Ablation‐Limited Erosion Rates of Permafrost Riverbanks

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.

A Model for Thaw and Erosion of Permafrost Riverbanks

“…We use a heat transfer coefficient, C h , from Yaglom and Kader (1974) that has performed well compared to numerical experiments (Kuwata, 2021), natural environments (McPhee, 1992), and frozen riverbank-erosion flume experiments (Douglas, Miller, et al, 2023). The parameterization is valid for fully turbulent flow in which the Reynolds number, Re = UH/ν > 10 3 , where ν is the fluid kinematic viscosity (m 2 /s).…”

“…Despite these important theoretical advances, existing models appear to substantially overpredict bank erosion rates, drawing into question their validity in forecasting landscape-change scenarios. While rates of several meters per day are possible over limited spatial and temporal scales (Costard et al, 2014;Gautier et al, 2021), thaw-limited models predict rates that are too large by a factor of 10 2 to 10 3 when applied over an entire melt season (Douglas, Miller, et al, 2023;Douglas, Dunne, & Lamb, 2023). This discordance between observation and theory suggests that another process beyond pore-ice melt must limit riverbank erosion rates in permafrost regions (Debol'skaya & Ivanov, 2020;Shur et al, 2021).…”

“…Thus, modeling the thawed layer in permafrost riverbanks poses a distinct challenge that is not fully addressed by models focused on coastal erosion (e.g., Barnhart et al, 2014;Kobayashi & Aktan, 1986), river flooding (Zhang et al, 2023;Zheng et al, 2019), or talik development (Ohara et al, 2022;Roux et al, 2017). Likewise, Douglas, Miller, et al (2023) and Douglas, Dunne, and Lamb (2023) modeled permafrost riverbank erosion for the thawand entrainment-limited end members, but the model is incomplete because they did not dynamically track the pore-ice front nor the thawed-layer development.…”

“…One possibility is that erosion rates are limited by sediment entrainment rather than thaw (Costard et al., 2014; Douglas, Miller, et al., 2023; Douglas, Dunne, & Lamb, 2023; Gautier et al., 2021). This aligns with field observations of a layer of thawed sediment tens of cm thick formed on eroding riverbanks on the North Slope of Alaska (Scott, 1978).…”

Disparate permafrost terrain changes after a large flood observed from space

“…Surface water can increase annual ground temperatures at 5 m depth by >5 , so that a talik commonly forms under river channels of sufficient depth 3–5 . Ice‐rich banks can retreat by 10 m due to thermal erosion 6–11 . Floodplains are also prone to permafrost thaw following enhanced energy transfer into the soils during 12,13 and after a flood through, for example, disruptions to the organic matter 14–16 .…”