Army Corps flood study offers a range of costly options to reduce Garden City flood risk

Garden City — City engineers and consultants updated the Garden City City Council on a U.S. Army Corps of Engineers general investigation (GI) flood study that used a new two‑dimensional hydraulic model of the Boise River and presented five alternatives to reduce flood risk.

The city engineer, Joe Canning, and Nick Krause, principal with QRS Consulting, told the council on Jan. 27 that the Corps’ newest 2D model is more detailed and, in their assessment, less conservative than the Corps’ 2016 draft mapping. The 2016 draft would have placed roughly 73% of Garden City in the 100‑year floodplain; the Corps’ latest model shows more localized street flooding in many areas instead of widespread property inundation.

The study’s nut graf: the Corps has produced a more precise hydraulic model and a draft integrated feasibility report that identifies several approaches to lowering flood risk. Those options range from a comprehensive structural plan of levees and walls to narrow, targeted barriers in the highest‑damage locations — and a hybrid that relies on temporary flood barriers that would require rapid city deployment during a flood event.

Most important findings and context

Joe Canning, the city engineer, said the difference between the 2003 FEMA maps and the 2016 draft maps was substantial, with base flood elevations (BFEs) rising by about 1 to 3 feet in places and greatly expanding the area inside the 100‑year floodplain. Garden City negotiated seclusion after the 2016 maps, meaning FEMA has not adopted those draft boundaries while the city pursues mitigation measures.

Nick Krause said the Corps’ 2D model (based on 2019 survey data and updated bathymetry) is “the best model…