11:30 〜 11:45
[HGM02-04] 上高地における2020，2021年に豪雨イベントによって生じた地形変化
キーワード：群発地震、豪雨、土石流、河道地形変化、上高地、中部日本
The Kamikochi Valley is located in the Northern Japan Alps, central Japan. The upper Azusa River runs through the Kamikochi Valley forming braided channel with thick bouldery sediments. This area is one of the most geomorphologically active area in Japan. Yearly mapping of micro-landforms of the riverbed in the observation site in the upper Azusa River revealed that channel migrations and landform changes in the active riverbed occurs once every one or several years during a bankfull flood in the Baiu rainy season from June to July.
The recent two years, 2020 and 2021, were special period for landform changes. Intense swarm activity took place in Kamikochi Valley and surroundings from April to August 2020. Three times of heavy rainfall events occurred in June and July 2020 and August 2021. These events caused landform changes such as channel migration of the riverbed, landslides on the valley side slope and debris flows. This study aimes to discuss the characteristics of the landform changes occurred in the recent two years in the Kamikochi Valley.
The swarm activity began in April 2020. More than several hundreds of earthquakes might cause many small landslides and that seems to have made many fractures in bedrocks of valley side slopes which would be a cause of slope failure. On 14 June 2020, the automated weather station in Kamikochi (AMeDAS Kamikochi) recorded 117mm rainfall in one day. This heavy rain event caused valley head landslides in steep slope streams on the valley side slopes. These landslides originated debris flows. Some of the debris flows including huge boulders flowed directly into the Azusa River. That is the important cause of debris supply from tributaries to the Azusa River. Some debris slows stopped on the floodplain of gentle slope, which is covered with dense riparian forest. These debris transport processes did not contribute debris supply to the Azusa River at the present moment.
From 6 to 8 July 2020, more than 350mm rainfall was recorded in three days. The heavy rain event caused the bankfull discharge in the upper Azusa River. At the observation site of the riverbed of the Azusa River, where a geomorphological map of riverbed was made every year by surveying, major landform changes with channel migration and intense degradation occurred. Consequently deep channels formed. Also bank erosion occurred in other parts. The June and July events occurred during the Baiu rainy season in which slope surface deposits were saturated enough to occur landslides and debris flows. Thick sediments of the floor of the Kamikochi Valley also were saturated, therefore water level rose high enough to change riverbed landforms.
On 13 and 14 August 2021 more than 300mm rain fell in this area in two days. In the observation site channels migrated again and deep channels formed in 2020 event were buried. According to the previous studies in this observation site, after the end of the Baiu rainy season, usually the beginning of August, water level does not rise enough to move the riverbed sediments, therefore major landform change do not occur. In this case because of the extraordinary rainfall for two days bankfull flood occurred and sediment transport took place in the upper Azusa River.
The recent two years, 2020 and 2021, were special period for landform changes. Intense swarm activity took place in Kamikochi Valley and surroundings from April to August 2020. Three times of heavy rainfall events occurred in June and July 2020 and August 2021. These events caused landform changes such as channel migration of the riverbed, landslides on the valley side slope and debris flows. This study aimes to discuss the characteristics of the landform changes occurred in the recent two years in the Kamikochi Valley.
The swarm activity began in April 2020. More than several hundreds of earthquakes might cause many small landslides and that seems to have made many fractures in bedrocks of valley side slopes which would be a cause of slope failure. On 14 June 2020, the automated weather station in Kamikochi (AMeDAS Kamikochi) recorded 117mm rainfall in one day. This heavy rain event caused valley head landslides in steep slope streams on the valley side slopes. These landslides originated debris flows. Some of the debris flows including huge boulders flowed directly into the Azusa River. That is the important cause of debris supply from tributaries to the Azusa River. Some debris slows stopped on the floodplain of gentle slope, which is covered with dense riparian forest. These debris transport processes did not contribute debris supply to the Azusa River at the present moment.
From 6 to 8 July 2020, more than 350mm rainfall was recorded in three days. The heavy rain event caused the bankfull discharge in the upper Azusa River. At the observation site of the riverbed of the Azusa River, where a geomorphological map of riverbed was made every year by surveying, major landform changes with channel migration and intense degradation occurred. Consequently deep channels formed. Also bank erosion occurred in other parts. The June and July events occurred during the Baiu rainy season in which slope surface deposits were saturated enough to occur landslides and debris flows. Thick sediments of the floor of the Kamikochi Valley also were saturated, therefore water level rose high enough to change riverbed landforms.
On 13 and 14 August 2021 more than 300mm rain fell in this area in two days. In the observation site channels migrated again and deep channels formed in 2020 event were buried. According to the previous studies in this observation site, after the end of the Baiu rainy season, usually the beginning of August, water level does not rise enough to move the riverbed sediments, therefore major landform change do not occur. In this case because of the extraordinary rainfall for two days bankfull flood occurred and sediment transport took place in the upper Azusa River.