Hydrology
Hydrology
(OP)
Performing a hydrology study on a 40 acres commercial site. Municipality requires showing 1 year storm. I have a small drainage basin (1.7 acres) that becomes 0.9 acres with a driveway n the post-developed condition. I show approx 0.4 cfs increase to the 1 year storm. I've tried relooking numbers (ie: sheet flow length, slopes) that I could possibly, interpret as larger or smaller, but I cannot get my one year to fall below pre-developed conditions. Does this mean I have to detain for 1 year storm (0.4 cfs)?? Seems unreasonable. What kind of detention do you recommend? What tips or suggestions are there. I just want to use some common sense and be comfortable with my decision. thanks to all.





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But if the reviewing agency also expects the larger storms to be detained or infiltrated then you might still have to look at some form of basin.
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How does a 1.7 ac basin become 0.9 ac? Paving wouldn't make the basin smaller. If the basin splits you still have 1.7 ac, it's just two basins that combined must produce no more runoff than pre-developement single basin.
And 0.4 cfs? Seems like the water quality facility for the major storm would address this issue.
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As for the peak flow times... I am researching other reports and text books on hydrographs. Generally, I think I agree about them happening roughly at the same time. Any reference material on this topic would be helpful. I have at most a 6 minute spread in peak times.
I seem to think the decrease in Tc (pre to post)along with different peak times cause this 1 year increas / 2,5,10,25,100 decrease. I can't think of anything else. Thanks to all who are commenting, this is helping me.
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Michael
www.revisionblock.com
Import Hydraflow Into Excel
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-Have you tried adjusting the time step? Sometimes I have found that it can lead to more favorable results.
-Report everything to the nearest CFS.
-A different model such as HEC-HMS might make such a small difference disappear.
-Can you slope a portion of the parking lot to collect the water (detain on the pavement) and have it outlet through a small curb opening? I wouldn't think You wouldn't need much volume to show that you can take care of 0.4 cfs.
-Is there room for a shallow retention swale? Mosquitoes need a place to live too.
It's a shame that so many reviewing agencies have such a shallow understanding of hydrology or real-world experience that they would make a big deal out of 0.4cfs and have us play these games.
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