Hydrodynamics in meandering compound channels with varied emergent floodplain vegetation densities: a 3D numerical modeling study
Date
2021
Authors
Brouillard, Nicolas P., author
Morrison, Ryan, advisor
Nelson, Peter, advisor
Wohl, Ellen, committee member
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Abstract
Emergent floodplain vegetation can influence the hydrodynamic interactions between floodplain and main channel flows during floods in meandering compound channels. These interactions impact the flow and boundary shear stress fields in the main channel, which govern sediment transport, channel morphodynamics, and the capacity to convey flood flows. These processes are important to sustaining aquatic habitats, understanding geomorphic change, and predicting flood severity. However, the effects of emergent floodplain vegetation density on flow phenomena in meandering compound channels are poorly understood. Therefore, this study had three objectives: 1) accurately numerically model three-dimensional (3D) flows at different relative depths (ratio of floodplain to main channel flow depths) in a meandering compound channel with a fixed rectangular main channel cross section and a smooth floodplain using data from published physical experiments, 2) use the numerical model to simulate varied emergent floodplain vegetation density conditions, and 3) analyze the effects of different emergent floodplain vegetation densities on the main channel and floodplain hydrodynamics. Specifically, the effects of floodplain vegetation conditions on primary flows, secondary flows, and boundary shear stresses in the main channel were explored. This study also looked at how floodplain vegetation density affected total discharge capacity as well as inbank and overbank layer-averaged flow patterns. Smooth floodplain, low floodplain vegetation density, and high floodplain vegetation density scenarios were modeled with uniform arrays of emergent cylinders with non-dimensional vegetation densities (portion of the control volume occupied by vegetation) of 0, 0.00946, and 0.0368, respectively, based on natural floodplain forests. These scenarios were modeled for eleven relative depths ranging from 0 to 0.80. Previous research in meandering compound channels with smooth and roughened floodplains has shown that minimum average streamwise velocities and boundary shear stresses in the main channel occur at a given threshold value of overbank relative depth. Therefore, a major focus of this research was to examine the relationships between vegetation densities, overbank relative depths, and minima in average main channel streamwise velocities and boundary shear stresses. The 3D numerical model accurately replicated the results of previously published physical experiments (objective 1) based on calibrated error metrics comparing free surface elevations and main channel streamwise velocities. Results from the calibrated numerical model show that as floodplain vegetation density increased, the initial minimum values of average main channel streamwise velocities and boundary shear stresses were lower in magnitude and occurred at greater relative depths and discharges (objectives 2 and 3). Unlike in the smooth and low vegetation density floodplain scenarios, these average main channel values generally did not increase with relative depth and discharge above the initial minimum case for the high vegetation density scenario. Furthermore, the main channel boundary shear stress field had strong gradients and had greater variations in magnitude in the vegetated floodplain scenarios compared with the smooth floodplain scenario. Additionally, increasing floodplain vegetation density greatly reduced the discharge capacity as well as the average main channel streamwise velocities and boundary shear stresses above the lowest relative depths. Finally, the character of the main channel primary and secondary flow structures as well as the inbank and overbank layer-averaged flows were also affected by floodplain vegetation density. As vegetation density increased, floodplain flows deviated further from the valley-wise direction and plunged more steeply into the main channel below the bankfull level, thus increasing interactions between inbank and overbank flow layers. The strength of separation between inbank and overbank flow layers at an imaginary bankfull level horizontal plane is believed to influence energy losses in the flow, which helps to explain trends in the flow velocity and boundary shear stress fields. In conclusion, this study illustrates why river scientists and engineers should consider the effects of floodplain vegetation density on main channel hydrodynamic processes in similar meandering compound channel systems.
Description
Rights Access
Subject
geomorphology
restoration
stream
overbank
flood
river