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[HCG20-05] Waterfall retreat processes due to abrasion erosion by sediment
Keywords:waterfall, retreat process, plunge pool, cyclic steps
Riverbeds are usually covered by sediment transported from upstream. In recent rivers in Japan, however, the bedrock has been exposed due to the reduction of sediment supply caused by the over-extraction of gravel, the construction of erosion control dams, and the unprecedented forest saturation. Knick points such as waterfalls are sometimes formed on the exposed bedrock. The migration rate is usually thought to be a few millimeters to a few centimeters per year. However, Cook et al. (2012, 2014) observed that a large amount of sand and gravel generated by an earthquake in Taiwan in 1999 flowed upstream of a waterfall, and the waterfall migrated several tens of meters per year. In Japan, there have also been reports of waterfalls and other transition points formed on relatively soft bedrock moving upstream at an unbelievable speed. Such rapid migration of waterfalls causes a rapid lowering of the river bed, degrading the river structures and the river environment. Therefore, studying the retreat processes of waterfalls formed on bedrock is essential.
Scheingross et al. (2017, 2019) experimentally demonstrated the spontaneous formation of a group of waterfalls on an initially flat slope. On the other hand, Inoue et al. (submitted) found from their experiments that when the initial drop of a waterfall is relatively small, a group of waterfalls forms upstream and downstream of the waterfall, but when the initial drop is somewhat high, the process is such that the upstream side of the plunge pool is eroded and the waterfall moves upstream. However, in Inoue et al.'s experiments, the bedrock model was incised deeply, so it was unclear what the incision process inside the model was like.
Therefore, in this study, we conducted similar experiments to Inoue et al.s using an experimental flume that allows observation of the interior of the waterfall. The flume used in the experiments was as follows. Assume a flume is divided into two symmetrical parts by a vertical plate along the center line of the flume. The vertical plate along the center line is a transparent acrylic plate so that the interior of the waterfall can be observed. The experiment revealed the following waterfall retreat process.
The rotation of sediment by the vortex in the plunge pool, which is formed directly downstream of the waterfall, erodes the steep portion of the waterfall. At the same time, a small step is formed on the steep part of the waterfall, and erosion proceeds in the vertical direction. The small steps develop and erode, becoming integrated with the plunge pool. The waterfall moves in the upstream direction by repeating these processes.
It is also found that when the amount of sediment supply is high, the upstream migration of the waterfall is controlled by the retreat of the plunge pool, as described above. However, when the sediment supply is low, many steps are formed upstream of the waterfall. This may be due to the following reasons. The sediment transport capacity is high on the steep portion of the waterfall due to the high gradient, while the sediment transport capacity is low upstream of the waterfall due to the low gradient. More sediment covers the riverbed, and erosion is suppressed when the sediment transport capacity is low relative to the amount of sediment supply, while less sediment covers the riverbed, and erosion is accelerated when the sediment transport capacity is high relative to the amount of sediment supply. Therefore, because the sediment transport capacity is low in the upstream reach of the waterfall, erosion is suppressed when the amount of sediment supply is high, and erosion is accelerated when the amount of sediment supply is low. This suggests that many steps were formed in the upstream reach of the waterfall when the sediment supply was low.
Scheingross et al. (2017, 2019) experimentally demonstrated the spontaneous formation of a group of waterfalls on an initially flat slope. On the other hand, Inoue et al. (submitted) found from their experiments that when the initial drop of a waterfall is relatively small, a group of waterfalls forms upstream and downstream of the waterfall, but when the initial drop is somewhat high, the process is such that the upstream side of the plunge pool is eroded and the waterfall moves upstream. However, in Inoue et al.'s experiments, the bedrock model was incised deeply, so it was unclear what the incision process inside the model was like.
Therefore, in this study, we conducted similar experiments to Inoue et al.s using an experimental flume that allows observation of the interior of the waterfall. The flume used in the experiments was as follows. Assume a flume is divided into two symmetrical parts by a vertical plate along the center line of the flume. The vertical plate along the center line is a transparent acrylic plate so that the interior of the waterfall can be observed. The experiment revealed the following waterfall retreat process.
The rotation of sediment by the vortex in the plunge pool, which is formed directly downstream of the waterfall, erodes the steep portion of the waterfall. At the same time, a small step is formed on the steep part of the waterfall, and erosion proceeds in the vertical direction. The small steps develop and erode, becoming integrated with the plunge pool. The waterfall moves in the upstream direction by repeating these processes.
It is also found that when the amount of sediment supply is high, the upstream migration of the waterfall is controlled by the retreat of the plunge pool, as described above. However, when the sediment supply is low, many steps are formed upstream of the waterfall. This may be due to the following reasons. The sediment transport capacity is high on the steep portion of the waterfall due to the high gradient, while the sediment transport capacity is low upstream of the waterfall due to the low gradient. More sediment covers the riverbed, and erosion is suppressed when the sediment transport capacity is low relative to the amount of sediment supply, while less sediment covers the riverbed, and erosion is accelerated when the sediment transport capacity is high relative to the amount of sediment supply. Therefore, because the sediment transport capacity is low in the upstream reach of the waterfall, erosion is suppressed when the amount of sediment supply is high, and erosion is accelerated when the amount of sediment supply is low. This suggests that many steps were formed in the upstream reach of the waterfall when the sediment supply was low.