Consultant outlines drainage analysis and possible capital projects to reduce flooding risk

West Consultants presented the city’s interior drainage analysis (IDA) developed to support levee accreditation and FEMA flood mapping. Project manager James Hyen described the approach — a 72‑hour, 100‑year rainfall event for hydrology and a two‑dimensional HEC‑RAS hydraulic model — and explained how interior drainage, pump capacity and groundwater levels combine to determine mapped inundation depths.

Hyen said the modeling showed some local opportunities for improvement but stressed the limits of a flat, low‑gradient system with a high groundwater table. He outlined an example capital improvement — adding a localized drainage channel and a new pump station in the northern UGB (estimated capital cost roughly $3.5 million) — that models indicated could reduce peak interior water surface elevations by about 0.6 feet and shrink the mapped 100‑year inundated area by roughly seven acres. Hyen advised that additional planning and targeted projects (neighborhood drainage master plans, selective pump upgrades) combined with land‑use actions would provide the best value in areas like downtown where gravity and groundwater limit pump effectiveness.

Commissioners asked about alternatives such as dredging of Alder Creek, timing of pumping operations, and how the model accounts for failing riprap and pile dikes. Hyen noted operations such as pre‑pumping can be limited by high groundwater and that some options (e.g., extensive dredging) may produce limited returns given the system’s low gradient. The city directed staff to retain consultants’ outputs for use in capital planning and potential grant applications.