7 March 2017
Oroville dam: multiple riverbank failures on the Feather River after the flow was abruptly stopped
The San Francisco Chronicle has an excellent article about what happened next on the Feather River when the flow down the Oroville Spillway was abruptly stopped last week to remove debris from the channel:
When state water officials scaled back their mass dumping of water from the damaged Oroville Dam this week, they knew the riverbed below would dry up enough to allow the removal of vast piles of debris from the fractured main spillway. But they apparently did not anticipate a side effect of their decision to stop feeding the gushing Feather River — a rapid drop in river level that, according to downstream landowners, caused miles of embankment to come crashing down. With high water no longer propping up the shores, the still-wet soil crashed under its own weight, sometimes dragging in trees, rural roads and farmland, they said.
“The damage is catastrophic,” said Brad Foster, who has waterfront property in Marysville (Yuba County), about 25 miles south of Lake Oroville.
The farmer not only saw 25-foot bluffs collapse, but also lost irrigation lines to his almonds. “When the bank pulled in,” he said, “it pulled the pumps in with it. It busted the steel pipes.”
In the article, farmers describe losing substantial sections of their riverbank. For example one, Philip Filter at Live Oak describes losing most of the banks in his section, which extends over a kilometre. This image shows some of the damage:
Meanwhile on Twitter, Mike Luery of KCRA3 has provided another example of a riverbank collapse on the Feather River:
Riverbank collapse during rapid draw down is a well-known phenomenon – indeed riverbank failures are most likely on the falling limb of the hydrograph. As the river level rapidly falls the banks are left in a saturated state, which means that they are weak. But more importantly, they also have a hydraulic imbalance, which drives flow of water through the soil towards the river. This provides an additional force that reduces stability.
The problems of riverbank failure are well-described in the literature, and there is even a Wikipedia page that provides a detailed description. The tragedy is of course that riverbank failures represent a permanent loss of land, and the input of sediment into the river can have implications for the ecosystem. Drawing the river down more slowly means that the hydraulic instability is avoided, and the riverbank failures should not occur.