Monitoring of retrogressive thaw slumps in the Arctic Network, 2010 baseline data: Three-dimensional modeling with small-format aerial photographs
Dates
Year
2010
Citation
Swanson, David K., and Hill, Ken, 2010, Monitoring of retrogressive thaw slumps in the Arctic Network, 2010 baseline data: Three-dimensional modeling with small-format aerial photographs: National Park Service Natural Resource Program Center National Park Service: Fort Collins, Colorado, v. NPS/ARCN/NRDS—2010/123.
Summary
Retrogressive thaw slumps (RTS) are caused by thaw of massive ground ice on slopes and combine subsidence, mass movement, and water erosion. They can expose several hectares of bare soil that is susceptible to erosion into nearby water bodies. In the summer of 2010, oblique aerial-photographs of 24 selected RTS in Noatak National Preserve (NOAT) and Gates of the Arctic National Park and Preserve (GAAR) were taken with a hand-held, 35-mm digital camera. This photography was used to create high-resolution three-dimensional topographic models with photographic overlay. Accurate ground control was obtained at 14 of the slumps by surveying the location of temporary targets that were captured on the aerial photographs and then removed; this [...]
Summary
Retrogressive thaw slumps (RTS) are caused by thaw of massive ground ice on slopes and combine subsidence, mass movement, and water erosion. They can expose several hectares of bare soil that is susceptible to erosion into nearby water bodies. In the summer of 2010, oblique aerial-photographs of 24 selected RTS in Noatak National Preserve (NOAT) and Gates of the Arctic National Park and Preserve (GAAR) were taken with a hand-held, 35-mm digital camera. This photography was used to create high-resolution three-dimensional topographic models with photographic overlay. Accurate ground control was obtained at 14 of the slumps by surveying the location of temporary targets that were captured on the aerial photographs and then removed; this allowed us to scale the models and obtain measurements of elevation, area, and volume. Ground control will be obtained at the remaining slumps in future years. These 3D models are the baseline that will be used with models created in the future from new photographs to track the rate of growth or stabilization of the slumps, the volume of material displaced, and the rate of revegetation. The study slumps ranged in size from less than 1 ha to over 4 ha and were generally oval shaped with a steep main scarp 2 to 10 m high along the uphill side. Most of the slumps exposed ground ice that was actively melting, causing material to fall from the main scarp and also producing a zone of liquefied mud below the scarp that flowed downhill. Within about 10 m of the main scarp the mud had lost sufficient water to become solid and resist the flow of liquefied mud above, though often buckling up to form a low ridge. The lower halves of the slump floors were dry, hard, and apparently quite stable, though still sparsely vegetated. Several of the slumps had alluvial fans below them consisting of sediment eroded from the slump. Sediment from the slumps caused a noticeable increase in turbidity in a few of the adjacent lakes. A laterally extensive, debris-rich ice layer, interpreted as relict Pleistocene glacial ice, was exposed near the base of the main scarp in many of the study RTS. In moraines of the Itkillik II glaciation (the last glacial maximum, 14 to 25 thousand years ago) the ice was typically overlain by about 1.5 m of cobbly glacial till, and main scarps were about 2 m high. Some of the RTS in older deposits had large, Pleistocene ice wedges in addition to glacial ice. RTS with ice wedges had higher main scarps (8 to 10 m) and larger volumes of displaced material.Examination of c. 1980 aerial photography showed that similar slumps had occurred prior to that time. The slumps that were active in 1980 are visible on 2008 satellite imagery (and at our 2010 visit), but most had revegetated after expanding little beyond their 1980 extent. Some of the currently active slumps studied in this report were advancing across land that had slumped in the past and then stabilized. Other slumps were advancing across land with no obvious signs of prior slumping. Only two of the study slumps appeared likely to have formed by continued migration of an escarpment that was active in 1980. The main scarps of the study slumps have migrated between 100 and 300 m in 30 years or less, indicating minimum migration rates of 3 m to 10 m per year. The goal of future monitoring of these slumps will be to determine their rates of growth or stabilization and revegetation, and to understand the factors that influence their growth, including the effects of ground ice types, geological setting, and weather.
