Sediment Entrainment and Slump Blocks Limit Permafrost Riverbank Erosion

Abstract

Climatic warming and permafrost thaw are predicted to increase Arctic riverbank erosion, threatening communities and accelerating sediment, carbon and nutrient cycling between rivers and floodplains. Existing theory assumes that pore‐ice thaw sets riverbank erosion rates, but overpredicts observed erosion rates by orders of magnitude. Here, we developed a simple model that predicts more modest rates due to a sediment‐entrainment limitation and riverbank armoring by slump blocks. Results show that during times of thaw‐limited erosion, the river rapidly erodes permafrost and undercuts its banks, consistent with previous work. However, overhanging banks generate slump blocks that must thaw and erode by sediment entrainment. Sediment entrainment can limit bank and slump block erosion rates, producing seasonally averaged rates more consistent with observations. Importantly, entrainment‐limited riverbank erosion does not depend on water temperature, indicating that decadal erosion rates may be less sensitive to warming than predicted previously.

Scale‐Dependent Influence of Permafrost on Riverbank Erosion Rates

“…This retreat occurred as thawing chunks of bank material spalled off the subaerial face and dropped into the flowing river and were carried away from the bank face (Figure 1a). Along banks with extensive thermal niches on the Yukon River, massive failure blocks that were 10 m wide and several meters thick were observable in July 2009 (Figure 1b), with similar observations in 2022 by Douglas et al (2023). Despite their size, the blocks appeared to thaw rapidly, and rarely appear in high-resolution imagery persisting from 1 year to the next.…”

“…Subaerial frozen bank sediments exposed by ice melt water and/or surface runoff can continue to thaw and retreat independent of fluvial erosion (Kanevskiy et al, 2016;Lawson, 1983;Shur et al, 2021;Stettner et al, 2018). Douglas et al (2023) argued that sediment entrainment and slump-block erosion could control riverbank erosion rates in some cases, rather than pore-ice thaw.…”

Ablation‐Limited Erosion Rates of Permafrost Riverbanks

“…Most Arctic rivers are much deeper than our experiments (meter vs. cm-scale channel depths), which indicates that top-down seasonal thaw by warm air should not significantly alter permafrost bank erosion rates. Instead, we hypothesize that thaw by warm air may actually slow riverbank erosion at low water levels by thawing upper layers of the banks and causing slump block failure that subsequently insulates the submerged portion of the riverbank (e.g., Douglas, Dunne, & Lamb, 2023).…”

“…For a 4‐month long open water season between ice break‐up and freeze‐up, continuous ablation‐limited erosion would produce an unrealistic amount of bank erosion. Thus, a mechanism, different from pore‐ice ablation, must limit bank erosion for large parts of the year (Douglas, Dunne, & Lamb, 2023). Such a limitation could come from sediment entrainment (Roux et al., 2017; Scott, 1978; Shur et al., 2021), the collapse of cohesive slump blocks (Barnhart et al., 2014; Parker et al., 2011), or root reinforcement of bank sediments (Ielpi et al., 2023).…”

“…In this scenario, the erosion rate is assumed to be ablation‐limited, such that heat transfer and pore‐ice melting set the erosion rate, and sediment is assumed to be immediately entrained following thaw (Figure 2). It is also possible that bank erosion is limited by sediment transport or slump blocks (Douglas, Dunne, & Lamb, 2023; Kanevskiy et al., 2016), but our focus here is to evaluate the ablation‐limited end member. For ablation‐limited erosion, bank erosion rates should depend on river flow velocity and water temperature because these parameters are the primary control on heat transfer from the river to the bank (Costard et al., 2003).…”