HGL for Major Storm when there is an allowable depth above gutter

[LincolnPE]

I work in a region where the storm sewer pipe system is initially sized for the Minor Storm event (2yr or 5yr depending on municipality). In the Major Storm event (100yr here) streets are allowed to have a depth above the gutter flowline of 12" in order to convey the major event as an open channel on top of the street (as long as all bldg lowest openings are 1 foot above 100yr WSEL,etc.). If that street capacity is still not enough during the Major Storm, the storm sewer would then be upsized to provide the additional required capacity.

The effects of the Major Storm on the storm sewer need to be shown. Some reviewers are mandating that the 100yr HGL is required to be 12" below flowline at the inlets. How can this ever be if the equilibrated 100 yr water suface elevation (WSEL) in a balanced design for the Minor and Major storms is one foot above gutter flowline?

The reviewers say to put what is going into the storm sewer from the inlet during the major storm (so that would be using the 1 foot of head above the gutter), and upsize the pipes if needed to keep this captured flow "HGL" 12" under the gutter flowline. Then analyze the system to ensure this "100yr captured HGL" is below manhole covers, etc.

The problem I see is that this is not the true HGL (static pressure) at the inlet location, that should be used to subsequently analyze the system. The true HGL and static pressure for the equilibrated 100 yr condition is always going to be relavent to the WSEL that is 12" above the gutter flowline.

The HGL that the reviewer would like to use to analyze the system is really missing 2 feet of head (1' below ground + 1 foot above ground)at these gutter inlet locations. This under estimates the true HGL for the subsequent system analysis.

Any thoughts?

 

[beej67]

The effects of the Major Storm on the storm sewer need to be shown. Some reviewers are mandating that the 100yr HGL is required to be 12" below flowline at the inlets. How can this ever be if the equilibrated 100 yr water suface elevation (WSEL) in a balanced design for the Minor and Major storms is one foot above gutter flowline?

It can't be. Your reviewers are on crack. Bring them a fluids text book and explain to them what a "piezometer" is, and how all your structures are essentially piezometers. You seem to understand the concept perfectly to me.

Of course, a more civil approach will earn you better returns in the long run, but you've really only got two options:

1) Teach them fluid mechanics somehow, which may be difficult if they're being adamant about the flawed procedure you describe, or

2) Do whatever they ask you to do, even though it's blatantly wrong, then go back and check your own design and satisfy yourself that your design meets the intent of the law and is safe for the public / etc.

It might help to explain the situation to the client too, so he knows your frustration in case you have to bill him any adds.


The only thing I can possibly think your reviewers might be thinking, if they aren't actually (as stated above) on crack, is they might want you to reduce the amount of capacity you're crediting yourself for in piped flow at the 100 year storm due to blockage or something, and the lowered HGL is merely what they've decided to use as a factor of safety. In that case, I guess you fiddle with your Qs until the HGL is a foot below grade, then note that as 100 year piped flow, then subtract and determine your 100 year gutter flow for your open channel analysis.

A much simpler analysis, if they'd accept it, is just determine the capacity of the pipe and the capacity of the street at the maximum allowable depth, add them, and show that the capacity is higher than the design flow. They don't get their fancy HGLs on the profile in that case, though, which they may want for whatever reason.

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

http://www.campbellcivil.com

 

[gbam]

Sounds like they are requesting that you design the stormdrain pipe system for the 100-Year event. However, the inlets are sized for a more frequent event 2 or 5-Year event. Simply compute the inlet capture for the 100-Year event based on your 2 or 5-Year inlets. Then make sure the HGL is a foot below the ground. This way your manhole lids won't "blow off" per say.

 

[HEHurst]

My take is that the hydraulic design criteria is "whatever gets into the storm sewer from the inlets during the 100-year design event must result in a calculated HGL 1' or more below gutter elevation." Inlet inflows would be calculated based on surface depth of flow during 100-year event (max 1' head; might be less at some locations). It does seem misleading to call this the 100-year HGL, as it seems to be an artificial profile. As I think everyone has noted, the true HGL is some depth above the ground surface. I think I'm thinking along the same lines as gbam. They want the capacity of the storm sewer system during the 100-year event to be limited by inlet capacity, not pipe capacity.
It's impossible to have the actual 100-year water surface below the gutter elevation because of the head needed to force flow into the inlets, no matter how big your pipes are.
What I'm wondering about, and I'm not sure if it's relevant or just an interesting engineering/physics question to ponder, is what the actual water surface and pressure conditions are in the inlet shafts during this condition. Does the free surface actually exist as calculated by the pipe hydraulics analysis? Or does the fact that the inlet opening is completely covered with water, some of which is dropping down into the shaft, have a major effect? Seems like a complicated rapidly-varied flow with both air and water fluids. I feel like I should be able to understand what would happen but I can't without doing some research.

 

[beej67]

Guys, the pipe design criteria is for capacity flow at the 5 year storm. There's no way that the inlets aren't going to take enough water to surcharge that pipe when there's a foot of water in the street.



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

http://www.campbellcivil.com

 

[gbam]

Beej67 - I agree; however if the inlets are standardized, it is possible that they will be oversized for the 5-Year event resulting in an increased capture rate during the 100-Year. Sometimes it is just less stressful to accommodate the reviewer. With today's software packages to evaluate stormdrains it can be pretty quick to evaluate a check storm.