A New Method for In Situ Measurement of the Erosion Threshold of River Channels

The vast majority of alluvial deposits have some degree of cohesion, typically due to the presence of clays and/or organic matter. Determining the threshold fluid shear stress, τ c , necessary to entrain these sediments is essential for predicting erosion rates and morphodynamics of rivers, tidal channels, and coasts. Cohesive sediments present a greater challenge than noncohesive sand and gravel beds due to the sensitivity of τ c to such factors as compaction, aggregation, and particle surface chemistry. All of those factors may be altered if bed and bank sediments are extracted for later analysis in the laboratory. Environments with mixed cohesive and noncohesive materials are common, such as sand‐bedded rivers with muddy banks; it is therefore desirable to have a method for in situ measurement of τ c across a very wide range. We present a novel instrument that is capable of reproducibly determining τ c for submerged cohesive and noncohesive sediments in situ. The instrument has several advantages over alternative methods, including ease of implementation in the field, and a fluid shear that is more representative of natural flows. The device incorporates common and low‐cost components integrated with an Arduino microcontroller, which may receive commands from a mobile phone. We demonstrate our instrument's capabilities in gravel, sand, and clay‐sand mixtures prepared in the laboratory, and present a proof of concept field deployment in a wadeable stream. Additionally, we provide the necessary schematics, parts lists, code, and calibration procedures for the interested reader to build their own version of the instrument.