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Commissioners revisit channel lining, Sixth Street box culvert and floodplain mapping; planning recommended before state remap
Summary
County staff urged commissioners to consider replacing the undersized Sixth Street box culvert and to begin planning channel lining and wetlands mitigation before the state's updated floodplain model is finalized, citing potential effects on hydraulics and property mapping.
County staff brought a multi-part update on stream-channel work through town, urging the Fallon County Commission to start planning now for repair and lining projects that could affect the hydraulic assumptions used in the state's updated floodplain model.
Staff said the Sixth Street box culvert is undersized and is the main near-term concern for localized flooding. The original plan discussed replacing box culverts and lining a portion of the downtown channel, with the intent of alleviating cattail-related backups that create maintenance and flooding problems. Staff noted existing concrete channel sections installed in the early 2000s and indicated county-owned channel liners are limited in quantity; they recommended prioritizing replacement between Fourth and Sixth streets and that the Sixth Street culvert be addressed first.
Staff warned that if the county waits until after the state and FEMA release new floodplain mapping, hydraulic models may not reflect proposed channel improvements. They recommended initiating a delineation of wetlands in the channel for mitigation planning, considering the potential need to acquire mitigation acreage or coordinate with the fire department for use of floodplain areas. Staff also said partnering with the city on lining through city property will be necessary and suggested beginning the public and intergovernmental conversations now.
Why it matters: sequencing construction and floodplain remapping affects the model results used to set regulatory floodplains; replacing undersized culverts or lining channels may lower flood elevations in some areas and reduce localized flooding but requires mitigation and coordination with state agencies and the city.
Next steps: staff recommended placing the item on a future commission agenda for full discussion, arranging site visits for commissioners, and initiating wetlands delineation and budgeting/solicitation for culvert/lining work.
