Bidirectional River‐Floodplain Connectivity During Combined Pluvial‐Fluvial Events

Abstract

Hydrologic connectivity controls the lateral exchange of water, solids, and solutes between rivers and floodplains, and is critical to ecosystem function, water treatment, flood attenuation, and geomorphic processes. This connectivity has been well‐studied, typically through the lens of fluvial flooding. In regions prone to heavy rainfall, the timing and magnitude of lateral exchange may be altered by pluvial flooding on the floodplain. We collected measurements of flow depth and velocity in the Trinity River floodplain in coastal Texas (USA) during Tropical Storm Imelda (2019), which produced up to 75 cm of rainfall locally. We developed a two‐dimensional hydrodynamic model at high resolution for a section of the Trinity River floodplain inspired by the compound flooding of Imelda. We then employed Lagrangian particle routing to quantify how residence times and particle velocities changed as flooding shifted from rainfall‐driven to river‐driven. Results show that heavy rainfall initiated lateral exchange before river discharge reached flood levels. The presence of rainwater also reduced floodplain storage, causing river water to be confined to a narrow corridor on the floodplain, while rainwater residence times were increased from the effect of high river flow. Finally, we analyzed the role of floodplain channels in facilitating surface‐water connectivity by varying model resolution in the floodplain. While the resolution of floodplain channels was important locally, it did not affect as much the overall floodplain behavior. This study demonstrates the complexity of floodplain hydrodynamics under conditions of heavy rainfall, with implications for sediment deposition and nutrient removal during floods.

Floodplain Surface‐Water Circulation Dynamics: Congaree River, South Carolina, USA

“…For example, the flow reversals of Figure 2 could not be reproduced in model configurations using the steady‐ or quasi‐steady boundary conditions of Tull et al. (2022), Czuba et al. (2019), and Meitzen (2011).…”

“…Furthermore, sensitivity testing revealed that a realistic and unsteady modeling approach using boundary conditions developed from observations is necessary to accurately represent complex floodplain flow processes. For example, the flow reversals of Figure 2 could not be reproduced in model configurations using the steady-or quasi-steady boundary conditions of Tull et al (2022), Czuba et al (2019), andMeitzen (2011).…”

“…Similarly, Tull et al. (2022) used quasi‐steady boundary conditions to demonstrate that during sub‐bankfull river stages complex floodplain flows can result due to prior flooding. This approach improves on the work by Czuba et al.…”

“…If constrained with suitable field data, model results can provide near-realistic predictions of flows with high spatial and temporal resolution, which allow for hydrodynamic assessments across the model domain. Recent floodplain modeling efforts provide insight into floodplain flows, but the model constructs lack sufficient calibration to account for realistic, time-varying flow conditions (e.g., Czuba et al, 2019;Pinel et al, 2020;Tull et al, 2022). For example, Czuba et al (2019) highlight how floodplain channels affect hydraulic connectivity under steady boundary conditions.…”

“…This approach precludes the assessment of inundation dynamics of floods with highly variable stages. Similarly, Tull et al (2022) used quasi-steady boundary conditions to demonstrate that during sub-bankfull river stages complex floodplain flows can result due to prior flooding. This approach improves on the work by Czuba et al (2019) but it provides limited insight on flow processes because the boundary conditions fail to represent actual drivers of system dynamics, hence advances in floodplain science through numerical modeling require the application of unsteady forcing conditions.…”