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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]

During major floods, rivers erode their banks and thus recruit large wood pieces from the riparian zones. There is still a lack of knowledge about the transport of large wood, the volumes involved and the flux distribution, i.e. the large wood connectivity at catchment scale. During storm Alex (October 2020), the French Roya catchment (394 km2) experienced a paroxysmal morphogenic flood involving massive bank erosion. The riparian vegetation was largely recruited, with large wood contributing to logjams and bridge destruction. This paper presents a methodology for volumetric assessment of the large wood fluxes involved. Simple approaches are used to (i) quantify the inputs from stand density data from the national forest inventory and from source areas based on diachronic analysis of active channels highlighting the erosion of 87 ha of wooded areas; and (ii) quantify the volumes deposited via an exhaustive manual digitisation of 16,846 pieces of large wood deposited on 59 km of channels on the Roya and its tributaries. This catchment-scale, large wood connectivity analysis shows that the flood recruited and transported downstream a volume of around 14,000 m3 of large wood (uncertainty range: 7000–29,500 m3). Drone observations of the Roya River mouth in Italy and satellite images showing a raft of driftwood, several km long, drifting off the Roya River mouth in the aftermath of the flood corroborate our findings. • Storm Alex (Oct. 2020) triggered dramatic geomorphic changes in the Roya River (FR). • We quantified the volume of large wood (LW) eroded, conveyed and deposited. • Large wood production is deduced from eroded forest maps and forest stand density. • Large wood deposition is estimated by digitizing logs on aerial pictures. • Circa 14,000 m3 of LW were recruited, 900 m3 deposited, the rest went to the sea. [ABSTRACT FROM AUTHOR]

A new river discriminant system is presented based on resistance law. A parameter characterizing river morphology and indicators reflecting activity of bed and bank are derived. They are defined as river morphology parameter and river activity indicators respectively. By relating river morphology to river activity through a comprehensive resistance factor, the discriminant curves characterizing river patterns, namely resistance thresholds, were derived, which make it possible to calculate the activity of bed and bank by just a few easy-obtaining hydraulic variables. In addition, the effect of riparian vegetation on riverbank strength is incorporated into this study. The discriminant method proposed in this paper has proved to be applicable in distinguishing river patterns by selected data sets of the model and natural rivers and in improving the understanding of patterning process. [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]

The erosion of sediment through the exchange of momentum and energy transfer within the debris flow affects the unstable motion of the debris flow. Many models have been established to study the impact of erosion on debris flow motion, but most of the models were based on bed erosion. This paper analyzed the process of unstable motion of debris flows through an experimental flume to contrast bank erosion-dominated conditions and bed erosion-only conditions. The experiments showed that bank erosion enhanced the formation and propagation of debris flows. The volume of sediments eroded by water and the debris flow mass for the bank erosion-dominated conditions was much greater than that for the bed erosion-only conditions. The height and velocity of the debris flows fluctuated, and the total basal normal stress and pore pressure increased unsteadily along the path under both conditions. However, bank erosion increased the velocity of the debris flow and made the motion fluctuation more obvious. Physical equations were established and the analyses suggested that bank erosion-dominated debris flows had increased resistance and gradient enhancement than that of bed erosion-only debris flows. [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]

River confluences are key nodes in fluvial systems. Past studies on large river confluences were focused on the Paranà, Amazon, and Yangtze rivers, while the Yarlung Zangbo River (YZR), the longest plateau river in China, has not been investigated yet. This paper investigates a large multi-channel confluence between the YZR and one of its major tributaries, the anabranching Niyang River (NR). A remote-sensing study of this area demonstrated that the confluence planform is very stable compared with its upstream and downstream reaches that suffered from sandbar migration, meandering, and the development of a head-cut. A field survey using Acoustic Current Doppler Profiling (ADCP) was conducted at the confluence in June 2019, when the YZR and NR had discharges of about 1393 m3/s and 1299 m3/s in total, respectively. Although the discharge ratio of this confluence attained a value of 0.93, the existence of an estuary delta with a large area of floodplain largely weakened the impact of the NR on the dynamics of the YZR. The inflows of the multiple channels of the NR separated by the delta created an evident and long mixing interface and produced the left-side secondary cells in the YZR. In contrast, the curved riverbank played a more important role in the formation of deep scour holes, generating heterogeneity of the velocity distribution and the fast-mixing of flows. This study leads to an advancement in our current knowledge about multi-channel confluences. • Remote sensing images show that the plateau river confluence planform is very stable. • A field survey on flow and morphological features of the confluence was conducted. • The estuary delta with large floodplain largely weakened the confluence dynamics. • The curved river bank played an important role in river bed scours and flow mixing. [ABSTRACT FROM AUTHOR]

Vegetation growing in river bars and banks determines the morphological processes in river channels. The presence and dynamics of riverine vegetation depend on the magnitude and frequency of floods able to change morphology and uproot plants, and on the plant's resistance to pull-out. A series of pull-out experiments were carried out with the objective to determine the resistance of riverine vegetation to be uprooted and its effects on river-bed dynamics. Nine riverine species were used for the experiment: seven of them are native from Chile, and the other two are exotic and invasive in Chilean environments. A total of 200 specimens were transplanted in a substrate simulating a bar of a gravel-bed river, and after 7 months they were individually pulled-out, and the force and time needed to uproot the plant quantified. Data were analysed by means of generalised linear models (GLM) and linear regressions, and force-time curves were interpreted and related to the root morphology of the species. GLM results showed that resistance values were dependent on root and plant physical characteristics, and by the factor species. Among the species studied, the three species showing higher resistance values were native, while the less resistant was an exotic invasive species. The time needed to uproot the individuals did not seem to be influenced by plant parameters nor by the factor species. Root architecture seemed to have an effect on plant pull-out resistance and uprooting rapidity, being heart-shaped roots the ones that registered higher resistance values. Despite this, when incorporating flow strength on the findings, the exotic invasive ones seemed to be some of the more resistant to uprooting. The findings of this paper contribute to the better understanding of river vegetational and sedimentary dynamics, and are useful for parameterising the modelling of fluvial landscapes evolution and for the design of river restoration projects. Unlabelled Image • Evaluation of pull-out resistance of species growing on Chilean fluvial environments. • Root metrics and biomass influence the maximum resistance to pull-out. • Root morphology seems to affect plant resistance to pull-out. • Exotic invasive species did not show higher resistance values than native ones. • Considering flow strength, the exotic species seem to be more resistant than native. [ABSTRACT FROM AUTHOR]