“Dams Keep Flooding to a Reasonable Level,” Doug Darlington, Cottage Grove Sentinel, 1996
If it had gone strictly by the numbers, last week’s flood could have been the worst to hit South Lane in two centuries, according to inflow tallies recorded at local reservoirs during the event.
As it was, the dams on area rivers and a relatively short flood duration helped keep damage to a minimum, said Herschel Henderly, area manager for the U.S. Army Corps of Engineers.
“These dams are designed to retain flow levels of 100-year floods, but we were getting 200-year flows on Monday,” Henderly said. “What saved us was that the flows dropped off fast. They spiked quickly and they went down quickly.”
The inflow total at 10 p.m. Monday, Nov. 18, at Dorena Lake was 38,142 cubic feet per second. Runoff into the reservoir only the day before had been 1,388 cfs. At Cottage Grove Lake, inflow peaked at 20,146 cfs at 9 p.m. Monday.
The deluge resulted as more than 7.3 inches of rain fell in the area in less than 48 hours, that on top of a landscape already saturated from several days of drizzle.
“In 16 years, I’ve never seen higher than a 16,000-cfs inflow,” Henderly said. “When you have flow levels of 200-year floods and it’s still raining, that’s a little scary.”
Discharge at Dorena Dam was pushed to 7,000 cfs at the height of the event to prevent an unregulated flood over the dam’s spillway. In less than 48 hours, the reservoir at Dorena went from only 9 percent to more than 84 percent full. Cottage Grove Lake climbed to 74 percent of capacity in the same period despite sustained flows of approximately 2,000 cfs.
Inflow at Cottage Grove Lake was more than doubled that of the 1964 flood, while that at Dorena Lake was nearly equal to the 1964 totals. Flood pool levels at both reservoirs were expected to be near normal this week.
Threat of heavy flooding downstream prompted the City of Cottage Grove to declare a state of emergency Tuesday, Nov. 19, in order to receive sandbags and assistance from the Corps and the National Guard.
The city’s efforts were focused on protecting the waste-water treatment facility, which sits on the 100-year flood plain of both Row River and the Coast Fork of the Willamette. The facility was not damaged as flood waters remained lower than anticipated.
Unaffected, also, was the city’s water supply, tapped from areas of the upper Row River and Layng Creek. According to Bon Sisson, public works director for the city, the primary intake at Layng Creek had become clogged with debris, but there was no threat to water quality or quantity.
The basement of City Hall flooded, but damages were minimal.
Scattered power outages, including about 60 homes in the Mosby Creek area, were reported throughout the region, but all had been repaired by Friday.
