AP 2 - Project

Sediment dynamics and evolution of the Illgraben fan.

Aims and Objectives

Debris flows are a primary geologic hazard in mountain landscapes and play a crucial, but poorly understood, role in erosion and sediment transfer. Accurate assessment of sediment fluxes in mountain landscapes depends on a proper knowledge of both (1) the mechanisms and patterns of sediment transport, particularly in debris flows, and (2) likely frequency-magnitude relationships.
Debris-flow fans, such as the Illgraben fan in southwestern Switzerland, provide attractive targets for both of these objectives, as they are ubiquitous in mountainous regions; are built up by deposition of repeated flows over geological time scales; and preserve, in their geometry and surface morphology, a direct record of past debris-flow occurrence. Despite this, there have been almost no attempts to constrain sediment dynamics through quantitative study of fan surfaces, in part because of the lack of sufficiently high-resolution topographic data over large areas of fan surface.

The aim of this AP is to address this gap by assessing patterns and magnitudes of channel change after individual debris-flow events that occur at the Illgraben key site, and by comparing those with the record of past debris flows derived from the fan surface.
We will accomplish this through two linked objectives:

  • monitoring of short-term spatial patterns of erosion and deposition in the active Illgraben channel using ground-based laser scanning techniques,
  • assessment of depositional patterns and resulting sediment fluxes in pre-historic (Holocene) flows preserved on the fan surface. 
Within the larger context of SedyMONT, IP1 is aimed at understanding the time scales of sediment liberation and discharge at the Illgraben key site, while IP6 is focused on determining the long-term, three-dimensional architecture of the Illgraben fan. This AP complements both of these efforts by addressing the short-term record of erosion, deposition, and sediment transfer on the Illgraben fan surface and within its active channel. The AP thus directly supports SedyMONT Objective 2 (assessment of short-term rates of topographic change), through detection of landscape change and assessment of sediment fluxes.

Research methods

The first objective involves monitoring of spatial patterns of channel change on the Illgraben fan during and after debris-flow events. The Illgraben, described in more detail in IP1, has experienced 36 debris flows since June 2000, generally in response to summer (May-October) convective storms. The Swiss Federal Institute WSL maintains a network of rain gauges and flow sensors that allow real-time measurement of front velocity, flow depth, volume, bulk density and pore pressure [9], and flow composition (e.g., grain size, fluid content). We have identified two reaches of particular interest on the Illgraben fan. The first, or control, reach is directly upstream of the confluence with the Rhone River. This reach is straight and confined by concrete check dams, so that expected morphological changes are limited to aggradation adjacent to the dams. The reach also houses WSL cameras and sensors, allowing direct comparison between our scans and flow characteristics. The second reach is near the head of the fan where the channel bends to flow around a large rockfall deposit. Here, the channel position has shifted laterally by several tens of metres over the past 5 years, and the amount of channel incision has varied irregularly from 0 to 6 m. This reach is unconstrained by check dams and so represents a chance to observe morphological changes in a natural channel.

Monitoring will occur through repeat measurement of the channel using ground-based laser scanners. The use of repeated laser scanning to monitor surface change in active debris-flow channels is a novel and exciting application, with several advantages over traditional survey techniques. Ground-based scanners allow rapid construction of 3D topographic models with point spacings on the order of 1-10 cm over areas of several hundred square meters. Comparison of scans after individual flows enables mapping of changes in channel morphology at a comparable resolution. Crucially, laser scanning does not require personnel to occupy the area being surveyed, a key consideration in steep, often unstable channels. Finally, the laser reflectivity, although uncalibrated, enables estimation of surface colour and roughness of successive flow deposits, potentially allowing us to link morphology with grain size.

The second objective will use this active monitoring as a guide to interpreting the surface geometry of older flow deposits. This requires quantitative analysis of the older (Holocene) surface of the Illgraben fan using high-resolution (1 m) digital topography, available from the WSL and described in IP1. In this work, we will identify paleo-debris flow snouts and levee-channel complexes, and will focus on determining patterns of erosion and deposition and volumetric estimates of older debris-flow deposits. Prior work by PI Densmore has shown that paleo-flow geometry and volume change can be reconstructed from DEM data of comparable resolution, even for fan surfaces that are several 10s of ky old. This is important, because there are indications that paleo-flow runout distance and snout position – both of which may be retrieved from high-resolution DEMs – may be robust indicators of flow magnitude.

 
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