Engineers fear operators of the tallest dam in the United States are running out of time to eliminate flood risks resulting from damage to a vital spillway.
The stricken Oroville Dam in California has suffered severe erosion damage to its primary and auxiliary spillways – passages for excess water – in the past few days following weeks of heavy rain.
This increased the risk of severe flooding – forcing state officials to issue an evacuation order to 180,000 people on Sunday (12 February).
The immediate threat to affected households has now subsided as water levels settle below the auxiliary – emergency – spillway but engineers continue to battle to contain the problem. The main dam structure is sound.
“I would make sure that the auxiliary spillway didn’t operate again,” British Dam Society chairman Andy Hughes told New Civil Engineer, adding that the emergency spillway was “easily erodible”.
He suggested using sandbags to address damage to the auxiliary spillway, while plugging the hole in the primary spillway with concrete and large rocks to stabilise it.
Oroville Dam primary spillway damage
Source: Kelly M Grow/California Department of Water Resources
The erosion of a concrete section midway down the primary spillway was discovered last Tuesday (7 February), forcing engineers to temporarily close it.
This led to water breaching the reservoir limit of 275m elevation above sea level so that it flowed down the auxiliary spillway – which then also developed signs of erosion on Sunday (12 February).
It was the first time the auxiliary spillway has been used in the dam’s 48-year history. It is the tallest in the US at 235m in height.
Engineers now have to repair the damage to the primary spillway before the next spell of bad weather hits the area, around 100km north of Sacramento, forecast for later this week.
The California Department of Water Resources, which operates the reservoir, reportedly dropped rocks into the auxillary hole by helicopter during the incident.
DWR plans to use helicopters to drop rocks to fill in the gouge in the Oroville Auxilliary Spillway to stabilize.— CA - DWR (@CA_DWR) February 13, 2017
Hughes said that a long term solution was needed, including repairs to the defects which caused the damage. He stressed it was most likely a construction issue such as a joint failure, rather than a design fault.
Hughes added that decisions were needed to ensure the auxiliary spillway is less vulnerable to failure so early in an incident, possibly using more hard engineering solutions.
On Sunday (12 February) California governor Jerry Brown issued an emergency evacuation order following the discovery of erosion at the head of the auxiliary spillway, which could have led to a devastating volume of water running into the river below.
Oroville Dam aerial view
The California Guard was mobilised during the incident, reportedly the largest response from the guard since the Los Angeles riots in 1992.
Brown said on Monday that events over the weekend meant the situation was “complex and rapidly changing” and he remained in close contact with the emergency team on site.
- Tuesday 7 February California’s Department for Water Resources temporarily suspended flow from the Oroville Dam’s primary spillway after significant erosion was uncovered midway down the concrete chute. At the time, 20% of the dam was reported as being empty. Around 1,557 cubic metres per second of water had been released via the spillway just prior to its closure.
- Wednesday 8 February Engineers tested the spillway by releasing up to 556 cubic metres per second over a two-hour period. At the same time, trees and debris started to be cleared from the auxilliary spillway path as a precaution.
- Thursday 9 February Engineers allowed water to flow overnight, monitoring the spillway and testing its structural integrity in the morning. Following reports of more storms, the flow rate was increased to 991 cubic metres per second as officials tried to find the maximum safe limit. Later that night the flow was increased to 1,840 cubic metres per second – but it could not match the 5,437 cubic metres per second entering the reservoir.
- Friday 10 February The higher flow rate caused further erosion, and was reduced to 1,557 cubic metres per second.
- Saturday 11 February Water levels in the reservoir breached the 275m limit and started to run down the auxilliary spillway – the first time it had been used in the dam’s 48-year history. The flow was relatively low at 141 to 283 cubic metres per second.
- Sunday 12 February An emergency evacuation order was issued after erosion damage was found at the head of the auxilliary spillway, potentially undermining the concrete weir. At this stage, release from the main spillway was increased from 1,557 to 2,832 cubic metres per second.
- Monday 13 February The immediate danger appeared to have passed, and water had stopped flowing from the auxilliary spillway.