## Win TR-55 Question

## Win TR-55 Question

(OP)

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?

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?

## RE: Win TR-55 Question

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

www.hydrocad.net

## RE: Win TR-55 Question

depthinstead ofintensityin the rational equation. I would expect your intensity to be much higher, resulting in higher runoff predicted from the rational method## RE: Win TR-55 Question

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.

## RE: Win TR-55 Question

Peter Smart

HydroCAD Software

www.hydrocad.net