Engineers outline Coconino Wash dam and large basins; Middle Wash excavation would require tree removal

Consultants described the Coconino Wash Flood Retarding Structure (FRS) as a major upstream element intended to reduce peak discharges so the Town’s new storm drain system can handle flows. "A large‑scale 25‑foot‑tall dam creating a basin that can impound 500–600 acre‑feet of water" was presented as the conceptual capacity, and the FRS is being pursued on a separate permitting and design track requiring state and federal approvals.

Presenters said the Middle Wash basin would be an excavated basin across the valley width, requiring substantial dirt removal and tree clear‑cutting — a measure with major flood‑reduction impact but significant environmental disturbance. The Long Jim Basin would expand existing storage with limited changes to the outlet, and Water Tank Wash work would raise an embankment and modify the outlet with less need for large excavation.

Consultants cautioned that while the combined system yields large reductions in flood depths and extents, a true 100‑year watershed event can still produce localized flooding; the projects are intended to substantially reduce depths and protect critical facilities rather than guarantee zero flooding.