10:45 〜 11:00
[HDS07-07] Geomorphic Connectivity Patterns within in a Landslide Scar in Hokkaido Across Seasonal and Annual Timescales Based on UAV-SfM photogrammetry
★Invited Papers
キーワード：landslide, geomorphic connectivity, UAV
Understanding the connectivity of all geomorphic processes within landslides is necessary for estimating landslide internal erosion, sediment (re)deposition, and sediment delivery to rivers. There is a need for further research on how temporal changes affect geomorphic connectivity, particularly in terms of sediment and hydrological processes. Current studies often focus on spatial patterns; however, the temporal component, which relates to how connectivity persists or changes over time, is less understood.
The scar of the Yahata landslide, located in Hokkaido, was chosen to provide seasonal- and annual-dependent perspectives on connectivity. The landslide scar developed in serpentinite and consisted of the main 2.8 ha large section and four small sections ranging between 0.5 to 0.8 ha. All these sections supply sediment to the fan head of the otherwise dormant fan. UAV imagery was taken over three years on 10.2018, 05.2019, 10.2019, 7.2020, 10.2020, 07.2021, and 10.2021 and was used to calculate high-definition DEMs and orthoimages. Process interpretation was facilitated by imagery from 3 intervals cameras and rainfall data measured in the landslide scar.
In this complex landslide, multiple geomorphic processes occurred: rockfall on bare rock slopes, debris flow, sediment collapse caused by undercutting debris flows, and gully erosion. Process analysis revealed a distinct seasonal connectivity pattern. During winter, material weakened by freeze-thaw weathering was transported by rockfalls into the channels of all sections. In terms of sediment connectivity, hillslopes are connected to the channels in winter, but the channels are disconnected from the fan, as no sediment deposition on the fan was identifiable. However, in summer, rockfall activity is limited throughout the scar. Therefore, slopes remain coupled with the channel but at a lower degree of connectivity in summer. In the small sections, no or only one small-scale debris flow occurred during the summers over the three years, forming deposits in the lower channel. Individual rockfall-derived boulders reached the same location, thus creating a deposit developed by the two processes. As only a limited degree of connectivity to the fan head occurs in summer, the lower channel infill indicates a reduction in landslide activity. However, in the main section, gully erosion in the upper channel reaches and debris flow originating in the middle reaches occurred more frequently, scouring the channel deposits and resulting in storage depletion before the next freeze-thaw season. The transferred sediment was deposited on the fan apex or exported to a lesser extent from the surveyed area. Therefore, in contrast to the smaller sections, the main section is connected from the slopes over the channels to the fan head.
On an annual scale, the connectivity pattern for the small section remains the same; slopes are connected to the channel with the main accumulation in the lower reaches. For the main section, however, in-channel storage can be skipped on an annual scale, as channels are typically storage-depleted after the debris flow season. This leaky emergence reduces the connectivity pattern to a slope-fan connectivity system.
The elucidated connectivity patterns might change over seasonal, annual, or even longer time scales when infrequent, high-magnitude rainfall events or earthquakes impact the landslide scar and cause different connectivity degrees and patterns throughout the landslide.
The scar of the Yahata landslide, located in Hokkaido, was chosen to provide seasonal- and annual-dependent perspectives on connectivity. The landslide scar developed in serpentinite and consisted of the main 2.8 ha large section and four small sections ranging between 0.5 to 0.8 ha. All these sections supply sediment to the fan head of the otherwise dormant fan. UAV imagery was taken over three years on 10.2018, 05.2019, 10.2019, 7.2020, 10.2020, 07.2021, and 10.2021 and was used to calculate high-definition DEMs and orthoimages. Process interpretation was facilitated by imagery from 3 intervals cameras and rainfall data measured in the landslide scar.
In this complex landslide, multiple geomorphic processes occurred: rockfall on bare rock slopes, debris flow, sediment collapse caused by undercutting debris flows, and gully erosion. Process analysis revealed a distinct seasonal connectivity pattern. During winter, material weakened by freeze-thaw weathering was transported by rockfalls into the channels of all sections. In terms of sediment connectivity, hillslopes are connected to the channels in winter, but the channels are disconnected from the fan, as no sediment deposition on the fan was identifiable. However, in summer, rockfall activity is limited throughout the scar. Therefore, slopes remain coupled with the channel but at a lower degree of connectivity in summer. In the small sections, no or only one small-scale debris flow occurred during the summers over the three years, forming deposits in the lower channel. Individual rockfall-derived boulders reached the same location, thus creating a deposit developed by the two processes. As only a limited degree of connectivity to the fan head occurs in summer, the lower channel infill indicates a reduction in landslide activity. However, in the main section, gully erosion in the upper channel reaches and debris flow originating in the middle reaches occurred more frequently, scouring the channel deposits and resulting in storage depletion before the next freeze-thaw season. The transferred sediment was deposited on the fan apex or exported to a lesser extent from the surveyed area. Therefore, in contrast to the smaller sections, the main section is connected from the slopes over the channels to the fan head.
On an annual scale, the connectivity pattern for the small section remains the same; slopes are connected to the channel with the main accumulation in the lower reaches. For the main section, however, in-channel storage can be skipped on an annual scale, as channels are typically storage-depleted after the debris flow season. This leaky emergence reduces the connectivity pattern to a slope-fan connectivity system.
The elucidated connectivity patterns might change over seasonal, annual, or even longer time scales when infrequent, high-magnitude rainfall events or earthquakes impact the landslide scar and cause different connectivity degrees and patterns throughout the landslide.