Stone outlet protection, or riprap, is designed to release water from management practices at non-erosive velocities. An apron of angular stone is placed at the outlet to prevent erosion by dissipating the energy of water as it flows over.
Advantages
Disadvantages
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:
$$ L=1.7*(\frac{Q}{{D_o}^\frac{3}{2}})+8D_o $$ $$ W=3*D_o $$ $$ T=D_{50}*2 $$
where
L = Apron length (ft)
W = Apron width (ft) at the end wall
Q = Flow rate (cfs) for the 10-year design storm
Do = Pipe diameter (ft)
T = Apron thickness (in)
D50 = Median stone size (in)
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 Velocity (ft/sec) | D50 Cubical (in) | D50 Spherical (in) |
---|---|---|
10.8 | 12 | 14 |
9.9 | 10 | 12 |
8.8 | 8 | 10 |
7.6 | 6 | 8 |
6.3 | 4 | 6 |
5.0 | 2 | 4 |
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.
Percent Passing by Weight | Diameter (in) |
---|---|
100 | 2 x D50 |
60-85 | 1.5 x D50 |
25-50 | D50 |
5-20 | 0.5 x D50 |
0-5 | 0.2 x D50 |
Stone Outlet Protection to a Flat Area (Left) and Well-Defined Channel (Right)
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.