Mixing Treated Stormwater with Untreated Stormwater - calculating TMDLs

[kar33]

So if you are proposing a design that treats water in a separate pond and then discharges into a larger pond so that that water mixes with untreated water before it all discharges to the outfall - how do you treat the lbs/removed for total phosphorous and nitrogen? Is it the same straight calculation for that volume that was treated or is there some dilution effect going on when it mixes with untreated water? Has anyone addressed this properly via research and testing?

 

[beej67]

Does the second pond remove pollutant or not?

If not, you already have your answer.

Do your best not to wade into the highly confusing morass of whether the discharge criteria should be in pounds per acre per year, or in concentration of pollutant. I did a project in South Florida where we were required to discharge water that was eight times cleaner than the undeveloped site, because we were held to a lb/ac/yr threshold instead of a pollutant concentration threshold, so our increase in volume of runoff hurt us instead of helping us. Lost that argument with SFWMD.



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

http://www.campbellcivil.com

 

[kar33]

No the second pond is just a dry detention pond - It's a large existing dry detention pond and they are converting a portion of it to bioretention - the argument being that 80% of the runoff flowing into the pond will go directly into the bioretention facility and then join with the other 20% in the larger pond. So - how will credit work for that do you think?

 

[beej67]

If it's two BMPs in series, both properly sized for their respective watersheds, then do it by watershed. Presuming your local regs do not give credit for dry detention, you'd get full credit for the watershed going into the bioretention pond and no credit for the watershed that bypasses the bioretention pond.

If you're talking about converting some but not all of a single existing large detention pond into a bioretention facility, I'd say take some caution. When you overload bioretention facilities, they can turn into a real mess.

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

http://www.campbellcivil.com

 

[kar33]

Right, the pond is really large - more than is needed for the incoming flow. The designer is proposing turning a portion of it into a bioretention to treat 80% of the inflow - so NO SERIES - the rest would bypass the bioretention and they'd both join in the remainder of the pond before flowing to the outfall. I figured it can work as long as the bioretention pond can hold the Water Quality volume and can either overflow the 10 and 100-year into the remaining portion of the existing facility or hold it in the bioretention facility. Right? The second inflow pipe that is bypassing the bioretention pond is no where near that pond so there is no danger of that water going into the pond and causing it to fail b/c it was only sized for 80% - it will never get there. So, all that said, What do you think?

 

[beej67]

How frequently would the bioretention portion of the pond be flooded, and to what depth?

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

http://www.campbellcivil.com