[H-CG28] Earth surface processes related to deposition, erosion and sediment transport

[HCG28-06] Behavior of plucking of bedrock with fully developed joints and its sensitivity to surface roughness

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

Keywords:plucking, flume experiment, riverbed roughness, bedrock river, bedrock erosion

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 at weakly bonded joints. While much experimental researches on abrasion have been performed from decades ago, only few experimental researches on plucking have been conducted, and the effect of roughness of rock riverbeds is unknown. In this study, we observed the influence of roughness on plucking, using a physical model in a circulating flume of 12 m long, 0.4 m wide, and 0.4 m deep. The modeled riverbed was examined while increasing the average flow velocity by gradually lowering the downstream weir, with a flow rate and channel gradient constant at 30 L/s and 0.1 %, respectively. A gypsum (specific density 1.32 to 1.44 g/cm³) block was used as the analog material for the bedrock. Five conditions were tested: the blocks were arranged by differentiating the wavelength and height with regard to roughness of the bed surface. Results showed that plucking is more likely to occur for bedrocks with sufficiently developed joints if there is roughness. Plucking is more likely to occur in a block with a larger height of unevenness. This result simply means that the larger convex part receives a larger lift. In addition, however (somewhat opposite to intuition), as the wavelength of roughness decreases, the faster flow velocity is needed to make plucking occur. This result suggests that the flow modulated by the presence of the downstream concave portion reduces the lift from the flow applied to the block of the upstream convex portion on the other hand, our observation showed that plucking in a bedrock with roughness does not occur like a chain-reaction, i.e., following plucking after plucking is difficult to happen, because the detached blocks are likely to deposit in a concave potion and suppress the next plucking. Therefore, in the field, large fluctuations in hydraulic conditions and weather in a short period of time are necessary to promote plucking by breaking a temporarily stable arrangement of detached blocks, and these fluctuations are considered to control the long-term erosion rate in nature. Thus, it is likely that not only the time average value of hydraulic conditions but also the range and frequency (namely, pattern) of the fluctuation are also important.