Scalable Flux Metrics at the Channel-Floodplain Interface as Indicators of Lateral Surface Connectivity During Flood Events

Abstract

Lateral surface connectivity (LSC), defined here as the surficial component of hydrologic exchange flows perpendicular to longitudinal flow, is vital to biogeochemical, geomorphological, and ecological processes on floodplains. Because rivers throughout much of the world have been subjected to anthropogenic manipulation, LSC has been greatly altered. While qualitative descriptions of LSC exist, only a limited number of studies quantify this process. The objective of this study was to quantify mass and momentum flux as proxies for LSC. This objective was met through the analysis of hydrodynamic model outputs along a 10-km reach of the Middle Rio Grande near Albuquerque, New Mexico, USA. Mass and momentum fluxes were quantified for two synthetic floods across three spatial scales: geomorphic sub-units, units, and reaches. In addition, three floodplain surface elevations were defined: inset, restored, and historical. Mass and momentum fluxes displayed distinct hydrodynamic signatures at each floodplain surface elevation. Inset floodplains were subjected to the greatest and most widely distributed flux magnitudes, an indication of the anthropogenic alteration along the reach. Flux heterogeneity on inset surfaces was also greatest, and increased flood magnitude did not result in comparable heterogeneity at higher floodplain elevations. Unsteady results showed that peak flux magnitudes can occur on the rising and falling limbs of a flood hydrograph, providing evidence of hysteresis in channel-floodplain connections. Ultimately, the results are indicative of complexities associated with LSC processes along geomorphologically altered channel-floodplain systems. The scalable methods are applicable to a wide variety of river-floodplain systems and are complementary to empirical flux measurements.