Tuesday, November 22, 2011

High River Levels Bring More Than Flood Risk

With heavy rains there is the risk of river flooding. But the another risk is channel migration. That is the risk that the channel of a river will shift. I recently assessed a reach of a northwest river for channel migration risk. Pretty simple evidence was visible to show that the river has been recently migrating.

Concrete foundation stem wall and concrete post supports

Undermined structure

undermined trees

Another home foundation

Utilities leading to where homes sites are now occupied by the river

It was not the first time I had assessed this particular river. Other reaches of this river have migrated much more dramatically over the past 50 years. At one reach upstream of the pictures above the river did a complete course shift in one storm event moving to a new channel nearly one mile away from the previous channel. The particular reach I was investigating was on a reach I first looked at ten years ago. I was on the opposite side of the river on that project and it was very clear the river was starting to migrate away from the property - good news for one side of the river is not so good for the other side.

The risk of flooding along this particular reach of river is low. The 100-year flood inundation area does not extend over much of the valley. But the risk of channel migration, a much more catastrophic problem, is obviously much greater.

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