Defining and characterizing the phenomenon of river-bottom tearing scour (RBTS): A case study of the Middle Yellow River

River-bottom tearing scour (RBTS) in natural rivers refers to the incipient motion and transport processes of clay blocks formed by fine cohesive sediment after deposition and consolidation in riverbeds. The RBTS phenomenon can lead to significant channel erosion and changes in river planview morphology, and has, thus, attracted the attention of hydrologists and engineers. In the study, a new determination measure (K) for the occurrence of a RBTS event is derived based on the theoretical expression for the critical incipient velocity of the clay block, $K={\alpha }^2\left(C_L+2{\lambda }_2^2\right)$, which is a combination of the coefficients of the velocity (i.e., $\alpha$ is a local velocity coefficient), lift force (i.e., C_L is the lift coefficient), and clay block size (i.e., ${\lambda }_2$ is the ratio of block thickness to length). Furthermore, to explain river behavior during a RBTS event, the Fugu, Xiaobeiganliu, and Weihe river reaches in the Middle Yellow River (MYR) were selected as study areas. Analysis of hydrological data from 1950 to 2023 in the examined reaches implies that a single discharge or sediment threshold cannot predict the occurrence of RBTS. The cross-sectional erosion and deposition volume (CEDV) and the migration intensity of the channel thalweg (MI) also were calculated at the section and reach scales for the examined reaches during the RBTS events. It was observed that RBTS typically causes significant channel erosion originating from the most upstream portion of the study reach with CEDV values ranging from 61 to 6034 m², while MI values during the RBTS events were close to the multi-year average for the study reach. Finally, a flume experiment simulating RBTS was done to discuss and verify the threshold value of K. Field survey and experimental results indicated that RBTS occurs if K > 0.5, thus, providing theoretical support for the prediction and prevention of RBTS.