Upstream off-site detention

[EITZM]

Hello All,
Was hoping to get some feedback on an idea I was attempting to achieve.
A site with a stream originating and flowing right through the middle of the property. Have two different approaches to comply with the stormwater management
First approach: Lose 4 lots total 2 on each side of the stream to provide stormwater management (Runoff Reduction and Detention).
Second approach: Provide stormwater treatment on-site, while achieving peak flow attenuation through off-site detention upstream of the site (i.e. pick up off-site and instead of by-passing through site, detain to bring total post-developed peak flows to the pre-developed peak flows at study point. The purchase of the off-site property will not be as costly as losing the other 4 lots in approach 1
What issues do I need to consider if any regarding the second approach. The only concern I have in the second approach is should I model the areas draining to the pond as they are now or should I consider those areas fully developed, any feedback would be greatly appreciated.

 

[Twinkie]

If I understand correctly, you are proposing to completely overdetain an upstream area, and let your downstream developed area rip?

Your downstream development should be modeled as it will be constructed, and shown as a bypass area.

 

[beej67]

If you can make the numbers work on Idea 2, then there's no direct engineering problem with it. Your biggest engineering challenge for that will probably be making sure your upstream pond doesn't flood neighboring land further upstream from it. State water quality rules might hang you up on the idea, though. If I were to try that in Georgia or North Carolina, for instance, I'd still have to provide water quality treatment for the onsite runoff somehow.

There's an ownership issue with #2 as well, that you need to consider, and inform your client of. I almost always strive to bypass upstream water around my BMPs, because pollutants from upstream might end up in my BMP, and then my client will be legally on the hook for remediating that pollution. Common here is sediment. If I did what you describe on a site in Georgia, and the pond filled with silt from an upstream construction project, there'd be a tremendous legal hoopla about who has to pay to clean the silt out. Worse, if the pond filled with silt purely from the natural bed and suspended load in all alluvial channels, my client would be on the hook to clean it out at his expense with no recourse to seek compensation from upstream properties. Then there's issues of other pollutants as well. If some nitwit dumps an old transformer in the creek somewhere upstream, suddenly you've got PCBs in your pond.

Sounds like your client is a subdivision developer, which are often temporary LLCs who dissolve after the project and pass maintenance responsibilities on to an HOA or to the municipality. Check to see what sort of legal framework forces the owner to maintain the BMP, and think through the scenarios above.

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