If the coming winter in the Bay Area is as rainy as long-range forecasts have been predicting, Atherton will have problems handling storm-water runoff as it makes its way to and out of the Atherton Channel.
The City Council met Sept. 2 to hear a report from San Jose engineering firm NV5 Inc. on an update to the town's 2014 drainage master plan -- specifically, how to store storm-water runoff and prevent flooding in the channel.
Three issues topped the priority list:
● A water flow bottleneck exists in the form of a culvert on Marsh Road east of the intersection with Middlefield Road. Is it feasible to temporarily detain storm water in upstream basins, perhaps at Holbrook-Palmer Park, or the private Menlo Circus Club, or Las Lomitas Elementary School?
Of the three locations, the school is closest to the channel, engineers said. A basin at the Circus Club could hold 5.6 million gallons of water, while the park could hold 3.9 million gallons and the school 1.1 million gallons, according to the NV5 report. Two basins would be needed "to really address" the flood potential, City Manager George Rodericks told the Almanac.
● The Las Lomitas Elementary School District is an independent agency. The town should keep track of district actions to address flooding at the school, including plans to divert water around the school and/or detain it.
● Water from the Horse Park at Woodside passes through the Atherton Channel. The town should get a reading on the water quality, including bacterial content, of storm water coming from the Horse Park.
Atherton's problem with runoff is complicated because of the multiple sources of the water. About half of it has sources west of town, with 36 percent coming from Menlo Park and unincorporated San Mateo County, and 19 percent from Woodside and Stanford University, according to NV5 engineers.
The Marsh Road bottleneck is a box-shaped culvert that can handle about 85 percent of what a 10-year storm would produce. Widening the culvert would be more problematic than building detention basins, Mr. Rodericks said, chiefly because the culvert is located in an area covered by multiple jurisdictions.
Complaints in town about flooding are markedly lower since the 2001 flooding analysis, with just 17 complaints in 2013 compared with 97 in 2001, the engineers said. The council's concern, for now, is a 10-year storm in that the Marsh Road culvert can handle flows for a seven-year storm only, engineers said.
Detention is key to reducing flow, and cooperation among the players is the best approach, said Nona Espinosa, a senior engineer at NV5.
Mr. Rodericks said the basins under discussion have two forms: a lawn over a concrete structure installed a few feet beneath the surface, making it all but invisible; or a field or lawn enclosed by berms, permeable and capable of holding maybe 4 feet of standing water. A key difference: an above-ground pond presents a drowning hazard and would have to be fenced.
The city had 48 drainage-system improvement projects on the table in 2001 and finished 26 of them to date, including 10 swales and six storm drain systems, at a cost of about $44 million, Councilwoman Elizabeth Lewis said. It's not clear what future infrastructure investment costs would be, town officials said.
Councilman Bill Widmer said he would not be 100 percent behind an above-ground detention pool at Holbrook-Palmer Park. "This is a park," he said. "It's not a water reservoir."
Such a basin becomes a safety issue, added Mayor Rick DeGolia.
The town should be talking about storm water with San Mateo County regarding the situation at the box culvert, and with officials from the Las Lomitas district and the Horse Park, Mr. DeGolia said.