Atherton: Too much water causing problems Atherton: Too much water causing problems (October 26, 2005)

By Andrea Gemmet

Almanac Staff Writer

Water, water everywhere -- at least, that seems to the case in parts of Atherton west of Alameda de las Pulgas.

The Atherton City Council, which has a surfeit of attorneys but nary a hydrologist, is seeking expert advice on handling increasing complaints about drainage problems.

The council has been getting complaints from residents saying that new construction by uphill neighbors is causing flooding and damages on their properties.

Other drainage issues include massive quantities of groundwater being pumped into roadside storm drains to accommodate basement construction, and a mysteriously rising water table in the western part of town.

While council members said they needed more information before they can make a decision, they indicated at the October 19 meeting that they will likely change the town's drainage policy and add new construction rules to require soil percolation tests in some areas of town.

Current drainage rules require the installation of French drains or other water-detention systems that collect water during heavy rainfall and slowly release it, preventing huge amounts of water from overwhelming storm drains and flooding the Atherton Channel.

The town also requires runoff to follow "historic drainage patterns."

"People downhill have to accept some amount of water, but they don't have to accept a tsunami," said Councilman Alan Carlson.

Atherton's drainage policy was put in a place to grapple with new residential development, said Building Official Mike Hood.

"Time was, Atherton was primarily moderately sized houses on one-acre lots. In the past 10 years or so, we've seen development for considerably larger houses with considerably larger impervious areas," he said. "If we continued to allow it, the Atherton Channel could not handle that (water) flow."

Nothing in the town's policy addresses protecting neighboring properties from increased runoff, he said.

In order to change drainage rules to keep water flow at historic levels on adjoining properties, town officials will have to figure out what to do with the additional runoff, he said.

Rex Upp, a Campbell-based geotechnical engineering consultant, said that Atherton's methods for dealing with storm runoff only work in areas where the soil has good drainage, or percolation.

Mr. Upp, who was hired by two Atherton residents to investigate drainage problems, said that in some areas of town there is only a shallow layer of soil over bedrock. Detention pits dug in bedrock don't allow water to slowly seep through the ground, but instead function like bathtubs that quickly overflow during storms, he told the council.

Soil percolation tests would be one way of finding out if other drainage measures are needed, he said.

Janice Ross, a resident of the Lindenwood neighborhood, said that a construction project on Greenoaks Drive that includes swimming pools and a basement has been pumping water into the storm drains 24 hours a day for the past seven months. She urged the council to consider what such projects are doing to the health of Atherton's entire water system.

"When I looked at it, water was pouring out at a ferocious rate," said Councilwoman Kathy McKeithen, who visited the site. "I've never seen such velocity."

The council directed town staff to compile a list of similar pumping projects over the past two years, and provide a list of alternatives to pumping.

The council also asked for a map showing the parts of town where the water table has been rising.

The area west of Alameda de las Pulgas between Atherton and Stockbridge avenues has extremely high groundwater, and the water level has been getting higher in recent years, Mr. Hood said. Town staff doesn't know what is causing the rise, he said.

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