VIRTUALLY REAL: MONITORING CUTBANK EROSION MECHANISMS WITH SFM IN BUTTERNUT CREEK, NEW YORK

Several natural processes erode stream cutbanks including bank collapse, fluid bank shear stress during high flows, burrowing and bioturbation, and freeze-thaw events. Which of these contribute most to meander migration rate? Butternut Creek in upstate New York provides a case study for unraveling this question. Migration rates were determined for several meander bends based on aerial imagery. Cross correlation between migration rate and the number of exceedance days for discharge reveals a vaguely inverse relation, suggesting that higher flows stymie migration. One might hypothesize that higher flows enhance bank shear stress which removes a more uniform thickness from the bank, causing a reduction in bank collapse frequency, whereas lower flows may tend to undercut more effectively and enhance bank collapse. To identify and quantify the bank erosion mechanisms for Butternut Creek, air photos captured at low altitudes using a drone were collected. However, this view from above did not capture sufficient detail to elucidate cutbank erosion mechanisms. Surveys in three successive years with overlapping photographs of the stream banks were taken from a canoe and from the ground. PhotoScan, a structure-from-motion software by Agisoft, was used to construct three dimensional object models of the stream banks.Temporal changes were mapped by computing the distance between scaled, aligned point clouds using CloudCompare, an open source software designed to detect nonuniform changes in 3D object models. The processing steps and initial results will be presented, and the utility of highly detailed (0.005 to 0.010 m/pixel) process studies will be discussed.