HIGH RESOLUTION CHANNEL SURVEYING TO EVALUATE THE HYDRAULIC AND EROSIONAL CHARACTERISTICS OF A BEDROCK KNICKPOINT IN AN APPALACHIAN RIVER

We use conventional surveying and low-altitude balloon aerial photography to document the channel morphology of a water gap on the Maury River, Virginia, where the slope is approximately 8 times greater than other gaps in similar rock types. Stereo images with a resolution of several centimeters are used with ground control points from laser total station surveying to make channel topography for input to hydraulic models, which are further constrained by high water indicators and field photos. The aerial photos also provide a platform from which to document the bedrock geometry, clast size and orientation, erosional mechanisms, and degree of bedrock exposure. The images are sufficient in detail to allow the calculation of channel changes at an annual to decadal scale, given that flows of sufficient magnitude warrant repeat photography. However, observations following a 4 to 5-year flood that occurred during surveying generally show a lack of bedrock scour and limited transport of median and larger size clasts. The morphology of this water gap is formed therefore by low frequency events. Bedrock reaches form at the head of separate steps or knickpoints in the water gap, and scour in these reaches is strongly conditioned by boulder accumulations downstream. Enormous clasts in these “rock gardens” can deflect flow across alluvial channel margins and hence, limit bedrock erosion. Although erosion at this location is likely heightened because of a translating knickpoint, the channel morphology creates increased roughness that mitigates the effective stream power in this steep, narrow channel.