Engineer Richard Moll, who lives in the area and scrutinized the project, told the board on June 25, "I believe approval of the basin at San Antonio should be delayed until significant technical issues are resolved."
The board approved a 15-foot deep basin for Rancho San Antonio, which replaces a parking lot near the Hammond-Snyder trail. The trail will be closed during construction next summer as it runs through the basin site.
Joe Simitian, the north county representative on the Board of Supervisors, proposed postponing approval for six weeks to allow more study, which water district officials said could delay construction for a year.
"At the risk of stating what is perhaps fairly well known, I must say over the years I've had more than a little healthy skepticism about the water district," Simitian said. "So I'm a little frustrated to be sitting here at the end of this lengthy process with twelfth hour concerns and a front page story in the Mountain View Voice on the topic," he said, referring to last week's article about local civil engineer Jerry Clements' claims that the project would be unnecessary if a restriction were removed from a diversion channel behind Blach School.
The Santa Clara Valley Water District has already spent $15 million designing the project and would spend another $34 million building it, much of which goes toward digging two flood basins, one at Rancho San Antonio and the other at McKelvey Park in Mountain View. The project is designed to prevent damage from a 100-year flood, a severe event that has a 1 percent chance of happening every year. Such a flood is not known to have happened since Mountain View was developed.
It would also allow hundreds of residents to go without paying for flood insurance and put $9 million toward constructing a new neighborhood park and baseball fields at McKelvey Park.
"I think your credibility would be greater on this issue if there hadn't been other sites that were previously suggested or recommended or identified as being essential to the project, which were then removed, as I understand it," Simitian told water district representatives. He was referring to a previous proposal for four flood basins, including ones at Blach Middle School in Los Altos and the Cuesta Annex in Mountain View. He asked if that was a "fair analysis."
"It's unfair, I think, because there was one time when the hydrology for the project was updated in 2010. And based on that hydrology update, the proposed project was changed," said Afshin Rouhani, the water district engineer on the project since 2002. "So that was a one-time change. Projects, during the design phase, you study them in far greater detail than during planning. So it's not unusual for there to be changes to the proposed project, you know, through the process."
"It's absolutely true that originally there was going to be four detention basins. Now we think the (project's goals) can be met with two," he said.
Moll said the proposed Rancho basin may be almost twice as big as it needs to be. He cited a USGS study from 1986 that showed flow into the creek at half the levels predicted by the water district. He added that the water district failed to "calibrate" its model for creek flows against the USGS numbers, as has been done for other creeks.
Water district engineer Liang Xu said Moll was wrong about the usefulness of the USGS data. "The district always calibrates our models when data is available and reliable," Xu said in an email. "USGS doesn't have a hydrologic model for Permanente watershed and just had a few high flow measurements in 1986."
Moll also expressed concern about the basin at Rancho being "a settling or collection pond for selenium" — the toxic mining waste that's been flowing into the creek from the Lehigh cement quarry in the hills just above Rancho San Antonio. A cleanup is underway, thanks to a settlement agreement in a lawsuit filed by the Sierra Club.
After the meeting, Rouhani said that the area already sees flooding from the creek and would continue to have a 10 percent chance of flooding every year.
A neighbor of the creek in Los Altos who said she would benefit from the project's flood protections but was concerned about losing access to trails during construction at Rancho San Antonio. Rouhani told the Voice that access to the Hammond-Snyder trail would be blocked by construction and that the trail would have to be re-routed after the flood basin is built. The parking lot will be rebuilt next to the basin.
Addressing the concern about construction impacts, Simitian made it a condition of the board's approval to have "as many as two public hearings after construction has started" if requested by a county supervisor in response to complaints from park users.
Construction will begin on the Rancho San Antonio basin as early as next summer, Rouhani said. Construction on the Mountain View City Council-approved McKelvey Park basin could begin in the summer of 2015.