# time of concentration



## Calixico (Aug 30, 2009)

In the exam, when solving problems which utilizes tc calculation (ex. Rational method), how does one know which tc equation to use for the most accurate value which is then used to find intensities. In CERM several different equation are presented to find tc.


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## Dleg (Aug 30, 2009)

I think the CERM (I have the ENVRM - same chapter as CERM for hydrology) does a pretty good job of explaining when to use each. The usual method, that I see at work, is to follow the sheet-shallow concentrated-channel flow progression for each segment, say for entry into TR-55. Using that with the Rational method would be completely acceptable, in my opinion, in virtually any circumstance - the rational method is not exactly known for its accuracy. Remember to keep sheet flow limited to less than 300 feet (if you're using TR-55 you are required to limit sheet flow to 100 feet, I think).

The other formuals given in the CERM are for more specific circumstances. I have never seen any of those used in practice, but then again I live in BFE so maybe they are more common in the real world.

Oh - and I wouldn't worry about which to use in the exam. If you get a tc question, chances are it will be broken down in a way that makes it apparent which equation you should use.


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## Fluvial (Aug 30, 2009)

Agreed, *Dleg*.

Also, if you calculate a Tc less than five minutes, it's acceptable to use 5 minutes instead of the smaller number.

A good rule of thumb for larger watersheds is to use a velocity of 2 fps and calculate Tc.

I typically use the DOT method for calculating Tc for a small (5 acres or less) drainage area when I'm using the Rational Method.


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## tymr (Aug 30, 2009)

Can either of you lend some insight on the hydrographs? I was told the CERM gives a good explanation on them but it doesn't make much sense to me. Thanks.


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## Fluvial (Aug 30, 2009)

What kind of insight?


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## RIP - VTEnviro (Aug 31, 2009)

> Oh - and I wouldn't worry about which to use in the exam. If you get a tc question, chances are it will be broken down in a way that makes it apparent which equation you should use.


Unless the main goal of the problem is to solve for the Tc, I wouldn't be surprised if it was just given to you.

And I also suspect the variables will be given to you in a way that makes the correct equation apparent.


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## EnvEngineer (Aug 31, 2009)

The main thing to remember about unit hydrographs its the measured value (flow in a stream or channel) divided by the rainfall in inches. Having the unit hydrograph will answere most questions however you need to be able to develop the unit hydrograph from runoff data. In that case the area under the hydrograph is the sum of the total rainfall. If you spread that rainfall over the land area (Cubic feet of runnoff/ acres of land x conversion) that tells you how many feet of rain fell. Now you can develop the unit hydrograph. Go back to the discharges and divide by the inches of rain. To get the area just add the peak discharge values together and multiply by the time interval between the samples (usually 1 or 2 hours) multiply by 3600 seconds per hour.

Do note that you have to subtract groundwater flow from the total flows if it is included, see CERM for a better explaination of that.


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## tymr (Aug 31, 2009)

Fluvial said:


> What kind of insight?


The enlightening kind. 



EnvEngineer said:


> The main thing to remember about unit hydrographs its the measured value (flow in a stream or channel) divided by the rainfall in inches. Having the unit hydrograph will answere most questions however you need to be able to develop the unit hydrograph from runoff data. In that case the area under the hydrograph is the sum of the total rainfall.


I think that's where I'm having my problems. Thanks, I'll look it over one more time and may be back with more questions.


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## NCcarguy (Sep 3, 2009)

Typically they didn't really give you problems that had random formulas that you had to decide how to solve. It's tricky, but not that tricky. If you know what Tc is, and how it's generally applied, then you'll be fine.


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## tymr (Sep 24, 2009)

Okay, I need to have this confirmed/clarified. On a hydrograph, if Q=0 at t=0 and the peak is at Q=200 cfs when t=5 then the peak runoff is 200 cfs. But when Q=100 cfs at t=0, and the peak of the graph shows Q=300 cfs at t=5 then the peak runoff is 200 cfs. Is this right? Also, how do I extract data out of a hydrograph to create a unit hydrograph? Thanks for the help.


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## pinkpig (Oct 1, 2009)

tymr said:


> Okay, I need to have this confirmed/clarified. On a hydrograph, if Q=0 at t=0 and the peak is at Q=200 cfs when t=5 then the peak runoff is 200 cfs. But when Q=100 cfs at t=0, and the peak of the graph shows Q=300 cfs at t=5 then the peak runoff is 200 cfs. Is this right? Also, how do I extract data out of a hydrograph to create a unit hydrograph? Thanks for the help.


simple answer to first question: yes.

more: by saying yes, you are assuming at t=0, Q=100 is pure baseflow and peak runoff you are talking about is actually precipitation excess after baseflow/interception/infiltration,....

unity hydro is equal to (total hydro)/(precipitation in inches), where both hydro here mean precipitation excess, or pure storm water runoff after loss.

There is another concept of dimensionless unit hydro,make sure you are not confused by these two.


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