Abstract
Glacial recession in Alpine Valleys uncorks low-altitude tributary valleys that spill trapped sediments in the sediment cascade, in turn generating sediment-related hazards and flood hazards. Within this complex, the present contribution investigates the transition zone between the tributary and the main valley at Fox Glacier, New Zealand, where glacier recession has allowed the development of large debris flow fans. As the valley is narrow and its geomorphology is rapidly evolving, 3D point cloud technologies based on aerial photogrammetry from UAV or ground and airborne laser technologies are essential to understand the geomorphology and generate boundary conditions to run hazard simulations such as debris flows. Using airborne structure from motion photogrammetry with ground control points collected by RTK-GNSS, we investigated (1) the geomorphology of the valley to understand its evolution and (2) the role of this recent evolution on the flood hazards at the Yellow Creek debris flow fan. The geomorphology of the Fox River valley is a typical U-shape valley with steep valley walls that command the valley by more than 1000 m. The valley walls are connected to the bottom of the valley by active sediment aprons and debris flow fans at the exit of the tributaries. The slopes present several topographic steps or noses on both the aprons and the debris flow fans. By comparison with the glacial recession, they can be related to past locations of the limits of the glacier. On Straight Creek fan, the ridges are perpendicular to the valley and divide the fan between an overgrown half downstream the Fox Valley and an upstream section thinner and smaller. This pattern was also created by the presence of the glacier that stopped the development of the fan on its right (upstream half) while developing the true left hand side of the fan. On the other side of the valley, the debris flow fan of Yellow Creek presents a series of deep trough that control water flows. Flood simulations show that these controls can be overcome and instead of being channeled in other sub-channels, the flow tends to spread on the true right (downstream) end of the fan as a sheet flow. In the 2014 state of the fan, the safest location was thus on the true left half of the fan towards the glacier. Such information is essential as tourists occupy the valley and walk towards the glacier the whole year.
Highlights
Presentation of the problem The southern half of the South Island of New Zealand is home of Alpine Valleys connected to the West Coast, where the landscape is dominated by Quaternary Glaciation products
The slopes are characterized by bedrock eroded by the Fox Glacier as well as a mantle of debris deposited on the side of the glacier
Geomorphological interpretation of the results The present contribution is organized around two steps: (1) a geomorphologic description and analysis of the valley and (2) example of flood simulations on Yellow Creek fan using the geomorphologic data as boundary conditions
Summary
Presentation of the problem The southern half of the South Island of New Zealand is home of Alpine Valleys connected to the West Coast, where the landscape is dominated by Quaternary Glaciation products. As Quaternary scale large glaciers have “disappeared,” a postglacial rebound, or debuttressing, has been occurring through the temperate mountain environments, generating mass movements and sediment releases in the valleys (Fig. 1) This effect is accentuated with glacial retreat, which impacts sediment transport and downstream hydrology. As the glaciers are receding to their lowest extend for centuries, the sediments trapped in the tributaries are released, sucked in the main valleys, because of the sudden lowering of the base level—the previous base level was the glacier surface elevation, it is the river bed— and eventually because of the unwelding of rocks and ice, due to ice melting These processes in turn create issues downstream with sediment accumulation and transfer, threatening infrastructures and settlements like at Franz Josef (Davies et al 2013). It is critical to be able to generate topographic dataset of those areas in a very short period of time
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