Sean C. Morgan
Ever wondered what would happen if Foster Dam were to crumble?
Dam failure is highly unlikely, but if it were to occur, some parts of Sweet Home would become temporary lakefront property, while others would be submerged, according to recent calculations by the U.S. Army Corps of Engineers.
The Corps recently updated its dam failure inundation maps, which were previously updated in 1980. Modeling for the current maps was completed in 2010 and 2011. Corps officials discussed the maps with local media following publicity this year in Lane County.
The maps depict two scenarios, failure during full pool on a normal (nice) summer day and full pool on an uncharacteristically stormy summer day with a lot of water flowing in and out of the Foster and Green Peter reservoirs, which is the worst possible case.
If Foster Dam were to fail, Sweet Home would have about 11 minutes notice. For Green Peter, the warning period would be about 20 minutes. The inundation would reach its peak and recede within four to eight hours. Downstream, receding waters would take days in some places. Flooding would be more widespread in Lebanon, with flooding also appearing along the Willamette River north from Albany.
“This portrays an extremely unlikely event,” said Matt Craig, Portland District Dam Safety Program manager.
In the event of failure under dry conditions, Foster Reservoir would inundate property below Nandina Street in the downtown area. It would rise around Ames Creek, flooding the west end of the downtown area. Under the storm scenario, flooding could reach the west edge of Oak Terrace, the hillside at Sankey Park, and leave the Avenues above that hill dry. The water would reach to just north of Green River Drive in the Midway area on a clear day at full pool and would cover Green River on a stormy day.
On a normal summer day at full pool, with Foster remaining intact, a Green Peter failure would flood a couple of blocks south of Oak Terrace and everything below. On a stormy day, it doesn’t get much worse, just reaching Ironwood along Fourth Avenue but reaching Elm Street from around Eighth Avenue to around 14th Avenue.
Oak Heights Elementary, Hawthorne Elementary, the Junior High and everything east of Mountain View and above the railroad stay out of the water in all scenarios.
A Green Peter failure would cover much of North River Drive. Flooding crosses Highway 20 east to about the 2900 block and covers the railroad tracks all the way to Foster.
The two scenarios do not include failure of both dams at the same time.
Among potential threats are aging infrastructure, Craig said. Some components are old, but “we have been able to go through and upgrade parts.”
THIS INUNDATION MAP depicts how the failure of Green Peter Dam would affect Sweet Home. The striped blue area illustrate the effect of failure at full pool but under dry conditions. The red areas depict flooding under a failure at full pool with high river flows.
Green Peter is slated for work on the spillway gates this fall, said Spokesman Scott Clemans. That work, similar to the work recently completed at Foster Dam, should be finished next spring or the following year, depending on which construction alternatives are selected.
The dams are inspected annually, with a comprehensive inspection every five years, Craig said.
In earthen dams, such as Foster, the concern is water seepage and piping through the dam, which can create voids, Craig said. The concern with concrete dams, like Green Peter, is movement, possibly in an earthquake.
“The historical performance of dams in seismic events has been exceptionally good,” Clemans said. Few dams have failed from quakes.
Dam failure is usually more incremental than catastrophic, he said.
And the Corps is funded to keep up maintenance on the dams, Craig said. As the Corps identifies needs, it takes care of them.
“Dams kind of prove themselves,” Craig said. “If there is an issue, they show up early.”
Perhaps the most notable dam failure in Corps history was the Teton Dam in 1976 in Idaho.
That earthen dam failed as the reservoir was being filled, Craig said. Dam failure is “incredibly rare.”
Emergency planning for failures is handled locally, Clemans said.
The biggest tool for local emergency planners is “reverse 9-1-1,’ said Joe Larsen, Linn County Emergency Services coordinator. In the case of dam failure, “we would send out a message.”
Reverse 9-1-1 can send messages to every land line based on ZIP code or other parameters to suit the emergency.
Cell phone users can sign up for alerts through Linn-Benton Alert through Everbridge, ww2.everbridge.net, Larsen said. In the case of dam failure, an alert would include evacuation routes.
“Obviously, if the dam breaks, they have a short period of time,” Larsen said, then it’s a matter of dealing with the aftermath.
Larsen noted that the maps depict only the worst-case scenarios, with reservoirs at full pool.
As with other types of emergencies, residents should have some basic preparations in place, including a “go bag” with important papers and resources for survival, he said. Residents may need to leave in a hurry.
Even though dam failure may not be a significant threat, having such preparations in place is a good idea this year, he noted, with anticipation of a dry summer and high fire danger.