What’s So Important About Riprap Outfall Protection?

No Photo Available By: Charlene Harper

It may not be glamorous, but erosion control design is a critical component to managing water quality both during and after site construction.  The transition between a manmade drainage system and a natural channel is one area of potential erosion that must be given special attention during design.  A structure is needed at this point to dissipate both the velocity and the energy of the concentrated storm flows and to protect the downstream channel from scour.  Although it is not the only measure available, the most typical transition to alleviate potential erosion is a riprap outfall protection (ROP) apron.

The process of designing and specifying ROP is well detailed in every state erosion control manual; however, one requirement that is often overlooked is the need to install the apron at a zero percent slope.  It may seem like a small detail; however, the absence of slope is absolutely essential for the ROP to function properly.  The pictures below show a drainage system failure resulting from the installation of ROP  at a slope greater than zero percent.

Timmons Group was asked to investigate the cause of the issue and identify potential solutions. The original riprap apron was installed at a slope of approximately three percent at an outfall which drained into a stormwater management basin.  Stormwater began to scour the downstream end of the riprap and then undercut the entire section, causing it to drop over three feet in less than five years.  After consideration of several possible alternatives, the sandy soils and failure magnitude led us to make the following recommendation:  install a drop manhole with a new outfall pipe, construct a flat (zero percent slope) riprap apron with a cutoff wall at the downstream end, and re-grade and stabilize the failed slopes.

The problem shown above is not uncommon, but it is easily avoidable.  Whether using riprap outfall protection or alternative methods (e.g., concrete or gabion aprons or reinforced turf mats), designing this delicate transition from manmade elements to natural systems is vital to the protection of water quality in our receiving waters and the overall success of your project.