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Engineers: storage west of Lake Munson unlikely to stop south‑side flooding; city highlights decades of work
Summary
City stormwater staff and consultants presented analysis showing limited benefit from expanding storage near Silver Lake and detailed prior investments that prevented far more flooding in the April event; consultants recommended alternative conveyance and trash‑capture measures rather than further upstream storage.
City stormwater staff and outside consultants told the commission an analysis of the Jay Gaither neighborhood shows limited benefit from adding modest storage in and near Silver Lake and recommended targeted conveyance improvements and trash‑capture upgrades instead.
Stormwater Manager Jody Cahoon reviewed decades of investment in the Lower Lake Munson basin and said about $24,000,000 has been spent on projects in the Jay Gaither area over several decades, including a large channel reconstruction, box culvert upgrades and the East Drainage Ditch work that city crews completed upstream. That earlier work, Cahoon said, reduced what would otherwise have been widespread household flooding during a severe April storm event.
Consultant findings: Brett Cunningham of Jones Edmonds said the team modeled several alternatives. Small‑scale restoration of a historic lake footprint (about 0.8 acres) or modest expansions of storage (19 acres) would produce little to no reduction in peak flood stages because the downstream system is driven by very large watershed inflows and by “tailwater” effects from Lake Munson and Munson Slough. A much larger storage alternative (roughly 54 acres) would modestly reduce stages but would be costly and difficult to permit; consultants cautioned that large storage expansion would inundate wetlands and provide limited reduction for home flooding.
Conveyance and trash‑capture options: Consultants said that alternatives that enlarge or reconfigure the slough conveyance could reduce peak stages in the immediate Silver Lake area but would transfer higher stages downstream — creating new impacts in other neighborhoods. The city is moving forward with a recommended trash‑trap improvement (30% design complete) to reduce debris and trash that can block conveyances and worsen flooding.
Cahoon told commissioners most of the flood‑prone parcels south of Bragg Drive are already preserved or owned by the city, by conservation easement or public ownership, and that built homes in low‑lying areas remain at some flood risk by geography and hydrology.
Ending: The commission and staff agreed further work is needed to refine options that avoid moving flood risk downstream; staff said they will provide cost estimates and permitting assessments for the recommended trash‑trap project and a package of conveyance upgrades.
