Help asmo build road/bridge -- want ideas and thoughts

[68Charger]

OK, you've cropped the topo to "protect the guilty"... But what is REALLY upstream is critical.

Decide what percentage of wash across the road is acceptable... (Or what 'year' flood).
Because unless you're willing to put an 18' culvert down there and bring the level to 4995-5000' ASL, it will eventually cross that road... Been there, done that.

That being said, there are lots of things you can do to mitigate water flow across the road, as rare as it will be...
Make the lowest wide, flat and level... Think 30+ feet wide, dead flat with real heavy boulders on the downhill side.
Concentrated water flow destroys, if you give a small area for the water to flow through, it will cut through just about anything.

Don't worry about what is on the uphill side, that water will be slow, except right near the culvert and across the road.
The downhill side will be eroded FAST in the first major storm if it's not reinforced, flat, level, and WIDE. the wider it is flat, the slower the water.

I lost 4'-5' of road when a major storm hit because an 18" culvert couldn't keep up (not even chose), mitigation is to refill with 3-4' boulders, level and flatten crossing, and plan for when it will be under water.

If you want 100% access, it will get very expensive... I settle for 99+%