DYNAMICS OF RIVER-GROUNDWATER EXCHANGE DRIVEN BY LARGE WOODY DEBRIS: NUMERICAL AND LABORATORY FLUME EXPERIMENTS

Wood debris is a natural component of rivers. By obstructing channel flow, wood debris can induce a large zone of river-groundwater mixing. Using laboratory flume experiments and coupled turbulent channel (CFD)-groundwater flow simulations, we quantify river-groundwater exchange driven by the flow of river water around channel-spanning logs. In flume experiments, dye streaks in the sediment delineate a deep zone of river-groundwater exchange beneath channel-spanning logs. River water downwells into the bed upstream of the log and upwells downstream. These flow paths are consistent with streamlines from numerical simulations. Combined numerical and flume experiments show that exchange increases with log blockage ratio (the ratio of log diameter to channel flow depth), sediment permeability, and river velocity. Exchange decreases with gap ratio (the ratio of vertical distance between log and riverbed to channel flow depth). The combined data will be used to develop a quantitative model for predicting river-groundwater exchange due to channel-spanning logs. Model applications include quantifying the loss of river-groundwater exchange associated with historic wood removal practices and optimizing the response of river-groundwater exchange to wood reintroduction projects.