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. We apply structure from motion (SfM) photogrammetry with imagery from anunmanned aerial vehicle (UAV) to measure bank erosion processes along amid-sized river reach. This technique offers a unique set of characteristicscompared to previously used methods to monitor banks, such as high resolutionand relatively fast deployment in the field. We analyse the retreat of a1.2 km restored bank of the Meuse River which has complex vertical scarpslaying on a straight reach, features that present specific challenges to theUAV-SfM application. We surveyed eight times within a year with a simpleapproach, combining different photograph perspectives and overlaps toidentify an effective UAV flight. The accuracy of the digital surface models(DSMs) was evaluated with real-time kinematic (RTK) GPS points and airbornelaser scanning of the whole reach. An oblique perspective with eight photooverlaps and 20 m of cross-sectional ground-control point distribution wassufficient to achieve the relative precision to observation distance of ∼1  : 1400 and 3 cm root mean square error (RMSE), complying with therequired accuracy. A complementary nadiral view increased coverage behindbank toe vegetation. Sequential DSMs captured signatures of the erosion cyclesuch as mass failures, slump-block deposition, and bank undermining. AlthoughUAV-SfM requires low water levels and banks without dense vegetation as manyother techniques, it is a fast-in-the-field alternative to survey reach-scaleriverbanks in sufficient resolution and accuracy to quantify bank retreat andidentify morphological features of the bank failure and erosion processes.Improvements to the adopted approach are recommended to achieve higheraccuracies.

Bank erosion processes measured with UAV-SfM along complex banklines of a straight mid-sized river reach