COM 0560.020 2002-2004 - Laserfiche WebLink

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iJrlivers~t~ Te~rac~ for Hilo and Puna, where between 1980 and 1994, 18 flood events occurred. The most recent draft update of the Hawai i County General Plan devotes several pages to flooding. "Present drainage and flood problems are mainly due to the development of vacant lands, which are often subject to serious flooding without any commensurate, coordinated development of new drainage systems or expansion ofthe existing drainage systems," the plan states. While planners rely on Flood Insurance Rate Maps - FIRMs - developed by the federal government to steer development away from flood-prone areas, these maps are notoriously inaccurate. Current FIRM maps "are not very accurate as to the location, position, and formation of geographic and geologic attributes," the draft plan says. "Furthermore, there are many areas where there is no data to determine the flood potential." After the November 2000 flooding, the county, with Federal Emergency Management Agency support, hired a contractor to redraw FIRMS for the areas around Waiakea and Palai Streams. Typically, FIRMS indicate areas that will be underwater in a 100-year flood and a 500-year flood -floods that have a ]percent and a 0.2 percent chance, respectively, of occurring in any given year. But what constitutes a 100-year flood and how is it figured? The USGS' Fontaine described a variety of ways that hydrologists use to calculate expected 100-year peak discharges in a given stream. Federal agencies generally agree on something called alog-Peazson type III approach. "It's just a type of statistical analysis," he says, that considers stream-flow data from monitoring gauges, the watershed area that drains into a stream, and rainfall patterns within that area -among many other factors. Should one element in the equation change - if, say, a subdivision is built whose roads and houses increase the impervious azea in a watershed, or a highway is laid that interrupts natural drainage, or a forest is cut -the hydrology of the azea changes as well. And it's back to the drawing boazd to calculate peak 100-year flows. Even in the best of circumstances, determining the 100-year flood levels is more art than science. "You're trying to estimate something that's a very rare quantity," says Fontaine. "It's something that over the long term will occur just every 100 years. Of course, you could get two 100-year floods back to back." And if you are trying to predict floods for streams where sampling information is meager, the calculations aze all the more imprecise. Were all this not bad enough, estimating floods in Hilo was made tougher yet when the USGS discovered that one of its Hilo employees was submitting falsified data on stream flows over a period of several years. The employee was fired, but the loss ofyeazs of data is still being felt. Antedeluvian Records Some of the worst damage in the November 2000 flood occurred along the areas drained by the Palai and Waiakea streams. The Palai watershed had