Dennis, Cynthia, and I did an emergency survey for a washed out bridge in rural Grant County, near the town of Carson. A tributary drainage of the Cannonball River washed out a county bridge. Here's the carnage we came across. Thankfully no one drove into this gaping hole!
The bridge is a "choke point" where the stream channel narrows, which causes the force of the water to increase as the area it can pass through is decreased. The north abutment wall and wings, seen here on the left, remained intact. The south abutment was on the outer meander bend, where the highest erosive forces were in action. The abutment wall and the upstream wing are completely gone, except for one framing timber lying in the water. The downstream wing remains, but it was twisted and torn about 90 degrees from how it was originally built. It looks like someone took a box of matchsticks, wet them, then bent them over sideways.
When the south abutment wall washed out or collapsed, the bridge deck fell straight down into the stream...it's possible the south abutment wall is trapped under the deck. Overland flooding also eroded out the gravel road bed behind the standing abutmen. The road bed and gravel top were scoured out down to the level of the stream, roughly 10-ft deep. I'd guess 20-30 cubic yards of fill were scoured out.
This last photo shows the downstream view of the bridge remains. The large pile of boulders in the stream, in geomorphic terms, form a channel bar deposit. Those large stones once anchored the base of the abutment walls, and some likely formed the substrate of the washed out road grade. Yikes!
This was the first time I'd ever been to a washed out bridge in person, and it was truly awe inspiring. The power created by that water boggles my little primate brain. Again, thankfully, no one was hurt by the collapse.
Best wishes all, Mike
p.s. -- On the drive down to this project area, I got the opportunity to revisit some old friends. Carrie and I first met them in 1996. They are the Tin Family...the original piece of artwork on the Enchanted Highway. It's the highway between Gladstone (ca. 10 miles east of Dickinson ND) and Regent ND.
My favorite part? Here curly hair is barbed wire!
Archeological self portrait. Doing survey work in northeast North Dakota near a coulee FULL of water.
Me doing survey work in the badlands in an area once part of Theodore Roosevelt's Elkhorn Ranch, in the Little Missouri River valley.
Sometimes, you have to wait for a flower to open its petals. And if it doesn't open it petals tonight...
