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Slope Drain

General

A temporary slope drain is a flexible pipe that is designed to carry concentrated runoff from the top of a slope to the base of the slope without causing erosion. Runoff is intercepted upslope of a disturbed area and is routed to the slope drain, which carries the runoff to a stable outlet, where it is released at a non-erosive velocity into a sediment trap or basin.

Temporary slope drains are applicable on sites with a maximum drainage area of 5 acres and on slopes 3 percent or steeper that have not yet been stabilized. This practice is used in conjunction with several other BMPs such as temporary diversions, stone outlet protection, and sediment traps and basins.

Advantages

  • Prevents gully erosion
  • Relatively easy to install
  • Inexpensive

Disadvantages

  • Maximum life span of 25 years
  • Maximum drainage area of 5 acres

Design

Pipe Selection and Installation

The pipe should be constructed of a durable, flexible, corrugated plastic with secure, watertight joints and a flared inlet portion. The size of the pipe will vary depending upon the drainage area of the site, but must not have a diameter that exceeds 30 inches. To prevent failure of the device during large storm events, the soil surrounding the pipe must be hand compacted, with the portion of the diversion or berm above the pipe at least twice the height of the pipe. In cases where the normal berm or diversion height is less than this, the portion above the pipe will be returned to the previous height by descending with a slope of 3:1 or flatter, see Temporary Diversion.

The pipe should be anchored to the slope according to the manufacturer’s installation instructions. However, regardless of the model selected, it must be secured to the slope with grommets in at least 2 places spaced no more than 10 feet apart. At the base of the slope, a minimum of 4 feet of pipe should have slope of 1% or flatter before discharging in order to reduce the velocity of the runoff.

Proper Sizing of Slope Drains

Maximum Drainage Area (ac)Pipe Diameter (in)
0.512
0.7515
1.518
2.521
3.524
5.030
Source: NRCS Planning and Design Manual

Inlets

To prevent erosion of the diversion structure, the inlet of the slope drain should be underlain with geotextile filter fabric. The type of filter fabric that is selected will vary upon the individual characteristics of the site, but must extend at least 5 feet from the inlet, with the edges keyed at least 6 inches into the ground.

Outlets

While all outlets must discharge runoff at non-erosive velocities, the outlet structure used will vary depending upon the amount of cover that is present on site. Unstabilized sites must be discharged into a sediment trap or basin, while stabilized sites may drain into a stone outlet.

Proper Slope Drain Installation

Source: NRCS National Catalog of Erosion Control and Storm Water Management

Example of Temporary Slope Drain

Source: NRCS National Catalog of Erosion Control and Storm Water Management

Construction & Maintenance

  • Construction of slope drains should be completed before any disturbance begins
  • Slope drains should be inspected weekly and after each rainfall event and repairs made immediately

Method to Determine Practice Efficiency

The efficiency for this practice is derived from the reduction in slope length that it provides. To calculate the efficiency, simply use the new, reduced slope length in place of the pre-existing one in the USLE and recalculate. The difference between the two completed equations is the efficiency for the practice.