You could say the rain in California falls mainly on the plain. You could also say the mapping of the floodplain is a huge inter-governmental effort causing angst for thousands of property owners in the state.

The Federal Emergency Management Agency is unveiling new digitized maps to show the risks of 100-year floods in California and identify new groups of owners required to buy insurance to cover their high risk of flood.

This is keeping staff busy in many California cities and counties, cross-referencing federal flood maps against parcel maps and determining which property owners are affected.

“It’s a daunting effort,” said Bob Beaumont, assistant public works director in Marin County, to coordinate masses of geographic and hydrologic information.

Controversy struck in Marin County with hundreds of properties added to flood hazard areas. In some cases, properties were properly added because of improvements in computer modeling and housing developments built near San Francisco Bay.

But, Beaumont added, other parcels were apparently added in error, sometimes with homes built on high ground while undeveloped parts of the property are at low, flood-prone levels.

For FEMA, an initial step is to ask local authorities to help set up a notification process.

“In Marin County, this process has been going on for almost a year and a half,” said Kathy Schaefer, an engineer with the National Flood Insurance Program in FEMA’s regional office in Oakland. “We rolled out a new digital map for the communities and held outreach meetings.”

FEMA was flexible, said Schaefer, where owners and municipalities pointed out inconsistencies and better topographic information. The agency then delineated more accurate shoreline boundaries, she said.

In 2003, Congress appropriated money with a goal of effective digital mapping of floodplains for 90 percent of the U.S. population by 2010.

“Paper maps are hard to use, hard to access and hard to update,” said Schaefer. “The new digital maps are a GIS (Geographical Information System) database of flood hazard information.”

Once fully implemented, the digital maps are easier for homeowners to use and for FEMA to update, she explained.

In California, the FEMA updates started with the populous counties of the South, then working up through the Central Valley and Bay Area. Next up are the less populous counties of Northern California. Counties where the digital maps take effect in the first half of 2009 include Fresno, Monterey, San Benito, Tuolumne, Marin, Solano, Tehama, Santa Clara, Contra Costa, Kings and Tulare.

When the federal government identifies a parcel as being in a floodplain, an owner needs to buy expensive, federally regulated flood insurance, with implications for mortgages, building permits and property values.

Beaumont recommended that staff be aggressive in encouraging active communication processes among local government staff, FEMA and property owners.

Contra Costa County is coming up on a June 16 deadline for digital FEMA maps to become official, which locks in higher rates for flood insurance for those in flood-prone areas.

The county is notifying many property owners that new digitized maps show them in a floodplain. In many cases, the county believes this is in error, when staff goes in tight on the digital maps and finds the boundaries have shifted by 50 or 100 feet out of alignment with a watershed contour, said Rich Lierly, flood plain manager for the county’s Public Works Department.

“We’re still working with FEMA, hoping this will get resolved,” said Lierly.

In Visalia, the city is taking an aggressive stance. On May 4, the City Council directed staff to get proposals from engineering and surveying firms to demonstrate FEMA map inaccuracies — the city is hoping for a volume discount for a firm surveying many properties, thus securing a more reasonable charge-back to residents. The council also authorized city staff to work with Tulare County staff to update watershed information and demonstrate dam improvements.

The city staff is drafting a letter to federal representatives seeking authorization directing some FEMA flood map appropriations to subsidize local flood control initiatives, said City Engineer Chris Young.

Some counties and cities have set up web pages to help residents check new digital floodplain maps by FEMA.