11:00 AM - 11:15 AM
[HGM02-02] Decadal-scale development of an earthflow reactivated by gully incision determined from historical aerial photographs, North Island, New Zealand
Keywords:earth flow, gully
Earthflows can dominate landscape evolution and control hillslope morphology of steep, erosion-prone slopes. Coupling of earthflows and the fluvial system has received increasing attention within the last years. Here, the evolution of a typical earthflow in the Managaoporo catchment on the North Island in New Zealand is analyzed, as understanding of such coupling is limited for a region, where removal of native forest for pastoral farming at the beginning of the 20th century initiated a major, landscape-scale erosion phase. Historical aerial photographs of 06.1939, 04.1957, 09.1971, 06.1988, 05.2005, and 01.2012 were ortho-rectified using Agisoft Metashape and ERDAS IMAGINE Photogrammetry to map earthflow movement and gully incision.
Results indicate that an incipient gully eroded along an 0.7km2 dormant earthflow on the earliest photography of 1939. The toe of dormant earthflow was destabilized by gully incision leading to shallow landslides along this main gully. The instability propagated upstream and failures retrogressively extended into the earthflow displacing increasing sediment volumes. This intermitted sediment supply overwhelmed gully reaches by infilling gully channels, while the landslide deposits were re-incised by gully erosion at earthflow-distant locations. While the main gully increased in length and volume, a gully network was formed and extended into the earthflow. This network contains two gully types: smaller axial gullies and gullies eroding along scars or in cracks of the shallow landslides. Instability also reached a head scarp section near the main gully where slumping occurred supplying limited sediment into the gully network. Ultimately, the gully network serves to destabilize hillslopes and to reactivate dormant earthflows retrogressively, controlling the evolution of earthflow terrain.
Results indicate that an incipient gully eroded along an 0.7km2 dormant earthflow on the earliest photography of 1939. The toe of dormant earthflow was destabilized by gully incision leading to shallow landslides along this main gully. The instability propagated upstream and failures retrogressively extended into the earthflow displacing increasing sediment volumes. This intermitted sediment supply overwhelmed gully reaches by infilling gully channels, while the landslide deposits were re-incised by gully erosion at earthflow-distant locations. While the main gully increased in length and volume, a gully network was formed and extended into the earthflow. This network contains two gully types: smaller axial gullies and gullies eroding along scars or in cracks of the shallow landslides. Instability also reached a head scarp section near the main gully where slumping occurred supplying limited sediment into the gully network. Ultimately, the gully network serves to destabilize hillslopes and to reactivate dormant earthflows retrogressively, controlling the evolution of earthflow terrain.