Storm Sewer Systems and the Energy Grade Line 1

[655321]

Many jurisdictions have a storm sewer system design requirement that the calculated energy grade line be below the rims of manholes and the throats of inlets. This is in additional to the requirement that the hydraulic grade line be 2' (or something close to that) below rims and throats, which I understand completely. What is the purpose of the separate Energy Grade line requirement? Wouldn't the HGL line criteria alone be enough? Is there a reason the EGL should be considered separately?

Thanks

 

[cvg]

if you have supercritical flow, than yes - if some blockage causes a hydraulic jump than you will be popping manhole lids

 

[655321]

Thanks cvg. do you think this requirement was developed only to mitigate the effects of debris and blockages or could there be some other issues it relates to? Just trying to understand the motivations and thinking behind the guidance. Our local drainage manual is under revision and this question came up. I'd appreciate any commentary you might have.

 

[cvg]

lot of assumptions in the analysis such as flow rate and pipe roughness, friction losses. Unless the froude number is very high, a small change in flow, slope, roughness or a slug of air could cause the flow to go subcritical. Suggest that froude number not be near 1.0 in order to avoid this unstable flow regime

 

[beej67]

In my practice area, municipalities aren't going by EGL, but they often require a design storm HGL to be shown in the pipe. The truth is, the only reason we have to show these things now, is many engineers started using our tools improperly back in the 90s.

When I started engineering, everyone sized their storm drains with 'the wheel,' such that all your storm pipes were sized for Manning's open channel flow capacity. We also stuck to a minimum slope of 1%, and we matched crowns. If you follow those three guidelines, you basically never have a problem in your storm sewer unless there's a tailwater issue. But then along came computer models such as StormCAD, which allowed engineers to justify undersizing pipes based on an HGL analysis and pressure flow, which in turn caused sinkholes and failures from putting non-watertight pipes into pressure conditions, which in turn caused the regulators to start imposing additional restrictions on the HGLs, EGLs, and the like. So now, even if you wanted to, you can't size your pipes with 'the wheel.'

Sad to say, this is a layer of complexity that the private engineering firms brought upon themselves, at least in this region.


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

http://www.campbellcivil.com

 

[655321]

In my preliminary designs I also use manning's formula, I match crowns, and I try to stay over 1% with my pipes. I've also found that it almost always works when I run the HGL.

Our Design criteria specifically says no pressurize pipes (unless we have a really really good reason), so we do it that way and sumbit the old manning's pipe spreadsheet showing the pipes meet the minimum slopes, and the the hgl calcs. Is running the HGL calcs completely unnecessary?

 

[cvg]

showing HGL on the plans is sometimes invaluable to the next engineer that comes along years later and wants to extend the storm drain. usually the design report is long gone, but the as-builts show the HGL. and for a flatter, surcharged system like most are, this information is invaluable

 

[beej67]

I find that the HGL information is extremely valuable for storm systems that discharge into detention ponds as well, if you use the proper tailwater, because it will identify problems in your system you didn't know were there to start with.

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

http://www.campbellcivil.com