Drainage Before and After Development 1

[Althalus]

I've never had this happen before. I'm wondering if anyone had this happen.

I've just done a hand calculation of the drainage before and after development.

Factors:
1) I used the rational method to determine Qp.
2)I started off with about 15% prairie and 85% Forest.
3)I have 15% impervious cover after development.
4) Runoff coefficients change with slope and soil type per our County manual.

The two pre-dev areas have different slopes than the four post-dev areas​

The composite C(pre) = 0.28​

The composite C(post) = 0.29​

5) Total A(pre) = 7.73 acres; Total A(post) = 9.49 acres.

Methods:
1) I've used McCuen's formula for sheet flow and the TR-55 formula for shallow concentrated flow (and the.
2) I used the formula for velocity found in Appendix F example for unpaved flow. (I determined that the longest Tc would be from the unpaved areas rather than the paved areas of the same shed).
3) I used that velocity to find the Tt for shallow concentrated flow (Eq 3-1).

Results:
1) Tc before (12.6min & 12.5min)
2) Tc After (17.1, 14.9, 13.5, 12.4 min)
3) Q(pre)= 38.9cfs
4) Q(post)= 39.6cfs

C went up, Area wend up. Somehow I didn't think Tc would have that big of an impact. But I had to shorten the flow length in the calculations to get the Q(post) to be higher than the Q(pre). The reason is that after development, the flow had to go through an intricate path of swales which increased the travel time. But I thought that the reviewer would never accept that if they saw it. So, I fudged the numbers to be more conservative.

Has anyone ever seen this before?

 

[fel3]

Yes. In fact, creating a longer path of travel is one design strategy that is used to get Q(post) to be less than Q(pre) when using the Rational Method or TR-55. This may not be intuitive because most Intensity-Duration-Frequency curves are plotted as straight (or nearly straight) lines on a log-log chart, so in your mind you might think it's a linear relationship. However, plot the IDF curve on a linear-linear chart and you can readily see how much the rainfall intensity decreases with only a small increase in Tc. A typical IDF curve has the form I = a*Tc^-b. My local 10-year storm has this IDF curve (rounded off for this example): I = 8.00*Tc^-0.60
for Tc = 5 min, I = 3.05 in/hr
for Tc = 10 min, I = 2.01 in/hr --> so, increasing Tc from 5 min to 10 min reduces the rainfall intensity and Q(peak) by 34%
for Tc = 15 min, I = 1.58 in/hr --> so, increasing Tc from 5 min to 15 min reduces the rainfall intensity and Q(peak) by 48%
for Tc = 20 min, I = 1.33 in/hr --> so, increasing Tc from 5 min to 20 min reduces the rainfall intensity and Q(peak) by 56%

As you can see, the low-hanging fruit is to add minutes to very short Tc's (e.g., 5 minutes to 10 minutes), which is the bane of small sites. Adding the same number of minutes to a long Tc (e.g., 60 min to 65 min) has a smaller effect, but it's still worth doing.



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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[beej67]

I would not prove compliance with a pre/post match by monkeying with your Tcs in rational method, because it's disingenuous.

In rational method, Tc is merely dictating which storm you use for your analysis, which means doing a pre/post analysis with different Tcs in rational method means you'd be comparing the results of different storms, not an apples to apples comparison of the same storm.

Rational is good for culvert design and storm conveyance design. It is not good for pre/post hydrology. For a pre/post hydrologic analysis, choose a method where you can compare the same design storm for both pre and post conditions, such as SCS method.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com