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Time of Concentration

Time of Concentration

Time of Concentration

I have been told that the time of concentration can be lengthened by 5-15 min. when storm water is routed through a catch basin. I have not been able to find any technical data discussing this and was wondering if anyone has information on this subject.

RE: Time of Concentration

Sounds bogus to me, I would make you prove that.  I would guess water takes somewhere around a nanosecond or two to enter and pass through a catch basin.

Engineering is the practice of the art of science - Steve

RE: Time of Concentration

Sniff - Sniff...smells like fertilizer.  The incoming water has to reach a point where the catch basin will discharge (if there is any storage).  Depending on the rate of inlow, this could be a couple of seconds to a couple of minutes.  Once the flow is going out, your Tc through the basin is essentially zero.

RE: Time of Concentration

Do you mean catch basin or detention/retention basin?  Detention basins can increase times of concentration for hours.

RE: Time of Concentration

Crystal has a point - catch basin is usually an inlet.
The detention/retention basin or pond will hold water, and could make an impact.

Your pipe flow time may be used in the Tc, but it won't be very much time.  

RE: Time of Concentration

Thanks for the input. I was in a teleconference on detention basin design and the presentor specifically stated that 5-15 min. inlet concentration times could be added to the time of concentration. I had not ever heard that before and it seems like is it not a reasonable assumption.

RE: Time of Concentration

The travel time for a lake or reservoir can be estimated by:

Tt = L / V

where V = sqrt( g D )

Tt = Travel time (seconds)
L = Flow length
V = wave velocity
g = gravitational constant
D = mean depth

RE: Time of Concentration

I wonder if the presentor was refering to the minimum Tc of 5 minutes commonly used in Rational method calculaitons.

RE: Time of Concentration

The overland travel time (time to reach the inlet) for a one acre parking lot will be about 100 ft/2fps = 50 seconds. For this reason computer models that require a minimum Tc of 5 minutes are inherently invalid.
The Rational Method was never intended to be used with Tc --this misconception was invented in 1890.
The Rational Method uses the IDF correlations that estimate the ability of a raincloud to generate rainfall. This ability is determined by rain gage data.
The minimum duration for IDF correlations is 5-minutes. This duration can be used as long as the pipe network travel time is less than 5 minutes. For example, a 1000-ft long storm sewer flowing at 3.3 fps has a travel time of 5 minutes.
Rational Method Runoff coefficients can be reduced to allow for ponding and/or the longer travel time for heavily vegetated areas.

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