How will bank erosion rates in Arctic rivers respond to a warming climate? Existing physical models predict that bank erosion rates should increase with water temperature as permafrost thaws more rapidly. However, the same theory predicts much faster erosion than is typically observed. We propose that these models are missing a key component: a layer of thawed sediment on the bank that buffers heat transfer and slows erosion. We developed a 1D model for this thawed layer, which reveals three regimes for permafrost riverbank erosion. Thaw‐limited erosion occurs in the absence of a thawed layer, such that rapid pore‐ice melting sets the pace of erosion, consistent with existing models. Entrainment‐limited erosion occurs when pore‐ice melting outpaces bank erosion, resulting in a thawed layer, and the relatively slow entrainment of sediment sets the pace of erosion similar to non‐permafrost rivers. Third, the intermediate regime occurs when the thawed layer goes through cycles of thickening and failure, leading to a transient thermal buffer that slows thaw rates. Distinguishing between these regimes is important because thaw‐limited erosion is highly sensitive to water temperature, whereas entrainment‐limited erosion is not. Interestingly, the buffered regime produces a thawed layer and relatively slow erosion rates like the entrainment‐limited regime, but erosion rates are temperature sensitive like the thaw‐limited regime. The results suggest the potential for accelerating erosion in a warming Arctic where bank erosion is presently thaw‐limited or buffered. Moreover, rivers can experience all regimes annually and transition between regimes with warming, altering their sensitivity to climate change.
Permafrost warming and degradation is well documented across the Arctic. However, observation‐ and model‐based studies typically consider thaw to occur at 0°C, neglecting the widespread occurrence of saline permafrost in coastal plain regions. In this study, we document rapid saline permafrost thaw below a shallow arctic lake. Over the 15‐year period, the lakebed subsided by 0.6 m as ice‐rich, saline permafrost thawed. Repeat transient electromagnetic measurements show that near‐surface bulk sediment electrical conductivity increased by 198% between 2016 and 2022. Analysis of wintertime Synthetic Aperture Radar satellite imagery indicates a transition from a bedfast to a floating ice lake with brackish water due to saline permafrost thaw. The regime shift likely contributed to the 65% increase in thermokarst lake lateral expansion rates. Our results indicate that thawing saline permafrost may be contributing to an increase in landscape change rates in the Arctic faster than anticipated.
more » « less- NSF-PAR ID:
- 10475491
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 22
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Permafrost thaw is hypothesized to increase riverbank erosion rates, which threatens Arctic communities and infrastructure. However, existing erosion models have not been tested against controlled flume experiments with open‐channel flow past an erodible, hydraulically rough permafrost bank. We conducted temperature‐controlled flume experiments where turbulent water eroded laterally into riverbanks consisting of sand and pore ice. The experiments were designed to produce ablation‐limited erosion such that any thawed sediment was quickly transported away from the bank. Bank erosion rates increased linearly with water temperature, decreased with pore ice content, and were insensitive to changes in bank temperature, consistent with theory. However, erosion rates were approximately a factor of three greater than expected. The heightened erosion rates were due to a greater coefficient of heat transfer from the turbulent water to the permafrost bank caused by bank grain roughness. A revised ablation‐limited bank erosion model with a heat transfer coefficient that includes bank roughness matched our experimental results well. Results indicate that bank erosion along Arctic rivers can accelerate under scenarios of warming river water temperatures for cases where the cadence of bank erosion is set by pore‐ice melting rather than sediment entrainment.
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