Stormcad Velocity - Average vs. Velocity in/out

[mbbaldwin]

I am designing a drainage system down the side of an extremely steep embankment, where there was a slope failure due to a leaking block unit catch basin. The slopes of the pipes are about 20%. When I run the analysis through Stormcad, I get the average velocity, velocity in and velocity out. I understand the velocity in is at the upstream end of the pipe, just downstream of the structure, so is the velocity out the downstream velocity just upstream of the structure? The average velocity is not even near either the velocity in or out. I need to make sure the velocities within the system do not exceed 10ft/second. Can anyone shed some light on whether I should be focusing on the average or velocity out?

 

[beej67]

The easy answer is 20% slope pipes aren't ever going to pass a 10fps velocity constraint, and you need to build them at shallower slopes, perhaps with one or more drop structures in your run.

The more complicated answer probably lies in your Hydraulic Grade Line. If you're using a fairly modern version of StormCAD it will identify locations for hydraulic jumps, which will drastically change the velocities in pipes. Pull up a profile and look at the HGL, see if it does a funny hop in the middle. Then pull up the help file and see how "average" is calculated - it may be a weighted average along the pipe instead of just averaging the in and the out, or alternately it may be the normal velocity of a pipe at that slope given that flow, which wouldn't necessarily have any correlation to the in or out velocities in a steep, gradually varied flow situation.

But the simple answer sticks. If your reviewer is holding your feet to the fire about a max velocity constraint then 20% won't cut it for any reasonable design storm. Look at lowering the slope, and look at alternate pipe materials that are rougher.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[beej67]

Something else worth mentioning, is that the very derivation of the Manning's Equation itself (which I'm sure StormCAD is based on) assumes slopes are flat enough to where you can replace the sin of the bed angle with the tan of the bed angle to make the math easier. This assumption probably blows up at 20% slopes anyway, although nobody really pays attention to that in the engineering field. Just an interesting tidbit you might be able to use to win a court case some day.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com