CSDMS 2014 annual meeting - Uncertainty and Sensitivity in Surface Dynamics Modeling

Predicting the influence of floodplain vegetation on the geomorphic effects of large floods

Mariela Perignon

University of Colorado, United States

The spatial distribution of vegetation along the banks and floodplains of a river can drastically affect its geomorphic response to large floods. Plants influence sediment transport dynamics and the resulting patterns of erosion and deposition by steering the flow, changing the scale and intensity of turbulence, and increasing the effective cohesiveness of surface material. Efficiently simulating these interactions over river reaches requires simplifying the small-scale processes into measurable parameters that can reproduce the large-scale behavior of the system.

We present simulations of the evolution of the morphology of vegetated, mobile sand-bed rivers during this flows that were obtained by coupling the existing hydrodynamic model ANUGA with modules for sediment transport and vegetation. This model captures the effects of vegetation on mean flow velocity by treating plant stems as cylinders of specified diameter and spacing and calculating the drag they impart on the flow.

The outputs of this model were tested against a well-constrained natural experiment to determine the accuracy of the model predictions. Multi-temporal airborne lidar datasets capture the topographic change that occurred along a 12-km reach of the Rio Puerco, New Mexico, as a result of a large flood in 2006. The magnitude of deposition on the floodplain was found to correlate with vegetation density as well as distance from the primary sediment source. This relationship is reproduced by the model using only the simplest drag formulation. The local variability in deposit thickness was seen to depend strongly on the dominant species present, suggesting that plant-scale processes are reflected in the patch-scale behavior of the system. This indicates a need for more complex parameters that reflect the changes in turbulent energy and shear stress that result from different plant characteristics.

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Of interest for:
  • Terrestrial Working Group
  • Hydrology Focus Research Group