Modified Rational Versus TR-20

[TheEnginerd]

Hello All:

My company has to review a site plan for a small 10 unit towhhome development. So I’m reviewing the plan and almost everything on the job is tight, but meets the ordinance requirements. The only real concern I have is with respect to stormwater.

The site is about an acre. The existing lot is completely vacant and pervious. The new project proposes 8/10ths of an acre of impervious surfaces. The applicant’s engineer proposes (2) – 10 foot wide swales totaling 2300 Cubic Feet to as he claims, fully detain the 100 year storm. There is no outlet from these swales, they use infiltration only.

The engineer used the MODIFIED RATIONAL method to do his basin sizing calculations. I do not have a lot of experience with this method. I was always taught to use the NRCS TR-55 methodology (by hand) or TR-20 using software (hydrocad, hydraflow, etc.)
In NJ where I work, the NRCS 100 year design storm is 8.9 inches for the 24 hour event. Over 8/10ths of an acre, that comes out to roughly 20,000 cubic feet of water. That’s about 9 times more than this engineer calculated.
Basically all of the engineer’s calculations are one 2 or 3 sheets of paper and look like they were done in about an hour. For many reasons I can’t believe that this methodology is acceptable not just for this project but for any project where it will be used to size a detention basin.

The problem that I have is that nowhere in the municipal ordinance does it prohibit the use of the modified rational method.
So what can I do here? I have a real concern that not only will these tiny swales not contain a large storm event, but according to the NRCS methodology the swales would not even contain a normal 1 year storm event. The flow would then overtop and head directly into the neighboring houses. Everywhere I look I read that the mod. Rational method underestimates stormwater volumes, but I can’t find anything that definitively says that it’s incorrect or irresponsible of an engineer to use only this method.

I would appreciate and be grateful for your thoughts on this.

 

[psmart]

Your concerns are well justified. At the very least, check the rainfall duration that was used for the Rational volume estimate. If the intent was to retain the entire 24-hour event, make sure they used a 24-hour duration, and not something shorter. The 24-hour depth should be the same regardless of the runoff procedure that was employed. My guess is they used a shorter duration, perhaps for pipe sizing, and then calculated the corresponding volume, instead of the full 24-hour volume.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Drew08]

When it comes to a constant discharge systems (like infiltration) as appose to head controlled spillways, longer, high volume storms like the 100-year, 10-day storm may be more critical. I design a lot of store and pump systems, and like to use a 10-day frequency distribution event because it includes both a short duration peak and long duration volume, but still has the same probability of a 24-hour event.

 

[TheEnginerd]

Thanks PSMART. So basically the engineer uses a formula that is 0.5 * Q * TC * (60 sec/min) to arrive at an answer in cubic feet. Which i assume is supposed to represent the volume under a triangle with TC on the X axis, and Q on the y axis. They previously calculated Q using the rational method.
Seems way to short of a duration to me. how do you think i should respond psmart? do you know of any documentation that says to use a 24 hour duration for the modified rational method? I thought this was only an NRCS, TR-55, TR-20, etc. storm duration.

Drew08, I appreciate your comment and agree with you. But i can only make the engineer/applicant adhere to what the ordinance requires even though something else may actually be more accurate and lead to a more complete design.

 

[psmart]

That approach will only provide the volume for the peak-flow event, where the duration is equal to the Tc. The 24-hour volume will be much larger.

Do the stormwater regulations provide any guidance on the detention calculations? If they require the retention of a 24-hour rainfall, then you need to use the 24-hour rainfall depth. Since the lot is mostly impervious the 24-hour runoff volume will be nearly equal to the 24-hour depth. The peak flow is irrelevant - just look at the volume directly.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Twinkie]

If your ordinance does not contain language that prohibits the use of modified rational, there is probably something that says the Twp Eng reserves the right to reject any design that may create adverse conditions.

 

[cvg]

with no outlet structure, these really should be treated as "retention" basins not "detention". generally infiltration is very slow, so attempting to route a storm runoff hydrograph through a retention basin results in the basin being sized to hold all the runoff from the entire storm. Typically around here, a basin is sized to hold the entire 2-year storm plus freeboard. Direct infiltration / drywells / pumps are required dispose of the entire thing in 36 hours or less. there is no Tc calculation necessary and no need to use a modified rational method triangular hydrograph, since the entire storm runoff is being captured.

V = P x C x A

where
V = storm runoff volume to be retained
P = 2-hr rainfall depth
C = runoff coefficient
A = area

 

[fel3]

As others have pointed out, it sure sounds like the swales are woefully undersized per your town's criteria, regardless of method. You mention that the design engineer is relying on infiltration. Does your town allow this for retention basin design, and if so have there been any geotechnical studies to confirm that the site is suitable for infiltration and to provide criteria for infiltration design? Regardless, I doubt that actual infiltration will bridge the gap between a basin incorrectly sized to store only a peak flow and a basin correctly sized for a true 24-hour event.

BTW, There are some agencies here in Central California that do not allow infiltration to be considered when sizing a retention basin. The reason is that infiltration decreases over time due to silting. Sure, the basins can (and should) be mucked out every few years, but that presumes (often falsely) that there is sufficient funding for regular and timely maintenance. Any infiltration that does occur is considered a extra benefit. Other agencies allow partial consideration for infiltration based on geotechnical studies.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[TheEnginerd]

thank you all for you great responses.
fel3, yes - our state DEP requires us to EXCLUDE infiltration for the very same worst case/ no maintenance scenario.
Regarding the soils in general, like everything else on this project, the applicant has reached the bare minimum. These swales have no outlet and no way to drain other than through infiltration. The state and ordinance minimum infiltration rate is 0.5 inches per hour. The borings all show the soil to be almost exactly 0.5 inches/hour. Seems suspicious to say the least!

 

[MrBachelor]

While the applicability of the NRCS Method of runoff calculation can be debated for sites that small, I always question the use (Modified) Rational Method for volume calculations. Rational Method was designed to calculate peak flow rates. NRCS Method was designed to calculate runoff volume from agricultural lands. Choose your assumptions wisely.

However, if the ordinance allows the Modified Rational Method for calculation of the storage volume it should at least be applied correctly. It sounds as if the designer calculated the peak flow for one given Tc, and calculated the hydrograph volume for only that instance. Remember that i in Q=CiA is related to the storm duration, not the time of concentration. When using the Rational Method correctly to determine the peak flowrate there is no need to increase the duration beyond the Tc as there will be no additional contribution of drainage area and therefore no increase in flowrate. However, when seeking a volume from the Modified Rational Method it is vital that the calculation be performed at multiple durations with corresponding intensities to produce a series of trapezoidal hydrographs. Once the allowable discharge is subtracted the hydrograph with the greatest volume is then your critical storm duration for that analysis.

You mentioned being from NJ and I happened to find this example:

http://njscdea.ncdea.org/The Modified Rational Method.pdf

Nate the Great

http://www.ceieng.com

 

[beej67]

Although I have never seen anyone use Rational method to determine a capture volume, the means MrBachelor outlines are exactly correct for doing so.

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

http://www.campbellcivil.com

 

[psmart]

BTW, this is the same procedure used by HydroCAD to calculate the "critical duration" for a pond. It will handle all scenarios including outlet devices, exfiltration, or 100% retention.

Of course, the math is trivial for the current situation with 100% impervious area and no outflow: The required storage is essentially equal to the rainfall depth for whatever duration and return period you're trying to retain, as specified by the applicable stormwater regulations. The Tc doesn't enter into this calculation at all, and you certainly shouldn't use the (smaller) volume associated with the peak-flow duration.


Peter Smart
HydroCAD Software

http://www.hydrocad.net