigation, including augering and shallow

excavations to identify soil profile hydraulic characteristics, soil texture layering, water table depth,

etc 

• Maximizing wetland functions and values to the extent practicable 

• Subsoiling or ripping of soils with contrasting texture layers to improve internal drainage. Where this

treatment is needed use NRCS CPS Deep Tillage (Code 324) 

• Installations in a dry soil profile to minimize problems of trench stability, conduit alignment, and soil

movement into the drain 

• The effects to surface water quality 

• Using measures to reduce the risk of drain water contamination from surface applications of

manure (e.g., temporary flow-blocking devices) 

• Using NRCS CPSs Drainage Water Management (Code 554), Constructed Wetland (Code 656),

Saturated Buffer (Code 604), or Denitrifying Bioreactor (Code 605) in conjunction with this standard

where removal of nitrate nitrogen in subsurface drainage is needed 

• Including a tailwater reuse system that conforms to NRCS CPS Irrigation and Drainage Tailwater

Recovery (Code 447) in conjunction with this standard where excess soil water will be reused for

irrigation 

• The potential existence of a hazardous atmosphere in junction boxes or manholes 

PLANS AND SPECIFICATIONS 

Prepare plans and specifications for installing subsurface drains according to the applicable criteria that

describe the requirements for implementing the practice to achieve its intended purpose. 

At a minimum, plans and specifications must include, as applicable— 

• Location and plan view of the drainage system 

• Conduit lengths, grades, spacing, sizes, and type of materials 

• Requirements and typical cross sections or details for the subsurface drain, filter, envelope, and

bedding 

• Structure locations, dimensions, and elevations 

• Outlet locations, elevations, and protection required 

• Location of utilities and notification requirements 

• Construction specifications describing site-specific installation requirements of the subsurface drain 

OPERATION AND MAINTENANCE 

Provide an operation and maintenance (O&M) plan with specific instructions for operating and maintaining

the system to ensure proper function as designed. At a minimum, the O&M plan must address— 

• Necessary periodic inspection and prompt repair of system components (e.g., structures for water

control, underground outlets, vents, drain outlets, trash, and rodent guards) 

• Winterization protection from freezing conditions (if applicable) for drainage systems in cold

climates 

• Protection requirements during manure applications (if applicable) 

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REFERENCES 

USDA NRCS. 2008. National Engineering Handbook (Title 210), Section 16, Chapter 4, Subsurface

Drainage. Washington, D.C.https://directives.sc.egov.usda.gov/

USDA NRCS. 2017. National Engineering Handbook (Title 210), Part 633, Chapter 26, Gradation Design

of Sand and Gravel Filters. Washington, D.C. https://directives.sc.egov.usda.gov/

USDA NRCS. 2005. National Engineering Handbook (Title 210), Part 636, Chapter 52, Structural Design

of Flexible Conduits. Washington, D.C. https://directives.sc.egov.usda.gov/

USDA NRCS. 2021. National Engineering Handbook (Title 210), Part 650, Chapter 14, Water

Management (Drainage). Washington, D.C. https://directives.sc.egov.usda.gov/

USDA NRCS. “PLANTS Database.” Accessed February 17, 2021. https://plants.sc.egov.usda.gov/

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