Satish C. Gupta, Associate Fellow

Quantifying Riverbank Erosion Using Airborne Scanning Laser Altimetry

Riverbank sloughing along the Blue Earth River, Minnesota.

The Minnesota River flows through a relatively flat agricultural landscape, but is fed by tributaries that are incised with steep and unstable stream banks. It is not clear what proportion of the sediment and nutrient pollution in the Minnesota River is from upland erosion vis-á-vis stream bank collapse. This study evaluated the feasibility of using an airborne scanning laser to estimate riverbank erosion. Specifically, this project used a helicopter-mounted Topeye laser system to construct detailed digital elevation models of a geologically active river valley twice on an annual basis. A 56-km length of the corrider of the Blue Earth River in Minnesota was scanned April 2001 and April 2002. The database includes X, Y, Z coordinates of laser return from the river valley plus return intensity. Distance between footprints on the ground was generally between 60 and 100 cm. A bare earth model for both scans was made by stripping vegetation laser returns. One-meter-square grids were constructed for each annual scan and differenced to determine volume change over time. The sum of grid cell differences was multiplied by average bank material bulk density to estimate mass wasting. The percentage of sediment in the river sourced from bank materials was determined as the proportion of mass wasting to sediment load measured at a downstream gauging station. The percentage of sediment from bank erosion varied from 23 to 56 depending on the range of textural material that is transportable once in the river. Based on analysis of riverbank samples, 201 tons/yr of materials comes from bank erosion and slumping.