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Stone Outlet Protection


Stone outlet protection is designed to release water from management practices at non-erosive velocities. An apron of heavy stone is placed at the outlet to prevent erosion by dissipating the energy of water as it flows over the stone.


  • Cost-effective
  • Relatively easy to construct
  • Prevents erosion to receiving structures


  • Large storms may displace rock
  • Removal of accumulated sediments is difficult
  • Not applicable on steep slopes



Stone outlet protection must be capable of handling the peak outflow of the structure.

For outlets which discharge into channels, the riprap apron should extend across the channel bottom and up the sides. If the outlet discharges onto a flat area, the upstream portion of the apron should be at least three times as wide as the pipe diameter.

To determine the necessary length and width of the stone outlet protection, the following equations should be used, respectively:

In these equations, L is the apron length (ft), W is the apron width (ft) measured at the end wall, Q is the flow rate (cfs) for a 10-year storm event, and Do is the pipe diameter (ft). The minimum thickness of the riprap apron is determined by multiplying the D50 by 2.


Stone outlet protection should consist of clean, angular stone that is resistant to weathering. Recycled concrete may also be used provided it has a density of at least 150 pounds per square inch and is clear of any steel or reinforcing agents. The stone should be sized by using the median stone size (D50).

Once the D50 has been selected, 50% of the stone, by weight, should be larger than the D50. The diameter of the largest stone should not exceed 1.5 times the D50 size. The remaining portion of the stone should be well graded with a sufficient amount of smaller stones to fill the voids between the larger stones.

Maximum Velocities for Various D50 Sizes and Shapes

Maximum Velocity (ft/sec)D50 Cubical (in)D50 Spherical (in)
A geotextile fabric barrier should be installed underneath the stone to prevent the stone from settling and to prevent erosion of the outlet structure. Care should be taken during construction to ensure that the fabric is not torn, cut, or punctured. Any damage should be repaired before proceeding with construction.

Recommended Stone Gradation

Percent Passing by WeightDiameter (in)
1002 x D50
60-851.5 x D50
5-200.5 x D50
0-50.2 x D50


  • Side slopes should be 2:1 or flatter
  • Bottom grade should be equal to zero
  • The apron should be straight, without any bends
  • Construction should be completed before any water is allowed through the outlet
  • In applications where damage is possible to the outlet structure, the stone should be hand placed
  • The apron must be at least 1.5 feet below the invert elevation of the outlet structure

Manning's "n" Values for Various Rock Sizes

Diameter (in)"n" Value

Stone Outlet Protection to a Flat Area (Left) and Well-Defined Channel (Right)

Source: Adapted from the Georgia Stormwater Manual


  • Stone outlet protection should be inspected after all storm events for displaced stones – all necessary repairs should be made immediately
  • Accumulated sediments should be removed periodically

Method to Determine Practice Efficiency

Stone outlet protection prevents erosion of the receiving structure by dissipating the energy of water. This practice is required with many management practices and, as a result, no efficiency is given for this practice.