Modified rational formula

[SMIAH]

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!

 

[SMIAH]

* for a 10 000 sqf lot (not the area of the basin).

 

[psmart]

When calculating peak runoff, the rainfall duration should be equal to the Tc. This is the "critical duration" that produces the highest peak runoff.

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

http://www.hydrocad.net

 

[cvg]

generally the duration for retention or detention design is specified by the regulator and may be 1, 2, 6 or 24-hour storm. NRCS even designs large reservoirs for a 10-day storm. you will need to either capture and infiltrate or route the entire storm through your basin without overtopping

 

[SMIAH]

For larger pond designed for urban development (we as the regulator) specify a 3-hours duration storm (modified Chicago)(we deal mostly with relatively small urban basin).

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.

 

[SMIAH]

My point is... what we want to protect is often the network downstream of the small detention facility on the lot.
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.

 

[beej67]

They handle that down here by making all the engineers do an analysis of the regional system out to where your basin is ten times the size of your development parcel. That way you can tell if a timing issue is making the downstream system worse instead of better.

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

http://www.campbellcivil.com

 

[SMIAH]

This can't be done for detention of small lots (e.g. 10 000 sqf lot) ?!

 

[beej67]

Why not?

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

 

[SMIAH]

I thought it would be costly for every engineer in charge of "small projects".

It's probably the way to do it but you rely a lot on a regulator to make sure that every project.

 

[cvg]

the problem with exempting small lots is that there is no such definition for what small is. is it 1/4 acre, 1 acre, 10 acre or what? and why pick on the 1/2 acre lots and ignore the 1/4 acre ones? And is it really that much more difficult for a 1/4 acre lot to provide a retention than a larger lot? it can be done quite easily in the yard or parking lot. the real problem with individual small lots is that the property owners tend to fill in the retention area after the city inspector does the final inspection.

 

[SMIAH]

So true.

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.

 

[beej67]

I thought it would be costly for every engineer in charge of "small projects".

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

 

[bugler]

On site detention prevents runoff that exceeds the predevelopment rate from a particular water shed. Most authorities having jurisdiction will allow runoff up to the predevelopment rate to be passed through immediately, without being detained. Only the runoff rate that exceeds the predevelopment rate must be detained. I developed a curb inlet system that diverts the excess runoff into a reduced size detention pond through a weir plate at the back side of the curb inlet. The weir is sized for a specific height so that no matter what the inflow rate is that comes into the curb inlet, only the predevelopment flow rate will discharge into the downstream environment and the excess flow goes into the pond. After the event, a flapper gate installed on the weir plate at the back side of the curb inlet allows the pond to empty by gravity flow. I want to patent the system, but my funds are low, and this invention has limited commercial value.

 

[cvg]

while buglers method is certainly logical, effective and been done many times - most agencies require the first flush to be managed to meet NPDES / SWPPP requirements. I'm not aware of any agencies in this area that would allow first flush to be discharged without any sort of BMP to capture the sediment. Peak scalping basins are not allowed around here for that reason, except for large regional flood control projects.