# Tc Equation from my NSPE CD-ROM review course



## Blu1913 (Oct 17, 2006)

Not sure if anyone cares, but the CD-ROM review course I bought uses the following to calculate Tc when P2 is unknown:

Tc = (.0078 X Length^0.77)/(Slope^0.385)

This is Kirpich's equation, useful in urban and suburban applications.

L is in ft, slope is in ft/ft, and your answer comes out in minutes.

Maybe useful to just write down...


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## JasonT33 (Oct 17, 2006)

Dont forget the adjustment factor you need to use based on type of flow (overland (soil and asphalt), channel, etc)

k= 0.2 (concrete/asphalt channel)

k=2.0 (overland grass)

k= 1.0 (bare soil or ditch)

k = 0.4 (overland asphalt/concrete)


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## 3gorgesdam (Oct 17, 2006)

I have never seen this equation before. What is this for? Is this for traveling time for some kind of particles within a flow?


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## GTScott (Oct 18, 2006)

Nah..it is a standard formula for time on concentration (time is takes flow to reach a basin outlet from the HYDRAULICALLY furthest point in the basin). Isn't this in LB? I went through that section a bit quick so I could not remember.


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## RIP - VTEnviro (Oct 18, 2006)

The one I saw in 101 Solved Envl problems, and the practice tests was the Federal Aviation Agencies Eq.

Tc = [3.64*(1.1-C)*L^0.83]/H^0.33

C = rational method runoff coefficient

L = flow length

H = difference in elevation

The book I've got is "Hydrology and Hydraulic Systems" by Gupta. It has 5 different Tc estimates, and they're all pretty close. If you've got a good equation you like, it'll probably get you in the ball park.


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## Blu1913 (Oct 18, 2006)

I mostly work with stormwater at my job. Tc can vary up to 100% from different methods. The one used most often is the one in TR-55. Thats the one in the CERM where you break up Tc between sheet flow, shallow concertrated, and channel flow. If your given the rainfall depth for a 2-yr storm, (P2) make sure you use the TR-55 method.


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## JasonT33 (Oct 18, 2006)

Can you show me an example of 100% variance between Tc methods? I've been dealing with stormwater management and design for 10+ years and never encountered that.

Just curious


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## Blu1913 (Oct 18, 2006)

Most stormwater is completely up to engineering judgement. If you and I were to design a site, we could potentially have a 20 - 30% difference in peak flows and runoff volume, and we would both still be correct.

Differences of Tc can vary from Kirpich, FAA, Lag method, TR-55 and on and on. Not to say any method is wrong, but if used correctly, a good stormwater engineer can maniplate his data to produce a shorter pre-developed Tc and a longer post-developed Tc. (Especially useful if you have to design to the Critical Storm) There-by saving his client money and still meeting regulations. In our City, we make it very clear as to which methods are acceptable so there is consistancy and ease of review. Hence, we currently use TR-55 methodology.

I dont have a specific example, and dont have the time to start one, but there are many, many different methods for doing stormwater calculations. The lastest and greatest is a methodology produce by Huff and Angel. Their rainfall depths for different storms are generally higher than the tradition ones introduced by Technical Paper No. 40.

I think, in all, were moving away from what this forum is based on. While Im sure this is all good info for those reading it, many probably dont give a hoot.


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