Starting HGL in the storm drain basin (pond) 6

[lidabc]

I don't have much experiences on storm drainage basin calculations. So it would be great to get some advices from the experts in this forum.

A local storm drain specs includes the following criteria.

"The starting point for hydraulic grade calculations in collection system design shall be the water surface in the pond generated by the volume of 1 10yr-48hr storm"

Although there is a controlled pump station in the basin, should the HGL in the basin start from that high surface water? HGL can't start from the basin bottom or pipe crown?

I assume that there would be some back-to-back storms to increase water elevation in the basin during pump is operating. But I thought that this is TOO conservative to design the storm drainage systems. (e.g., increasing pipe size, raising lands & building footings, etc.)

Other conditions FYI
1. The criteria for storm drain basin is a 10yr-48hr storm (equivalent to 100yr storm). It sounds like the assumption of the above criteria is that the pump doesn't operate until the basin is full.

2. Pump Capacity: 10yr-48hr storm should be evacuated within 48 hours.

Any comments or advices on Starting HGL in the basin would be highly appreciated.

Thank you.

 

[cvg]

how have you determined that a 10-year storm is equivalent to a 100-year storm?

you need to follow the local regulations, regardless if you think it is too conservative. What happens if your pump fails to run because you lose power? How do you expect a 10-year storm drain to function if you don't also start with the 10-year tailwater condition?

 

[TerryScan]

cvg has a good point regarding pump failure. Otherwise, with a typical detention basin (with a gravity outfall rather than pumped) we use the ROUTED peak elevation in the pond for the tailwater of the drainage system for the storm event criteria required for system design (typically 10yr).

 

[lidabc]

Thanks for your comments.

I just want to make sure if I have understood the design criteria correctly.

"The starting point for hydraulic grade calculations in collection system design shall be the water surface in the pond generated by the volume of 1 10yr-48hr storm"

If,

- Composite C = 0.70
- 10yr-48hr storm runoff = 4.0 inch
- Tributary Area = 30 acres

Volume in the basin = CRA/12 = 0.7 x 4.0 x 30 / 12 = 7 ac-ft

So water surface elevation in the basin storing 7 ac-ft or 304,941 cubic feet storm water should be used for "Starting HGL" for hydraulic design of the drainage system upstream.

I believe that this is based on the worst scenarios like pump failures, no power, etc. for several hours.

 

[beej67]

I presume we're talking about gravity storm drain sewers discharging to a detention pond with a gravity outlet control structure, not a retention pond / cistern / etc with no discharge for certain storm events. The answers to your question might change quite a bit depending on that presumption.

This bit here:

***
"The starting point for hydraulic grade calculations in collection system design shall be the water surface in the pond generated by the volume of 1 10yr-48hr storm"
***

...was probably INTENDED to mean the routed peak water surface elevation from the 10 year 48 hour storm in your pond, but what it actually SAYS is that they want you to drop the whole slug volume of 48 hours worth of runoff into the pond, discharging none of it, figure out that elevation, and start your HGL there.

The guy who wrote the regulation probably doesn't have any idea what the difference is, so your first task is to get clarification from the person doing your review, and make sure that clarification is documented. And you'll want to ask the question in a way that allows them to easily agree with letting you route the storm to determine that elevation. Say on the phone, "That means the routed 10 year elevation, right?" and they'll say "Right," and then you document that in an email conversation record, to point at later.

I've seen the required detention volume for a pond, and the necessary routing assumptions go drastically out of whack because one guy phrased a regulation too sloppily, and another guy interpreted that sloppy phrasing too strictly.

That said, the routed 10 year elevation isn't a terrible assumption for a starting HGL in a storm drain pipe. It's conservative, but not as conservative as you might think. Sometimes I've seen engineers that designed using HGLs at the crown of the discharge headwall, and they had problems when the pond filled up and then spilled out of inlets in the storm sewer network. You have to make some sort of assumption if you plan on mixing methods.

The problem between science and practice is engineers typically use a steady state approach to design pipe systems and a time series approach to design detention ponds. So when you do your "modified rational method" pipe design, you're looking at each stick of pipe, figuring out what the watershed and Tc is to that pipe, generating a Q based on that watershed and Tc, and designing the pipe for that Q. Then the next pipe down the run might have a higher Tc, which means a different intensity, which affects Q. Essentially, you're looking at each element in your storm sewer network and designing for the worst case design storm's effect on that particular pipe, and the design storms are all a little bit different in your network depending on travel time.

Then with your pond, you're picking one storm, that isn't the same as any of the "storms" you used for the rational method in the pipe system, and then claiming that as a fixed tailwater for your rational method analysis.

What you get when you're done, and you show on the plans, is an HGL that cannot ever be. It's the product of a 10 year 5 minute storm hitting your furthest pipe, a 10 year 6 minute storm hitting a downstream pipe, a 10 year 8 minute storm hitting a pipe closer to your pond, and discharging into a pond that's at its peak from a 10 year 48 hour storm. In many ways it's nonsense. But it's conservative nonsense, which is what civil engineering design is all about. We don't want to get things exactly right, we want to design them so they don't fail. And if your local municipality has a set of things in their regs that in their experience don't fail, its your job to follow those regs, because doing so is your legal defense.


note:

If this thing is a retention pond or a cistern or some other sort of water capture device that doesn't drain freely during the storm, then there's nothing to "route," and they want you to figure that slug volume and set your HGL with that.



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

http://www.campbellcivil.com

 

[beej67]

gtg -

You're mixing terminology there, as well as mixing methods a bit.

***
- Composite C = 0.70
- 10yr-48hr storm runoff = 4.0 inch
- Tributary Area = 30 acres
***

"Runoff" is the amount of water that runs off a watershed during a rain event. In the rational method, C is the fraction of *rainfall* that becomes *runoff*, so presuming you meant "10 year 48 hour *rainfall* is 4 inches," you'd have 2.8 inches of runoff, or seven acre feet of runoff volume for the storm. Some folks like to use the NRCS TR-55 method for watersheds of that size instead of the rational method, check with your reviewer and use the simplest method they'll accept.

If they'll allow you to use the routed elevation (see my previous post) then you don't want to use Rational, you want to get your hydrologist to route the 10 year storm through the outlet works of the pond and see what level that comes up to. That could be quite a bit lower than 7 acre feet, because the pond is continually discharging during the storm. You'd be using your peak storage volume to set your HGL, not the total volume that passes through the pond during the whole storm.

Hope that made sense.


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

http://www.campbellcivil.com

 

[jgailla]

beej67,
Your summary of stormwater design in the last two posts are probably the most succinct and accurate that I've ever seen, both in the physical and regulatory aspects.

 

[beej67]

Thanks!

I was worried it was too wordy, actually.

For the record, pipe design in Georgia tends to vary between 10 year and 25 year storm frequencies, with 100 year sometimes required for pipes conveying offsite flow through a development. Tailwater assumptions are usually required to match the pond stage of the design storm in the detention pond from the hydrology study, although routing assumptions for the pond vary by who your regulatory agency is. Gets sort of convoluted, with no state agency to manage it.

Where you run into hurdles in Georgia is a lot of reviewers use the HGL as an eyeball indicator of whether or not a pipe is properly sized, and many municipalities have regs written that state explicitly that the HGL must stay "within the pipe." This was a regulatory reaction to engineers designing pipes for pressure flow in the late 90s using software instead of the good ole mannings equation, justifying their design with HGLs in the ground, but failing to make the pipes water tight in the specs. Bunch of blowouts led to regulators revising the rules, and HGLs were their canary-in-the-mine for poor design.

Well the problem with that is if you've got a pipe discharging into the bottom of a pond (because that's the best place to discharge it to avoid erosion) and you're using a tailwater half way up the pond bank as your starting tailwater, then the HGL is out of the pipe no matter how big you make the pipe. And some reviewers don't understand that. And boy oh boy some reviewers really do not like to be educated in the middle of a review meeting in front of their boss. SO.. ..what I've done in the past is used the discharge crown as my starting tailwater for the plan set, but then used the peak stage in the pond as a check number for tailwater to make sure there's no flooding in the system, just internally to be sure there's no problems.

Alternately, feel out the personality of your reviewer and if he/she is a good candidate, explain to them the science, and they'll appreciate you for it later. That takes tact though. Often not an engineer's strong suit.

Sometimes the most responsible thing to do as an engineer is give the reviewers what they want, instead of explaining to them what they need, but then do what they probably needed anyway on the side to make sure everything's going to work.



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

http://www.campbellcivil.com