[Duncan]

I third the motion for culverts.
Easy to do, and three pieces of 2.5" ABS pipe will handle all the water you could put in there...
Ain't nuthin' gonna put down so much water that it wouldn't drain out.
'Cept amaybe a Cattergory 5 hurry-cane.
But yer screwed then, anyway...

Otherwise, I'd go with a triple wide trestle. Your depth isn't too much for that (and they still use trestles of sorts in modern day stuff, JC...)

 

[Duncan]

Doc,
Use ABS pipe...
It's UV impervious (unlike PVC), and cutting it on an angle sort of approximates (from The Department of Redundancy Elimination Department) the look of the real thing...
I've got some pictures somewhere, and will try to remeber to find them/post them of what Ralph & I have done.

 

[Duncan]

Doc,
Here's a couple of shots of one of the culverts we've got going for flood control.
This one has "open" ends, where most of te others have some bigger rocks stacked over the openings.
Enough space is available between the rocks for the water to flow through...

Hope it gives you some ideas...