Win TR-55 Question 1

[edduran]

I am using Win TR55 to analyze a drainage area. The parameters are as follows:
Area 10.61 Ac,
Weighted CN 87
Tc 0.188 (hr) (100' Sheet flow at 2%, 662' at 28.4% unpaved, 208' at 2.16% Paved and 593 feet channel at 6.589 f/s.
This site is in the Caribbean and the rainfall type is II,
I have a PF Tabular Rainfall Data of:
5 min 0.847
10 1.16
15 1.49
30 2.38
60 3.53
2 HR 5.29
3 HR 6.04
6HR 8.94
12 HR 12.2
24 HR 15.6

When I analyzed the site in Win TR55 it returns a peak flow of 118.88 cfs.
That number seems to be off by about a factor of 2 based on an analysis of the site using the rational equation as a check with the 10 min intensity. Q=CIA, Q=0.87 x (1.16 in/10 min x6)x 10.61 acres Give me a Flow of 69.4
What am I missing?

 

[psmart]

There are a multiple reasons I wouldn't expect a good match:

1) They're different runoff procedures. Rational is linear with respect to rainfall, SCS is not.

2) CN=87 does not necessarily equate to C=0.87

3) When you use a Type II rainfall and set the 24-hour depth, the 10-minute depth is effectively given by the Type II curve, and may not match the 10 minute depth you quoted. In fact, if I scale a 24-hour Type II rainfall to your 15.6 inches I'm seeing a 10-minute depth of about 2", vs your value of 1.16".

Item 3 alone would explain most of the difference.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

unless I am mistaken, it appears that you are using a 10 minute rainfall depth instead of intensity in the rational equation. I would expect your intensity to be much higher, resulting in higher runoff predicted from the rational method

 

[edduran]

cvg, The 6 in the rational calculation is to convert the rainfall depth to intensity. I am using an intensity of 6.96 inches per hour intensity of the storm.
The C value calculated for this area is .80 based upon the land use so that actually pushes the numbers further apart so that is not a contributing factor.

I did run the calculations by hand for TR-55 using the worksheets and I still found the numbers to be much closer to the rational numbers and not the Win TR-55 numbers. To do this I interpolated off of Exhibit 4-II to get a Qu number which is the only place where I can see any wiggle room on the calculations, but that should not be lead to at 58% changes in the peak flow.

 

[psmart]

I think you'll find that the Type III distribution is a better match for your PF data than Type II.


Peter Smart
HydroCAD Software

http://www.hydrocad.net