Modeling Floodplain Inundation, Circulation and Residence Time Under Changing Tide and Sea-Levels

Sea-level rise will have unknown effects on the structure and function of valuable tidal freshwater floodplains. One reason for this knowledge gap is our poor constraint on the physical controls on complex floodplain inundation and circulation processes. Here, a high-resolution light detection and ranging (lidar) digital elevation model (DEM) is applied to fine-scale numerical simulations of flow and tracer exchange in a 0.43 km² river floodplain in Southeast Florida, USA. The sequence of inundation and associated circulation patterns is assessed at 1-hour intervals of the rising and falling tide in the context of floodplain geomorphic structure. The depth averaged velocity vectors show concomitant flow divergence and convergence over small spatial scales, and this complexity arises from the submergence and emergence of subtle floodplain topography over the tidal cycle. Tracer exchange and associated residence times highlight the controls of floodplain topography on water storage at the end of the ebb cycle, or during low river stages. The effects of a 0.2 m and 0.5 m increase in mean sea-level on inundation extent and water retention times were also assessed. Percent change in inundated area and associated e-folding times reveal greater lateral inundation extent and a 20% increase in water retention times with up to a 0.5 m increase in mean sea-level. This work reveals the topographic influence on how, when, and where sea-level rise will impact the freshwater floodplain through increased hydro period and salt-water intrusion, and the importance of evaluating floodplain restoration benefits in the context of fine-scale surface flow processes and sea-level rise.