Hydrodynamic Modeling of Flood Dynamics and Restoration Potential of Lower Missouri River Floodplains

Lower Missouri River floodplains have the potential to provide multiple ecosystem services including agricultural production, floodwater storage, nutrient processing, and provision of habitats. In this research, a 2-dimensional hydrodynamic model of a representative looped floodplain bottom of approximately 20 km is utilized to explore how floodplain inundation contributes to ecosystem benefits and costs. High resolution 2-dimensional hydrodynamic modeling provides insights into the way velocities, flood stages, residence times, and transported constituents (sediment, nutrients, and fish larvae, for example) are affected by levee geometry, floodplain vegetation patterns, and flood magnitude and duration. The utility of 2-dimensional numerical hydraulic models to represent the channel and floodplain are demonstrated at a scale relevant to understanding processes that control channel/floodplain dynamics. The sensitivity of model response to alternative land use scenarios, including levee setbacks and variable overbank roughness, is quantified using hydraulic parameters such as velocity, water level, conveyance, and residence time. The 2-dimensional models are calibrated to existing 1-dimensional modeling solutions and field measurements of water surface from 1993 and 2007 for the 2-year, 5-year, and 10-year recurrence intervals. Calibration runs with current levee configurations are matched to approximately ±0.1 meters. Simulations of alternative land use scenarios demonstrate the tradeoffs between ecological restoration and flood risk reductions. Levee setbacks with low hydraulic roughness associated with traditional row crop agriculture on the floodplains have the greatest potential for flood stage reductions, while native plant communities with higher roughness can negate the effects of the setbacks by increasing water levels due to enhanced frictional resistance. Residence times, which are presumed to be related to ecosystem services, demonstrate increasingly complex flow paths as levees are setback. Model results indicate that high end-member roughness values on connected floodplains decrease residence time relative to lower roughness characterizations, suggesting compatibility between floodplain restoration and traditional agricultural practices. The 2-dimensional representation of channels and floodplains captures the spatial heterogeneities in water levels and inundation patterns, demonstrating the importance of preferential pathways and ecological hotspots in restoration design. Spatially variable roughness patterns representing a mix of vegetation communities are modeled to explore design scenarios that optimize flood risk reduction, ecological, and agricultural objectives. Evidence of floodwave attenuation is negligible for floods of approximately the 10-year recurrence interval for all land use scenarios because of the small scale of the study domain relative to the contributing area.