"Evidence of Plucking at Balcony Falls on the James River, Virginia"
James Freeman, Thesis 2016
Abstract: Bedrock plucking is an effective form of erosion in rivers characterized by bedrock channels. Through the concentration of a variety of forces including turbulence, sheer stress, velocity and lift, plucking is capable of lifting enormous blocks from a river bottom and transporting them downstream. Due to the relative infrequency of plucking events, little is known about this process. This study evaluates the presence and parameters surrounding plucking at Balcony Falls on the James River, Virginia. We aim to gain a further understanding of a complex process by focusing on one component, specifically the effects of channel velocity on the erosional process. After surveying the river geometry and assessing the discharge generated during flooding events at Balcony Falls, we were able to model average channel velocity through HECRAS. These values were compared to the threshold velocity needed to initiate plucking for several observed boulders at Balcony Falls. Overall, we found that the velocity generated during the 100-year flood is insufficient to pluck blocks of the sizes we measured at the bedrock step. However, simplifications within the equations used imply that channel velocity is incapable of describing plucking alone, and other factors contribute to the process. These include river geometry, the distribution of turbulence, and the spatial variation of velocity, among others.
Full thesis available. Contact the Geology Department at 540-458-8800.