Large Wood Retention in Forested River Corridors During Overbank Flood Events: Insights From Flume Experiments

Understanding large wood (LW) dynamics on floodplains is important for closing wood budgets and predicting hazards associated with LW. We conducted 38 experiments in a fixed-bed flume to examine how forest stand density (FSD), wood transport regime, and overbank flood magnitude affect floodplain LW retention. We found that during low flood experiments at the lowest FSD, most wood pieces were transported via rolling/sliding in the floodplain and a high percentage of wood pieces were retained on the floodplain (38%). Increasing FSDs resulted in deeper floodplain flow. During low flood experiments, this resulted in a transition to floating transport, and we observed lower wood retention on the floodplain at high FSDs (17%). We interpreted these results as indicating that more wood is retained on the floodplain when wood is being transported in contact with the floodplain surface versus floating. During high flood experiments, when wood pieces were floating at all FSDs, floodplain retention increased from low to intermediate FSDs (19%–38%) but then declined from intermediate to high FSDs (38%–22%). Across all experiments, denser forests resulted in more in-channel retention. From these findings, we interpreted that when wood is floating, denser floodplain forests can help trap wood, but if too dense, it can impede access to the floodplain. We found little difference in wood retention between uncongested and semi-congested wood transport regimes. Our findings can inform river management efforts to increase wood loads in river systems and underscore the importance of viewing floodplains and channels as integrated corridors.