Time-of-Conc Question
Time-of-Conc Question
(OP)
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.
Damon
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.
Damon





RE: Time-of-Conc Question
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.
Ches
RE: Time-of-Conc Question
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
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.