Reason for Hope, but Danger Persists at Oroville Dam
One thing is certain, this story will continue to unfold for at least a week, because the rain forecast for later this week will exacerbate the situation.
Over the weekend the situation at Oroville Dam went from bad to worse. The Sacramento Bee headline said it best: “Nature is in Control.” In response to revelations that the emergency spillway might fail catastrophically, the Butte County Sherriff heroically took charge and ordered an evacuation Sunday afternoon that eventually expanded to include three counties and 188,000 people.
One thing is certain, this story will continue to unfold for at least a week, because the rain forecast for later this week will exacerbate the situation. The worst case scenario would be for the evacuated population to return to their homes before it is safe. Thousands, perhaps tens of thousands of lives would be at risk. Given the extensive damage to both the emergency and main spillway, it would be unsafe for evacuated people to return home before the Department of Water Resources (DWR) is able to evacuate the reservoir to safe levels. DWR says they want to lower the reservoir by 50 feet, and this would be necessary to comply with the US Army Corps of Engineers rule curve for the dam.
Yes, there is a rule curve (figure 1 below) for the dam and thanks to the spillway fiasco, DWR is radically out of compliance with the rule curve as shown in figure 1 below. To prevent a disastrous downstream flood, it requires DWR to keep the reservoir level relatively low in the rainy season to make sure there enough space to accommodate large flood events. During wet years like this one when the soil is saturated, DWR is required to maintain 750,000 cfs of empty space in the reservoir through April 1 to capture flood flows. If you really want to geek out on how we should adjust rule curves to better adapt to a changing climate, check out the this UC Davis research paper on the subject.
Figure 1: The grey area shows the maximum allowable flood stage (top of conservation pool). According to the reservoir rule curve, the remainder of the reservoir space, the area between the grey zone and the top pink line (gross pool) is reserved for capturing floods (flood pool). Note how reservoir storage rocketed above the allowed conservation pool and over the gross spill (emergency weir crest) in the last few days due to temporary suspension of releases from the damaged spillway. The size of the allowed conservation pool (the grey area) gradually declined since the beginning of the water year, because under the rule curve (see below) the size of the required flood pool is proportional to antecedent precipitation. Click on this link to get and update on this graph.
As of last night, DWR had zero extra space in the reservoir and the math for DWR’s prospects of complying with the rule curve in the next ten days is not good. DWR is currently releasing 100,000 cfs down the main spillway, while inflows continue at 40,000 cfs. The net outflow of 60,000 cfs will evacuate about 120,000 acre feet per day (1 cfs per day equals 2 acre feet), but rains are expected to return by Wednesday evening. At the current rate, they only will be able to evacuate 360,000 cfs before inflows start to spike again.
Heavier rains starting Friday and predicted throughout the weekend could fill the reservoir to the brim by the end of next week. If inflows rebound to 130,000 cfs and DWR holds releases at 100,000 cfs, the reservoir will fill within 7 days. During the six peak days associated with the last storm, the daily average was 104,000 cfs, which provides some hope that DWR will be able to stop the reservoir from filling. Fortunately, the forecast is for lighter, cooler precipitation then the last week’s storm.
Given the moderate storm forecast through next Monday, the best case scenario is for the weather to dry out next week enabling DWR to comply with the rule curve by next Thursday or Friday. While this would avert a catastrophe, it means that the evacuated population will have to wait until the later part of next week to safely return home. Allowing people to return before the DWR complies with the rule curve risks the need to re-evacuate if the weather takes a turn for the worse, and the second evacuation attempt would inevitably meet resistance, increasing the chance that many people refuse to leave.
Fortunately, there is now reason for hope that the worst is behind us, but the probability of the situation deteriorating again is still too high. If the rains this weekend turn out to be significantly greater than forecasted, or if the wet weather persists, some very dangerous flood scenarios could still arise. I will describe some of those scenarios in my next blog before it rains again.
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