I have a proposed stormwater design which will necessitate some very large diameter pipe installed near the surface to minimize constructability issues. However, near the end of the run, it will be necessary to install a structure to allow the stormwater to drop its elevation more than 7', which I'm concerned will risk erosive forces scouring out the bottom due to the fall. Based on the standard equation of standard equation of v=a*t, the vertical velocity is in the range of 21.4 feet/second upon impact with the bottom of the structure.
Generally, I'd like velocities for pipe flows to be less than 15 feet/second to prevent scouring and 10 preferable (though not always feasible). This scenario where water is falling is a little different, but given the quantities being on the order of approximately 170 CFS for the 25-year design storm, I don't want to neglect the kind of erosive potential this could have on the structure's bottom.
To mitigate this issue, I'd like to propose a sump at the bottom of the structure to help dissipate some of the energy. However, I can't figure out how deep to spec this sump area.
I've reviewed my state's Soil Erosion Control Manual, but it doesn't really specify measures for this kind of circumstance. Is there an alternative equation or source I can use to size the sump area for something like this?
Sketch for clarification: