Freeboard Beneath a Bridge

[GAH9]

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?

 

[cvg]

you aren't missing anything, there is no requirement for freeboard 100-feet upstream of the bridge. There should be HEC-RAS section at each face of the bridge and that would be a typical location to check the bridge freeboard.

 

[SMIAH]

Interesting.

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.

 

[cvg]

interesting sketch, however that sketch illustrates only one of many possibilities for the hydraulic conditions at the bridge in question. I don't know what point there is to checking a wsel at some arbitrary distance upstream of the bridge. Why not set the distance to 50 feet or 200 feet or a few meters? Is there any logic to this?

 

[SMIAH]

I'm wondering...
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?

 

[SMIAH]

I'm trying to find why someone would take this level 100-feet upstream of the bridge...

 

[cvg]

probably wouldn't matter because you would not have any freeboard. Either way I would check the RAS section at the face of the bridge and compare.

 

[SMIAH]

Sorry for asking so much questions! But this is interesting.

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).

 

[cvg]

again, this is just a sketch of a theoretical condition and not to scale. Is section 3 a few meters or a few hundred meters upstream? What is happening to the width of the channel? How deep and how wide is the channel? How wide is the bridge? Is there a levee which constricts the 50-year channel width? Is there any change in the bed slope? Is there any change in the channel cross section? Is there any change in roughness? Where and how are these changes happening? Are we looking at a 50-year or 500-year water surface profile? what is the difference in elevation between section 3 and the face of the bridge? Is it .1 foot, 1.0 foot or 10 feet? Without reviewing the entire situation, this question cannot be answered.

 

[RWF7437]

Model your bridge in HEC-RAS.
Use the HEC-RAS Hydraulic Reference Manual for guidance.
Plot and submit the profile with your bridge plans.

good luck

 

[SMIAH]

cvg i know it's a theorical condition...

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.

 

[msquared48]

I have seen videos of a lot of floods at various bridges over the years and was amazed at the debris washed down these rivers in 50 and 100 year events. Two feet seems real tight to pass a lot of the material I've seen, to include intact houses, even though it is code, to prevent or limit impact on the structure, and log jam development against the structure.

Just a thought.

Mike McCann
MMC Engineering