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Large Wood Deposition in Forested River Corridors During Overbank Floods: Insights from Flume Experiments

Large wood (LW) in the river corridor (channel/s and surrounding floodplain) influences hydraulics, sediment deposition and erosion pattern, provides habitat and nutrients for biota, and influences local soil moisture. However, LW can cause damage and exacerbate flooding when it is transported downstream and deposited on bridge piers and on the spillways of dams. Our ability to predict when and where LW will be deposited vs. transported downstream during large, overbank floods is limited by our incomplete understanding of the complex interactions between hydraulics, floodplain vegetation, geomorphology and LW. In this work, we conducted a series of flume experiments at St. Anthony Falls Laboratory to quantitatively describe the effect of forest stand density (FSD), flood magnitude, and transport regime (how much wood is in transport at one time) on LW deposition quantity and pattern across the river corridor. We conducted a total of 36 experimental trials across four FSDs, two flood magnitudes, and two transport regimes. We quantified the deposition quantity for each trial by calculating the retention capacity (volume of wood deposited / volume of wood added to flume) for the entire flume, the floodplain only, and the channel only. We quantified the deposition pattern of each experiment by calculating the proportion of total LW deposited that was on the floodplain, the number of jams, and jam characteristics such as size and proportion of volume on the floodplain. Preliminary results suggest that LW deposition is primarily affected by a complex interaction between FSD and flood magnitude, while transport regime showed little effect. These results represent a step towards improving our understanding of LW behavior in river corridors and our ability to predict when and where wood will be deposited vs. conveyed downstream. Ultimately, results from these flume experiments aim to support river managers who want to leverage wood for ecological benefits while minimizing the risks associated with its movement.