A Friday night deluge that saw .79 inches of rain fall in just 20 minutes flooded parts of downtown The Dalles and put the city’s sewer treatment plant offline for five and a half hours.
“We had flows at the wastewater treatment plant increase from a 2.6 million gallons a day rate up to 13.2 million gallons a day in seven minutes,” said Dave Anderson, public works director for the city of The Dalles.
“We do know that the intensity of this storm was in excess of a 100-year storm event in terms of just how hard it was raining, not necessarily the total volume of water produced but how hard it rained, so that’s what affected our systems the most: an instantaneous demand on our system,” he said.
“We had no issues at our water treatment plant; there are no issues with the drinking water quality; there were no adverse impacts to the drinking water system at all,” he said of the water treatment plant, located on Mill Creek about six miles outside of town.
But downtown, at the wastewater treatment plant along First Street, power fluctuations and repeated power outages kept pumps from operating properly, which allowed water to back up and flood portions of the plant itself, Anderson said. The water was up to two feet deep in places, he said.
“We submerged and shorted out some of the electrical system, including the ultraviolet disinfection system,” he said.
Because the disinfection system was inoperative for almost six hours, the city posted notices at the Columbia River about the untreated sewage being dumped into the river.
“We took samples from the river and all those samples came back good” and were well within standards for recreational water, Anderson said.
The disinfection system has been restored, but the initial sampling at the wastewater treatment plant did not pass, Anderson said. That failed testing is what triggered the sampling and posting of signage at the river.
It takes 24 hours to culture the bacteria in each sample from the sewage treatment plant, so the new test that was done after the first one failed had not been returned yet by presstime Monday afternoon.
The warning signs on the river will stay up until the test results come back good from the treatment plant, he said.
Flooding occurred elsewhere downtown, but the public works department was still gathering information about the scope of it by late Monday afternoon, when the Chronicle went to press.
Several manhole covers were dislodged around town.
“The city basically has two generally separate systems. Stormwater is collected in the city and just piped directly to the river without treatment and it doesn’t have sewage in it,” Anderson said. “So much rain fell that the capacity of our storm water pipes were exceeded and water was coming up out of the manholes in various places in town.”
Sixth and Webber was closed due to the flooding. “We often have to close that undercrossing during high water events and this was no different,” he said.