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 I’ve seen in the Boise River” and yields different flood footprints than the earlier 1D work. Krause and Canning showed maps indicating the Corps’ options would remove many of the shallow, low‑depth street flooding areas from the highest‑damage classification, while channelized high‑depth flow locations would remain the priority for permanent protection.

What the Corps proposed

- Structural alternative: continuous levees and flood walls along the river that Corps staff estimate at roughly $170 million in total construction costs. The Corps’ estimate includes a local cost share; the presentation cited a Garden City local share figure in the tens of millions (the Corps’ cost‑share accounting and some land‑acquisition costs are called out separately by the presenters).

- Light structural alternative: reduced length of permanent walls/levees combined with limited roadway and channel modifications. Estimated total cost and local share presented in the briefing were substantially lower than the full structural plan (presenters cited a range that placed the city’s share in the mid‑eight‑figure range depending on options).

- Ultralight structural alternative: similar to the light structural plan but substituting temporary or deployable barriers (for example, HESCO baskets, sandbags or water‑filled inflatable dams) in some locations to reduce permanent construction. Presenters stressed that temporary measures do not change FEMA flood maps or flood insurance requirements because they are not permanent measures.

- Most damages / targeted alternative: Corps staff produced a lower‑cost option that installs permanent protection only at a few high‑risk, channelized outflow locations (three levee/flood‑wall segments the Corps identified in midtown and downstream of Riverside). That alternative carries a smaller local share (presenters referenced an example Garden City cost share near $14–15 million) but would protect fewer parcels.

- Do‑nothing: includes the option of accepting the 2016 draft FEMA maps (and their higher BFEs) or pursuing ad hoc local measures.

Costs, insurance and operational limits

Krause and Canning emphasized tradeoffs: permanent structural work can change the FEMA maps and thereby potentially lower flood insurance obligations for properties that become protected; temporary measures generally reduce risk only during any given event if deployed successfully and do not alter flood insurance designations. As Krause put it during the meeting, “That is correct. Yeah. And flood insurance cost would not change,” when asked whether temporary barriers would result in lower premiums.

Presenters also discussed operational challenges for a temporary‑barrier approach: stockpiling materials (HESCO barriers fold flat but require gravel to fill), storage space, refill or one‑time use issues, the speed of deployment relative to forecast lead time, training and turnover of staff, and the cost of replenishing disposable materials after an event.

Technical benchmarks and examples

- The Corps’ 2D model and maps were produced from updated surveys and 2019 data, and presenters praised hydrologic modeling work by Corps staff and contractors. - Nick Krause identified the 100‑year flow at the Glenwood gauge as about 16,600 cubic feet per second (cfs); 2017 peak flows reached about 9,800 cfs in Garden City, and historic high flows (1983) approached 25,000 cfs in the valley.

Local next steps and decision points

Presenters and staff said the Corps is moving toward peer review and a decision about a tentatively selected plan. The Corps is looking for a city preference among the alternatives as it finalizes the integrated feasibility report. City staff indicated they want access to the Corps’ model so the city can run its own scenarios and refine options. Mayor Evans and staff said the Corps and city have a high‑level meeting scheduled the week of Feb. 13, and staff (including Canning and internal staff) anticipate returning to the council with updates in mid‑February.

Why it matters

Council members and staff repeatedly stressed the stakes: a change to FEMA‑adopted base flood elevations would affect development rules and flood insurance obligations across Garden City. The Corps’ modeling refines—but does not eliminate—the risk, and the council will need to weigh public cost share, permanence of protection, impacts to riverside vegetation and greenbelt access, and operational feasibility of any emergency deployment plan.

What the Corps study did not decide

There was no council vote on the Corps alternatives on Jan. 27. Presenters and staff recommended further technical work, access to the Corps’ model, and additional council discussion before the city commits to any option. Corps staff are constrained by federal program rules, including criteria for benefit‑to‑cost and allowable cost shares.

Ending note

City staff said they will continue to coordinate with the Corps, seek model access for local scenario analysis and return to the council with specifics about funding options, potential land or easement impacts, and refined cost estimates.