After $7 million in federal aid was directed to Coconino County for post-fire flood mitigation earlier this month, city staff told the Flagstaff City Council and impacted residents that the city is also moving forward on several projects.
City staff said they are nearing the final stage of design on three projects, including a massive detention basin on the grounds of Killip Elementary School and improvements to the culvert at Dortha Avenue.
But for all that work to be successful, interim city water services director Andy Bertelsen said it was critical that the county’s efforts north of the city find a way forward.
Those county-led projects are largely designed to limit the amount of sediment and debris that is swept into the city during flooding events, clogging storm water infrastructure and worsening floods.
“The water is important, the sediment is even more important -- we want to keep that up on the mountain,” Bertelsen said.
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But with the U.S. Forest Service and the Natural Resources Conservation Service both providing $3.5 million for those county-led projects, staff said projects inside Flagstaff's city limits are also closer to reality.
A portion of that federal aid will be going to support one of those city projects: the creation of four detention basins along the Spruce Wash channel just above Linda Vista, said city water services engineer Gary Miller.
Miller said the county flood control district is also expected to cover about a quarter of the cost for that project, which may total $4 million. Efforts further within the built environment of Flagstaff will be paid for by the city alone.
Working with Natural Channel Design, Miller said they are planning to build four sediment basins on the 3.6-acre section of city-owned land that runs behind several of the homes on Paradise Drive, with the lowest basin lining up with Park Way to provide access to city maintenance crews.
Earlier this fall, a project was also completed at the top end of Paradise Drive to stabilize and widen the Spruce Wash channel, which was particularly narrow and threatened several nearby homes.
City Stormwater Manager Ed Schenk said they are also planning work to further increase the carrying capacity of the Spruce Wash channel farther downstream between Cedar Avenue and Dortha. That work will include an effort to enlarge the 60-inch pipe under Dortha that the channel feeds into, Schenk said, although exactly how big that pipe will become has yet to be determined.
Schenk said the cost of that effort had originally been estimated at about $1 million, but that has since increased to about $2 million. Schenk said the doubling of that cost comes partially as the scope of the project has expanded, but also because the cost of construction has risen recently.
Lastly, interim Public Works Director Scott Overton said they are nearing design completion on the Killip Elementary detention basin.
Overton said it now appears that the basin, as designed, will be capable of storing between 8 and 11 acre feet of water before metering that out slowly. That equates to a capacity of between 2.6 and 3.5 million gallons of water.
“We hope this will do a lot of work by not only storing that water, but slowly releasing it versus a sheer flood of water,” Overton said.
Water from that detention basin will flow down a new underground channel that will take the water away from the basin, largely benefiting Sunnyside residents living south of Sixth Avenue.
Adrian Skabelund can be reached by phone at (928) 556-2261, by email at email@example.com or on Twitter at @AdrianSkabelund.