WASHINGTON CITY – With homes along the city’s Main Street getting hit twice by floods within a five-week span, Washington City Council members and staff reviewed the circumstances that contributed to the flooding and what might be done to mitigate them.
“The last eight years we’ve had five floods that have significantly damaged people’s homes,” City Councilman Daniel Cluff said during Tuesday night’s council work meeting.
This year, the two monsoonal rain storms that hit July 13 and Aug. 11 resulted in a large amount of water rushing down Buena Vista Boulevard northwest of Main Street that overwhelmed drainage infrastructure.
More than an inch of rain fell in a 30-40 minute span during both storms, Clint Merrell, a project manager for Bowen Collins & Associates, told the City Council. Bowen Collins is an Idaho-based engineering firm specializing in water-related projects, including flood control.
“What really hurt us is where (the rain) fell,” he said.
Part of the 1.3-square-mile area that drains toward Main Street on the northern side of Interstate 15 consists of bedrock and outcroppings where the only course the water can take is downhill. There’s no place for the water to pool or be otherwise contained. Too much rain in too short a time in that area spawned a flash flood.
Despite retention basins and various drainage pipes that divert water toward Mill Creek, some of which were built in the wake of previous flooding, the system was still overloaded.
The largest storm drain pipe in the area, which goes from Buena Vista Boulevard and drains into Mill Creek, carries about 235 cubic feet of water per second. One cubic foot per second is equal to 449 gallons per minute.
Following the July 13 flooding, City Manager Roger Carter said that over 300 cubic feet of water per second had hit the storm drains.
The existing system was built to handle what are commonly called “100-year events,” Todd Olsen, the principal of Bowen Collins, told the council. He said that means the event has a 1 percent chance of happening at any time, not necessarily once a century.
“There’s always potential for a large flood.”
A review of the drainage system found that Main Street had the “largest deficiency,” Olsen said.
Part of the road slopes to the east where it lacks a curb and gutter that could act as a barrier to keep water from running into homes.
Councilman Doug Ward said there were parts of Main Street where a curb and gutter had been installed and the water simply went right over it.
Olsen agreed, adding, “Main Street doesn’t have the capacity to carry a flood event.”
One of three priority solutions proposed by Bowen Collins involves rebuilding Main Street so it could handle flood events better. Among the reconstruction concepts proposed is inverting the street so it slopes toward the middle of the roadway instead of away from it. The proposed improvements would handle an estimated 800 cubic feet per second of water flow, Olsen said.
Council members questioned where the water would go after it was funneled down Main Street. However, Bowen Collins’ primary focus was how to improve Main Street, so it did not touch on where the water would go once it intersected with Telegraph Street at its southern end.
The Main Street reconstruction was counted as the third of three priority options as they are projects the city could begin immediately, Olsen said.
The other two options, which were highly recommended by Olsen and Merrell, are building new retention basins and a flood channel.
The first option calls for two detention basins to be built higher on the watershed where the flooding originated. This would be accompanied by a system that would help slow the water’s race toward Main Street.
The second option recommends building a flood channel that parallels a 60-inch pipeline that runs from Buena Vista Boulevard to Mill Creek. It would help divert additional water toward Mill Creek in case the original line is overwhelmed.
“In our opinions the detention basins provide more bang for your buck,” Olsen said. “They protect everybody (from flooding)…. A whole point of the two priorities is to cut off everything at the I-15 underpass on Main Street.”
While having the detention basins, flood channel and accompanying infrastructure in place wouldn’t completely stop water from running down Main Street, Olsen said he believes it would prevent a major flood event in the area.
There were a handful of lower-priority recommendations as well that include putting flood walls in front of the homes routinely impacted by the floods, or the city buying those homes outright. Neither of those options was given much attention.
“I appreciate some real solutions that don’t break our bank,” Cluff said.
While securing funding for the projects overall would need to be addressed, the city’s Public Works Director Mike Shaw said, “We can do something rudimentary right now before the next monsoon season.”
The City Council expressed support for moving forward with the two main priorities and may vote to approve plans moving forward on the recommendations at its next meeting in September.
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