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Minimum time of concentration

Minimum time of concentration

(OP)
Could someone please tell me why some engineers and agencies use and specify a minimum time of concentration when using the rational method? Does it have something to do with intensities or time of concentrations of past rainfall events not being accurate at short concentration times? I notice that on IDF curves the intensities rise sharply, almost exponentially as time of concentrations become shorter.

Also is it still necessary in industry to use time of concentration calcs now that rainfall intensities are available from sites such as NOAA and other sources.

Thanks in advance.
Civil "G"

RE: Minimum time of concentration

A minimum Tc is sometimes specified because available rainfall data (i.e. the IDF curve) does not typically extend below a duration of 5 or 6 minutes.

The availability of updated rainfall data, such as the NOAA site, does nothing to reduce the need for an accurate Tc value. The rainfall data indicates how the rainfall is distributed over time, while the Tc is a physical characteristic of the site, indicating how quickly runoff will occur from a given drop of rain.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Minimum time of concentration

(OP)
I agree with what you posted; however, when an entire watershed is contributing to an point of interest (i.e. drainage inlet) wouldn't the intensity of a given storm event be the same as the intensity derived from the time of concentration from the most distant point in the watershed?

RE: Minimum time of concentration

Tc can vary between pre and post, within the same watershed POI.
IDF curves are intended as use in reasonable basis for design, (FDOT, Drainage Hydrology Handbook, 2012).
"For drainage calculations, it is desirable to divide the storm into convenient time increments and to determine the average intensity over each of the selected periods,"(FDOT, Drainage Manual, Vol 3, 1987).
"For drainage calculations, two design storm calculations can be important: the first provides a basis for estimating a peak discharge, while the second provides a basis for evaluating volumes. For peak flow calculations, the storm duration should be selected to correspond with the critical time characteristic of the watershed (i.e., time of concentration). The duration for storage calculations may be established by regulation (e.g., 24 hours) for permitting purposes (regulatory duration, however, may not be appropriate for hydraulic design calculations)," (FDOT, Drainage Manual, Vol 3, 1987).

RE: Minimum time of concentration

unless a storm is extremely intense or the surface is steep and entirely impervious, initial abstractions and depression storage will limit the amount of runoff occuring in very shore times of concentration. after 5 - 10 minutes, runoff from the entire area begins to run off. The IDF curve gets very steep at short Tc's and estimating the actual intensity is prone to error, using short Tc's will usually over estimate actual runoff

RE: Minimum time of concentration

Quote (CivilG)

when an entire watershed is contributing to an point of interest (i.e. drainage inlet) wouldn't the intensity of a given storm event be the same as the intensity derived from the time of concentration from the most distant point in the watershed?

The Tc is calculated from the most hydrologically distant point in the watershed.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Minimum time of concentration

I have seen two flavors of regulatory minimum Tcs.

One flavor is basically "the chart ends here," which usually means the minimum is either 5 minutes or 0.1 hours, depending on your method. The other flavor is basically "hey, guys, quit cheating," which is often a regulatory minimum set by a municipality after seeing too many engineers tweak the SCS Method spreadsheet to produce 5 minute Tcs for existing undeveloped watersheds. Cobb County Georgia, for instance, states explicitly that the minimum allowable Tc for an existing undeveloped watershed is 10 minutes no matter what your site looks like, for the purposes of hydrograph analysis to set your allowable discharge. There's no scientific basis for the reg, but the practical engineering basis is ostensibly to prevent engineers from cheating their allowable discharges up and undersizing their ponds.

The fundamental problem with Tcs, at least in site hydrology, is that the lions share of your time of concentration is in sheet flow, and the sheet flow at the most hydraulically remote point doesn't truly have much of a bearing on the overall reaction of the watershed. If I were a regulator, and drawing up my own rules, I'd scrap the SCS Tc method entirely and require everyone to use Kirpich. Most of the time I see the exact opposite by reviewers - they prohibit Kirpich and require SCS.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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