How would you fix this? Road washed out...

[Bocephous]

We had a hundred year flood a couple of years ago and the culvert serving the creek that flows through our property was blocked by debris, and the water overwhelmed/eroded the earth over the pipe. I had the metal pipe, which was corroded badly, replaced with a larger plastic one, and sourced two large (ton each) concrete blocks that act as an anchor on each side of the pipe upstream. Large rock was stacked on each side and over top of the concrete blocks and the pipe was then buried with dirt/rock. I'll take a pic when I can and post it. I would go much bigger with the pipe the OP is replacing, as debris blocking the mouth of it will cause overflow next time and the same thing will probably happen again.

 

[Bocephous]

 

[fruitcakesa]

We had a hundred year flood a couple of years ago and the culvert serving the creek that flows through our property was blocked by debris, and the water overwhelmed/eroded the earth over the pipe. I had the metal pipe, which was corroded badly, replaced with a larger plastic one, and sourced two large (ton each) concrete blocks that act as an anchor on each side of the pipe upstream. Large rock was stacked on each side and over top of the concrete blocks and the pipe was then buried with dirt/rock. I'll take a pic when I can and post it. I would go much bigger with the pipe the OP is replacing, as debris blocking the mouth of it will cause overflow next time and the same thing will probably happen again.

When Hurricane Irene hit Vermont in 2011, the majority of damage was caused by either blocked culverts as you mention, or trees jamming against bridges which caused erosion around the abutments and washed out the entry and exits of the bridges. Afterwards, all the affected culverts were pulled and replaced with much larger units.
They have not yet been tested

 

[kneedeep]

You need at least a 24" pipe