.
Two notes in this one (a bit of a late response)...
1) A link to the latest version of HY8 can be found at "
as well as links to other software to aid in energy dissipation analysis. The site “
has a table of what software application does what stuff. HY8 is excellent software for culvert analysis.
2) Somewhat over 10 years ago I pretty much stopped using rip-rap aprons for energy dissipation at culvert/pipe outlets (as part of a general process to move away from rip-rap except where absolutely necessary). You might consider using a device that I call a "compact weir outfall". It sounds fancy, but is really not much more than a yard inlet at the end of the culvert. You can find a detail for one of these at “
A picture of one of these operating during a storm event is at “
The picture is of a compact weir outfall at the edge of a yet to be stabilized construction site.
You can see how the system works by checking out the picture. Some advantages of this outlet type include:
1) You can “turn the flow” (redirect the water) in a very small space, which is perfect for culverts exiting at large intersection angles to downstream channels.
2) The cost is generally less than a rip-rap apron (depending upon your location and market, of course).
3) It works well where aesthetics are important - it is easily hidden behind shrubbery or placed in an open yard/lawn and doesn't need weeding and trash removal.
4) It takes up much less space than a rip-rap apron - which can be huge!
5) The outlet water is “instantly subcritical” with minimal horizontal velocity.
When first mulling these creatures over, some folks are concerned about clogging. The systems are self-flushing of sediment. You might can imagine low velocity flow outside the pipe and higher velocity in the pipe which results in the sediment being fluidized - stirred up - in the pipe. Some sediment will drop out at the tail end of the runoff curve (hydrograph), but that gets swept out at the rising limb of the next stormwater runoff event. I don't suggest placing these where lots of sediment is expected to enter or pass, but I have been surprised at these haven't clogged even under very high sediment loadings (fully denuded watershed being graded). As long as the inlet openings are about 1/2 the size of the minimum outlet opening (read as “use a grated inlet of some sort”), then debris clogging is not a problem either. I have not seen any of these fail to date (10+ years in the ground).
.
tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation
