New data collected by the U.S. Geological Survey suggests the December 2007 flood was a 500-year event that smashed previous records. The designation means the likelihood of an equally strong flood is one in 500 during any given year.
According to USGS hydrologist Mark Mastin, the preliminary figures collected from river gauges in Doty and Boistfort could mean a reassessment of 100-year floodplain levels across the watershed and could make new development more difficult.
New flood maps are now being devised by the Federal Emergency Management Agency for the first time since the late 1970s. Many have questioned the accuracy of the maps, considering the last 17 years saw two 100-year floods in 1990 and 1996, and what now appears to be a 500-year level in 2007.
December was devastating event, but its actually kind of convenient that it happened recently so these new maps can take new data into account, Mastin said.
The new maps would replace the previous height standard for building in the floodplain. It would also be used to determine flood insurance rates. State and federal law generally requires new and existing developments to be built at or above the 100-year level.
With this big flood event we had, the floodplain will be more restrictive, Mastin said, adding that 100-year levels will be based on river gauge data in specific locations.
In Boistfort, at the Chehalis Rivers south fork, and Doty on the main stem, the flood discharge was more than two times the previous record in 1996, Mastin said. At Grand Mound, the flood level was not significantly different than in 96.
We still have to run that data through the computer, Mastin said. The new points will be put on a graph and will probably be outliers so the 100-year level may not increase as much as you might think.
According to a USGS press release, scientists made the calculations using a field survey technique in which they identified high-water marks and determined the channel geometry with surveying equipment.
Usually, USGS scientists measure streamflow directly with current meters, but during a flood, it is often impossible or unsafe to use this measurement method, so a post-flood field survey technique is used.
We havent seen anything like it on the Chehalis River in 67 years. It seems like it was a spectacular flood, Mastin said, referring to USGS data at Doty going back to 1939. I wont say youll never see it again, but it was quite rare.
Dan Schreiber covers county government and environmental issues for The Chronicle. He may be reached at 807-8239 or dschreiber@chronline.com.
