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Cottonwood culvert fix starts process toward system repair

City buckles down to fund multi-million public project

Ryan Rehder, senior project engineer with Mountain Waterworks, shows the Cottonwood culvert repair and bypass project plans at last Thursday’s Oct. 12 special city council meeting.

Photo by David Rauzi
Ryan Rehder, senior project engineer with Mountain Waterworks, shows the Cottonwood culvert repair and bypass project plans at last Thursday’s Oct. 12 special city council meeting.



— City officials are in action to resolve a long-standing flooding problem for Cottonwood’s downtown business district.

“We’re going to do a temporary fix to hold things together, and start the ball rolling to come up with a long-term solution,” said Cottonwood City Councilor Ron Grant.

What he’s talking about is an approximate $20,000 grouting project, currently under way, to repair a section of culvert diverting Cottonwood Creek underneath a portion of downtown. The fix is expected to provide three to five years extra life to the section, providing a little breather as the city gathers its funds and funding options for a major culvert renovation project.

How major? Estimates range from $2.5- to $4 million-plus to rehabilitate the approximate 580-foot-long system that stretches from Front and Lewiston streets to where it exits into Cottonwood Creek underneath Riener’s Grocery east of King Street. The option under consideration is replacing the most damaged section at Lewiston and Front streets, and then lining the remainder with plastic pipe, along with installing a four-foot bypass culvert underneath Main Street to handle overflow from the main system.

How soon? Pending all the funding pieces are found, construction could start as early as 2019, but as yet the timeline is still a large unknown.

“It’s one of those problems that everyone’s walked by over the years, because they knew it was going to be expensive,” said Councilor Jack Duman, “but one day it rears its ugly head, and here it is.”

The ball on this started rolling last Thursday night at an Oct. 12 special council meeting where officials approved application for Federal Emergency Management Agency Pre-Disaster Mitigation (PDM) grant funding. PDM funds 75 percent of a project up to $4 million, and as part of its application the city tentatively committed to $801,851 in cost-share.

For its part, the city is currently exploring its funding options, including block grants (up to $500,000) and through the Local Highway Technical Assistance Council (up to $100,000) to meet its pledged financial commitment. Already committed has been assistance from the Idaho Transportation Department with its inspection and evaluation of the culvert system, and as-yet undetermined project funding that may free up in 2020.

The FEMA grant timeline puts notification of making the short list by January 2018 and announcement of final awards that December.

“We’re getting this started now as we know the process for these things takes two to three years,” Duman said.

“We finally realized we can’t ignore this anymore,” Grant said. “We’ve got to do something. We have to bite the bullet.”

Public attendance at last Thursday’s meeting was attended only by mayoral candidate Seth Harmon and a Free Press reporter. That night’s presentation on the project included information on several alternatives – as prepared by Mountain Waterworks, represented by senior project engineer, Ryan Rehder – for the culvert problem. Those not considered were due to potential FEMA objections to constructing a new culvert that would divert the creek from its historic path; of the problems in excavating to depth and navigating city services by constructing a new culvert down Main Street; and of the high costs from planning and operation/maintenance in developing an upstream high-flow reservoir.

“The bypass recommendation is the most viable option,” Grant said. “Rerouting the whole thing is not feasible.”

Positives going for the alternative being considered include the mitigation factors to prevent future damage, its staying within the existing creek system, and the threat flooding poses to historic downtown structures, most of which in the century-old range.

“Everyone felt this was a very viable project,” Duman said. “There’s a very strong chance it will be funded, but don’t put that money in the bank just yet.”

Flooding is not uncommon for Cottonwood’s downtown, due to flash events such as from a summer downburst to seasonal rain and snowmelt runoff.

“And whatever doesn’t go down the culvert goes down Main Street,” Duman said.

Last winter, the city prepared sandbags, straw bales and other diversion efforts in advance of what was expected to be an unusually high snowmelt runoff. However, the city avoided that onrush as the melt occurred more gradually, and storm drain capacity was sufficient. But as the water receded, Duman said a vortex developed, revealing a huge hole at the headwaters to the culvert that was damaging to and diverting around the system. Repairs under way will be a short-term fix to this issue pending a long-term solution is put in place.

“It’s the highest number-one priority we have,” Grant said.



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