FIELD AND FLUME EXPLORATION OF CHANNEL BEDROCK EROSION BY PLUCKING

Two dominant erosional processes in bedrock rivers are thought to be abrasion and plucking. Plucking, or quarrying, is the erosional process in which loosened blocks of bedrock are lifted out of the riverbed and transported downstream during high flow events. This study examines the erosion process at bedrock steps of multiple settings where plucking is important in bedrock channel incision, as well as how plucking influences and connects to river profile evolution upstream and downstream of the step. HEC-RAS modeling of bedrock steps using channel survey data collected from the James River and the Cowpasture River in Virginia establishes flood-flow conditions that estimate the stream power available for plucking. At each step, plucked blocks were measured to determine the frequency of the flow conditions needed to create lift and shear forces great enough to move the blocks. However, mean flow conditions poorly quantify the plucking process by local flow properties due to the complexity of flow over a bedrock step. Therefore, flow structure and initiation of motion were visualized using a 15 cm-wide x 2.4 m-long plexiglass flume to improve the understanding of the process.

In both rivers, resistant sandstone beds of varying thickness create the river steps that contain evidence of both plucking and abrasion across the channel. The nature of the plucking action differs depending on the dip of the beds relative to river flow. Abrasion appears to erode the rock more frequently, when the river is actively carrying sediment and especially in the channel corridors where rounded clasts are more abundant on the bed. Plucking is more episodic, only occurring during large flooding events, as suggested by freshly plucked blocks of sandstone with scour marks on their pre-plucking lee-side as well as plucked clasts that become pot-holed and fluted in the less competent reach downstream of the step. Large scour holes downstream of each step interact with the step to create powerful eddies during high-flow events.