Abstract
Abstract. We apply structure from motion (SfM) photogrammetry with imagery from an unmanned aerial vehicle (UAV) to measure bank erosion processes along a mid-sized river reach. This technique offers a unique set of characteristics compared to previously used methods to monitor banks, such as high resolution and relatively fast deployment in the field. We analyse the retreat of a 1.2 km restored bank of the Meuse River which has complex vertical scarps laying on a straight reach, features that present specific challenges to the UAV-SfM application. We surveyed eight times within a year with a simple approach, combining different photograph perspectives and overlaps to identify an effective UAV flight. The accuracy of the digital surface models (DSMs) was evaluated with real-time kinematic (RTK) GPS points and airborne laser scanning of the whole reach. An oblique perspective with eight photo overlaps and 20 m of cross-sectional ground-control point distribution was sufficient to achieve the relative precision to observation distance of ∼1 : 1400 and 3 cm root mean square error (RMSE), complying with the required accuracy. A complementary nadiral view increased coverage behind bank toe vegetation. Sequential DSMs captured signatures of the erosion cycle such as mass failures, slump-block deposition, and bank undermining. Although UAV-SfM requires low water levels and banks without dense vegetation as many other techniques, it is a fast-in-the-field alternative to survey reach-scale riverbanks in sufficient resolution and accuracy to quantify bank retreat and identify morphological features of the bank failure and erosion processes. Improvements to the adopted approach are recommended to achieve higher accuracies.
Highlights
Bank erosion is a fundamental process in morphologically active river systems, and much research has been devoted to understanding, quantifying, and modelling it from disciplines such as engineering, geomorphology, geology, and ecology
Afterwards, we show the precision of the digital surface models relative to other techniques, an analysis of the particular errors that may arise from the use of unmanned aerial vehicle (UAV)-structure from motion (SfM) to survey long riverbanks, and the features and processes identifiable from the bank profiles
This study showed the UAV-SfM capabilities to produce extensive 2.5-D digital surface models (DSMs) from a 100 m high nadiral view, which achieved 0.11 m mean error and 0.33 m root mean square error (RMSE) compared to terrestrial laser scanning (TLS)
Summary
Bank erosion is a fundamental process in morphologically active river systems, and much research has been devoted to understanding, quantifying, and modelling it from disciplines such as engineering, geomorphology, geology, and ecology. Predicting and monitoring bank erosion is necessary for sound river management strategies and important for both socio-economic problems, such as preventing material losses The techniques that identify the temporal change in vertical bank profiles detect and quantify the different phases of the erosion cycle (Thorne and Tovey, 1981). This characteristic helps distinguishing the factors influencing bank erosion and their relative role in the whole process A simple record of sequential mass failure events (see Fukuoka, 1994, for a graph of failure-driven retreat) is suf-
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