Modified rational formula
Modified rational formula
(OP)
Isn't there something with this particular method about the fact that the duration of the storm has to match or be near the time of concentration of the watershed ?
Calculating small detention basin, I see that the duration that control the required volume is sometimes way more than the time of concentration.
E.g. Is it acceptable to design a detention basin (small, say 10 000 sqf) for a duration of 2 hours when the time of concentration will be 5-10 minutes?
Thanks!
Calculating small detention basin, I see that the duration that control the required volume is sometimes way more than the time of concentration.
E.g. Is it acceptable to design a detention basin (small, say 10 000 sqf) for a duration of 2 hours when the time of concentration will be 5-10 minutes?
Thanks!





RE: Modified rational formula
RE: Modified rational formula
But for pond sizing purposes, the maximum detained/retained volume will generally occur at a somewhat longer rainfall duration - this is the "critical duration" for that pond.
So it's quite possible that the your critical duration for the pond will be 2 hours, even though the runoff Tc is just 5 minutes. It all depends on the exact storage and discharge characteristics of the pond.
Of course, there is the entire issue of whether Rational method should be used for pond sizing at all. But if/when it is used, it's essential to use the (longer) critical duration and not just the Tc.
This has been discussed in many previous threads. Try a search...
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Modified rational formula
RE: Modified rational formula
But this is for retention on lots. I was just wondering about that critical duration being so far away from the tc but reading psmart, I get it.
RE: Modified rational formula
We deal with fairly small time of concentration.
Isn't an overkill to design a small pond with a critical duration of, let's say, 90 minutes when the peak flow in the downstream network will be seen after 15-20 min.
RE: Modified rational formula
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Modified rational formula
RE: Modified rational formula
They do it on any size lot that trips the detention requirement. In Georgia, single family residential lots are often exempt from stormwater management, but small urban commercial lots aren't, and must provide detention. A 10,000 sf lot would have a detention vault, and the analysis for it would include surrounding lands up to a limit of 100,000 sf, to show not only that the lot met the discharge requirements for its own parcel, but that in so doing it didn't cause further problems to any downstream structures.
They've been doing it that way in most municipalities down here for about a decade.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Modified rational formula
It's probably the way to do it but you rely a lot on a regulator to make sure that every project.
RE: Modified rational formula
RE: Modified rational formula
In here we just changed the regulation to waive off detention on lots under < 13000 sqf with everything under a 4-housing (except commercial/industrial). It was < 10,000 sqf only for single housing.
Then, we just released a new rainfall intensity-duration-frequency curve taking in account rainfall data measured from 1960 to 2013.
Most of the engineers here were using 1960-1990 curve. Just to show, the 100-year 24-hour rainfall moved from 4'' to 6'' (it might be small for you guys down there, but here in the north (Canada) it's pretty high! Everything under 60 min didn't changed much (e.g. 50-year 15 min stayed the same).
Using modified rational formula, we were dealing with critical duration near time of concentration more than often. Using the new IDF curve, we see a drastic increase in required volume and critical duration is mostly higher than 60 min. I guess that we can't go against nature and we might see more detention of roofs (it's not a problem here with the snow charge) and more underground detention, less pavement, etc.
Thing is, we were thinking (bosses were asking) about introducing a "maximum critical duration" of, let's say, 60 minutes... but I think it pretty random and far from the premises of the modified rational formula.
RE: Modified rational formula
Yep.
But to be fair, the smaller the project, the smaller the regional analysis necessary. For a one acre project, you're only analyzing ten acres.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Modified rational formula
RE: Modified rational formula