CAN WE SEE THE FOREST FLOOR THROUGH THE TREES? QUANTIFYING SEDIMENT STORAGE VOLUMES FROM REMOTE SENSING AND STREAMBED MAPPING

A critical component of measuring long-term fluvial sediment fluxes is determining volumes of sediment stored in valley-bottom landforms, especially at major process transitions, such as the transition between dominance of sediment flux by debris flows and fluvial transport. Determination of stored sediment volumes from valley cross-sections and longitudinal channel profiles measured in the field is time-consuming due to dense understory vegetation, and therefore spatial resolution of cross-sections is limited by time available for field work. A novel technique is developed to quickly measure valley-bottom sediment volumes using LiDAR-derived digital elevation models (DEMs) and field-determined bedrock channel locations. LiDAR and channel-bottom bedrock location data were collected in portions of two Oregon Coast Range basins (Knowles Creek and Wassen Creek). Bedrock surfaces below stored sediments are inferred by linear interpolation between bedrock points and assumption of horizontal bedrock cross-sections perpendicular to valley centerlines. Bedrock surfaces are bounded on valley sides by adjacent hillslopes, which approximate bedrock side slopes because soils are generally thin. Bedrock surfaces are then subtracted from the DEMs to determine stored sediment volumes in 1-m-by-1-m cells composing the valley bottoms. Results are compared with field-measured sediment volumes.