High‐resolution topography reveals that floodplains along meandering rivers in Indiana commonly contain intermittently flowing channel networks. We investigated how the presence of floodplain channels affects lateral surface‐water connectivity between a river and floodplain (specifically exchange flux and timescales of transport) as a function of flow stage in a low‐gradient river‐floodplain system. We constructed a two‐dimensional, surface‐water hydrodynamic model using Hydrologic Engineering Center's River Analysis System (HEC‐RAS) 2D along 32 km of floodplain (56 km along the river) of the East Fork White River near Seymour, Indiana, USA, using lidar elevation data and surveyed river bathymetry. The model was calibrated using land‐cover specific roughness to elevation‐discharge data from a U.S. Geological Survey gage and validated against high‐water marks, an aerial photo showing the spatial extent of floodplain inundation, and measured flow velocities. Using the model results, we analyzed the flow in the river, spatial patterns of inundation, flow pathways, river‐floodplain exchange, and water residence time on the floodplain. Our results highlight that bankfull flow is an oversimplified concept for explaining river‐floodplain connectivity because some stream banks are overtopped and major low‐lying floodplain channels are inundated roughly 19 days per year. As flow increased, inundation of floodplain channels at higher elevations dissected the floodplain, until the floodplain channels became fully inundated. Additionally, we found that river‐floodplain exchange was driven by bank height or channel orientation depending on flow conditions. We propose a conceptual model of river‐floodplain connectivity dynamics and developed metrics to analyze quantitatively complex river‐floodplain systems.
Floodplain inundation has been viewed as a type of binary process set by the relative elevation between river stage and levee crest. However, recent reports in the literature show that this perception may have limited applicability. In particular, through‐bank channels, conduits that cross the main river levees or banks, facilitate conditions for an “inundation continuum,” or inundation for a range of sub‐bankfull flows. Moreover, through‐bank channels and their networks provide a direct hydraulic connection between the main river and the floodplain interior. We analyzed through‐bank channel structure and floodplain topography and compared them to river surface elevation to provide greater insight on floodplain inundation processes. Results show that well‐developed levees with through‐bank channels facilitate frequent through‐bank inundation. Where levees are poorly developed, floodplain inundation occurs by overbank flow. Therefore, for a given discharge through‐bank and overbank inundation may occur simultaneously. For the Congaree River floodplain, we infer that this dichotomy of inundation processes leads to temporally and spatially complex inundation flow paths for a given river stage. Further, our analyses reveal that the inundation continuum concept should be considered in the context of having vertical, longitudinal, lateral, and temporal components.
more » « less- Award ID(s):
- 1751926
- NSF-PAR ID:
- 10370900
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 57
- Issue:
- 12
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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