You've got to overbuild there for the once a decade flood....
THAT is what is not done on a regular basis around here.....planning on that 'little creek' turning into a raging torrent once or twice in the owner's lifetime.
Look real close at how much watershed is upstream, and how much it could drain down your creek before you build.
Guy down from me moved here from Nevada. Built a bridge way too narrow, and low. I told him it would wash out eventually....well, it took 15 years, and he was dead already, but that's what happened. Bunch of limbs/trees/etc washed down against his obstruction, formed a great dam, then backed up and cut across the road where the creek hadn't run in a thousand years.
Where the bridge used to be: (Normally, you could walk across that creek and not get your shins wet)
Lifted this guy's house, on the other side of the road, and washed it out in the road.
Another guy, other side of the mountain, put two LARGE ( like 6' diameter ) culverts in parallel across the creek on that side of the mountain. Told him the same thing...."you're building a dam". At the time, there was maybe 6" of water running thru them. He looked at me like I was nuts.
Sure enough, same storm sent 5" of rain in about 2 hours down 4,000ac of National Forest watershed, washed a bunch of trash up against his culverts, cut one of them out and pushed it down stream to the next dam.
His solution was to haul it back up there, install it again, and this time pour a 4" concrete driveway to hold them in. That won't work either. Just that next time, he will have to remove a broken concrete slab out of the creek in addition. I told him to use option 3 (below), but he didn't do it.
The key to building one that stands a better chance of not washing out IF you have a lot of watershed up stream is: (and this would seem like common sense, but apparently isn't
)
1. Build it high and wide enough that stuff will wash under it. (expensive)
2. Build it right down at creek level...put a series of small culverts in the creek, and pour a thick concrete roadway over the top. When the creek gets up, blocking the culverts with trash ( it will ), it then simply washes over the top, leaving you unable to cross until it goes back down, and you have to clean out your culverts after the flood. (less expensive)
3. Build a small bridge. THEN cut a swale, or dip, in the property on one side or the other, so when the bridge does block up, the water has a nice, wide, gentle path to divert around the dam without cutting it out. (mid range on expense) You might have to repair some drive after the water runs across it, and clean out the floated trash up against the bridge, but it's cheaper than a new bridge.
