NATURAL RESOURCES CONSERVATION SERVICE

CONSERVATION PRACTICE STANDARD

SUBSURFACE DRAIN

(Ft.)

Code 606

# DEFINITION

A conduit, such as corrugated plastic tubing, tile, or pipe, installed
beneath the ground surface to collect and/or convey drainage water.

# PURPOSE

-   Improve the soil environment for vegetative growth, reduce erosion,
    and improve water quality by:

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-   Regulating water table and ground water flows

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-   Intercepting and preventing water movement into a wet area

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-   Relieving artesian pressures

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-   Removing surface runoff

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-   Leaching of saline and sodic soils

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-   Serving as an outlet for other subsurface drains

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-   Regulating subirrigated areas or waste disposal areas

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-   Collect ground water for beneficial uses

-   Remove water from heavy use areas, such as around buildings, roads,
    and play areas; and accomplish other physical improvements related
    to water removal

-   Regulate water to control health hazards caused by pests such as
    flukes, flies, or mosquitoes

# CONDITIONS WHERE PRACTICE APPLIES

This standard applies to areas having a high water table where the
benefits of lowering the

water table or controlling ground water or surface runoff justify
installing such a system.

This standard applies to areas suitable for the intended use after
installation of required drainage and other conservation practices. The
soil shall have enough depth and permeability to permit installation of
an effective and economically feasible system.

In areas where an outlet is available, either by gravity flow or by
pumping, the outlet shall be adequate for the quantity and quality of
water to be discharged.

# CRITERIA

**General.** Subsurface drains shall not be used to convey or dispose of
any waste or process water.

Wells or sinkholes shall not be used as outlets for subsurface drains.

The design and installation shall be based on adequate surveys and
investigations.

**Outlets.** Natural occurring channels or constructed ditches in good
condition may be used as outlets for subsurface drains. The outlet of
the subsurface drain shall be placed a minimum of 1.0 foot above the
seasonal high base flow elevation or a minimum of 1.0 foot above the
bottom for channels/ditches that only flow water during surface runoff
events.

The outlet must be protected against erosion undermining of the conduit,
entry of tree roots, damaging periods of submergence and entry of
rodents or other animals into the subsurface drain. A continuous section
of rigid pipe without open joints or perforations shall be used at the
outlet end of the line. Standard corrugated plastic tubing is not
suitable for the outlet section.

Continuously submerged outlets shall be permitted for water table
control systems if planned and designed according to Drainage Water
Management (554).

The outlet pipe and its installation shall conform to the following
requirements:

-   If burning vegetation on the outlet ditch bank is likely to create a
    fire hazard, the material from which the outlet pipe is fabricated
    must be fire resistant. If the likelihood is great, the outlet pipe
    must be fireproof

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-   Two-thirds of the outlet pipe shall be buried in the ditch bank, and
    the cantilever section must extend to the toe of the ditch side
    slope or the side slope protected from erosion. The minimum length
    of the pipe shall be\
    20 feet

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-   If ice or floating debris may damage the outlet pipe, the outlet
    shall be recessed to the extent that the cantilevered part of the
    pipe will be protected from the current in the ditch

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-   Headwalls used for subsurface drain outlets must be adequate in
    strength and design to avoid washouts and other failures

Existing subsurface main(s) in good condition may be used as outlets for
new subsurface laterals if the in situ mainline is positioned such that
installed laterals meet all applicable criteria found within this
standard. Existing subsurface main(s) to be utilized as an outlet shall
have a minimum capacity for the greater of either:

-   ⅜ inch per day for the sum of the existing tiled area and the area
    drained by the new laterals, or

-   1 inch per day for the area(s) draining into surface intakes (i.e.:
    Terraces and Water and Sediment Control Basins)

**Capacity.** ~~The drainage coefficient (rate of water removal in
inches per 24 hours) for all new mains and new laterals shall be a
minimum\
of ½ inch.~~

The above paragraph was inserted into the April 2008 606 standard and
the following paragraphs removed with the following comments:

"Based upon current industry recommendations and citing the concerns of
plant health and potential yield reduction affiliated with inadequate
drainage it was logical to make all new subsurface drainage systems meet
a minimum drainage coefficient of ½ inch."

The fact that this statement conflicts with the Iowa Drainage Guide was
overlooked. The following guidance is in the national standard and was
in the 2002 Iowa standard, but removed from the 2008 standard following
the insertion of the above paragraph. To eliminate the conflict with the
drainage guide, we'll put this guidance back in the standard.

Guidance for the selection of an appropriate drainage coefficient can be
found in the Iowa Drainage Guide and NEH Part 650 (EFH) Chapter 14. For
new installations, the minimum drainage coefficient shall be 3/8 inch
per day. If surface water enters the system, the minimum drainage
coefficient shall be 1 inch per day for the area contributing surface
runoff.

Alternatively, one or more of the following shall be used to determine
the required capacity:

~~Application of a locally tried and proven drainage coefficient to the
acreage drained, including added capacity required to dispose of surface
water entering through inlets.~~

-   Yield of ground water based on the expected deep percolation of
    irrigation water from the overlying fields, including the leaching
    requirement.

-   Comparison of the site with other similar sites where subsurface
    drain yields have been measured.

-   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.

-   Estimates of lateral or artesian subsurface flow.

**Size.** The diameter of corrugated plastic tubing, clay, and concrete
drain tile for a known acreage of land can be obtained from the
nomographs within NEH, Part 650, Chapter 14; NEH,\
Part 624, Chapter 10; or the Iowa Drainage Guide. The diameter of
subsurface drains made of other materials may be calculated using
Manning's Formula along with guidance from NEH, Part 650, Chapter 3.

All subsurface drains shall have a nominal diameter that equals or
exceeds 4 inches. All\
4-inch subsurface drains designed on grades flatter than 0.4 percent
shall be limited to the following installation length restriction:

-   Do not exceed 1,800 feet for clay or concrete drains

-   Do not exceed 1,500 feet for plastic tubing drains

Larger diameter subsurface drains may be designed and installed if any
of the following conditions exist or will exist:

-   Potential groundwater movement from lower depths upward into the
    drainage zone

-   An indefinite collection area of side hill seepage

-   Potential loss of capacity from misalignment, non-uniform drain
    grade or build-up of sedimentation within the drain

Use a minimum 6-inch nominal diameter segmented non-flexible drain pipe
or minimum 5-inch nominal diameter flexible non-segmented drain pipe in
peat and muck soils greater than\
4 feet deep.

Use a minimum 5-inch nominal diameter drain for collecting spring or
side hill seepage from an area greater than 1.0 acre or when draining
non-cohesive loess and sandy soils, including soils that contain pockets
or layers of sand in the upper 5 feet.

**Depth, Spacing, and Location.** The depth, spacing, and location of
the subsurface drain shall be based on site conditions, including soils,
topography, ground water conditions, crops, land use, outlets, and
saline or sodic conditions.

Designers may use Table 2-2 "Drainage Guidelines for Iowa Soils" from
the Iowa Drainage Guide for determining drain spacing and depth
parameters.

The minimum depth of cover over subsurface drains, after initial
settlement, shall be a minimum of 2.4 feet in mineral soils and 3.0 feet
in organic soils. A minimum 2.0 feet of cover shall be allowed for
sections of pipe near the outlet laid through minor depressions where
the conduit is not subject to damage by frost action or equipment
travel.

The minimum depth of all drains in normal field conditions shall be 3.0
feet. Mains or laterals crossing under roads, railroads, canals, ditches
or other man-made structures shall be designed and constructed of a
watertight material that is engineered for the load and environmental
condition to which it is exposed.

The maximum depth of cover for subsurface drains shall comply with the
Table 3 parameters of Underground Outlets (620). Installations at depths
beyond those listed in Table 3 will require an engineering analysis in
accordance with NEH, Part 636, Chapter 52 "Structural Design of Flexible
Conduits" and specific installation criteria.

All subsurface drains shall be designed with connections that are placed
center to center or higher.

**Minimum Velocity and Grade.** In areas where sedimentation is not a
hazard, minimum grade shall be based on site conditions and a velocity
of not less than 0.5 feet per second (ft/s).

If a hazard exits, a velocity of not less than\
1.4 ft/s shall be used to establish the minimum grade if site conditions
permit. Otherwise, provisions shall be made 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.

Minimum permissible grade shall be in accordance with the
recommendations set forth in the Iowa Drainage Guide.

**Maximum Velocity Without Protection.** Design velocities shall not
exceed those given in Table 1 unless special protective measures are
installed.

  ----------------------------------- -----------------------------------
  **Table 1. Maximum Velocities By    
  Soil Texture**                      

  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
  ----------------------------------- -----------------------------------

**Maximum Grade and Protection.** On sites where topographic conditions
require that drain lines be placed on steep grades and design velocities
will be greater than indicated in\
Table 1, special measures shall be used to protect the conduit or
surrounding soil. These measures shall be specified for each job
according to the particular conditions of the job site.

The protective measure shall include one or more of the following:

-   Enclose continuous perforated pipe or tubing with fabric-type filter
    material or properly graded sand and gravel

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-   Use non-perforated continuous tubing, a watertight pipe, or seal
    joints

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-   Place the conduit in a sand and gravel envelope or blinding with the
    least erodible soil available

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-   Select rigid butt end pipe or tile with straight, smooth sections
    and square ends to obtain tight fitting joints

-   Wrap open joints of the pipe or tile with tar impregnated paper,
    burlap, or special fabric-type filter material

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-   Install open-air risers for air release or entry

**Iron Ochre Control.** If drains are to be installed in sites where
iron ochre and manganese

dioxide problems are likely to occur, provisions shall be made to
provide access for cleaning the lines. Each drain line should outlet
directly into an open ditch and/or should have entry ports as needed to
provide access for cleaning equipment. Drain cleaning provisions should
be installed in such a way that the drains can be cleaned in an upstream
or rising grade direction. If possible, drains in ochre-prone areas
should be installed during the dry season when the water table is low
and the iron and manganese dioxide is in its insoluble form.

Where possible, in areas where the potential for such problems is high,
protection against their development can be provided by designing an
outlet facility to ensure permanent submergence of the drain line.

**Protection Against Root Clogging.** Problems may occur where it is
necessary to place drains in close proximity to perennial vegetation.
Roots of water loving trees, such as willow, cottonwood, elm, and soft
maple, or some shrubs and grasses growing near subsurface drains may
enter and obstruct the flow.

Where possible, use non-perforated tubing or closed joints through the
root zone area. Where this is not possible, water-loving trees should be
removed from a distance of at least 100 feet on each side of the drain.
A distance of 50 feet should be maintained from other species of trees
except for fruit trees. Orchards can often be drained by drain lines
located close to the fruit trees.

**Materials.** Subsurface drains include conduits of plastic, clay,
concrete, bituminized fiber, metal, or other materials of acceptable
quality.

The conduit shall meet strength and durability requirements for the
site. All conduits shall meet or exceed the minimum requirements of the
appropriate specifications published by the American Society for Testing
and Materials (ASTM), American Association of State Highway and
Transportation Officials (AASHTO), and the American Water Works
Association (AWWA). A detailed listing of these material standards and
specifications can be found in NEH, Part 650, Chapter 14.

**Foundation.** If soft or yielding foundations are encountered, the
lines shall be stabilized and protected from settlement by adding gravel
or other suitable materials to the trench, or by using long sections of
perforated or watertight pipe having adequate strength to ensure
satisfactory subsurface drain performance.

**Filters and Filter Material.** Filters will be used around conduits,
as needed, to prevent movement of the surrounding soil material into the
conduit. The need for a filter will be determined by the characteristics
of the surrounding soil material, site conditions, and the velocity of
flow in the conduit. A suitable filter shall be specified if:

-   Local experience indicates a need

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-   Soil materials surrounding the conduit have a plasticity index less
    than 7 or are dispersed clays

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-   Deep soil cracking is expected

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-   The method of installation may result in voids between the conduit
    and backfill material

If a sand-gravel filter is specified, the filter gradation shall be
designed in accordance with the procedural guidance in NEH, Part 633,
Chapter 26, Gradation Design of Sand and Gravel Filters.

Specified filter material must completely encase the conduit so that all
openings are covered with at least 3 inches of filter material. Not more
than 10 percent of the filter material shall pass the No. 60 sieve.

Artificial fabric or mat-type filter materials may be used, provided
that the effective opening size, strength, durability, and permeability
are adequate to prevent soil movement into the drain throughout the
expected life of the system.

**Envelopes and Envelope Material.** Envelopes shall be used around
subsurface drains if they are needed for proper bedding of the conduit
or to improve the characteristics of flow of ground water into the
conduit.

Materials used for envelopes do not need to meet the gradation
requirements of filters, but they must not contain materials that will
cause an accumulation of sediment in the conduit or that will render the
envelope unsuitable for bedding of the conduit.

Envelope materials shall consist of sand-gravel, organic, or similar
material. Sand-gravel envelope materials shall all pass a 1.5-inch
sieve; not more than 30 percent shall pass a\
No. 60 sieve; and not more than 5 percent shall

pass the No. 200 sieve. ASTM-C-33 fine aggregate may be used in lieu of
the aforementioned gradation.

Where organic or other compressible materials are used, they shall be
used only around a rigid wall conduit and above the centerline of
flexible tubing. All organic or other compressible material shall be of
a type that will not readily decompose.

**Placement and Bedding.** All subsurface drains shall be laid to a neat
line and grade. Conduits shall not be placed on exposed rock or stones
more than 1.5 inches in diameter for 6 inch or larger pipe and stones no
more than ¾ inch diameter for pipe less than 6 inches. Where such
conditions are present the trench must be over-excavated a minimum of 6
inches and refilled to grade with a suitable bedding material.

The conduit must be placed on a firm foundation to ensure proper
alignment.

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
necessary to prevent soil movement into the drain or plugging of the
envelope if installation will be made in such materials as quicksand or
a silt slurry.

For trench installations of corrugated plastic tubing 8 inches or less
in diameter, one of the following bedding methods shall be specified:

-   A shaped groove or 90 V-notch in the bottom of the trench for tubing
    support and alignment

-   A sand-gravel envelope, at least 3 inches thick, to provide support

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-   Compacted soil bedding material beside\
    and to 3 inches above the tubing

For trench installations of corrugated plastic tubing larger than 8
inches, the same bedding requirements shall apply except that a
semi-circular or trapezoidal groove shaped to fit the conduit will be
used rather than a V-shaped groove.

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

All trench installations should be made when the soil profile is in its
driest possible condition in order to minimize problems of trench
stability, conduit alignment, and soil movement into the drain.

For trench installations where a sand-gravel or compacted bedding is not
specified, the conduit should be blinded with selected material
containing no hard objects larger than 1.5 inches in diameter. Blinding
should be carried to a minimum of 3 inches above the conduit.

**Auxiliary Structures and Protection.** Structures installed in drain
lines must not unduly impede the flow of water in the system. Their
capacity must be no less than that of the line or lines feeding into or
through them. The use of internal couplers for corrugated plastic tubing
shall be allowed.

If the drain system is to carry surface water flow, the capacity of the
surface water inlet shall not be greater than the maximum design flow in
the drain line or lines. Covers or trash racks should be used to ensure
that no foreign materials are allowed in the drain lines.

The capacity of a relief well system will be based on the flow from the
aquifer, the well spacing, and other site conditions and will be
adequate to lower the artesian water head to the desired level. The
minimum size of relief wells shall be\
5 inches in diameter.

Junction boxes shall be installed where three or more lines join or if
two lines join at different elevations. In some locations it may be
desirable to bury junction boxes. A solid cover shall be used, and the
junction box shall have a minimum of 1.5 feet of soil cover.

Junction boxes, manholes, catch basins, and sand traps must be
accessible for maintenance. A clear opening of not less than 2 feet will
be provided in either circular or rectangular structures.

The drain system must be protected against velocities exceeding those
given in Table 1 and against turbulence created near outlets, surface
inlets or similar structures. Non-perforated or closed-joint pipe must
be used in drain lines adjoining structures where excessive velocities
will occur.

If not connected to a structure, the upper end of each subsurface drain
line will be capped with a tight-fitting external cap of the same
material as the conduit or other durable materials.

# CONSIDERATIONS

Consider installation of tile blocks, stoppable catch basins, or other
temporary flow blocking devices if the subsurface drain may pick up
polluted water from manure spreading.

Use grass filter strips around inlets to reduce downstream impacts of
water quality.

Where the roots of crops and grasses may cause trouble on drain lines,
facilities may be installed to provide a means for submerging the line
to terminate the root growth as desired or to maintain a water table
above the drain lines to prevent growth into the system.

# PLANS AND specifications

Plans and specifications for installing subsurface drains shall be in
keeping with this standard and shall describe the requirements for
properly installing the practice to achieve its intended purpose.

The following list of Construction Specifications is intended as a guide
to selecting the appropriate specifications for each specific project.
The list includes most, but may not contain all, of the specifications
needed for a specific project:

IA-1 Site Preparation

IA-5 Pollution Control

IA-6 Seeding and Mulching for Protective\
Cover

IA-11 Removal of Water

IA-21 Excavation

IA-23 Earthfill

IA-24 Drainfill

IA-27 Diversions

IA-45 Plastic (PVC, PE) Pipe

IA-46 Tile Drains for Land Drainage

IA-51 Corrugated Metal Pipe

IA-52 Steel Pipe Conduits

IA-61 Loose Rock Riprap

IA-95 Geotextile

IA-620 Underground Outlets

# OPERATION AND MAINTENANCE

A site-specific operation and maintenance (O&M) plan shall be provided
to, and reviewed with, the landowner(s) before this practice is
installed. The plan shall adequately guide the landowner(s) in the
routine maintenance and operational needs of subsurface drains. The plan
shall also include guidance on periodic inspections and post-storm
inspections to detect and minimize damage.

# REFERENCES 

[Iowa Drainage Guide]{.underline}, Iowa State University Special Report
13

USDA-NRCS, National Engineering Handbook (NEH), Part 624, Chapter 10

USDA-NRCS, National Engineering Handbook (NEH), Part 633, Chapter 26

USDA-NRCS, National Engineering Handbook (NEH), Part 636, Chapter 52

USDA-NRCS, National Engineering Handbook (NEH), Part 650, Engineering
Field Handbook (EFH), Chapters 3 and 14