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Time-of-Conc Question

Time-of-Conc Question

Time-of-Conc Question

Hello everyone.  I am presently crunching through some detention basin design calculations and I came across a minor snag.  I think it's a fairly common problem, but I was wondering if anyone had any feedback about how to deal with it.  Here's what I have.   
1) I set up pre-development hydrographs using the Rational Method, using one large watershed (about 25 acres) to model the pre-development runoff.  The max Q came out to 36.31 CFS for the 100 year storm with a 17.44 min Tc.
2) For post-development, I divided the pre-development watershed into routed and bypass sub-areas.  I set up my post-development hydrographs and modeled it without the basin routing to quantify how much the runoff would increase as a result of the added impervious surface area.
Here's where I run into a problem. The combined post-development hydrographs (unrouted) resulted in a REDUCTION in the peak runoff, rather than an increase.  For the 100-year storm, the max Q was 33.73 CFS with a 17 min. Tc.  It seems pretty clear what the problem is.  Simply put, the peaks of the sub-watersheds do not line up anymore.  The increased runoff from the developed area occurs at Tc=5 min, which is well before the Tc of the much larger bypass watershed, which occurs at 17 min.  The developed runoff is already off-site by the time the bypass runoff peaks.
Has anyone dealt with a similar problem before?  I don't think it would fly if my SWM report indicated that the development resulted in a decrease in runoff.  My instinct is to manually add the peak flows of the individual watersheds.  It's over-conservative and it does not accurately model the actual conditions. However, I cannot think of a better solution.  Suggestions anyone?  Thanks.


RE: Time-of-Conc Question

You shouldn't really be using the rational method to design a detention basin. That's problem number one. The SCS curve number method is much better. In the past, my company sized detention basins on the Rational Method and now that the City is requiring the SCS curve number method for volume calculations, any old detention basins that need to be recalculated end up much too small. The problem is that the rational method gives a peak flow, but not a volume, which the SCS CN method provides in addition to a peak flow.

However, your biggest problem is that you appear to be using a 5-minute design storm.  In that case, perhaps your peak is lower.  However, if you use a 15-minute design storm, and compare that to the pre-developed, you'll almost certainly get a different answer.  In this case, while your post-developed flow peaks within 5 minutes, because it keeps raining, the 5-minute time of concentration basins remain at peak flow, allowing you to add the peak flows together. This sum should be higher than than your pre-developed peak flow.

Remember that you have to make post-developed run-off equal to or lower than the pre-developed rate/volume/velocity for all storm durations and recurrances.


RE: Time-of-Conc Question

Of course! It doesn't make sense to add a 5-minute design storm in my routed watershed to a 17-minute design storm in my bypass watershed.  I will modify my calculations accordingly.  Thank you so much.

With regard to the use of the Rational Method, I should clarify.  I am actually using the Modified Rational Method to size the basin.  Thanks again.

RE: Time-of-Conc Question

You actually need to determine the critical duration.  This is the rainfall duration (and the corresponding intensity from the IDF curve) which produces the highest flow at a particular design point.

For a single subcatchment, the critical duration is equal to the time-of-concentration.  But for multiple subcatchments, the answer is not so simnple.  In your case, the critical duration for the combined flow can be anywhere between 5 and 17 minutes.  Note that the critical duration may be different at different points in the watershed.

The SCS/NRCS method is much easier to apply, and is far more appropriate for detention pond studies.

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