Quantifying channel-floodplain connectivity in the Trinity River, TX using numerical modeling and field observations

Rahman, Shazzadur, M.S. in Engineering
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River-floodplain systems play a fundamental role in fluvial ecosystem preservation, landscape evolution, and for building resilience against natural and anthropogenic modifications. Although floodplains are considered as relatively flat and featureless, high resolution imagery reveal that they are remarkably complex. This realization has led researchers to hypothesize that surface water connectivity can be observed between rivers and their floodplains over a range of discharges. This is contrary to the common belief that floodplains are only connected to the rivers via overbank flow during flooding events. This view of surface water connectivity calls for a reassessment of the physical processes in river-floodplain systems and their implications to landscape evolution. In this study, we focus on the quantification of channel-floodplain connectivity by studying a 23 km long sinuous reach of the lower Trinity River in Texas. This reach transitions from a quasi-normal zone upstream to a backwater-dominated zone downstream. We combine field observations and numerical modeling. High resolution lidar and bathymetric data are used to construct a 2D hydrodynamic model using ANUGA. We use the hydrodynamic model outputs to calculate channel-floodplain connectivity metrics - lateral exchange and residence time. Our initial results show that bidirectional channel-floodplain connectivity exists for a range of river discharges. We also find that this hydrological connectivity is primarily affected by discharge and floodplain vegetation in the form of hydraulic roughness. Sensitivity analyses show that residence time is more sensitive to changes in roughness whereas lateral exchange is more sensitive to changes in discharge