Today’s Erosion and Sediment Control Tip: #8

 


There are a few practices that require an open-channel auxiliary (emergency) spillway to allow excess runoff to safely bypass a small dam.  Such practices would be a Drop Structure with a dam, a Sediment Basin, or a Stormwater Detention (or Retention) Basin. An open top principal spillway is NOT considered an auxiliary spillway. Sometimes all that is shown on the plans is the elevation for the auxiliary spillway, but there should be details in the construction plans that show the cross-section and profile all the way through the auxiliary spillway including a stable outlet.

The best auxiliary spillway is a vegetated spillway (pictured) located around the end or abutment of the dam on undisturbed soil.  The Blue Book contains design guidance for this type auxiliary spillway.

However, a vegetated auxiliary spillway is not always possible, and a spillway “over the embankment” is often used.  In this case, the exit slope on the back of the embankment (dam) is usually where the system becomes unstable due to the high velocities associated with what is called super-critical flow.  Riprap is often the lining of choice and it often moves or dislodges at this location.  The rock must be properly sized to be “stable”.  And here lies the problem.  When the  outlet portion of the auxiliary spillway is placed on the same slope as the embankment (probably a 2.5:1 or 3:1), the riprap would probably have to be ALDOT Class III rock to be stable.  This is huge rock.  The outlet slope of the auxiliary spillway would need to be flattened to a 4:1 or 5:1 to use a smaller sized riprap (Class II).  It may be possible to use other manufactured armoring products for the auxiliary spillway too.  These would need to be properly designed for the flow and stress conditions.

A riprap auxiliary spillway is basically a chute spillway and should be designed as such.  So, put on your thinking cap and sharpen your pencil when designing an auxiliary spillway.  It is not just a bunch of rocks placed over a dam.  When installing a riprap auxiliary spillway ensure it has the cross-sectional capacity required in the plans.  Riprap is often placed too high (picture).  If excess water cannot go out the auxiliary spillway it could cause a breach in the dam and failure.

 

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