Modified Rational Versus TR-20
Modified Rational Versus TR-20
(OP)
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.
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.





RE: Modified Rational Versus TR-20
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Modified Rational Versus TR-20
RE: Modified Rational Versus TR-20
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.
RE: Modified Rational Versus TR-20
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
www.hydrocad.net
RE: Modified Rational Versus TR-20
RE: Modified Rational Versus TR-20
V = P x C x A
where
V = storm runoff volume to be retained
P = 2-hr rainfall depth
C = runoff coefficient
A = area
RE: Modified Rational Versus TR-20
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.
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
RE: Modified Rational Versus TR-20
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!
RE: Modified Rational Versus TR-20
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%20Modified%20Rational...
Nate the Great
www.ceieng.com
RE: Modified Rational Versus TR-20
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Modified Rational Versus TR-20
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
www.hydrocad.net