Starting HGL

[Jackster]

I have a pipe system entering a wet pond at the invert of the pond. Routing the 10 year storm through the pond the 10-yr WSE is 1 foot above the permanent pool elevation. Would my starting HGL be the permanent pool elevation or the 10-yr water surface elevation

Thanks, Jack

 

[TerryScan]

The starting HGL should be the 10yr WSE.

In tight situations, I may look at the pond hydrograph and set the pipe system starting HGL at the WSE at peak inflow time (often a bit less than peak WSE).

If it is crucial, I may do a detailed routing which includes the pipe system in question with the pond.

 

[gbam]

Jackster,
It depends on the size of the two watersheds. HEC-22 Table 7-3 shows Frequencies for coincidental occurence. It is possible that you should examine the 10-year in the stormdrain while looking at the 100-year for the basin. It will also dend on the governing entity so make sure that you check with them as well. For instance our city DOT requires the same storm frequency for both the storm drain and the receiving waters.

 

[LHA]

I always design for peak WS elevation for the design storm in cases like this. That way, you've erred on the most conservative side, and you head off endless quibbling back and forth with the reviewer. This tailwater will not increase pipe sized much, and usually doesn't affect any except the final pipe.

I also do a "check" run with the pond full, including design head on the emergency spillway. Not that you have to convey all that flow in the pipes, but it gives an HGL in all inlets. You can then address flood flow relief, if any is evident. I'm sure there is a requirement for safely conveying the 100-yr through the site, even though the design storm is less.