THE IMPACT OF DISCONTINUITY ORIENTATION ON FLUVIAL EROSION AND EVOLUTION: MODELING VALLEY AND RIDGE BEDROCK RIVERS INSIDE A FLUME

The Appalachian Valley and Ridge province is characterized by complex bedrock discontinuity networks which impart a distinct fabric to the landscape and clearly shape fluvial network evolution. Prevailing discontinuity orientations change with time as a result of exhumation through complex Alleghanian deformation structures, creating a dynamic boundary condition to which river networks must constantly adapt. To better understand the controls on Appalachian river evolution, it is therefore necessary to investigate the impact of discontinuity orientation on fluvial erosional dynamics, such as bedrock susceptibility to plucking, knickpoint evolution, and sediment tool/cover feedbacks. To this end, we conducted a series of flume experiments in which ceramic tiles were held at a range of orientations using 3D-printed trays to simulate fluvial incision into layered, dipping sedimentary bedrock, much like the river networks throughout the Valley and Ridge that drain anti- and synclinal ridgelines. We tested horizontal, shallow downstream-dipping, and shallow upstream-dipping cases and find that bedrock susceptibility to fluvial plucking varies as a function of dip angle: horizontal bedding being the easiest to erode and shallow upstream-dipping being the most resistant to erosion. We also observed that each dip angle produced distinct bedrock bedforms and knickpoint evolution patterns, which mirror our field observations of bedrock rivers in the Valley and Ridge of SW Virginia, and that the rate of downstream transport of eroded armoring material controlled total erosion rate more strongly than dip orientation. These results highlight the significance of the Valley and Ridge Province’s complex discontinuity networks in the evolution of Appalachian river systems and the landscape as a whole. Future work will further investigate this control using numerical modeling to simulate larger-scale river network evolution under varying discontinuity orientations.