As rain starts coming down in earnest in San Francisco and the Peninsula, public works officials are keeping tabs on decades-old rainwater systems, flood plains and landslide-prone areas that could be affected by the deluge.

San Mateo County, which the U.S. Geological Survey has called one of the most landslide-prone areas in the country, has a long history of slides, including a “prehistoric landslide” that continues to move slowly near La Honda to several coastline areas. Additionally, San Mateo and Burlingame have areas that are prone to flooding.

San Mateo County Office of Emergency Services spokesman Bill O’Callahan said his agency is keeping an eye on rising high tide expected later this week. The combination of high tide that follows an early-morning soaking could mean street flooding could occur in some areas, O’Callahan said.

“We’re just keeping an eye on it right now,” O’Callahan said.

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In San Francisco, low-lying areas or neighborhoods that used to be creekbeds, including parts of the Mission, Excelsior and Outer Sunset neighborhoods, typically suffer the most when the weather turns soggy in The City, according to SFPUC spokesman Tony Winnicker. Fourth Street and Stevenson Street in San Francisco already had reports of flooding as of Tuesday afternoon, Winnicker said.

SFPUC is in the middle of a $150 million capital project meant to upgrade the system in major flooding areas, a project that should wrap up in 2010, Winnicker said. San Francisco Public Works spokeswoman Gloria Chan said 10 sandbags per household are also available as free, minor flood protection.

tramroop@examiner.com