Incorporating sediment non‐stationarity into ice‐affected flood‐risk projections

Abstract

Sediment deposition and ice jams can both increase river stages. Ice and sediment both reduce channel conveyance, affecting the flow‐stage relationship and flood risk. Classic hydrologic statistics systemically under‐predict flood risk in depositional rivers or reaches with frequent ice jams. However, when both processes influence not only flood stage but each other, the interacting processes can complicate flood risk assessment. The Big Rapids reach of the Muskegon River developed ice jams in at least 16 of the last 81 years and is actively aggrading. In reaches like this, contemporary flood risk analysis must consider ice impacts (often without local gage data) and future flood risk analysis must consider of the non‐stationary impacts of deposition on channel capacity and ice‐jam frequency. This work used hydraulic, ice jam, and sediment transport models to calculate contemporary, ice‐affected flood risk (without local stage data) and quantify the impact of deposition on future ice jams and flood risk. Accounting for ice jams increased the flood stage associated with the 1% annual flood by 0.5‐1.1 m. Sediment deposition increased future flood risk between 0.06 m (on the topset slope of a reservoir delta) and 2 m (at the advancing foreset slope).