Affect of Minor Project on BFE 1

[cbear1]

Hi All,

I am working on a project where an existing bridge has been built within a mapped floodplain (Zone A). There is no FEMA BFE at this site, but it is clear from other indicators that the bridge is overtopped by up to 10-15 feet during severe storms. The client wants to laterally widen this bridge by 4 feet. Fema guidelines and this County's policies require that alteration of structures in a floodway requires that a civil engineer certify that the alteration will not cause any increase in the base flood elevation (100 year).

This is a small project for a bridge spanning 15 feet (exist. width is 12'). The civil engineer does not want to certify that there will be no increase in BFE without a full hydro study, but the client cannot afford this. Common sense says that a 4' increase (piers will be outside the channel, beyond top-of-bank) will not cause the BFE to rise, but is there another way of proving this without a full hydro study? Maybe assuming flow that overtops the bridge by a foot - if it doesn't make the BFE rise, then higher flows where there is just more water in the floodplain would not make it rise. Is this sound reasoning?

Thank you for reading. Any help is appreciated.

 

[Highway27]

There's no way I'd certify that the 100-year floodplain wouldn't increase in such a situation either without a full study. The only way such a widening wouldn't have an effect was if it was completely masked within the existing blockage of the bridge, and that is unlikely given that a widening will be at some cross slope, and therefore extend above or below the existing bridge. And even in that case, there are opening effects that would still change the flow characteristics.

I just don't think that a 'common sense' or 'by inspection' analysis is appropriate in this sort of case.

 

[cvg]

I don't believe there is any way you will find an engineer to certify this without doing some sort of analysis. If this is to be accepted by either the County or by FEMA I would be highly surprised if they would accept the certification unless it was backed by an engineering analysis. However, this doesn't have to be such a big deal. Estimate the flow using regional regression and do a quick HEC-RAS model and you are done. I would hardly think that this would be a budget buster.

 

[civilperson]

If the blockage cross section were made smaller by shrinking the profile of the guard rail or removing sidewalk/curb/median above the road I would certify that no effect on the flow during a flood event.

 

[mhiw]

Overtopped by ten to fifteen feet? How's the bridge holding up? This sounds like a dubious situation

 

[mhiw]

And if there's no FEMA BFE, how can there be a defined FEMA Floodway?