Rainfall Duration for Design Purposes 1

[ONENGINEER]

I am designing a dry well. The local authorities just recommend to use a 10 year return period for rainfall intensity. As the intensity depends on the rainfall duration i.e. 5, 10, 15, 30, 60 minutes, 1, 2, 6, 12, 24, 36, 72 hours. Which of these duration should be used for a small residential project. Thanks.

 

[beej67]

There's no such thing as a "ten year storm" without also specifying a duration for that storm. Ask the municipality.

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

http://www.campbellcivil.com

 

[psmart]

Assuming that we're talking about the Rational method...

If you're goal was to determine the peak flow, you would typically use a rainfall duration equal to the time-of-concentration. This is the duration that will produce the highest peak flow.

But for storage calculations (like a dry-well) you're also concerned with runoff volume. So you typically need to design for a longer duration, which will produce a greater volume than using the Tc.

It's also possible to determine the "critical duration" for your dry well, but you should check the regulations to determine the exact requirements.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ONENGINEER]

As indicated by psmart, the Tc is very small (less than 5 minutes). Thoughb the municipality expressed that it would be up to the engineer to choose a rainfall duration for design. I guess I should choose a standard duration but I do not know what that standard would be.

 

[psmart]

In that case, I would determine the duration that generates the highest water surface elevation in the dry-well. If you were retaining everything (i.e. no outlet or infiltration), increasing the duration would steadily increase the water surface elevation, so the worst case would be a very long (low-intensity) event, and you would need an infinitely large dry-well.

In reality, you'll have some kind of outlet or overflow, be it a pipe, infiltration, etc. With an outlet, there will be a specific "critical duration" which produces the highest WSE elevation, and therefore defines the dry-well design. A shorter duration will contain less volume, so the maximum WSE will be reduced. And a longer duration will have a lower intensity, allowing more time for the water to discharge, also giving a lower WSE.

You can approximate the critical duration by trial-and-error, but this requires doing a pond routing calculation for each trial, so you really need the right software to do the job. But once you determine the critical duration, it's easy to verify the answer, because any change in the duration (increase or decrease) will reduce the maximum WSE, demonstrating that you've found the worst-case duration.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

I the municipality isn't giving you direction, try 24 hrs and see if that doesn't make your dry well too big.

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

http://www.campbellcivil.com

 

[ONENGINEER]

psmart- I found the attached publication which is in line with your suggestion.

http://files.engineering.com/getfil...9929-0bcbb1246369&file=City_of_Eagle_Spec.pdf

Still am not sure about whta intensity to assume. The 24 hr gives unreasonably small dry well.

beej67- Do I nderstand correct that I should mulitiply the mm/hr for 24 hr duration by 24, multiply by the area and runoff coef. to obtain the voild volume in the dry well.

Thank you for your comments in advance

 

[gbam]

The runoff volume is equal to the precipitation depth x area x R/O coeff.

 

[psmart]

Pick a trial duration (lets say 12 hours), lookup the corresponding intensity from the IDF data, and do the math. Now repeat for 13 hours to see if you need to go longer - or shorter. Repeat the process (successive approximation) until you converge on the critical duration.

Remember "the math" involves routing this event through the dry-well. As stated above, calculating the total volume of each event is trivial, but this will not answer your question. You need to do a full hydrograph routing, including the outlet analysis, and this isn't practical to do by hand - you need runoff and routing software.

Of course, there are other ways to size a dry-well, but I believe this is the most correct approach when using the Rational method.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ONENGINEER]

psmart and gbam

Thank you. I am still not sure how to calculate the percipitation depth. If these are my IDF vales:

DURATION
10 year
5 min 70.2
15 min 43.0
30 min 28.9
1 h 18.6
2 h 12.1
6 h 8.1
12 h 6.5
24 h 4.3
48 h 2.9
72 h 2.2

Lets say I am considering 24 hr. Would it be 4.3 multiplied by 24 or just 4.3 mm. If 43 mm the dry well volume would be unreasonably small. T

 

[psmart]

Since your table provides intensity values, you need to multiply the intensity by the duration in order to get the total depth.

For example, your 24-hour depth would be 4.3 mm/hr * 24 hrs = 103.2 mm


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

you are confusing things a bit

dry wells are generally not sized to handle "peak flow rate", they are sized to handle the entire volume of runoff. It doesn't matter what the rational method Tc or intensity is. You don't use the rational method. You do as GBAM suggested, and use the 10-year precipitation "depth". This will be in inches. Estimate a runoff factor, apply the rainfall to the entire watershed and calculate the volume in cubic feet or acre-feet of water and size your drywell to hold all of it.

as far as the storm duration, that might be a policy decision made by the agency. If not, I would start with a 24 hour storm.

Typical in this region, we use 24 hour storm rainfall depth and design the retention basin to hold all of it plus freeboard. The drywell then has to drain all of that water within 36 hours. These dry wells are typically drilled wells into a granular layer, 100 feet deep or greater.

 

[ONENGINEER]

The confusion was about depth of percipitation which I understood.

cvg, thanks for the easy approch you introduced. The residential site is small and with considerably high water table. It seems if a reduction in volume due to water percolation during the 24 hours is allowed, the accumulated water volume can be reduced 41%. This saving is temting and I was considering it.

 

[beej67]

Lets take two steps back, since cvg and peter are both right.

For a given location, the worst 6 hour long storm you're likely to see in 10 years will be of some depth (in mm in your case). That's the "10 year 6 hour rainfall depth."

For the same location, the worst 12 hour long storm you're likely to see in 10 years will be of some other depth (in mm in your case). That's the "10 year 12 hour rainfall depth."

For the same location, the worst 24 hour long storm you're likely to see in 10 years will be of some other depth (in mm in your case). That's the "10 year 24 hour rainfall depth."

Follow so far?

Your municipality should have given you a duration when they said "10 year," because "10 year" by itself doesn't give you enough information to know the rainfall depth.

If you were purely using the Rational Method to determine a peak flow rate, and didn't care at all about volume, then you could make the additional assumption that the worst intensity you were likely to see corresponded with a storm short enough to exactly match the Tc of your basin. Then you'd go to your IDF relationship, read the intensity off based on the Tc, and use that in Q=CIA to determine peak flow. That's all those IDF curves are "supposed" to be used for. For peak flow rates, not for volume calculations.

However.

You can back figure what the total depth of rainfall is for a given duration using your IDF curves, because of how they were developed. If your IDF curve says your 6 hour intensity is 8.1 mm/hr, then you can multiply 8.1 mm/hr * 6 hrs = 48.6 mm of water that fell in that "10 year 6 hour" storm. So that can give you the rainfall depth you're looking for.

But you're not done yet. Two more pieces to the puzzle.

First, that's a depth, not a volume, so to figure out the volume of rainfall that falls over the watershed, multiply by the watershed area, and watch your units. You'll end up with a volume that's in mm/hectare or something (I'm American, so I get inches per acre). Then multiply by the conversion factor to turn it into cubic meters.

But that's still rainfall, not runoff. You have to figure some of it gets stuck in birdbaths and leaks into the ground and is carried away by gremlins, which is all rolled into your "runoff coefficient." All a runoff coefficient does is give you a ratio of runoff to rainfall. So for a C=0.7, that just means 70% of the rainfall turns to runoff, the rest gets carried away by gremlins.

So you multiply the rainfall volume by that coefficient and you get your total volume of runoff.

That should be enough math to size your dry well. What Peter's getting into with critical storm analysis is the sort of thing the Florida DOT makes you do, where you run a bunch of trial storm durations through a watershed model to figure out which one floods the worst, and use that as your design storm. I'm not sure that really applies for rainfall retention systems with no outflow, and if it does, I don't know how it would. Peter alludes to that above as well, by saying you'd eventually settle on an infinite duration event.

The safest thing to do is ask another engineer in the area what he does. A lot of times this stuff is just decided on by convention, and the old farts reviewing it don't even think so much about the science behind it. Alternately, just pick a storm you want to use and clearly call that out on the plans and calculations that you used a "10 year SIX HOUR storm," (or whatever) and if they approve it, then they've agreed with your duration assumption.



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

http://www.campbellcivil.com

 

[psmart]

My comments were directed at the situation where you are trying to optimize (minimize) the dry-well size including the infiltration or outflow effects. If you neglect any infiltration/outflow which may occur during the storm, then you're just calculating the volume to retain the entire event, and the calculations are much easier. But with full retention, you have to design for a specific duration, which brings us full circle to the original question...

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

around here, no reduction in volume due to infiltration during the storm is allowed

 

[psmart]

Design practices vary considerably. Some regulations require an infiltration allowance, while others exclude it.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ONENGINEER]

I am doing both ways. If I do not allow infilteration the dry well volume increase to almost double.

 

[ONENGINEER]

beej67 thank you for your clear explanations. I understood that the city is not providing any data on the rainfall duration and it would be my task to assume a value.

The area including the runoff coef. is 2800 sqft.
The volume of 10 year 24 hour rain (4.3/25.4 = 0.17 inch)is 2800x24x0.0141=948 cu.ft

This would require a drain rock volume of 2872 cu.ft which is too large to be implemented due to layout restrictions.

Can I treat the 10 year 24 hr rainfall similar to a 10 year one hour rain fall of 4" (24x0.17). If so, I have some formulations (City of Eagle Spec) to deduct the rate of infiltration, thus to decrease the size of the dry well.

Thank you again from all with valuable comments.

 

[psmart]

Retaining the entire 24-hour volume is asking a lot. If you were able to allow for infiltration during the storm, it would reduce the storage volume considerably.


Peter Smart
HydroCAD Software

http://www.hydrocad.net