A week after warm winter temperatures and 8 inches of rain fell on the Blue Mountains over a two-day period Feb. 6-7 causing a devastating flood, the U.S. Army Corps of Engineers has released details into how it managed all that water. According to USACE hydrologist Tracy Schwarz, four inches of rain fell on Thursday, and then another four inches of rain fell on Friday.
“What we saw on Thursday, it was looking like the jet stream was moving up into British Columbia, so we were filling the reservoir and mitigating flows down to a non-damaging level of about 1,400 cubic feet per second, diverting the excess into Bennington Lake,” Schwarz said.
The USACE believed that Thursday’s weather pattern was going to wind down. That didn’t happen. Schwarz said that during the night the jet stream moved and targeted another four inches of rain directly onto the Mill Creek watershed, as well as the Umatilla, Walla Walla, Touchet and Tucannon rivers.
“The difference being is we had four inches that were partially absorbed into the soil on Thursday and that resulted in a lot lower flows coming off,” Schwarz said. “What we saw on Friday, the ground was already saturated, and we had rain on top of the mountains. This brought in a whole bunch of water and we had a record inflow of 6,400 cfs. That was more than what we saw in 1996 or in the 1931 flood that justified the flood control project.”
With the high flow, Schwarz said the USACE had to do something to mitigate having already consumed part of the reservoir space on Thursday keeping flows down to a reasonable level through the community.
“Then we had to make a decision on how we’re going to manage the rest of our space in the reservoir,” Schwarz said. “So we made kind of a risk-informed decision. Our levee system is designed for 3,500 cubic feet per second, which allows a certain amount of freeboard on the levees for non-over-topping. We looked at the amount of space left in the reservoir and not knowing exactly how long the rain was going to last, but the intensity we were seeing, to mitigate some of that space we pushed the flows up to what we were targeting was 3,800 cubic feet per second through town.”
Schwarz said the USACE put people on the ground throughout the Walla Wall area, as well as having people monitoring water flows. If problems did arise, the USACE could immediately bring those flows down.