My parents own a lot of property and have a "river" going through it. They'd like to design a drawbridge they can build themselves to go over, but finding websites that do that are hard to find. Everyone seems interested in the toothpick design aspect and not in a real usbale version. It MUST be a drawbridge as people tube down the river in the summer!
The topic of bridge design has come up several times in this forum before, I suggest you do a search for "bridge" and see what was said.
Bridge design is a specialized field of structural engineering due to the presence of moving loads, and in your case flowing water. It is going to be difficult to impossible to get useful information over the Internet for the specifics of your bridge, especially since you seem to believe you need a draw bridge, which is even more specialized than a fixed span bridge.
In the end, you are likely to need to hire a local structural engineer who can evaluate the type of soils you are dealing with (this determines the foundation type), the loads you need to carry, and the effect of flowing water on the bridge. Sorry to be of so little help, but you are off the charts as far as a DIY site is concerned.
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