13 February 2017
An update on the Oroville Dam spillways – a new phase of erosion leads to evacuation of 130,000 people
An update on the Oroville Dam spillways
Somewhat unexpectedly the crisis on the Oroville Dam spillways deepened yesterday. As was widely reported, the emergency spillway was called into action as the water level within the lake exceeded the inflow. All seemed well – the quantities of water passing the crest were not considered to be particularly high, whist the erosion of the main spillway seemed to have eased despite the vast quantities of water flowing through the damaged structure. However, as CaDWR noted in a press release yesterday, erosion started to develop at the edge of the emergency spillway as well. Of course in this case there is no mechanism to control the situation – unlike in the primary spillway there is no sluice gate – meaning that there was the potential for a catastrophic release that would exceed the capacity of the downstream channels:
The concern is that erosion at the head of the auxiliary spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville.
Rich Briggs (@rangefront) tweeted these two images, taken from news reports, that show the problem. The first highlights the location of the headward erosion:
Whilst this one shows the magnitude of flows over the emergency spillway, and on the unprotected land to the side:
It is clear that the flow has exceeded the emergency spillway and has flowed over unprotected ground adjacent to it, which inevitably was vulnerable to erosion. To me this seems quite extraordinary – how did the design allow that to occur – but that is a question for another day. The response of the authorities was first to almost double the flow down the main spillway (from 55,000 to 100,000 cubic feet per second), which will have accumulated more damage as a result, and second to start the evacuation of 130,000 people downstream.
Reports suggest that the water level in the reservoir has now dropped sufficiently for the flow over the emergency spillway to have ceased. The authorities now have until Wednesday to both lower the water level and to patch up the spillways before the next storm arrives.
Fortunately the main dam remains undamaged and safe. The likelihood of a catastrophic failure of the Lake Oroville spillways should now be low, but there is a huge amount of work ahead.
I will be doing the Reddit AMA (Ask me Anything) today (Monday) at midday EST (5 pm GMT). Join in if you would like to discuss this event (or any other).