Stormwater Detention 1

[martin888888]

Hello,

We have a jurisdiction with a detention requirement that you only need to detain the 100yr storm. Their conveyance design storm is the 25 year storm. Has anyone ever ran into such a requirement? If your conveyance system is only going to handle the 25year storm you would need a secondary system to collect the 100year storm in order to convey it into a detention system.
Thanks

 

[beej67]

It is common in some municipalities I work in to require conveyance of the 25 year storm "without surcharging" in your pipe network, but detention of the 100 year storm. To prove that your site's runoff makes it to your pond, you have to show two HGLs on the plans, one for 25 yr and one for 100 yr, and you also must show the extents of 100 yr ponding at inlets. If the 100 yr ponding is enclosed, and the 100 yr HGL is in the ground (even if out of the pipe) then that's enough to prove your 100 yr water is conveyed to the pond. If neither of those is the case, you include a surface plan that shows how the 100 year flow spills around your site and makes it into the pond without bypassing it or ending up inside the building.

In Durham NC, which is notoriously extreme, they even made me put together a plan to show how the 100 year flow would make it into my detention facility if all of the pipes on site suddenly vanished, or were filled with concrete by some magic Pipe Stopper Fairy. We had to redesign all the grading to provide drainage of the 100 year storm through an entire apartment complex without pipes at all. Only time I've seen that anywhere.

I have opinions about why they have some of the rules they have, that I'll reserve for a more private setting. Email me if you're interested. Also, beware giving quotes for doing engineering in Durham without fully understanding what you're getting yourself into.

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

http://www.campbellcivil.com

 

[martin888888]

Thanks for the response. I have never seen that criteria before and was racking my brain on how to handle this. What you said is about what I envisioned. Crazy criteria if you ask me. Why not detain the 2-10-25 and you will still get most attentuation with the 100yr. Sounds like some crazy criteria you have down there in NC.

 

[SMIAH]

We do that here.

Thing is that most engineers are using the good old rational formula and they're designing the pipe system at 80$ full (Manning uniform flow) for the 25-year storm. This is sometimes over-design.
Then, when a major storm hits the system, some pipes are conveying more than the 25-year peak flow and they even flow full with heads in the manholes (conveying >> 25-year peak flow).
Plus, even if there is a peak flow attenuation in the system, the runoff volume (e.g. 100-year rainfall) will get to the detention system.
It all depends on the size of the watershed and the control of the detention system but honestly asking 100-year detention system even if pipe are designing for 25-year doesn't look unreasonable to me.

 

[LincolnPE]

That's not at all crazy... That is the way it is done in the industry.

Storm sewer is only designed for the minor storm event, and only upsized as needed where street capacity can not handle the major event, and at the bottom of the system as needed to convey the 100yr into the pond.

 

[martin888888]

Actually normally the system is designed to handle the design storm event (25yr, 10yr, etc). You then account for overland flow from the 100yr and larger storms to make sure the larger storms have a flow path that wont impact structures and facilities.

 

[LincolnPE]

No, you need to ensure your 100yr event in your OP Martin, is conveyed to the detention pond. It sounds like you're fuzzy on this...

 

[martin888888]

No not fuzzy. Yes, I agree, under this jurisdictions criteria I would need to make sure that the 100yr storm is conveyed to the pond either by pipe or overland sheet flow. "Normal" criteria is what I stated. Very rarely do you need to convey your 100yr storm to a detention pond.

 

[LincolnPE]

That's simply incorrect Martin.

 

[martin888888]

Whats incorrect about it? I have done this for 25yrs all over the world

 

[LincolnPE]

Maybe you do more international work then for the "all over the world" part where who knows what criteria there is... and then locally you may work in an area that has more than adequate downstream conveyance such as near rivers..large man-made storm channels. Then we can argue for the client that no detention is required.

Everyone else here is saying it's pretty common..

 

[martin888888]

No, do mainly domestic work. Pretty common to detain the 100yr storm?

 

[cvg]

our requirements are:
stormdrain can be 2-year, 5-year or whatever is required
must retain the 100-year, 2-year storm
yes 100-year must reach a retention basin
no it cannot leave the site or the right of way
the 100-year must infiltrate, or to dry well or bleed off line

 

[martin888888]

Ok, never seen that before. Its definetely not "industry standard" as someone proclaimed. My experience is storm drains are designed to the 25yr or 10yr and you must demostrate a drainage path for the 100yr storm to not cause damage to structures. If a detention pond is required it would be designed to detain the 25yr storm (plus 1' freeboard) and you would provide a weir at the top of the detention pond (or in control structure) to let 100yr bypass. Requirements do vary considerably by region.

 

[beej67]

Detention requirements vary widely across the country, so there's really no need to get into a pissing match over it. There is no "industry standard." The standards are local, or at best regional.

100 year storage used to be common across most of the southeast, but now many dixie states are dialing back to 25 year, with a downstream analysis to handle the effects of the 100 year without actually requiring detention. This to compensate for the increased storage volume on small storms to meet water quality goals.

In many areas of Florida, they plan on evacuating for the 100 year storm anyway, so only require the 25, and even then they require zero freeboard. Then if you're connecting to FDOT right of way, they require you to run not only a mess of different return periods, but also half a dozen different durations, with hyeotgraphs supplied directly from them containing multiple peaks and all sorts of shenanigans.

If a dam in Georgia is subject to the Safe Dams act, you're doing a 500 year and a quarter PMP analysis as well, although detention may not be necessary for those storms.

Stormwater management is very location specific, more than any other civil site related field, because every place has different storms, different topography, different soils, and different development challenges. All those inform the regs, and that's why the regs are different everywhere.



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

http://www.campbellcivil.com

 

[Twinkie]

I love how we, in the Mid Atlantic, regulate stormwater by municipality, not even by watershed. Some watersheds have consistent requirements, but not all.

 

[beej67]

If y'all want to hear a weird one, in South Florida, nearly the entire state is in the floodplain. The FIRM is just a big grey box with a number on it. When you develop, you set your building FFE at (that number +0.1 ft), detain for the 25 year 3 day storm, and then for the 100 year 3 day storm you do a 'glass box analysis.' That entails drawing an imaginary vertical boundary around your site, and using your site's proposed grades, drop a 100 year storm on it with the entire site as your pond, and zero discharge. Figure out what that stages to, and prove that it stages to less than the 100 year elevation on the FIRM. That way you can prove you're not pushing water onto your neighbor.

Recall their 100 year storm is basically a cat 5 hurricane.

Sorry for turning your thread into a "weird detention regulations" expose, Martin. But I do find it pretty interesting how they vary. For you northeastern guys, do the 10 States Standards spill over into stormwater?

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

http://www.campbellcivil.com

 

[brandonbw]

beej67: So what do you do to get a plan through?

A soils engineer told me the City of LA may require the retention, or detention, of any year storm eventually. I could actually believe it too for either case. Nothing is going to surprise me anymore.

B+W Engineering and Design | Los Angeles Civil Engineer and Structural Engineer

http://bwengr.com

 

[beej67]

It's not impossible, Brandon, but you have to think about creating 100 year storage with your overall grading plan. You push your parking lot as low as you can and put the building on a hill just big enough to keep the FFE clear of the 100 year BFE. Resign yourself to the fact that everything's going to flood but the building.

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

http://www.campbellcivil.com

 

[SMIAH]

Any year storm in LA is nothing.
The worst LA got over a year was about 30 inches.