Stormwater Detention
Stormwater Detention
(OP)
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
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





RE: Stormwater Detention
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
RE: Stormwater Detention
RE: Stormwater Detention
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
sometimesover-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.
RE: Stormwater Detention
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.
RE: Stormwater Detention
RE: Stormwater Detention
RE: Stormwater Detention
RE: Stormwater Detention
RE: Stormwater Detention
RE: Stormwater Detention
Everyone else here is saying it's pretty common..
RE: Stormwater Detention
RE: Stormwater Detention
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
RE: Stormwater Detention
RE: Stormwater Detention
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
RE: Stormwater Detention
RE: Stormwater Detention
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
RE: Stormwater Detention
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
RE: Stormwater Detention
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Stormwater Detention
The worst LA got over a year was about 30 inches.
RE: Stormwater Detention
RE: Stormwater Detention
They might do things differently down there, but the proof is in the pudding. It works for them, and they've been doing it that way for a long time, after putting a lot more thought in to their regs than anyone else at the time did. Only now are some portions of the rest of the southeast catching up to SFWMD's level of rigour.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com