INFLUENCE OF CATCHMENT PROPERTIES ON STABILITY OF FROZEN DEBRIS LOBES

Frozen debris lobes (FDLs) are mass wasting features that are potential threats to the Dalton Highway and Trans Alaska Pipeline System (TAPS) in the Brooks Range, Alaska. These features flow downhill from cirque-like catchments and are best known along the small tributary streams of the Dietrich River, which runs parallel to the highway. Larger tributary valleys to the Dietrich River tend to have alluvial fans, while the smaller, FDL-containing tributaries generally do not. This paper examines the hydro-thermal differences between catchments in order to explain the presence or absence of FDLs.

Erosion of mountain slopes is the key driver for sediment accumulation. For this analysis we assume that debris production rate is the same for all catchments in order to find differences in hill slope debris accumulation. Factors controlling sediment mobility analyzed in this study include local slope angle, permafrost stability, water flow, and vegetation. The young landscape in the Brooks Range is characterized by glacially carved valleys and actively moving, steep slopes, with permafrost underlying most of the vegetated slopes. Vegetation slows slope movement via root mats that bind debris together, thus reducing erosion from rain, and by shading the ground to cool the permafrost. South-facing slopes with little vegetation are expected to be permafrost-free or have deeper active layer depths. Most of the FDLs have debris catchments facing east or west in the highway corridor, and include both north- and south-facing slopes. Presence of permafrost in FDLs was confirmed by drilling and temperature observations on FDL-A, the closest FDL to the highway.

We observe that larger catchments have alluvial fans while smaller catchments are associated with FDLs. Moderately-sized catchments are at risk for massive degradation because they produce a large amount of debris, currently in the form of FDLs, and are likely to become unstable when warmer and wetter conditions prevail during the upcoming decades as a result of changing climate. As destabilization of FDLs occurs, we expect these catchments to develop alluvial fans from the large amounts of debris and sediment sliding and flowing downslope to the Dietrich River.