Even though the source of most of San Francisco’s drinking water, the Hetch Hetchy Reservoir, is poised to receive more precipitation this year than in the 95 years water levels has been measured there, public utilities officials are confident its spillway will not prompt a crisis like that seen at the Oroville Dam earlier this week.
The Hetch Hetchy Reservoir is at 92 percent capacity, and officials with the San Francisco Public Utilities Commission — which draws 85 percent of its drinking water from there — are releasing 1,000 cubic feet of water per second from the massive basin, less than the 7,000 cubic feet per second released several weeks ago, said Steve Ritchie, assistant general manager of the water enterprise for the SFPUC.
But that doesn’t mean Hetch Hetchy is out of the woods. With more rain predicted this water year, which runs Oct. 1 to Sept. 30, Ritchie said Wednesday he expects the reservoir will need to utilize its spillway in the coming months, especially as snow begins to melt into Hetch Hetchy.
“We’re just really trying to manage this huge amount of water that’s coming in this year,” Ritchie said.
Using a spillway, however, is exactly where the Oroville Dam got into trouble this month.
After Lake Oroville reached capacity following three major storms this year, officials utilized the dam’s spillway to reduce water levels. But on Feb. 7, that spillway — which is made of concrete — began to erode, causing officials to resort to the auxiliary emergency spillway on Saturday.
Officials then determined the auxiliary emergency spillway was in danger of failing, which could cause widespread and severe flooding. That prompted the evacuation of tens of thousands of residents, and for Gov. Jerry Brown to declare a state of emergency because of the dam.
But Ritchie said such events occurring at the spillway at Hetchy Hetchy are virtually impossible, because Hetch Hetchy’s spillway was carved out of the granite mountainside.
“Generally concrete will do a pretty good job if it’s installed property, but granite isn’t going anywhere,” he said.
And it’s a good thing, too, because in the 95-year hydrologic history in which the Hetch Hetchy water flow has been measured, this water year is poised to surpass the previous record set in 1983 of receiving the greatest amount of precipitation.