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Subsurface Drain

General

Subsurface drains (or underdrains) are tiles, pipes, or tubes installed below ground that collect and transport stormwater to an outlet. They are constructed from a variety of materials, including plastic, clay, and concrete and may be perforated to allow infiltration into the surrounding soil. They are used in areas with high water tables, poorly drained soils, or on slopes to prevent saturation and to remove the possibility of subsidence. They may also be used in areas where vegetation is needed to provide ground cover or as an outlet for detention areas and other structures with small drainage areas. Subsurface drains should not be relied upon to provide stormwater control by themselves, however, as they are designed as conveyance structures only and do not enhance water quality. As a result, they should be used in conjunction with other management practices.

Advantages

  • Reduces the thermal impact of stormwater
  • Improves drainage

Disadvantages

  • Limited effectiveness enhancing water quality
  • Cost
  • Has a tendency to plug

Design

Types

There are two types of subsurface drainage systems. Relief drains are used to lower the water table to allow the growth of vegetation or are used to remove surface water in the direction of the slope. Interceptor drains are used on slopes to prevent the soil from becoming saturated and to prevent slippage. They are placed across the slope and generally drain to the side of the slope.

Capacity

The capacity of subsurface drains will depend upon the individual characteristics of the site, but should be large enough to completely drain the basin it serves within 24 to 48 hours. To ensure proper drainage, the conduit should be at least 3 inches in diameter and provide a minimum flow velocity of 0.5 feet per second. However, in areas where sediment is present in the water column in large quantities, the flow velocity should be at least of 1.4 feet per second to prevent sediment accumulation in the drain. Maximum flow velocities vary depending upon soil type and are listed below.

However, these flow velocities may be exceeded if a continuous perforated pipe or tube, enclosed with a geotextile filter fabric, is used. Perforations should not exceed ½ inch in diameter, and slotted perforations should not be wider than 1/8 inch. The geotextile prevents the soil surrounding the drain from migrating into the conduit. The fabric selected will depend upon the size of the particles surrounding the conduit and should be durable enough to last the life of the practice. In applications where subsurface drains are used as outlets for detention areas, geotextile filter fabric is recommended for all discharge velocities due to the potential for high sediment loads.

Maximum Flow Velocities Without Protection

Soil TypeMaximum Flow Velocity (ft/sec)
Sand, sandy loam3.50
Silt, silt loam5.00
Silty clay loam6.00
Clay, clay loam7.00
Source: National Resources Conservation Service

Installation

Subsurface drains should be installed in a trench with a firm foundation to ensure proper alignment. The sections of the conduit should be supported on the sides with suitable bedding material, which should be properly placed and compacted. This prevents the conduit from collapsing during backfilling operations. All fittings and couplers should be compatible with the conduit material and should be installed following the manufacturer’s specifications. The trench should be free of stone or other materials that are larger than 1.5 inches in diameter. In applications where this is not feasible, the trench should be overexcavated by at least 6 inches and refilled to grade with ¼ inch pea gravel or gravel screenings as bedding material.

Envelopes are used around subsurface drains as bedding material or to improve the flow characteristics of groundwater. They are not required to meet the gradation requirements of the filter material, but should be large enough so that it does not accumulate in the drain.

In installations where it is necessary to install the drain below the water table or on unstable soils, special equipment, procedures, and bedding material may be required to prevent soil movement into the conduit or from plugging the envelope material.

Outlets

Subsurface drains should discharge through a rigid, continuous, non-perforated pipe that is free of joints and curves to a stable outlet that is designed to handle the maximum flow of the drain. The outlet should have the same invert elevation as the drain and should follow the design criteria for stone outlet protection.

Construction

  • Subsurface drains should be located at least 50 feet from trees to prevent damage to the structure by roots
  • Installation should take place when the soil is as dry as possible to minimize alignment and other problems
  • The ends of the drain should be covered with a trash rack and rodent protection to prevent clogging
  • All materials should be thoroughly inspected for quality before they are installed
  • Backfilling should be performed carefully to avoid displacement, deflection, or damage to the conduit

Maintenance

  • Drains should be inspected periodically to ensure that they are draining properly - all repairs should be made immediately

Method to Determine Practice Efficiency

Subsurface drains are designed as a conveyance practice and do not significantly reduce the amount of suspended sediment in stormwater runoff. As a result, no efficiency is given for this practice.