UTILIZING REPEAT TERRESTRIAL LIDAR DATA TO TRACK BLUFF EROSION IN AND AFTER A MAJOR FLOOD EVENT

Bluff erosion is highly episodic and difficult to measure, yet it poses a threat to infrastructure and can have a major impact on fine-grained sediment loading to rivers. To capture bluff topography through time and determine erosional volumes, we collected high-resolution terrestrial laser scanning (TLS) data on a series of river bluffs in Duluth, MN. A Faro Focus 3D phase shift laser was used to map bluff topography at cm-scale resolution. Consecutive scans collected from 2011-2017 were compared using Wheaton’s Geomorphic Change Detection (GCD) ArcGIS plug-in and CloudCompare to determine erosional patterns and volumes. Scans collected before and after a 500-year flood event in June 2012 give an average retreat distance for all of the surveyed unvegetated bluffs of 0.41 m during a single event, with local retreat distances exceeding 2.5 m. These data are being compared with repeat ~1m resolution aerial lidar data, collected in spring 2011 and fall 2012. Retreat rates coupled with surveys of bluff area were used to estimate a total load of 2,900,000 kg of silt and clay, 4 times greater than the average fine sediment load exiting the basin in a typical year. TLS data collected in the years after the 2012 flood show that average annual bluff erosion rates remained high, at 0.18 m/yr, with slightly higher rates on natural bluffs (0.23 m/yr) and lower rates on artificially-stabilized bluffs (0.04 m/yr). Efforts to stabilize bluffs through construction of a bankfull bench have led to an increase in local deposition rates, thus holding more sediment in place and reducing sediment loads to the river, even as bluffs continue to erode. Long-term analyses of the effectiveness of bankfull bench constructions on stabilizing high stream bluffs is on-going.