I like it. Have to ask; what is "sonotube"? Looks like maybe a square steel tubing? Are you thinking about raising the bridge high enough to get a small boat under it? I'd like to see your plan for the top structure when you get that far.
Maybe I should pump my pond down next August and do a similar foundation. I had been planing to use 4, 4"x6" pilings to support a 32' span, and I'm having trouble finding a contractor to do the job w/o costing an arm and a leg. Your approach looks like a good do it yourself project for a reasonable cost.
