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PUC staff recommends rehabilitation and downstream control for Mountain Tunnel; schedule, cost estimates presented
Summary
Staff recommended a rehabilitate‑with‑downstream‑control alternative for the Mountain Tunnel, citing feasibility, lower cost compared with bypass or reline options, and operational benefits; the work will require multiple outages and a multi‑year CEQA/NEPA process and construction schedule stretching into the mid‑2020s.
PUC staff presented an update Nov. 14 on the Mountain Tunnel — a roughly 19‑mile conveyance constructed between 1917 and 1925 — and recommended proceeding with the rehabilitate alternative with downstream control.
Dan Wade summarized inspection results from a recent shutdown and three evaluated options: rehabilitate, reline, and bypass. Staff and technical advisory groups recommended the rehabilitate option with downstream control because it meets 95 percent of conveyance capacity criteria, reduces hydraulic/erosion risks, improves access and maintenance, and is significantly less costly than the other alternatives.
Wade outlined key components of the recommended program: targeted lining repairs and contact grouting, up to 4,000 feet of invert smoothing in unlined sections to improve access, removal of trapped debris at known siphons, extension of the South Fork siphon to reduce seepage and water‑quality impacts, and construction of a new downstream portal and shaft at Priest Reservoir to install low‑flow control and isolation valves required for downstream control.
The proposed sequence includes immediate interim repairs already underway, CEQA and NEPA documentation beginning immediately with a roughly two‑year environmental and design effort, advertisement of interim repair construction, and an anticipated notice to proceed for major construction in spring 2018. The work plan calls for one 60‑day outage (to tie in downstream control) followed by four 100‑day outages for in‑tunnel repairs, with the final shutdown (and project completion) projected in 2025.
Wade provided preliminary cost ranges for options and said the recommended rehabilitate alternative with downstream control is $227 million for the base rehabilitate with downstream control option (recommended) and that other variants (e.g., full invert smoothing) are more expensive; staff estimated that the rehabilitate alternative could be $350M–$500M less than the reline or bypass alternatives when downstream control is included. Staff also estimated reduced maintenance and operations costs on the order of $20 million over a 100‑year life due to downstream control benefits such as reduced erosion and improved water quality.
Commissioners asked about impacts on fish and hatchery operations during outages. Wade said extended shutdowns will constrain water available to the Moccasin fish hatchery during those periods and that downstream control will ultimately allow dewatering for tunnel entry without fully draining Priest Reservoir in future outages. Commissioners also asked for additional information on potential hydropower benefits, which staff said are under evaluation.
