====== Stone Outlet Protection (Riprap) ======

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

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

=====Design=====
====Size====
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\\
D<sub>o</sub> = Pipe diameter (ft)\\
T = Apron thickness (in)\\
D<sub>50</sub> = Median stone size (in)

====Stone====
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 (D<sub>50</sub>).  

Once the D<sub>50</sub> has been selected, 50% of the stone, by weight, should be larger than the D<sub>50</sub>.  The diameter of the largest stone should not exceed 1.5 times the D<sub>50</sub> 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 D<sub>50</sub> Sizes and Shapes===

^  Maximum Velocity (ft/sec)  ^  D<sub>50</sub> Cubical (in)  ^  D<sub>50</sub> 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.

===Recommended Stone Gradation===

^  Percent Passing by Weight  ^  Diameter (in)  ^
|  100  |  2 x D<sub>50</sub>  |
|  60-85  |  1.5 x D<sub>50</sub>  |
|  25-50  |  D<sub>50</sub>  |
|  5-20  |  0.5 x D<sub>50</sub>  |
|  0-5  |  0.2 x D<sub>50</sub>  |

[{{ :wiki:images:correct_riprap.jpeg?direct&600 |Correctly installed riprap at pipe outlet}}]

[{{ :wiki:images:poorly_installed_riprap.jpeg?direct&600 |Erosion around riprap from improper shaping}}]

=====Construction=====
  *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

**Stone Outlet Protection to a Flat Area (Left) and Well-Defined Channel (Right)**
{{ :wiki:images:stone_outlet_protection.png?600 |}}

=====Maintenance=====
  *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.

===References===
  *[[https://efotg.sc.egov.usda.gov/api/CPSFile/31113/468_WI_CPS_Lined_Waterway_or_Outlet_2021|NRCS Conservation Practice Standard 468 - Lined Waterway or Outlet]]