KevinD wrote:The water was flowing pretty good, which makes me think the arch is still intact (which bodes well for faster restoration), but I would put my money on something happening down stream, like a fallen tree dam, a beaver dam, etc, causing water to back up to the point the embankment became super-saturated until it slipped down.
I respectfully disagree. Downstream of that culvert is the big open field visible in the news videos. I can't imagine that field being filled up so deep that it would saturate the fill from that side. It would have required a giant lake. And the culvert outlet does not appear to have been covered up with great amounts of soil. I suspect one of two other scenarios.
More likely I'd guess, would be the culvert was plugged with debris either in the middle, or just upstream of its inlet. This would allow a large amount of water to accumulate on the UPstream side of the fill, saturating it and eventually washing it away. In cases where the water actually tops the embankment (which may or may not be the case here), once the water starts cutting the top, it allows increasing volumes to flow through the little channel. The more water moving, the more force to cut the notch deeper until the whole embankment is cut down, leaving the track in mid-air.
Another possible issue, and harder to see beforehand, is a case where a part of the roof collapsed mid-culvert. Material can drop straight down into the culvert, and if there is a rapid flow of water it can get washed out. Eventually the void above can get so big that the track structure is affected. This can be hard to spot sometimes, as from outside the embankment looks normal and you may have no idea there is a big empty pocket inside.
Obviously this is speculation on my part, but obviously SOMETHING similar to this came into play. Fortunately the results, bad as they were, were not worse.