The Santa Clara Valley Water District will now build a Permanente Creek flood basin at the Annex that is 12 feet deep in some parts, with gentle slopes and a capacity of 32 acre-feet of water.
At its Jan. 17 meeting, the City Council voted 4-2 in favor of the flood basin, with council members Laura Macias and John Inks opposed. Member Jac Siegel recused himself because he owns property nearby.
The council made its decision despite a flood of skepticism from the community over the need for a basin. Of almost two-dozen public speakers, only two supported the project.
"To me it's obvious flooding is getting worse, it's getting worse all over the country," said council member Ronit Bryant, expressing concern about climate change. "I don't know why we would be the one place where flooding doesn't occur."
Whether such a change would ruin the Annex, an undeveloped former orchard on 12 acres of city-owned land next to Cuesta Park, is something residents have disagreed about.
"It's kind of an anarchist park," said Lex Nakashima on Monday as he threw a Frisbee to his two border collies that were busy sticking their noses into the area's many gopher holes. "The city sees it as an eyesore, everyone else sees it as a place to get away from everything."
Nakashima said he was concerned that additional landscaping done for the basin, costing the city $10,000 a year to maintain, would make it a place where "they wouldn't like dogs digging around."
At the meeting Mayor Mike Kasperzak said a main reason he supports the basin was because it "forever protects the area from any future development. Who knows if in 10 to 20 years the community doesn't come out and say, 'We need more ball fields, let's put them at the Annex.'"
Another frequent Annex user and dog owner, Sandra Barnett-Brook, said Monday that she might be OK with a flood basin in the front of the Annex, but she objected to further development of the Annex with a history museum at the rear, a concept the council has supported.
The museum project, however, is off the table. It was announced at Tuesday's meeting that the Mountain View Historical Association has stopped its effort to build a history museum in the Annex, citing financial difficulties in a letter to the city manager.
Larger flood protection plans
Along with basins at nearby McKelvey Park and upstream at the Rancho San Antonio open space preserve, the project is intended to prevent a rare 100-year-flood event from damaging 2,750 properties in Mountain View. The Water District passed on other alternatives, including a dam near Lehigh Cement Quarry that would have had larger environmental impacts. City staff said it would remove the need for flood insurance for those with federally backed mortgages in Mountain View's FEMA flood zone.
The Water District came up with the smaller flood basin at the Annex after Los Altos school district officials rejected plans for a fourth flood basin at Blach Middle School. Engineers re-examined water flows near Lehigh Cement Quarry and found 230 acres that did not runoff into the creek as originally thought. That reduced creek flows from 2,700 cubic feet per second to 2,400 cubic feet per second, which was enough to allow the Blach school basin to be removed and cut the size of the Annex flood basin in half from its original size of 65 acre-feet.
To put that in perspective, an Olympic-size pool uses only 2 acre-feet of water.
The city commissioned an independent study of the Water District's hydrology that concurred with its findings, though it estimated less water flowing along Permanente in a 100-year-flood — 2,317 cubic feet per second — versus the 2,400 cubic feet per second estimated by the Water District.
Local resident Mike Hayden said that if the 5-percent margin of error the Water District had given itself were applied to the 2,317 cubic feet per second, "It's quite likely that there won't be any need for improvements at all."
City staff recommended the flood basin because the alternative, a 4-foot diameter pipe under Cuesta Drive to catch flood waters, could leave 400 Mountain View properties subject to flooding. Under that alternative, the Water District reports that flood waters would overflow the Permanente Creek diversion channel, which flows to Stevens Creek, and flood north towards the El Camino Hospital area. Using the pipe instead of a basin, Water District modeling shows flooding about 1-foot deep around the hospital, with 2-foot-high waters on portions of South Drive and North Drive "restricting" ambulance access to the hospital's emergency room entrance at the rear of the building, while the hospital's buildings stay dry.
Hayden said that scenario was unlikely because "there's been no flooding in the diversion channel since it was built," except in 1983 because of "an anomalous event at the cement plant" which sent water rushing down the creek.
Council member Laura Macias said the hospital could find other ways to deal with the potential problem, while most council members were concerned. The hospital sent a letter of support for the Annex basin option, city staff said.
"I don't want to be the member of the council who decided that access to the hospital was really not important, that doesn't fit with my sense of responsibility," Bryant said.
While flood waters in a 100-year-flood might only be 1 to 2 feet deep, council member Tom Means said that was high enough during the 1998 flooding of Palo Alto's San Francisquito creek to render his friend's kitchen "effectively destroyed."
The Water District also plans to build flood walls along Permanente Creek north of Highway 101 to protect Google and other nearby businesses, remove cement channeling along portions of Permanente Creek, replace two bridges where Mountain View Avenue crosses the creek and widen portions of Hale Creek, which meets Permanente Creek south of Mountain View.