Corps maps show Green River flooding scenarios

Inundation maps that show how the Green River might flood communities from Auburn to Tukwila under four scenarios have been posted on the U.S. Army Corps of Engineers' Web site.

The maps show how the lower stretch of the river might respond if floodwaters were to reach stages ranging from 13,900 cubic feet per second to a worst-case-scenario 25,000 cfs. Those are the ranges of possibility that have generally been identified by the Corps if it ends up releasing water this winter to avoid stressing the troubled Howard Hanson Dam.

The heaviest-hit stretches in each case are just southeast of Auburn and immediately west of Highway 167 between Auburn and Kent, which could be under three to 10 feet of water even with the more minor flooding. At 25,000 cfs., a two- to four-mile wide swath of the entire valley, from Auburn to Tukwila, could be deluged.

The maps are not new. They've been posted at public meetings for months and have been available for viewing in city and county offices around the county. The Corps has now made them available electronically for easier viewing by the public.

In releasing the maps, the Corps cautioned that its predictions are very general: The river's actual response under any scenario could be dramatically different from shown. There also is no way to estimate what flood level is most likely, or by how much. There are just too many variables.

Since the dam's right abutment showed evidence of damage in a storm last January, the Corps has been working to shore it up. But the Corps' Col. Anthony Wright has said the possibility that he might be forced to release water causing a flood this winter is 1 in 3, though he recently refined his prediction to 1 in 4. How much water he would release would depend on the volume and frequency of rains.

King County officials presume the river's levees downstream of Auburn would mostly contain waters below 13,900 cfs. The county has been preparing government buildings for flooding of up to 17,600 cfs., but acknowledges that overflow could be far less — or far more.

Craig Welch: 206-464-2093 or cwelch@seattletimes.com