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HEC-2 Bridge Modeling 1

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Bayou

Civil/Environmental
Jan 7, 2002
88
I have a question on the modeling of parallel bridges in HEC-2. This is a standard boulevard set of two lane bridges. Each 200-foot, 3-span bridge is 35 feet across, seperated by 30 feet. The low chords are above the 100-year by 2 feet. However the floodplain is about 1500 feet wide.

What is the appropiate method of modeling?
The options I've come up with involve:
1) modeling the bridges as one 70 foot bridge using normal bridge, but this doesn't give a true representation of the
ineffective flow area at the true faces.
2) modeling the bridges as two decks with normal bridge, but does upset the program with that little of expansion
space
3) modeling with special bridge as one long bridge, but
the wsel doesn't hit the low chord

Any suggestions would be helpful.
 
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You probably need to model this as one bridge, 100 feet long, since you only have 30 feet between bridges. If the distance between the bridges was greater, you would want to model the partial expansion/contraction of flow between the two bridges.

You also need to use the ineffective area option to limit flow to the bridge openings until the bridges are overtopped. In your case, it sounds like they will not be overtopped at the 100-year event, so your initial estimates of ineffective elevations should not be critical, so long as they are above the 100 year. The ineffective area constraints are specified on X3 cards, one just outside the downstream bridge and the other just upstream of the upstream bridge.

You might consider using HEC-RAS, which has replaced HEC-2. RAS is a much easier program to work with and most agencies, including FEMA specify RAS over HEC-2.
 
True, but many agencies require use of existing models when adding bridges, i.e. if your stream is already modeled by FEMA using HEC-2, you must use HEC-2.
 
Unless your bound to use HEC-2, I'd seriously consider using HEC-RAS. Hec-RAS can model these as two seperate bridges with good accuracy, given that your passing the 100 year event. It will allow you to set you ineffective flow areas, and will allow you to have more control over your n values.

If you have to model in HEC-2....I'd model it as one bridge, and use X3 card to describe the ineffective flow.
 
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