I'm working on a bridge replacement project that is located in a county that used to have their own bridge crew and would build a lot of small bridges (10-30 ft) on low volume roads. The existing structure passes the 100-yr flood with no over topping of the roadway.
Current design standards given the roadway classification and ADT would allow for a replacement option that meets the headwater requirements for the 10-yr flood. The existing structure is located on a tributary that is in a FEMA floodplain labeled "Zone A."
Am I able to select a replacement option that would meet the 10-yr flood requirements, but raise the water surface profile more than 1 foot at the 100-yr? There would be no inverse impacts by raising the water surface profile. The increase is contained within a couple hundred feet and converges on the existing water surface profile. No property is effected other than a small amount of crop/grass land.
I have not dealt with the replacement of a structure that is much larger than what would be required. Usually they are over topping significantly at the 100-yr and the replacement option has been an improvement or doesn't change much in the higher frequency flows due to the amount of over topping occurring.
From an economical point of view it doesn't seem to make much sense to have a replacement option that is largely over sized just to not increase the water surface profile at the 100-yr more than 1 foot, if you can show no inverse impacts.
Any insights or comments would be greatly appreciated.
In general, the proposed replacement structure would not meet FEMA requirements. In a Zone A Special Flood Hazard Area, most jurisdictions require a Zone A replacement structure to result in no increase in the current base flood elevation. The actual CFR code says that the cumulative impact of all existing and future development cannot result in an increase in the base flood elevation of more than 1 foot.