HISTORICAL SEDIMENT DYNAMICS IN A DEBRIS FLOW IMPACTED WATERSHED

Photogrammetric methods have proven useful for geomorphic change detection across a range of spatial and temporal scales. Cuneo Creek is a 10.8 sq km watershed within the Humboldt Redwoods State Park that was the source of highly damaging debris flows in 1955, 1964, and 1997. Three parallel photogrammetric approaches were applied to constrain the historical sediment dynamics and inform future hazard assessments.

Active channel monitoring was conducted by producing annual point clouds of hydraulic cross sections for water years 2014, 2015, and 2016 near historically surveyed cross sections. A variety surface analytics were performed including particle size distribution and cross sectional rugosity. The three water years observed in this study were a period of extreme drought, characterized by low base flows, however several large discharge events occurred within this period scouring and reorganizing the channel. Particle size coarsened in each consecutive year.

Oblique photogrammetric models were produced to render stratigraphic details of debris flow deposit exposures between 1.2 and 6 meters high. These depict inherited profile weathering, particle size distributions, and the transition between matrix and clast supported facies.

A photogrammetric point cloud was produced from archival 1965 1:20000 air photos and 2007 Lidar. Derivatives from these coverages independently confirm sediment yield estimates from historically surveyed cross sections to within 0.2% as well as extend volumetric change detection to unsurveyed portions of the watershed. Degradation of the debris flow deposit represents 19 cm of watershed wide denudation. In addition a wave of incision has propagated upstream of the 1964 depositional surface. Channel incision has averaged 10.2 meters measured at 178 nodes distributed on channels throughout the watershed. This incision event suggests mechanisms of stream capture within Cuneo as well with the adjacent Bear River Watershed.