Tailwater Elevation

[bank]

The City's new guidelines state that on roadway projects the starting tailwater elevation for extreme event analysis should be the 10-year water surface elevation at the outfall. If the project location is a considerable distance from the outfall channel, the 10-year WSE can be assumed to be 48% of the elevation difference between the 2-year and 100-year water surface elevations.

We are using the soffit of the pipe where we tie into the drainage system as the 2-year elevation. Since the object of the extreme event analysis is to ensure that the HGL of the proposed system does not go above the right-of-way line, and since no 100-year WSE is defined at the project, would it be ok to assume the right-of-way elevation at the outfall as the 100-year water surface elevation? The starting tailwater elevation would then be 48% of the elevation difference between the pipe soffit and the right-of-way.

 

[beej67]

I wouldn't assume that. I'd do the hydraulics. What's your reasoning for assuming that?

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

http://www.campbellcivil.com

 

[francesca]

I agree with beej67, it's not okay to assume that because hydraulically speaking, the ROW line is completely arbitrary. You have enough information to determine the 2, 10 and 100-year hydraulic conditions for the pipe.

 

[bank]

In many cases there is no information on the hydraulics of the system you are tying into, other than the sizes and slopes of the pipes. Rather than require one to model the receiving storm sewer all the way from its outfall, which might be several miles away, the procedure I mentioned is my interpretation of what the City offers as a way to estimate the tailwater elevation in the new system.

It seems simple enough, which worries me. I'm not an H&H guru. We had one, but he left, and I'm having to finish one of his projects while we look for a replacement.

 

[beej67]

You shouldn't need to model something miles away to get a reliable tailwater elevation to go from. You should be able to make an assumption that makes more sense than elevations associated with property lines, though.

If your headwall discharges to open air and the discharge falls to a splash pad or stilling basin well below the discharge, assume critical depth at the exit of the pipe. If your headwall discharges to a defined open channel which only receives flow from your system, and the channel has some reasonable fall as it flows away from the culvert, calculate normal depth in the channel and use that. If you're tying to an existing piped storm drain system, get the HGLs from that design and use those. If they're not available, model the existing system to its outfall, and use one of the above methods. If you're discharging to floodplain, use the BFE to be conservative, or get the HEC-RAS model and run a 10 year steady flow analysis to determine your tailwater. If you're in a coastal area and things are very flat, you need to be a lot more careful of your tailwater, because effects miles away could indeed impact your culvert.

It's *important* to understand the tailwater issues of any existing piped system you're tying to, and to confirm and document that you've looked at that in detail, because if the road floods you get sued, not the last guy who designed some other piece of it, even if it was his fault that your part flooded. If there's an existing problem that's impacting your design, you need to know about it, and you can't know about it without modeling it.

If your "H&H guru" left, and you don't feel confident in your work, consider subcontracting to another H&H guru until you can hire a replacement.

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

http://www.campbellcivil.com