widen bridge vs detention
widen bridge vs detention
(OP)
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?





RE: widen bridge vs detention
RE: widen bridge vs detention
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.
RE: widen bridge vs detention
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.
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