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The outer bank slump blocks play a fundamental role in modulating the rate of bank erosion, and consequently, have a profound influence on many geomorphic and ecologic processes in meandering streams, e.g., channel evolution and habitat creation. To date, no adequate research has been developed regarding how these blocks, partially strengthened by roots of herbaceous plants, impact the near-bank flow velocity, bank erosion, and migration of streambanks. This paper analyzes 246 slump blocks in a meandering stream on the Marqu Meadow, China. Aerial surveys were conducted once a year from 2018 to 2021 and the lateral migration rates (M) of streambanks are estimated by superimposing the current-year drone imagery onto its previous-year counterpart. When compared to the outer bank segments lacking slumps, our research findings indicate that the bank segments exhibiting slumps demonstrate a reduction of 57 % and 43 % in the maximum and average bank erosion rates, respectively. Overall, the wider blocks provide stronger bank protection; nevertheless, they may also introduce or enhance local scouring in their vicinity, thereby increasing the heterogeneity of bank erosion rates. To elucidate this complex influence mechanism, we simulate the three-dimensional flow velocity field of a ~100 m meandering bend containing three slump blocks using the MIKE-3 model. Furthermore, we compare one-year bank migration distance with and without the blocks by correlating the lateral migration distance of outer banks to the longitudinal gradient of near-bank velocity. The paper provides valuable insights into the complex interplay between flow dynamics and streambank behavior in real-world settings, thereby enhancing the effectiveness of future management and restoration practices. • 246 slump blocks were analyzed in a meander on Marqu Meadow, China. • Slump blocks can persist for years but may be washed away in high flows. • The outer bank slump blocks play a vital role in modulating bank erosion rates. • Slump blocks protect outer banks by redistributing the high flow and secondary flow. • Slump blocks help protect bank overall but it may introduce scour in their vicinity. [ABSTRACT FROM AUTHOR]

Meandering river channels are often associated with cohesive banks. Yet only a few river modelling packages include geotechnical and plant effects. Existing packages are solely compatible with single-threaded channels, require a specific mesh structure, derive lateral migration rates from hydraulic properties, determine stability based on friction angle, rely on nonphysical assumptions to describe cutoffs, or exclude floodplain processes and vegetation. In this paper, we evaluate the accuracy of a new geotechnical module that was developed and coupled with Telemac-Mascaret to address these limitations. Innovatively, the newly developed module relies on a fully configurable, universal genetic algorithm with tournament selection that permits it (1) to assess geotechnical stability along potentially unstable slope profiles intersecting liquid-solid boundaries, and (2) to predict the shape and extent of slump blocks while considering mechanical plant effects, bank hydrology, and the hydrostatic pressure caused by flow. The profiles of unstable banks are altered while ensuring mass conservation. Importantly, the new stability module is independent of mesh structure and can operate efficiently along multithreaded channels, cutoffs, and islands. Data collected along a 1.5-km-long reach of the semialluvial Medway Creek, Canada, over a period of 3.5 years are used to evaluate the capacity of the coupled model to accurately predict bank retreat in meandering river channels and to evaluate the extent to which the new model can be applied to a natural river reach located in a complex environment. Our results indicate that key geotechnical parameters can indeed be adjusted to fit observations, even with a minimal calibration effort, and that the model correctly identifies the location of the most severely eroded bank regions. The combined use of genetic and spatial analysis algorithms, in particular for the evaluation of geotechnical stability independently of the hydrodynamic mesh, permits the consideration of biophysical conditions for an extended river reach with complex bank geometries, with only a minor increase in run time. Further improvements with respect to plant representation could assist scientists in better understanding channel-floodplain interactions and in evaluating channel designs in river management projects. [ABSTRACT FROM AUTHOR]

Long-term monitoring of river restoration and streambank stabilization projects has often been neglected, resulting in a poor understanding of the effectiveness of different types of restoration methods. This study utilized unmanned aircraft systems (UAS) to monitor and assess the effectiveness of streambank stabilization projects in the Cottonwood River valley, an incised river system in east-central Kansas, USA. Streambanks have been acknowledged as a dominant sediment source to downstream waterbodies in incised fluvial systems and are therefore high priority targets of restoration and stabilization strategies. Volumes of sediment eroded at 14 streambank stabilization sites along the Cottonwood River were analyzed to compare: (1) pre-construction and post-construction periods; and (2) stabilized sites and unmodified sites. As monitoring and assessment were not part of the original design plans for stabilization efforts, this paper outlines methods to generate appropriate datasets and models to assess stabilization effectiveness. Results from three years of UAS monitoring, together with historical pre-construction assessment, indicate that stabilization projects significantly reduced local-scale erosion at stabilized sites. However, evidence of continued bank erosion after construction, including erosion of the lower bank above installed toe protection, bank steepening, and mass wasting, suggests that the benefits of reduced erosion from stabilization might change over time. Overall, our investigation demonstrates the viability of UAS surveys for long-term monitoring of streambank stabilization projects along incised river channels. [Display omitted] • Unmanned aircraft systems provide an effective tool to monitor bank stabilization effectiveness. • Stabilization projects significantly reduced local-scale erosion at the stabilized site. • Substantial erosion of the lower bank above toe protection was observed. [ABSTRACT FROM AUTHOR]

Annual bank erosion was measured at multiple cross sections along the free-flowing meandering Powder River in the western United States from 1979 through 2019. Bank erosion was separated into two components—above water and underwater erosion. Above water erosion was measured as the annual bank retreat rate (0–15.4 m y−1). Underwater erosion rate (0–47 m3 m−1 y−1) was calculated as the volume eroded below the water level corresponding to the dominant annual peak discharge, Q p. This paper focuses primarily on the underwater erosion. A total of 491 annual erosion rates were calculated for 23 bank sites along a 90-km study reach in southeastern Montana. Sites were not just hotspots for bank erosion but represent the spectra of variables such as the radius of curvature divided by channel width, R / w (2–86), the peak discharge, Q p (22.7–314 m3 s−1), and the bank orientation (0–360°). Local annual bank erosion was extremely variable in time and space. It was episodic and unsynchronized along the study reach with the maximum annual bank erosion occurring in different years at different bank sites. The composite probability distribution of all 491 annual bank erosion rates was best modeled by a zero-adjusted Weibull distribution. Individual probability distributions for each of the 23 sites were all different from each other and from the composite distribution highlighting the extreme variability. The correlation of the annual underwater erosion with channel geometry and bank variables was low (R2 < 0.31) but the correlation was higher for peak discharge with 25% of the sites having R2 > 0.50. Time-averaging reduced the variability at each site and when grouped into five peak-discharge classes each class was correlated with R / w as a power law with an exponent of about −1. Reach-averaging also reduced the variability for each year, and when grouped by bank orientation (north-, east-, south-, and west-facing), bank erosion was linearly related to Q p with south- and west-facing orientations having about twice as much erosion per unit discharge (0.030 m3 m−1 y−1/m3 s−1) than north- and east-facing orientations. Bank erosion was found to be not just a multi-variate complex process with little correlation and high variability that suggests randomness, but also a process that was a function of a different combinations of variables at different sites at the same time. However, this high variability was reduced by time- and reach-averaging, which produced predictable results analogous to the central limit theorem. Annual underwater volumetric bank erosion for Powder River. Dotted lines are the time-averaged relations, { uwBV e }, versus ratio of radius of curvature and top width { R / w (Q)} for five discharge classes. Solid lines are the reach-averaged relation, | uwBV e |, versus dominant annual peak discharge Q p for four bank orientations. [Display omitted] • Annual bank erosion is extremely variable and characterized by frequent years with no erosion. • Variability at a site is a complex sum of mechanisms and variables, which differs from year to year and from site to site. • Time- and reach-averaging reduce the variability and produce predictable relations. • Time and reach-averaged relations depend on water discharge and bank orientation. [ABSTRACT FROM AUTHOR]