128-28 Validating a Numerical Model of Large Wood Transport in Forested River Corridors

Abstract:

Large wood (LW; >10 cm in diameter and >1 m in length) in river corridors (channels and adjacent floodplains) shapes hydraulic conditions, sediment dynamics, nutrient cycling, and habitat availability for diverse species. Thus, to fully understand how a river system functions, and how it will change through time, we must be able to predict why, when, and how LW is stored and transported.

We assessed the ability of IberWood, an existing 2D numerical model which couples hydraulics, sediment, and LW transport, to reproduce LW transport behavior in forested river corridors. We compared numerical simulations with results from physical experiments conducted in a 1:20 scale fixed bed flume with a sinuous channel inset into a planar floodplain. The flume contained vertically fixed dowels in a grid pattern to represent floodplain trees, and we varied the grid spacing across experiments to explore the role of forest stand density (FSD) on LW depositional patterns. We applied the IberWood model to reproduce the physical experiments, representing floodplain trees as holes in the computational mesh. We used flow depth and velocity measurements from the physical experiments to calibrate the modelled surface roughness parameters, then ran the model for multiple FSD scenarios to replicate the flume experiments. The simulated hydraulic conditions were compared with the experimental results to validate model performance. For each scenario, we compared curves for LW volume vs distance downstream and metrics such as the fraction of LW retained in the model overall and the fraction of LW retained on the floodplain only. Preliminary results show that LW accumulates against floodplain trees, consistent with flume experiments and previous field observations that identify trees as key retention sites for transported LW. However, IberWood generally underestimated LW retention for all FSD scenarios tested, aligning with previous studies, which observed IberWood underestimating LW deposition in other riverine environments. Still, the model is able to reproduce observed depositional patterns.

This work represents a step forward in understanding LW transport in complex forested environments and demonstrates the potential of IberWood as a powerful tool for exploring scenarios beyond those that can be readily tested in physical experiments. Future numerical simulations will leverage this capability to investigate how channel sinuosity, planform complexity, and forest structure influence LW transport and deposition dynamics.

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