Storm/Flood Engineering-Time of Concentration

[GleasonS]

When calculating time of concentration, how do you account for a detention pond?

My site drains to a small creek and the proposed development includes a small pond through which most of the site will drain. I'm thinking the most appropriate point at which to calculate the time of concentration would be where the creek leaves my site, but this then means that I'll have a pond intercepting a fair portion of my runoff. Would you add the time for the runoff to route through the pond to the overland flow?

 

[beej67]

You break the watersheds up, and each watershed gets its own Tc. One watershed goes into your detention pond, with one Tc, and the other watershed(s) bypass the detention pond, with their own Tcs. The flow at the property line is determined by summing the bypass hydrographs with the routed pond hydrograph.

Caveat: There are some other potential issues with geometry that might lend my answer to be slightly incorrect, such as if your site drains in two directions instead of one / etc.

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

http://www.campbellcivil.com

 

[GleasonS]

Thanks for the response. The site drains, more or less, in the same direction and you're answer confirms what I originally thought. One follow up question, the stream into which my site drains collects runoff from a much larger basin than just my site so would you generate basins for these additional bypass areas? In other words, I've got the on-site basin draining to my pond, my onsite bypass areas, would you then have an offsite-bypass area as well that would encompass the remaining drainage basin? The stream in question is my site's property line so these off-site areas don't actually drain through my site.

Thanks

 

[beej67]

Depends on the regulations, the size of the upstream basin, and how the reviewer is going to handle it. If the stream in question is the Mississippi River .. no.

In Georgia, you would be on the hook to analyze the upstream basin of the stream itself if that basin was no more than ten times the size of your site.

Then some municipalities would want you to follow their pre/post matching regs for "your site's contribution to the flow at the study point" and others would want you to follow their pre/post matching regs for "the entire flow at the property line." The latter could potentially allow you to hide some of your design flows inside a larger flood wave if the hydrograph peaks miss each other, or could also restrict you to additional detention if the hydrograph peaks line up better after detention than before detention. Then things get tricky.

By contrast, in many places in Florida your hands are tied not to pre/post matching at all, but to the capacity of the receiving waters as dictated by the WMD or FDOT. It all sort of varies on where you're at. Unlike ADA or building codes, stormwater regulations tend to be vastly different depending on where you are.

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

http://www.campbellcivil.com

 

[LincolnPE]

Just FYI>>

You mention site areas that bypass your pond to the creek.

If you have project site areas that in their developed condition are more impervious, and/or been graded more steeply affecting Tc and subsequently increasing peak flow off these areas, than their historical condition, and you are not intercepting and detaining these flows, you will have to do the compensating detention procedure. Not sure if you are aware of that?