Flagstaff council OKs grant submission for Big Fill Lake flood-mitigation project

Flagstaff City Council on Feb. 3 authorized staff to submit a state match application for the Saligman Sub Big Fill Lake project, the city’s proposal to replace undersized drainage beneath the BNSF embankment and Route 66 and reduce flood risk on the city’s east side.

Ed Shanks, Stonemar section director, told council the project at 100% design would replace two undersized culverts with five new tunnels — four 8-foot-diameter pipes and one 9-foot-diameter pipe sized to allow future pedestrian and bicycle access — and reconstruct inlet and outlet structures for both the railroad and Route 66. Shanks said the construction opinion of probable cost is about $13 million, with $1 million for project management and $5 million for contingencies, for a total near $19 million. The city is seeking roughly $15.6 million from a federal FRA grant and a state AZ SmartMatch contribution of about $3.9 million; BNSF would be asked to cover half the nonfederal share.

Shanks said the design intends to convey roughly 2,200 cubic feet per second through the new structures, lowering the base flood elevation behind the embankment by about 10 feet, removing roughly 200 acres from the FEMA floodway and allowing about 51 townhouses currently burdened by the floodway to be taken out of the designation. He said the project also aims to reduce rail-pedestrian conflicts at Mile Post 338, an area Shanks listed as having nine incidents and five fatalities.

Council members asked technical and sequencing questions. Council Member Aslan asked whether the design targets a particular flood frequency and how the Rio de Flag project affects the eastern floodway; Shanks said the project is not designed to a single “design storm” but seeks a reasonable conveyance (about 2,200 CFS) and that Rio de Flag’s downtown benefits would move water faster toward the east side, making a separate eastern mitigation project advantageous. Council Member Spence asked about a historic sinkhole (“the bottomless pit”) and whether it had been included in analyses; Shanks said FEMA’s analysis assumes that feature can pass about 54 CFS at full pool but that alone is insufficient for large flood events.

Council voted to adopt Resolution 2026-02 authorizing submission of the AZ SmartMatch application and acceptance of funds if awarded. Shanks said staff plans to submit the AZ SmartMatch application this week and to file the corresponding FRA application on Friday.

If the grants are awarded, next steps would include finalizing agreements with BNSF, securing required easements, coordinating with adjacent sewer and Wildcat interceptor projects, and scheduling construction procurement.