4:00 PM - 4:15 PM
[HGG03-09] Study on topographical changes to the landscape caused by heavy rainfall events.
-A case study of the artificially maintained Oirase Stream
Keywords:heavy rain, landrom change, caldera
The Oirase Stream in Aomori Prefecture is visited by many tourists every year because of its outstanding scenery. While the image of its rich natural environment is well established, many visitors are unaware that the Oirase Stream is a river whose flow is artificially regulated. The Oirase Stream is a river that flows out of Lake Towada, a caldera lake, and has a water intake near the outlet, and the period and time of discharge have been changed up to the present. However, it has been pointed out that in recent years sediment has accumulated on the river bed in places, the river width and flow path have changed and the landscape has deteriorated. The cause of this is thought to be the decrease in discharge in recent years, for which an urgent survey is required. In this study, a survey of sediment deposition in the Oirase Stream was carried out.
The original form of the Oirase Stream was eroded by the flood-related run-off of Towada-Hachinohe pyroclastic flow deposits deposited in the Towada Caldera, and it has been pointed out that the current river flow rate is small in relation to the size of the valley (Kataoka, 2011). High-precision topographic information generated from the 1 m DEM provided by the GSI can be used to decipher the topography of the gorge that characterises the Oirase Stream, the numerous waterfalls over it and the pyroclastic flow surface of the Towada Caldera origin in the upper reaches of the gorge. In the upper reaches of the waterfalls, linear valleys cut the pyroclastic flow depositional surface, forming a clear erosion front. In some places, distinct alluvial cones develop immediately below the falls, and the water from the falls flows through gullies that develop there.
During the period of high water levels, sediment deposition was checked and tuffaceous sediments including pumice were found at many locations in the upper reaches of the Oirase Stream (upstream of Kumoi no Taki). River cross section surveys and excavation surveys were carried out during the low water level period, and in areas where stagnation of the stream occurs during the high water level period, pumice, volcanic sand and tuffaceous silt deposits were found, many of which were more than 30 cm thick.
In the study area, torrential rainfall occurred in August 2022, causing flooding in the Oirase Stream. Comparison of Google Earth images before and after this period shows that many collapses occurred in the upper reaches of the falls. The overlapping of the distribution of the collapsed areas with the slope classification map indicates that the collapsed areas occurred near the breakn line at the end of the pyroclastic flow depositional surface, i.e. near the erosion front. In overlaying with the geological map, the collapse occurs at the boundary between the Kemanai pyroclastic flow deposit (AD915) and the Hachinohe pyroclastic flow deposit (15 ka), or at the boundary between the latter and the Oufudo pyroclastic flow deposit (32 ka). Based on the above facts, it is highly likely that the sediment deposition in the upper reaches of the Oirase Stream seen in recent years is the result of pyroclastic flow deposits that collapsed during the heavy rainfall in August 2022 and flowed into the rsteream.
What comments can geographers make on the Oirase Stream, which has such an ‘excellent landscape’ where flow is maintained by human management, if the landscape is damaged by very natural landform changes from a geomorphological developmental history perspective?
The original form of the Oirase Stream was eroded by the flood-related run-off of Towada-Hachinohe pyroclastic flow deposits deposited in the Towada Caldera, and it has been pointed out that the current river flow rate is small in relation to the size of the valley (Kataoka, 2011). High-precision topographic information generated from the 1 m DEM provided by the GSI can be used to decipher the topography of the gorge that characterises the Oirase Stream, the numerous waterfalls over it and the pyroclastic flow surface of the Towada Caldera origin in the upper reaches of the gorge. In the upper reaches of the waterfalls, linear valleys cut the pyroclastic flow depositional surface, forming a clear erosion front. In some places, distinct alluvial cones develop immediately below the falls, and the water from the falls flows through gullies that develop there.
During the period of high water levels, sediment deposition was checked and tuffaceous sediments including pumice were found at many locations in the upper reaches of the Oirase Stream (upstream of Kumoi no Taki). River cross section surveys and excavation surveys were carried out during the low water level period, and in areas where stagnation of the stream occurs during the high water level period, pumice, volcanic sand and tuffaceous silt deposits were found, many of which were more than 30 cm thick.
In the study area, torrential rainfall occurred in August 2022, causing flooding in the Oirase Stream. Comparison of Google Earth images before and after this period shows that many collapses occurred in the upper reaches of the falls. The overlapping of the distribution of the collapsed areas with the slope classification map indicates that the collapsed areas occurred near the breakn line at the end of the pyroclastic flow depositional surface, i.e. near the erosion front. In overlaying with the geological map, the collapse occurs at the boundary between the Kemanai pyroclastic flow deposit (AD915) and the Hachinohe pyroclastic flow deposit (15 ka), or at the boundary between the latter and the Oufudo pyroclastic flow deposit (32 ka). Based on the above facts, it is highly likely that the sediment deposition in the upper reaches of the Oirase Stream seen in recent years is the result of pyroclastic flow deposits that collapsed during the heavy rainfall in August 2022 and flowed into the rsteream.
What comments can geographers make on the Oirase Stream, which has such an ‘excellent landscape’ where flow is maintained by human management, if the landscape is damaged by very natural landform changes from a geomorphological developmental history perspective?