Both towns have asked for disaster relief from the governor's office through the San Mateo County Office of Emergency Services. None of the damage affects vehicle traffic, officials said.
In Portola Valley, the damage to upper Alpine Road was to the road's western shoulder along the bank of Corte Madera Creek. A tree fell over and took with it part of the shoulder, Town Manager Nick Pegueros said. The town geologist and public works director have inspected the site, and public works checks it regularly, Mr. Pegueros said.
Repairs could reach $300,000. The town "will repair it at some point in the future, but it's not a threat to life, safety or property," he added.
In Woodside, one of the damaged culverts carries water from Dennis Martin Creek under Portola Road near the intersection with Old La Honda Road; the other culvert is located near the intersection of Eleanor Drive and Stockbridge Avenue in the Woodside Heights neighborhood, just west and south of Woodside High School. Erosion compromised the shoulder of about two miles of unidentified roadway, according to a staff report.
The damaged culverts are 7 feet in diameter, Town Engineer Paul Nagengast said. The culvert along Portola Road is buried under 22 feet of earth and about 100 feet long. The town's culverts are generally made of corrugated steel and are at least 50 years old. "They probably need some attention," Mr. Nagengast said.
Town staff will be investigating alternatives to culvert replacement, including the use of plastic inserts. Staff will be bringing the Town Council a proposal for a town-wide analysis of the storm drain system.
The cost to replace the two culverts could reach $475,000, with another $112,000 to repair the equestrian crossing, $45,000 to strengthen a road shoulder, and $51,000 to remove debris, according to the staff report. "Obviously, if we can do it cheaper, we will," Mr. Nagengast said.
The damage to the culverts, he said, came not from the volume of water, but from the debris it carried. A chunk of lumber gets caught in the culvert and "the force and pressure of the water is enough to rip steel, and that's what happened," Mr. Nagengast said.