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Time Concentration

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  • 1.  Time Concentration

    Posted 04-24-2019 05:07 PM

    Hello fellow Civil Engineers,

    I am Ray, a newly practicing civil engineer in the field of Water Resources Engineering. I would like to ask for your advise about what would be the best empirical formula to use in determining the time concentration of culvert pipes?


    PS: Tried searching for the same topic here but got none from the search box.

    Best Regards,

    Ray



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    Ray Francis Ralota R.Eng,Aff.M.ASCE
    Civil Engineer
    Cebu
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  • 2.  RE: Time Concentration

    Posted 04-25-2019 07:43 AM
    Ray,

    I think you may be mixing up different concepts. Time of concentration relates to hydrology and it means the the longest time it takes for water to travel through a watershed. You can search the TR55 method from NRCS for a full explanation and computation methods.

    The other concept is pipe travel time which can be calculated if you know the flow in the pipe.

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    Luis Monzon M.ASCE
    Department Manager
    Fair Lawn NJ
    (201)334-6811
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  • 3.  RE: Time Concentration

    Posted 04-25-2019 11:10 AM

    Hello sir thanks for your response,

    At the moment, I'm still designing the size of the culverts, so I need to know the discharge (Qp).

    Cause as what I've read in Hydrologic Analysis and Design by Richard McCuen, it stated that I need to compute the travel time in the pipes first so that I can determine the rainfall intensity, which it will be used to determine the discharge.


    PS: Thank you for the enlightenment about the difference between the Time concentration and Travel Time.



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    Ray Francis Ralota R.Eng,Aff.M.ASCE
    Civil Engineer
    Cebu
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  • 4.  RE: Time Concentration

    Posted 04-27-2019 06:36 PM
    Ray, you can also go to Txdot.gov for the Hydraulic Design Manual.  It has whole chapters on runoff calculations, time of concentration, and culvert design.  It's also very practical versus a very theoretical approach.

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    James Justin Mercier, P.E., CFM
    Life Member ASCE
    Sr. Life Member IEEE
    Austin Texas
    512-442-4016
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  • 5.  RE: Time Concentration

    Posted 04-25-2019 10:29 AM
    There isn't any magical empirical formula.  There are so many variables.  You should try to get hydrograph information if it is available. For large watersheds the Kirpich Equation works well.  You can then adjust the result for the type of surface in the watershed.  Double for overland flow on grassed surfaces, multiply by .4 for paved surfaces and by .2 for concrete channels.  There is a book, Modern Sewer Design,  published by American Iron and Steel Institute that contains many helpful nomographs and tables.  Remember that the most common error in designing culverts is failure to determine the control conditions (inlet or outlet), submerged outlet or not.  The concrete pipe folks publish a book with very helpful charts in this regard.

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    James Anderson, MASCE
    Principal
    Anderson Consulting
    North Richland Hills, TX
    817-946-9943
    James
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  • 6.  RE: Time Concentration
    Best Answer

    Posted 04-25-2019 11:09 AM
    For pressurized flow, I would try to find a relatively cheap program that includes a water quality module.  20 years ago, Flownet worked great.  It may still be available.  Most of them just calculate the average residence time based on average flow demand.  There are some that are completely dynamic.  I used it to determine areas with dead ends in long runs of water lines without sufficient demand in order to determine whether we could remove those dead ends through system expansion, or we needed to continue to flush.

    It gets a little more complicated for non-pressurized systems.  If you want a very accurate answer, then you need to pull out partially full pipe equations, based on flow rates and Manning's Equations.  V. T. Chou has a great section in his red book.  (A must have for most water resource engineers.) Most jurisdictions allow you to calculate the TC by the travel time, without considering whether the pipe is partially full.  Velocity is Flow / Area of Culvert.  Travel time = length / velocity.   Make sure you keep your units consistent.  Every jurisdiction has their own rules, so make certain you get a good look at their design criteria prior to submitting things.  The travel time in the ditch is probably going to be much more significant for most projects unless this is a very long culvert.

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    Dwayne Culp, Ph.D., Ph.D.,M.ASCE
    Culp Engineering, LLC
    Rosenberg TX
    (713)898-1977
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  • 7.  RE: Time Concentration

    Posted 04-25-2019 11:10 AM
    Good Morning Ray,

    I assume that you are referring to a storm sewer system.  Most engineers use the Manning's equation, either through free flow or hydraulic grade line calculations, to determine the velocity in the pipe segment.  Once you have the velocity, the travel time is easily calculated.

    Good Luck in your newly chosen field,

    John

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    John Wood P.E.,M.ASCE
    Project Engineer
    Pittsburgh PA
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