The Federal Emergency Management Agency released new maps Wednesday night that more accurately show the nuances of flooding around the Newaukum River, including the fast-growing development at Exit 72.
The maps, from the Newaukum River's mouth near Chehalis to the confluence of the North and South forks near the community of Forest, updates data that was decades old and imprecise.
The new maps have been a long time coming: The process started in the late 1990s.
Ironically, despite massive flood damage in 1990 and 1996, official federal flood maps of the more developed Chehalis and Skookumchuck Rivers are still using old data from the late 1970s.
City and county officials simply use the flood levels from 1996 in their regulation, but the official FEMA maps for the Twin Cities won't be updated anytime soon. A proposed federal flood control plan would change the details of flooding anyway.
FEMA uses its flood control maps mainly for its flood insurance program.
Flood insurance is available to everyone in communities such as Chehalis, Centralia and unincorporated Lewis County, which participate in the national program.
Napavine, like the biblical city on the hill, hasn't needed the flood insurance, but after annexing land near Exit 72 several years ago, it has made moves toward signing up for the flood insurance program.
The maps produced by FEMA are of the "base flood level," sometimes known as the 100-year flood.
As people who waded through high water in 1986, '90, '96 and other years know, a 100-year flood can happen at any time. The standard is only a statistical measure that means in any given year, there is a 1 percent chance of this flood level occurring.
On the Newaukum River, the most visible and contentious issue has been filling on Rush Road, most notably a gravel dike 200 feet from the river and piles of fill dirt across the road.
Some neighbors have strongly opposed the fill, but Lewis County officials said the project doesn't have much of an effect on flooding. Interstate 5 constricts floodwaters and reroutes them, but the fill doesn't increase the water level significantly, officials say.
In 1996, and during other floods, the I-5 bridge over the Newaukum River upstream acts like an hourglass, constricting water, according to Lewis County Building Official Fred Chapman.
The water is pushed back, running parallel to I-5 near Bethel Church and through the Exit 72 underpass.
The fill on Rush Road is controversial, but county planners say the actual effect is minimal — perhaps so small it can't be measured even with complicated computer modeling.
At one time county officials said the fill at Exit 72 would increase flood levels by seven inches nearby, but Chapman said two separate studies of the development indicated water levels would rise just 1/4 inch, with effects tapering off 1/4 mile up and downstream.
Other filling in the Chehalis Valley has the same minimal effect on flooding, say county officials.
Putting rocks in a bowl does raise the water level, but the issue here, said Chapman, is that the bowl of the Chehalis Valley is incredibly vast compared to the amount of fill.
"This basin is so big it's like dropping a BB in a bathtub," said County Commissioner Richard Graham.
Brian Mittge covers politics, the environment and Lewis County government for The Chronicle. He may be reached by e-mail at bmittge@chronline.com, or by telephoning 807-8237.
