Freeboard Beneath a Bridge
Freeboard Beneath a Bridge
(OP)
I am currently reviewing a Hydraulic Analysis for a stream that passes beneath a bridge. According to the NYSDOT Bridge Manual, "Any stream structure that provides a minimum freeboard of 2 ft. for the 50 year flood shall be considered as satisfying normal hydraulic clearance requirements." In the analysis, it is shown that at the upstream face of the bridge, this 2 ft. of freeboard is met. However, the company doing the analysis has taken the freeboard 100 ft. upstream of the bridge, where the 2 ft. of freeboard is not met. Is there something that I am missing that states the freeboard shall be measured a distance upstream of the structure?





RE: Freeboard Beneath a Bridge
RE: Freeboard Beneath a Bridge
Attached is a picture of a bridge acting as a control.
Sometimes, to verify the freeboard requirement, i take the water level a few meters upstream. Even though, 100 ft seems unreasonable.
RE: Freeboard Beneath a Bridge
RE: Freeboard Beneath a Bridge
If the WSEL is at the level of the soffit elevation a few meters upstream of the bridge, would you still use the WSEL calculated at the face of the bridge (under the soffit) to determine the freeboard?
RE: Freeboard Beneath a Bridge
RE: Freeboard Beneath a Bridge
RE: Freeboard Beneath a Bridge
Imagine the case shown on the attached file.
What section would you use to calculate the freeboard? (section 3 or section at the face of the bridge).
RE: Freeboard Beneath a Bridge
RE: Freeboard Beneath a Bridge
Use the HEC-RAS Hydraulic Reference Manual for guidance.
Plot and submit the profile with your bridge plans.
good luck
RE: Freeboard Beneath a Bridge
Myself, I always take the section at the face of the bridge to determine if the freeboard requirement is met.
Freeboard = Soffit Elevation - WSEL
But i know that there might be some projects where this is not true. And this might be why the company doing the analysis has taken the WSEL 100 ft upstream of the bridge (or it's a mistake).
I guess that GAH9 will have to model the bridge and/or ask for explaination to the company doing the analysis.
RE: Freeboard Beneath a Bridge
Just a thought.
Mike McCann
MMC Engineering