Controls on Floodplain Inundation

Abstract:

Fluvial floodplain inundation is often assumed to be the result of overbank flow during high flow conditions. However, when surveying levees or banks, one can observe breaches that facilitate the exchange of surface water during below bank-full flow conditions (‘through-bank’ flow). These breaches may be interconnected with complex channel networks, which may facilitate hydraulic connectivity throughout the floodplain. Nevertheless, only a limited number of studies have focused on these floodplain channel systems. Therefore, deeper insight into the conditions and processes of floodplain circulation can provide substantial benefits to society.

This study combines extensive data collection, consisting of time series of water levels and velocity, with numerical modeling, to develop an integrated process-based understanding of controls on fluxes onto the floodplain by through-bank and overbank flow. The study site is the Congaree River and the adjacent low-relief and low-gradient floodplain consisting of the Congaree National Park, South Carolina, USA. A natural levee, which is breached at multiple locations by both permanent and ephemeral channels locally, separates the main river and the floodplain and allows for both through-bank and overbank floodplain inundation. Simulation results show that the model is capable of simulating the dominant features in flood wave propagation and floodplain inundation observed in the field measurements under a wide range of hydrologic forcing conditions. Realistic and hypothetical simulations provide insight into floodplain inundation and emergence pathways and the role of specific elements of the floodplain channel network. An important observation in the hydrodynamic simulations is that the flux through the levee (below bank-full flow) can lead to substantial and frequent floodplain inundation. This observation has important implications for floodplain hydraulic connectivity, sediment accumulation, and nutrient cycling as statistical analyses of discharge reveal that through-bank inundations appear to be the dominant material exchange process.