Sunday | August 20, 2017
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County unveils new S. Kona flood plain maps

Hawaii County is wrapping up a five-year process to revamp flood plain maps in South Kona.

The county’s Department of Public Works recently mailed letters to South Kona residents in about half of the district’s watercourses, or streams, officials said. Residents along the other watercourses should receive their letter informing them of their preliminary flood plain designation by the end of the summer.

Floodplain Manager Frank DeMarco said he’d like to hear back from property owners about flooding conditions they’ve observed.

“It all ties back to the maps, so it’s important the maps be reasonably accurate,” he said.

The flood insurance study, funded in part by a federal grant awarded in 2008, identifies changes to flood plains in the district, which DPW officials said has undergone significant residential and agricultural development since 1977. The flood maps are related to the National Flood Insurance Program, which is in turn run by the Federal Emergency Management Agency.

The flood plain designation determines a homeowner’s flood insurance costs, DeMarco said, and helps banks decide when to require a homeowner seeking a mortgage to obtain flood insurance coverage. Homeowners just outside flood plains should also consider purchasing the insurance, DeMarco said. For those homeowners, the insurance will be cheaper than for houses within the flood zones.

“Because of this young geology on this island, any mishap upstream, (water) could very well come out of a FEMA flood plain,” DeMarco said.

The county sent field crews around South Kona to study the topography and gather historical data about heavy rains and floods. The crews estimate the volume of water that may flow down a stream bed. County engineers used the data to create models to estimate what the water level of a base flood elevation would be. The documents use the phases 10-year, 25-year, 100-year and 500-year storms, but DeMarco said those terms really relate to the probability of a storm happening in a given year, not that such events happen every 10, 25 or 100 years. A 100-year storm has a 1 percent chance of happening in one year, but such an event could occur more than once in a year, or not happen for 100 years, he said.

The flood maps do more than just help determine insurance rates, DeMarco said. DPW uses the information when considering building permit applications within flood zones. In such a zone, a building’s lowest level must be built at least one foot above the base flood elevation, and the building must be sufficiently anchored to withstand flood waters and be rot resistant, he said.

Flooding in 2005 and 2006, which caused quite a bit of damage, prompted the flood map review, DeMarco said.

In addition to updating the maps, which the Hawaii County Council must adopt into the County Code, Hawaii County has also completed other tasks to earn points under a related community ratings program. That program allows communities to lower the cost of flood insurance for residents. Hawaii County has earned enough points to get a 10 percent discount on flood insurance premiums, DeMarco said.

The maps are available for review at the DPW office at the West Hawaii Civic Center or in Hilo. DeMarco said people who would like to see the flood plain designations overlaid on a tax map key map may also email him at or call him at 961-8042 and he will email a copy.