Modeling shows localized water‑level shifts if diversion dike is rebuilt, consultant says

Kent Johnson of Banner Associates described the hydraulic analysis Banner used to estimate how a reconstructed dike would change water levels in the diversion channel and the Big Sioux River.

"It's a 2‑D, HEC‑RAS model," Johnson said, explaining the model combined local survey data and lidar elevation data and used a 7‑day growing‑season wetland flow determination recommended by NRCS guidance (National Engineering Handbook). He said the model showed an immediate, localized water‑level decrease where the proposed structure would be placed — for example, a 3.5‑acre area showing about a 0.6‑foot drop — and broader areas with smaller drops, alongside mapped areas adjacent to the Big Sioux that would see small increases in water level in median flows.

Johnson emphasized that the project is designed to return the majority of normal flow to the main channel while allowing overtopping during high flows: "It's not going to impact the hundred‑year flood plain — the dike overtops during those high flow events," he said. Banner's figures included mapped acreages of modest increases (some areas up to 0.6 feet) and decreases (0.1–0.6 feet) under the modeled median growing‑season flow. The consultants noted that during extreme flood events the dike would be overtopped and the overall floodplain dynamics would remain controlled by larger events rather than the dike itself.

The modeling step prompted residents and some commissioners to ask whether the data — portions of the model dating to 2021 — reflected current channel conditions and recent changes such as sedimentation and land‑use change. Johnson responded that the team can update survey inputs, but also noted the model captures relative changes under the selected flow metric used for wetland/hydrologic assessment rather than storm‑frequency design for very large floods.