Rock Outlet protection

​Don’t let stormwater dig a hole you can’t get out of!

​Fast moving water pouring out of a pipe can quicky lead to erosion. Planning ahead and utilizing rock outlet protection can save future maintenance headaches. In this month’s article we will see where this practice is useful, consider design, and review maintenance considerations. Let’s start by identifying where the practice is helpful.
Rock outlet protection can be useful where discharge velocities from a channel, storm drain, or culvert are high enough to cause erosion. The practice can be applied for the following outlet types:

Stormwater Basin Outlets

Culvert Outlets

Constructed Channel Outlets

​For design of the rock outlet protection, assume the most severe soil and vegetation cover conditions. The size of the watershed and topography should also be given serious consideration. Rock outlet protection is not intended for slopes greater than 10% or at the top of cut or fill slopes.
Caution should be used if flow rates out of the discharge pipe will exceed 100 cubic feet per second (cfs) for a 10-yr.-fequency storm. Utilize the NRCS Technical Release 55 (TR 55) or other suitable method to determine peak rate of runoff. The outlet protection needs to be stable for the velocity of flow expected from a 10-year frequency storm event. While level spreaders are helpful in conjunction with outlet protections, we will focus on the design of the rock outlet. The width of the outlet should be the width of the headwall or 4 feet wider than the pipe diameter (2 feet on each side of the pipe). The elevation of the downstream end of the outlet protection needs to be equal to the elevation of the receiving stream. The necessary length of the outlet protection and the rock size can be determined from the following figure. 

Figure 1. Length of Rock Outlet Protection and Rock Size

Larger diameter pipes will require larger rock diameter, a greater thickness for the rock layer, and additional length for the rock channel. The rock riprap needs to be well graded and be placed to obtain a solid and compact layer. Filter or granular bedding is needed beneath all riprap to prevent underlying soil from eroding. Larger size riprap will likely require a thicker bedding layer or 2 different sizes of bedding. Geotextile use can help prevent piping of soil. Care should be taken to properly anchor the geotextile. A properly designed and installed rock outlet protection will help to reduce future maintenance needs.
 
Maintenance will help to protect the riprap, vegetation cover, and associated structural components. The following are key to proper maintenance.

• Determine a responsible party to inspect and maintain the outlet protection.
• Missing riprap should be replaced as soon as possible.
• Protect the outlet from damage by equipment and traffic.
• Fertilize and mow area vegetation to keep a healthy cover.
• Seed and mulch any bare areas that develop.
• Remove sediment and debris.

 
If properly designed, installed, and maintained the rock channel outlet protection should function for decades to come. Lack of proper outlet protection will require more extensive and costly repairs in the long run. Feel free to reach out to our office at (513) 695-1337 if you have any questions regarding rock outlet protection.
References

1. Urban Hydrology for Small Watersheds (TR-55), United States Department of Agriculture Natural Resources Conservation Service. June 1986. Web link for this publication is available at:

https://nationalstormwater.com/wp-content/uploads/2020/07/Urban-Hydrology-for-Small-Watersheds-TR-55.pdf

1. Rainwater and Land Development Manual (Chapter 6.1), Ohio Environmental Protection Agency. May 2025. Web link for this publication is available at: https://dam.assets.ohio.gov/image/upload/epa.ohio.gov/Portals/35/storm/technical_assistance/6.1_Outlet_Stablization.pdf

Article by Travis Luncan, Urban Technician