Floodplain Surface-water Circulation Dynamics: Congaree River, South Carolina

Abstract

A robustly calibrated and validated hydrodynamic model depicts flow patterns over a topographically complex floodplain that includes a heterogeneous main channel levee. Simulations highlight floodplain inundation dynamics for two conditions, the passage of a sub-bankfull flood wave, and of an overbank flood wave. Sub-bankfull inundation occurs primarily via through-bank channels and local levee breaches. During overbank inundation, through-bank channels continue to act as efficient pathways that produce high velocity zones several kilometers into the floodplain. Meanwhile, near-stagnant zones develop inland, between through-bank channel mouths, where water is stored and slowly routed to the through-bank channel networks. For the overbank flood wave considered here, more than 52% of the surface water enters the study area as flow from the upstream floodplain. Simulation results reveal that hydrograph shape in the main channel, flood timing and duration collectively influence the lateral exchange of water from the river to the floodplain. Given these findings, we propose that the concept of “hydraulic connectivity” be revised to more fully account for floodplain flow by acknowledging the through-bank delivery processes, intra-floodplain flows, and flow thresholds associated with the submergence and emergence of topography. Floodplain wetting and draining significantly influence flow direction, and characteristics of the flood wave such that a single stage in the main channel does not uniquely define floodplain flow hydraulics.