Hydrology 5

[mbherlihy]

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.

 

[ptmoss]

If you have suitable soils then the scenario you describe might be custom fit for one or several LID practices. Perhaps a rain garden, perhaps a few tree box filters, maybe the paved areas would be suitable for pervious pavement.

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.

 

[francesca]

Why do you think it's unreasonable to detain the extra volume of runoff your development is producing? If you want your client (and you) to stay clear of a civil law suit from downstream property owners you should also detain the 100-year volume.

 

[mbherlihy]

The only frequency of storm that has a post-developed increase is the 1 year storm, therefore I didn't think i needed to detain the 100 year storm.

 

[francesca]

How do you calculate that only the 1-year storm has an increase? If you are using the SCS methodology, the only difference between the 1-year and the 100-year storm is the rainfall depth. With all else held constant, an increase in one means an increase in the other.

 

[cvg]

in my book, 0.4 cfs increase is a wash... Seriously, you can't possibly hope to estimate peak runoff to the nearest 0.1 cfs?

 

[mbherlihy]

francesca, i often see certain frequency storms peak flows decreasing( pre to post developed)while others increase. For example, the 1,2,5 year storms increase, however the 10,25,50, 100 decrease. I believe is due to each frequency storm hydrograph peaking a different times. Therefore, as in my example, the 1 year shows and increase while all other storms decrease. FYI: I am using Pond Pack and the SCS method.

 

[rday]

I can't imagine the minor storm creating excess runoff and the major storm not.

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.

 

[ptmoss]

I've definitely seen the smaller storms present an increase in the post, usually the 2-yr, and the larger storms decrease from the pre to the post. However, these have been fairly large and complicated models. I suppose if a single subcatchment area had a sizable change to the Tc between pre- and post-developed conditions that might provide similar results.

 

[francesca]

I would take a long, hard look at how PondPack is calculating your hydrographs, because all 24 hour storms should peak at the same time if you're using the SCS method. The only difference should be the rainfall depth and the amplitude of the hydrograph; it should peak at the same time.

 

[ptmoss]

I also agree with cvg. When I summarize results I always round to a whole number. This hydrology stuff just ain't that precise, even if our computers are!

 

[RWF7437]

http://www.stormcon.com/sw_0607_voodoo.html

 

[mbherlihy]

The 1.7 acre sub basin becomes 0.9 acres, when you regrade the site in the post developed condition such that 1.7 acres no longer flows to your point of study. Part of this 0.9 acres is paved due to a driveway entrance.

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.

 

[mjwoodruff]

This happens to frequently in the jurisdiction where I work, that detention of the 1 year storm is not required. Often, jurisdictions will leave it up to the engineer's interpretation of what would be acceptable. In my experience, 0.4 cfs is negligible. Often times, that amount of flow can simply be due to the program's rounding.

Michael

http://www.revisionblock.com

Import Hydraflow Into Excel

 

[bob3306]

I've also run into this exact same thing. Here's a few suggestions:

-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.

 

[newnickname]

With a flow that insignificant, you may be able to use an inlet as detention with a weir/flow splitter. Use an underdrain to drawdown and infiltrate the volume of storage needed.