Improving flood preparedness using hydrodynamic levee‐breach and inundation modelling: M iddle M ississippi R iver, USA

There is growing concern in the United States over the ‘residual’ flood risk behind leveed floodplains, which is commonly assessed based on one‐dimensional (1D) river flow models. These models fail to capture the complex flow dynamics that typically characterise floodplain inundation. Here, we demonstrate methodologies for mechanistically simulating a levee breach and flow into a levee district along the Middle Mississippi River using a hybrid 1D/two‐dimensional (1D2D) flow model that combines a 1D model of the river channel with a two‐dimensional (2D) overland‐flow model for the leveed floodplain. This 2D modelling documents the complexity of flow over a large and topographically diverse leveed floodplain, including distribution and timing of inundation as well as the resulting public‐safety threats. In relation to safety threats, three distinct flood phases can be recognised: (1) a flash flood phase, (2) a flow redistribution phase and (3) an equilibrium phase. The flash flood phase presents imminent threats to human life and safety, whereas subsequent phases determine inundation areas and depths, and therefore infrastructure damages. 2D modelling of levee‐breach flooding provides input data necessary for: (1) specific local evacuation and mitigation planning in the Metro East area, and (2) methodologies for analyses of residual risk facing other levee‐protected floodplain areas.