Drainage Analysis - Brownfield Site

[martin888888]

We have a large (40ac) site where we are doing improvements in 3 locations that includes demo and adding facilities/infrastructure. We want to analyze the need for detention over all 3 areas combined. What would be the best way to do this. Could you just analyze the pre/post for just the disturbed area at all 3 locations and route them to a common point in the analysis? Basically model it as 3 different basins outletting to a common discharge location. Would this give you the same results as the time consuming task of analyzing the entire site pre/post?

The site is older with much of it just sheet flowing across the site. A small amount of storm drainage infrastructure does exist and that will be analyzed seperatley with each of the 3 areas.

 

[Twinkie]

I assume all three disturbed areas have a common discharge point?

If I were doing this I would analyze the entire site. 40 acres isn't really all that big, and getting Tc and CNs for this not all that encompassing.

I say this, because if your disturbed areas are upstream of the undisturbed, you will need to consider this in your analysis at the downstream point of interest. If you disturbed areas are far enough upstream of your analysis point, depending on your cover type, the need for detention may not even exist. If you only analyze the disturbed areas, you will need the detention because you are not gaining the added benefit of the increased Tc across your site.

 

[beej67]

I've done it both ways before, depending on the local regulations. What you really need to ask yourself, before you begin the analysis, is why you're doing the analysis.

If it's for municipal (/state/district/etc) approval, then follow their codes. Some codes, for instance, state that if you're redevelopming less than (say) 40% of your site, you only provide stormwater management for that portion, and if you're redeveloping more than 40% you must provide it for the entire site. In the former case, you'd be on the hook for three small ponds. In the latter, probably one big pond, or more depending on watersheds.

Some codes allow you to take credit for existing imperviousness when setting your allowable discharge. Some don't. If the former, you may not need detention at all if your imperviousness is staying the same or going down.

Some codes want you to look at downstream structures to identify potential flooding risk. In this case, you're on the hook to analyze the entire site, but you may not be on the hook to provide anything. Some codes make you analyze way downstream. In Georgia, you might be doing an analysis of as large as 400 acres.

If it's CERCLA/Superfund, you're exempt from local and state permitting categorically by virtue of the fact that you're dealing directly with the EPA. (even 404 wetland permits aren't required) In this case, you have the freedom to decide (in tandem with your EPA friends) what sort of criteria are relevant, if any. The owner may have an erosion problem they want you to address with detention. Etc.

So it really depends on the situation.


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