[H-GM03] Geomorphology

[HGM03-06] Flume experiments of threshold of plucking occurrence in bedrock with fully developed joints

*Masahiro Ishizuka¹, Noritaka Endo¹ (1.Kanazawa University)

Keywords:plucking, flume experiment, tensile strength, bedrock river, bedrock erosion, numerical simulation

In bedrock rivers, the dominated mechanisms of riverbed erosion are abrasion and plucking; in the former, bedrocks are abraded by transported detrital particles, while in the latter, rock masses are detached by river flow at weakly bonded joints. Many experimental studies of abrasion have been done, and qualitative and quantitative understanding of abrasion has been developed. By contrast, plucking is recently becoming better understood through laboratory experiments, but quantitative understanding remains an important research issue. In this study, the threshold of plucking occurrence is expressed in terms of tensile strength. Before the onset of plucking, the rock mass is either loosely bound to the base or completely separated by cracks but still interlocked with its surroundings. These conditions (degree of bonding and interlock) are quite diverse, which makes it difficult to quantify the ease (or difficulty) of plucking. In this study, the loose bonded state of the rock mass at the stage of preparation for plucking was modeled using magnets in a laboratory analog experiment, and the bonded state was quantified by the threshold force (tensile strength) when detachment by tension occurs. A circulating flume of 650 cm long, 15.3 cm wide 15 cm deep 0.5 % slope was used, and the riverbed was observed while the weir at the downstream end was lowered in steps to increase the flow velocity. Plastic blocks were used as an analog material for the rock blocks to be detached by plucking. Under each condition, the tensile strength and the orientation of the bond with the base were varied, and the experiments were conducted 10 times each to check the reproducibility. Parameters such as velocity profile, bottom shear stress and shear rate at the onset of plucking were determined by calculation using numerical simulation software (CADMAS-SURF) along with some measured quantities of the flume experiments. The result showed that a small difference in tensile strength contributed significantly to the suppression of plucking occurrence. This implies that plucking in nature occurs in rock masses with little or no tensile strength. However, when hydraulic or weather conditions change largely in a short period of time, plucking may occur even in rock masses that are in a state of some tensile strength. To predict bedrock erosion due to plucking, the relation between the threshold of plucking and flow conditions should be elucidated further. For this sake, the effort to develop the method of quantitative evaluation of pre-plucking states of bedrocks is needed in the future.