Fluvial Change Processes During an Exceptional Drought Punctuated by Atmospheric Rivers

Abstract:

Lidar-based topographic change detection (TCD) analyses are able to provide meter-scale detail over large spatial extents for understanding watershed sediment budgets, geomorphological processes, and links to ecosystem services. Most TCD analyses use a method of differencing two raster-based digital elevation models (DEMs) derived from interpolated point data. In order to maximum our understanding of topographic change processes, spatial uncertainty in the DEMs must be adequately accounted for before TCD results are tested against some hydraulic or geomorphic hypothesis. A TCD analysis was conducted from 2008 to 2014 for the ~37-km stretch of the lower Yuba River in Northern California. This time period experienced four floodway filling flow events ranging from 5.9 – 8.8 times bankfull discharge (corresponding to ~2.5-5 year recurrence intervals, respectively). The lower Yuba River provides an excellent site to study fluvial change processes as these moderate and frequent overbank flow events rework the gravels and cobbles left from a legacy of hydraulic mining. This study (1) develops a new method for characterizing DEM uncertainty by using a bootstrapping approach to create confidence intervals for each raster cell value based on the point density and surface variability (2) classifies the TCD results into fluvial change processes (e.g. channel downcutting, overbank scour, bar emergence…etc.) and (3) quantifies the sediment budgets at the segment, reach, and morphological unit scale. DEMs were created from a combination of airborne LiDAR with green and near-infrared lasers, single and multibeam sonar, and RTK-GPS surveys. Results show significant topographic change for the floodway area with a net erosional sediment regime and a slightly depositional sediment regime within the 2008 bankfull channel.