GEOMORPHIC RESPONSE OF A RECENTLY BURNED WATERSHED TO A WINTER RAINSTORM MEASURED USING TERRESTRIAL LASER SCANNING

Geomorphic processes are often controlled by boundary conditions that exist at various spatial and temporal scales, complicating the measurement of event-based changes in natural environments. For example, erosion and sediment transport are known to be significantly higher from recently burned watersheds than from unburned ones. However, the nature and degree of the influence exerted by the shape of natural surfaces on the patterns of erosion and deposition are inadequately conceptualized. Advances in terrestrial laser scanning (TLS) provide the ability to produce high resolution digital elevation models and measure changes in surface topography at multiple spatial scales. This information can be used to improve modeling and prediction of sediment transport processes. Here, we present the changes in pre- and post-rainstorm topography to assess the relationship between surface form and sediment transport processes in a sub-catchment of Wylie Canyon, a recently burned watershed near Santa Barbara, CA.

Two surveys were conducted: 28-30 September 2008 to document pre-rainfall conditions, and 18-21 December 2008, three days after 52mm of rainfall over a period of 22 hours. A Leica Geosystems ScanStation 2 TLS was used to generate 1 cm resolution DEMs, from which surface elevation changes were derived. Sediment transport was measured at three different spatial scales. At the coarsest scale, erosion rates were calculated for the entire catchment. Intermediate scale analysis compared nine sub-basins within the study area. The finest analysis scale assessed change at each point along the length of 35 flow paths extending from the drainage divide to the path confluences in the main channel. For the catchment, there was a net loss of 132 m³ of material which resulted in an average lowering of 1.2 cm/ m². At the sub-catchment scale, erosion depths showed positive correlations with gradient, surface roughness, and flow length. We identified complex interactions along flowpaths between sediment transport processes and contributing area, upstream gradient and local roughness. Relations between surface form and sediment transport were not consistent across spatial scales, suggesting that scalar dependencies are critical considerations for modeling and predicting sediment transport in recently burned watersheds.