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1. Ablation‐Limited Erosion Rates of Permafrost Riverbanks

   https://doi.org/10.1029/2023JF007098  

   Douglas, Madison M.; Miller, Kimberly Litwin; Schmeer, Maria N.; Lamb, Michael P. (July 2023, Journal of Geophysical Research: Earth Surface)

   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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2. Glacial Isostatic Adjustment Modulates Lateral Migration Rate and Morphology of the Red River (North Dakota, USA and Manitoba, Canada)

   https://doi.org/10.1029/2023GL103995  

   Kodama, Samuel_T; Pico, Tamara; Finnegan, Noah_J; Lapôtre, Mathieu_G_A; Willenbring, Jane_K (August 2023, Geophysical Research Letters)

   Abstract The lateral migration of a river meander is driven by erosion on the outer bank and deposition on the inner bank, both of which are affected by shear stress (and therefore channel slope) through complex morphodynamic feedbacks. To test the sensitivity of lateral migration to channel slope, we quantify slope change induced by glacial isostatic adjustment along the Red River (North Dakota, USA and Manitoba, Canada) and two of its tributaries over the past 8.5 ka. We demonstrate a statistically significant, positive relationship between normalized cutoff count, which we interpret as a proxy for channel lateral migration rate, and slope change. We interpret this relationship as the signature of slope change modulating the magnitude of shear stress on riverbanks, suggesting that slope changes that occur over thousands of years are recorded in river floodplain morphology. 

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3. Differential Bank Migration Limits the Lifespan and Width of Braided Channel Threads

   https://doi.org/10.1029/2021WR031236  

   Chadwick, A. J.; Steel, E.; Passalacqua, P.; Paola, C. (August 2022, Water Resources Research)

   Abstract Successful management of flooding and erosion hazards on floodplains depends on our ability to predict a river channel's shape and the lifespan during which it will continue to flow. Recent progress has improved our understanding of what sets the lifespan and width of single‐thread channels; the next challenge is to extend this knowledge to braided channels and their interwoven sub‐channels (threads). In this study, we investigate the lifespan and width of braided channel threads in a large experimental data set, coupled with particle‐image velocimetry‐derived measurements of riverbank erosion and accretion. We find that, unlike single‐thread channels, braided channels in the experiment do not exhibit an equilibrium between bank erosion and accretion. Instead, bank erosion outpaces lateral accretion, causing individual threads to widen and infill until they are abandoned. Thread lifespan is limited to the time it takes for threads to triple their width: tripling of the width yields enough bank material to aggrade more than half the channel depth, at which point flow is rerouted to a narrower thread. In consequence the width of active threads is limited to three times their initial width. Threshold channel theory accurately predicts the median thread width, which is roughly double the initial width and two‐thirds the limiting width. The results are consistent with existing field data and suggest that differential bank migration is sufficient to explain why braided channels show greater width variability and higher width‐to‐depth ratios than their single‐thread counterparts. 

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4. Modelling shear stress distribution in ice-covered streams

   https://doi.org/10.1144/SP540-2022-161  

   Koyuncu, Berkay; Le, Trung Bao (April 2023, Geological Society, London, Special Publications)

   Abstract Distribution of bed shear stress is the critical factor in regulating the meandering of single-thread rivers. However, the impact of ice cover on bed shear stress is largely unknown. In this study, we develop a theoretical model of cross-stream momentum balance to examine the distribution of bed shear stresses in ice-covered meandering rivers. To validate the theoretical model, field surveys were carried out in a river reach of the Red River in Fargo, North Dakota. Data monitoring was completed using an Acoustic Doppler Current Profiler to obtain time-averaged velocity profiles. Our theoretical model indicates that an ice covering develops high-shear zones near both the inner and outer banks, which might exacerbate sediment transport and enhance bank erosion. Velocity measurements confirm the results of the proposed model and demonstrate a clear impact of meandering river banks on velocity profiles and secondary flow patterns under ice cover. Based on our results, we hypothesize that ice cover increases turbulent stresses near banks, which in turn lead to the enhancement of the bed shear stress. Our work provides new insights into the impact of ice cover on bed shear stress distribution, which could play an important role in driving sediment-transport processes and the long-term morphodynamic evolution of meandering rivers seasonally covered by ice. 

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5. Quantifying bankfull flow width using preserved bar clinoforms from fluvial strata

   https://doi.org/10.1130/G48729.1  

   Greenberg, Evan; Ganti, Vamsi; Hajek, Elizabeth (May 2021, Geology)

   Abstract Reconstruction of active channel geometry from fluvial strata is critical to constrain the water and sediment fluxes in ancient terrestrial landscapes. Robust methods—grounded in extensive field observations, numerical simulations, and physical experiments—exist for estimating the bankfull flow depth and channel-bed slope from preserved deposits; however, we lack similar tools to quantify bankfull channel widths. We combined high-resolution lidar data from 134 meander bends across 11 rivers that span over two orders of magnitude in size to develop a robust, empirical relation between the bankfull channel width and channel-bar clinoform width (relict stratigraphic surfaces of bank-attached channel bars). We parameterized the bar cross-sectional shape using a two-parameter sigmoid, defining bar width as the cross-stream distance between 95% of the asymptotes of the fit sigmoid. We combined this objective definition of the bar width with Bayesian linear regression analysis to show that the measured bankfull flow width is 2.34 ± 0.13 times the channel-bar width. We validated our model using field measurements of channel-bar and bankfull flow widths of meandering rivers that span all climate zones (R2 = 0.79) and concurrent measurements of channel-bar clinoform width and mud-plug width in fluvial strata (R2 = 0.80). We also show that the transverse bed slopes of bars are inversely correlated with bend curvature, consistent with theory. Results provide a simple, usable metric to derive paleochannel width from preserved bar clinoforms. 

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