text: You'd have to hire a structural engineer to safely design a bridge for a car.  Also, you'd need to have an engineer assign a flow (Q) value for a given rain event based on the 200 acres.  The Q is a result of the type of landuse upstream.  Then given the dimentions of the channel you'll know how high the water elevation will be given a certain storm event, thus the high of the bridge.\n\nI'd contact the local municipality engineering department to find what the requirements (rain event, freeboard) are for flow and structures.  There might also be an easement on the ditch.  If that's the case, you might not be able to construct a thing.