[b]Calculating HGL's at Tailwater Depth[/b]

[24J]

I have an underground detention system consisting of twin barrells of 60" pipe with a concrete outlet structure. I have sized an 18" RCP outlet pipe to carry flows to a downstream storm drain. I have been asked to calculate the 100-yr hydraulic grade lines for the pond tailwater elevations. Any advice would be most appreciated.

 

[bltseattle]

Your question is not clear - are you looking for 100 year water surface in your 60" tanks? Or do you need to extend the HGL upstream of the 60" tanks to the upstream conveyance system?

If it's the 100yr WSEL in the tanks, A very simple approach would be to look at a culvert inlet nomograph using the 100-year flow and the 18-inch outlet with Ke for headwalls. This will give you HW/diam, the rest is simple math.

An alternate approach is to use a more sophisticated backwater model, e.g. HEC-22 calculations ala StormCAD, spreadsheet model, etc. - this would be the approach needed to extend the HGL into the upstream conveyances. In this case, you could start the analysis at the tank using a tailwater elevation to match the max design water surface in the tanks...

 

[LHA]

blt nailed it for finding the WS inside the tanks. This will also allow you easily extend the HGL up the system. But more often than not, the ability to drain these underground systems before they overflow is the concern.

I took the question to mean 24J needs the TW relative to the outlet pipe...ie, the reviewer is concerned (as I would likely be) that the water can get out fast enough into the recieving waterway.

If that is what is needed, you need an analysis (also the one described by blt) on the recieving system. If it gets surcharged during the 100-yr, it will limit what can flow out of the storage units. Most storm pipes are not designed to convey the 100-year, so that would probably be my concern, if I were the reviewer.