widen bridge vs detention 3

[wangeng]

a concrete bridge is widened with one more lane, and Reviewer comments we should consider about mitigation (detention) due to increased impervious area, but we don’t agree with it because this is a dry bridge (crossover) and the land (road under this bridge) is developed. What do you think?

 

[cvg]

I think you will need to show no adverse effects of the increased runoff. since this runoff is from a roadway, the first flush would typically need to be treated to remove sediment to comply with environmental regulations. (NPDES in the U.S.)

 

[froude]

Is the reviewer requesting it from a water quantity or quality standpoint? Is this a private development or a municipal/state/federal job?

I don't know all the specifics/forensics/history of your project, but it seems stringent to require detention for what you are describing from a runoff standpoint, unless there are some known flooding issues. However, if you had access to the watershed hydrologic model and stream hydraulic models, it may not be difficult to prove that the increase is negligible and that the hydraulic impacts are negligible. But, again, if there is already known flooding downstream, additional impervious could potentially make it worse.

 

[beej67]

I presume since you mention 'detention' the reviewer is concerned with rate or quantity, not quality.

Does the bridge drain through scuppers at the edges, or does it convey drainage down to the end of the span? That could be important.

If your bridge is spanning completely impervious area, the delta (existing-proposed) between your runoff on an overall watershed basis should be zero. That should be easy enough to show, with any methodology you choose. (rational method would work fine) But if you're changing where that impervious runoff *goes*, then that needs to be looked at, from a capacity standpoint.


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

http://www.campbellcivil.com