Sandy River: LiDAR Project
In September 2012, Watershed Sciences, Inc. (WSI) (Corvallis, Oregon) and Dewberry (Tampa, Florida) collected airborne topo-bathymetric LiDAR data for the Sandy River, Oregon. Data are being used to map channel and floodplain morphology and to evaluate the effectiveness of RIEGL’s VQ-820-G hydrographic airborne scanner in a Pacific Northwest riverine environment. Contributing partners for the test project were the Oregon Department of Geology and Mineral Industries, the Federal Emergency Management Agency and Bureau of Land Management.
The Sandy River in northwestern Oregon is a shallow river which runs from Reid Glacier on Mount Hood for 56 miles to the Columbia River and flows through areas of steep terrain and dense forest. The nature of the river makes it challenging for traditional transect or boat-based bathymetric surveys. The RIEGL VQ-820-G sensor was selected for its high pulse repetition rate, short pulse width, and narrow beam divergence.
The VQ-820-G was mounted in WSI’s Cessna Caravan and flown at 600 meters above ground level with 50% overlap between flight lines. With a flying speed of 105 knots, 7,000 acres of topo-bathymetric surfaces were mapped in less than 6 hours at a pulse density of 6 points per square meter. Analysis involved post processing and further line-to-line calibration to match collected ground survey data. Laser returns from water were corrected for refraction using software developed by Dewberry that utilized an extracted water surface. A depth model was calculated along with a confidence layer to assess depth penetration, coverage, and accuracy.
The clarity in the Sandy River was measured at a 2 meter Secchi depth on the day of the survey. With RIEGL advertising a depth finding range of 1 Secchi depth, it was expected that the majority of the river channel would be mapped. The mapped area was divided into high and low confidence categories based on density of bathymetric returns. High confidence areas accounted for 83% of the mapped channel and had a vertical accuracy of 0.18 meter RMSE when compared against channel cross section data. However, the accuracy of ground data collection was hindered by difficult wading conditions and GPS masking (terrestrial returns had a 0.03 meter RMSE).
Overall, the project produced encouraging results for the utilization of small footprint, topo-bathymetric LiDAR for mapping channel floodplains and shallow water river systems. The VQ-820-G successfully mapped the shallow river system including side channel and off-channel areas that are notoriously difficult to measure using conventional techniques. WSI and Dewberry have received strong positive feedback from project partners about the results of the survey and the future opportunities of airborne hydrographic scanning.
Report submitted with thanks by WatersWatershed Sciences, Inc. (WSI)