Permafrost Thaw Slumps
Background Info - Peel Plateau Lake Drainage
Permafrost Thaw Causes Lake Drainage on the Peel Plateau
- An acceleration of permafrost thaw in northwestern NWT is causing notable landscape changes. The Northwest Territories Geological Survey (NTGS) is collaborating with Universities, Governments and the Tetlit Gwich’in Renewable Resources Council to study landscape change and permafrost geohazards on the Peel Plateau.
- Permafrost thaw has caused development of very large thaw slumps; Individual disturbances can impact over 30 ha of terrain, displace millions of cubic metres of sediments to reconfigure slopes and impact downstream environments.
- One particular thaw slump about 20 km northwest of Fort McPherson has been growing for about a decade. It has caused gradual collapse of an entire hill, resulting in the rapid, partial drainage of a small upland lake about 1.5 ha in area.
- In anticipation of the drainage event, a geohazard advisory was re-issued by the NTGS in June 2015.
- The drainage event was captured by remote cameras. When the slump eroded to the edge of the lake, about half of the lake volume estimated to be about 30,000 m3 drained in about 2 hours by pouring over the slump headwall and forming a temporary waterfall 10 to 15 m high. Peak flows lasted less than 30 minutes and flow rates reached at least 10 m3/second. Water rushed over the slump debris and down a narrow valley before emptying into a larger lake about 5 km downstream in the Mackenzie Delta. The turbidity in the downstream lake was significantly elevated as a result of sediments transported by the drainage event. Two days following drainage, saturated debris flowed downslope at rates of up to about 50 m/hour for several days to enlarge a debris tongue deposit that has infilled about 2 km of valley.
- No one was in the vicinity when partial drainage occurred on the 15th of July, and the Renewable Resources Council in Fort McPherson was notified; We thank several individuals involved in research, regulation and industry activities for diverting flight plans to examine and report on the status of the lake. The lake partially drained so there is a chance that another release of water could occur.
- When the growing thaw slump encountered unfrozen sediments (talik) beneath the lake, thermal erosion stopped. This likely explains why drainage was only partial.
- In 2016 NTGS and University and Government partners will further investigate the impacts of slumps on landscapes, streams and lakes and explore the use of remote sensing to better monitor the landscape changes.
What is a thaw slump?
- Slumps develop when ice-rich permafrost thaws, causing ground to collapse and a crater like scar area to form. The headwall of thawing ground ice, which “eats away at the slope” can form a vertical wall of ice-rich permafrost up to 30 m height. Thaw of icy permafrost turns into a saturated slurry of mud which can accumulate or flow downslope to form a large tongue of debris. A debris tongue can fill a stream valley with millions of cubic metres of sediment (1 pickup truck = 2.5m3 so one million cubic metres of sediment = 400,000 pickup truck loads). Plugs of saturated sediment have been observed to ooze downslope at rates of up to 100 m/hr (this is fast, but not that fast).
Is thaw slumping and the changes we are observed today normal?
- Thaw slumping is a natural landscape process and these disturbances have always affected the certain landscapes. In northwestern Canada, ice-rich terrain that was deposited by the Laurentide ice sheet is particularly susceptible to thaw slump disturbances. Permafrost has preserved relict ground ice for over ten thousand years. The slumps we observe today develop in these landscapes and they are more abundant and much larger than they were in the recent past, so for northerners these are becoming increasingly important geohazards to be aware of.
- Slump activity on the Peel Plateau and other parts of the western Canadian Arctic underlain by ice-rich permafrost has become more common in the past two decades. This is in part being driven by rising air and ground temperatures and by increased rainfall. Intensification of rainfall means that mud flows will be more active and the slopes are less likely to stabilize. This allows slumps to enlarge beyond what was normal under colder or drier conditions.
What are the impacts of bigger slumps?
- Slumps can grow upslope over a period of several years or decades. Each summer an active headwall can grow up to 20 m upslope dramatically increasing the size of the crater-like scar area. The scar of active slumps and the debris tongue deposits can cover tens of hectares of terrain and can be hazardous because when saturated, the muds behave like quicksand. Extreme care is required around active disturbances.
- Some of the large slumps have displaced volumes of 5 to 10 million m3 of permafrost (ice and sediment) in the past decade and a half. The Rogers Centre “Sky dome and home of the Toronto Blue Jays” has a volume with roof closed of 1.6 million m3.
- The lake drainage we have documented is only one consequence of increasing slump activity. These disturbances are mobilizing previously frozen materials that have been stable for thousands of years. Other impacts include clogging up of stream valleys, development of debris damned lakes, elevated sediment loads in streams and rivers and changes to water chemistry. Stream ecologists from the NWT Cumulative Impact Monitoring Program and University of New Brunswick have been recently studying slump impacts on the biology of the streams and have found that sedimentation of the streams from slumps has a strong negative effect on benthic organisms.
- At NTGS we will continue working with Government, Academic and Community partners to investigate permafrost terrain sensitivity, develop tools that can help us monitor landscape change, and continue to disseminate geoscience information to northerners, project planners and regulators. Improving our geoscience information base will support northern decision making and resilient communities.