The Water District examined the area after Los Altos school officials pulled the Blach School overflow basin from the project, said Santa Clara Valley Water District engineer Afshin Rouhani. That spurred a new look at building a dam upstream near Lehigh Quarry. Hydrologists found 300 acres that that were assumed to drain into the creek but actually drained into the quarry pit.
"When the school turned us down we had to go back to the dam alternative," Rouhani said. "That was the only feasible alternative at that point. When they looked at the upper watershed in detail, they assumed all of it flowed downstream, but some of it actually drains to the Lehigh Quarry pit. Once they removed that area it made a big difference."
Water flow projections down the creek in a major, 100-year flood, have been reduced by 10 percent, or as much 300 cubic feet per second, Rouhani said. A so-called 100-year flood is a flood that has a 1 percent chance of occurring every year.
The reduction in flow not only is enough to make the dam unnecessary to meet project goals, but could also reduce the size of a planned flood basin at the Cuesta Annex, which has been a controversial use of a much-loved piece of open space. The Annex basin was proposed to be 21 feet deep in some areas, and cover 7.5 acres.
Detailed revisions to the project will be released in a draft environmental impact report in two to three months, Rouhani said. Plans for the City Council-approved flood basin at McKelvey Park, which would lower the park 15 feet and create new baseball fields, are not expected to change. And the proposal is likely to retain a third flood basin on county land near Los Altos at Rancho San Antonio Park, Rouhani said.
Dam still an alternative
Without a flood basin at Blach School, residents of Mountain View who live south of Cuesta Park may still be subjected to increased flooding if a dam is not built upstream. The Permanente Creek diversion channel, which runs east to Stevens Creek through residential areas near the city's southern border, could overflow in a major flood, Rouhani said. The Blach School basin would have prevented that.
Whether to better protect those homes near the diversion channel is up to elected officials. The 2000 bond measure that is funding the project only specified flood protection for 1,600 Mountain View properties in flood zones north of El Camino Real, but could pay for added flood protection south of El Camino Real if an acceptable way of doing so is found. It is a hard sell to residents however, as there is no record or memory of major flooding from the creek since the area became developed.