I'm trying to have a foot bridge built over a creek and will need a bridge with about a 30-40' span. Unfortunatly, the whole area is in a flood zone. What precautions should I take?
I'm planning on building a rainbow bridge since it just needs two supports and nothing in the middle. I also like the idea of how much higher it gets in the middle which I'm hoping will give it clearance during flood seasons if things are floating down the creek. The idea is to build really nice foundations on both ends and use rot resistant wood. The bridge might need to be replaced each year, but it would just need a cheap drop in bridge.
OK, tricky question since you don't mention anything but flood zone.
For any bridge to work correctly (as in not wash away in a flood) the ends must be anchored above the flood zone so to not have water washing against those anchor points.
You could build coffers to protect the anchors like they do for bridge supports located within a river but that is very time consuming and actually no guarantee against wash out.
All the bridges I know of in total flood zones have foundations made of concrete and rebar that are sunk into bedrock at least 3 feet.
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"Flood zone" has a wide range of conditions which will affect the answer.
How big is the watercourse, and how steep is the gradient? What are the banks like? Can water spread out in big floods, or is it all contained within the span you want to bridge? What is the worst potential tree that could be washed downstream in a big flood?
My creek has a seven-mile watershed, steep enough that the bed is rocky throughout my section, and carved down 6 to 10 feet below the valley floor and 25 to 40 feet wide. When it floods, the water is all confined until it gets to "catastrophic" scale, and it regularly carries respectable trees a foot or so in diameter, and in the two 500-year floods we had in 2006 and 2011, carried 80 foot white pines 2 to 3 feet in diameter, with all their roots and branches, and tumbled a two-ton boulder 20 feet downstream.
So you need to consider your situation and what could happen, versus how likely it is and what it would take to ensure against damage.
A rainbow bridge is really just a Archimedes Bridge renamed.
Since there are no connections, and it is little more then stacking timber in the right order, it would not be hard to rebuild in a flood. Still the foundation should be secure. That would be easy enough to do with a few purchased wire gabion baskets then filled with rock. For a couple hundred bucks a person could have a decent Archimedes Bridge I would think.
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