Rational Method Q vs TR-55 Q Results 2

[racv2]

I am trying to calculate runoff using the TR-55 Graphical Method for a 5.48 ac area. I used Type III Rainfall Distribution for a 2-years/24 hrs rainfall that gives me a rainfall P = 3.2 in, I also used a CN of 61 and a time of concentration Tc = .24 hr (.14 sheet flow and .10 shallow concentrated).
The Unit peak discharge I obtained was: 321.40 csm/in, the runoff Q=.37 in and the Qp = 1.01 cfs.
Just to verify these results I calculated the runoff Q using the Rational Method, C = .65, i = 3.2 in/hr A = 5.48 ac and the resulting Q = 10.68 cfs.
Now, Pardon my ignorance but shouldn't both Q be the same using these two different methods? What am I doing wrong?
Am I comparing the right Q's obtained from both methods?
Thank you for any help and please bear with me...asking is how we learn.

 

[ngneer]

without investigating more, i noticed the units are different for each q. hopefully it's not that simple but thought before time is spent...

 

[LHA]

I get 2 cfs from the TR55 DOS program. It is free from here:

ftp://ftp.wcc.nrcs.usda.gov/downloads/hydrology_hydraulics/tr55/tr55v21.exe

I get .65*3.2*5.48 = 11.4 cfs.

P=3.2 in and I=3.2in/hr are not the same as you've used. You need a local IDF curve for the Intensity I of a 2-yr event at .24 hours

.65 is way too high a "C" value for the conditions you've cited. Using Rossmiller's conversion, I get between 0.18 and 0.29, depending on impervious coverage and land slope. Rossmiller's nomograph is available a number of places, the PSU-IV flood manual on page 29 for one. These "C" values give between 3.2 and 5.1 cfs.

Even if do everything correctly, I have never had the two methods give the same result...but they should be close, with rational higher.

 

[racv2]

Thanks ngneer and Iha for your help.
Iha I read one of your replies to one of my previous posts and I understand you used to live in New England? cool huh?
The soil conditions I am assuming are Hydrologic soil B, good condition with a grass cover > 75%. So that gives me a CN = 61 from Table 2-2a of TR-55.
I used the Rational Formula just to verify and thanks to your correction I realized I was assuming P = 3.2" as being the same as I (in/hr).
I got 1.36 cfs using the software u forwarded for a Dist. Type III, P= 3.2",Tc=.24 hr, A=5.48 Ac, CN= 61, Pond and Swamp Areas= 0%.
How did you get the 2 cfs? Did you round up?

 

[LHA]

I have lived in Springfield, Waltham and Weston, MA at different times, my sife is from Pittsfield.

I always use the Tabular rather than Graphical analysis in TR55, because it gives a hydrograph you can then route through controls. I reads out to only a whole number, and it gave me "2". I didn't run Graphical, so 1.36 is probably also correct.

The difference is neglible in practical situations...the controls needed to convey and manage 5.5 acres to either 1.4 or 2 are going to be the roughly the same.

At most use 0.1 for reporting. Don't report the 0.01 decimal in runoff rate, it is far more precise than your initial "known" inputs.

 

[racv2]

cool! I am from the North Shore but have been to Springfield for work and know Waltham and Weston very well. I haven't been to Pittsfield yet.
Where are you located now?

I didn't use the Tabular method cuz I understood is more for subareas draining into a bigger area???

The whole story is that I am trying to use the Land Development/Civil Design (AutoCAD) hydrology module to do my calculations...and got kinda lost in it. I understand from our CAD provider that it is not the best thing to use for hydrology calcs. I used to use the Haestad methods software but where I work now we don't have any hydrology software so I had to re-educate myself and refresh my memory on how to do manual calcs and then how to use Land/Civil Design!!!

Thanks for all your help.