Banks constitute important areas for the river ecology since they provide a multitude of favourable conditions for flora and fauna. The hydromorphological diversity typical of these transitional zones between water and land, and the associated processes of erosion and accretion, make riverbanks vital for many aquatic and riparian plants and animals. In recent decades, the increasing awareness of the ecological significance of rivers and water bodies resulted in the gradual implementation of extensive stream, river and floodplain restoration. In the EU, these practices are regulated by the Water Framework Directive. An important and largely applied re-naturalization measure in highly trained watercourses is the removal of bank protections to reactivate erosion processes and promote habitat diversity. In rivers used as waterways, ship waves can be an important cause of bank erosion and ecological disturbance. The sediment yield from bank erosion may alter navigable depths, the water quality, and flood conveyance, for which enhancing the hydromorphology is a challenge in multifunctional rivers. Due to pressing needs to improve riverine habitats, large-scale restoration works have been implemented based on conceptual schemes without a comprehensive knowledge of wave erosion processes or a precise estimate of long-term bank retreat. The Meuse River in the Netherlands constitutes a remarkable example of systematic rehabilitation, where bank protections have been removed along 100 km between 2008 and 2020. Given that ship-induced erosion is still poorly understood, the management of navigable rivers and the planning of restoration measures would benefit from a solid and deeper understanding of natural bank dynamics induced by ship waves, for both economic and ecological reasons. Moreover, more precise estimates of long-term bank retreat would help to optimize different functions and reduce conflicts of interest within the river system. Therefore, the main objective of this investigation is to understand and predict erosion processes and the morphological evolution of natural banks in regulated navigable rivers. The research goal is pursued through the thorough investigation of a river reach that presents a wide range of erosion rates after the removal of bank protections. This main case study consists of a 1.2-km straight reach in the Meuse River, near Oeffelt in the Netherlands, the left bank of which was re-naturalized in 2010 by extracting the riprap. The Meuse is a midsize river with a pluvial regime, which has been canalized and is regulated with a series of weirs to enable navigation. Here, field techniques and complementary laboratory tests are utilized including topographic surveys with UAV, wave measurements with ADV, soil coring, geotechnical tests, and RTK GPS profiling. Processing and analysis of data are carried out with MATLAB. Four research steps are conducted. First, a methodology to quickly survey the 3D bank topography along a midsize river reach is determined to measure bank erosion processes. Second, distinct patterns of bank erosion that appeared along the Meuse River after protection removal are investigated. The aim is to disentangle the causes of the size, location and asymmetry of large embayments before analysing erosion processes at single river sections. Third, bank erosion processes in regulated navigable rivers are characterized and conceptualized. Fourth, a tool to estimate long-term or final retreat of re-naturalized banks in regulated navigable rivers is developed. The results of the first research component show that structure from motion photogrammetry applied to photos taken from an UAV is a practical and accurate method to measure riverbank erosion. By distributing ground-control points sufficiently spaced from the bank into the floodplain, digital surface models are georeferenced with sufficient accuracy to compare bank profiles between successive surveys. The identification of ground-control points in photographs is facilitated by placing oblique plaques on the floodplain, reducing the need for another perspective along banks. A single UAV flight with an oblique perspective of the bank becomes then sufficient to capture its three-dimensional complexity. Eight overlaps among consecutive images is the minimum number not to reduce the precision potential of a single UAV flight. The proposed methodology is fast to deploy in the field and surveys reach-scale riverbanks in sufficient resolution and accuracy to quantify bank retreat and identify morphological features of the complete erosion cycle, which enables the characterization of bank erosion at the process scale. Second, the oblique orientation of heterogeneous sedimentary strata with respect to the canalized Meuse River alignment explains the formation and asymmetry of large embayments. Depositional layers of varying compositions, structured by scroll-bar formation during former river meandering, led to wide-ranging erosion rates within a relatively short reach, which formed distinct bankline patterns across diverse lithologies and above the controlled water level of the river. The frequent occurrence of this water level and the persistent ship wave attack shaped bank profiles of varying strengths with a mild sloping terrace. The presence of isolated trees on the floodplain only locally delay erosion rates. Bank retreat rates at single cross sections primarily depend on the lithology near the minimum regulated water stage. Third, the evolution of bank profiles revealed the active role of ship waves in erosion progression, even at well-developed terraces. Currents initially contribute to all phases of the erosion cycle, but they gradually exert less shear stresses on the upper bank as the terrace elongates. Their later role at intermediate stages of development is reduced to the destabilization of steep high banks through water level fluctuations, without capacity to transport slump blocks. The resistance to erosion of the bank lithology defines the terrace geometrical proportions and the pace of morphological evolution of bank profiles. For instance, at a given time after protection removal, less cohesive banks can be present at intermediate stages of development while more cohesive banks remain at early stages. The latter present shorter and shallower terraces whereas the opposite holds for the former. Vegetation temporarily protects the upper bank from failure and toe erosion, but its permanence is subject to terrace stability and effectiveness to dissipate waves. Biofilms are able to partially cover well-developed terraces, changing entrainment thresholds. Fourth, based on the above conceptual framework of bank profile evolution, a model was developed which captures the observed non-linear morphodynamics driven by ship waves in regulated settings. This new tool estimates long-term retreat by accounting for the main erosion drivers and essential mechanisms. Equilibrium bank profiles are reached once wave-induced shear stresses fall below the threshold for entrainment of cohesive soils. Unlike previous models of ship-induced erosion, the process-based approach enables to distinguish the contribution of each factor to erosion. Primary waves are found to exert the highest loads on the terrace, shaping long-term profiles and defining ultimate retreat. To apply the model, it is necessary to measure or estimate the largest primary wave and the soil cohesion at the controlled level, preferably in the range -1.00 m to +0.50 m with respect to it. The above findings are based on cohesive banks in a straight reach of a regulated river. The presence of gravel layers in the bank changes the morphological response to ship waves due to the armouring of lower strata. In such cases, the bank terrace can reach a transverse slope in dynamic equilibrium defined by grain size, as long as longitudinal currents do not transport the gravel to the lower bank. The lower non-cohesive layer of composite banks responds in a similar way, eventually reaching a dynamic equilibrium, after which a final retreat of the upper cohesive layer is possible. The position of banks in the river planform affects the magnitude and duration of the contribution of currents to upper bank erosion. Their direct impact, especially during high floods, can dominate bank retreat during long periods if the flow is persistently steered against the upper bank, as at outer bends. Unregulated rivers present higher shear stresses than those with controlled stages. Their sandy strata of composite banks are normally exposed to currents and waves, creating larger morphodynamics and more challenging conditions for vegetation growth. The new model to estimate final retreat of cohesive banks may be used to prepare a reach scale strategy that defines the most convenient approach for stretches with similar morphological behaviour and available space to develop. In this way, the eventual need to reduce or stop erosion at sections with future excess retreat is determined in advance. In order to make the most of re-naturalized banks in terms of their benefits for ecological processes and habitat diversity in navigable rivers, the advantages of shallow areas with less perturbated zones should be sought where possible. Two phases of interventions are recommended, a first phase where ship waves freely reach the bank for terrace creation, responding to local lithologies, and a second phase with lowered erosive loads, facilitated by slightly submerged pre-banks. The latter phase increases the possibilities for vegetation, and likely other living organisms, to develop. The knowledge and tools now available create new possibilities for improved management of re-naturalized banks in navigable rivers. The progress made helps to better understand the contribution of different drivers to bank erosion and to identify which factors control retreat at different bank types, stages of development, and settings. The new insights explain how to apply SfM-UAV to monitor bank erosion processes along river reaches, interpret bankline patterns, assess the role of isolated trees in bank retreat, and manage expectations regarding bank retreat and the role of vegetation to control erosion. The understanding of erosion processes in regulated navigable rivers and the possibility to estimate final erosion magnitudes open future opportunities to analyse the river system from a holistic perspective and to find creative ways to balance diverse river functions.@en

Vessel-induced waves affect the morphology and ecology of banks and shorelines around the world. In rivers used as waterways, ship passages contribute to the erosion of unprotected banks, but their short- and long-term impacts remain unclear. This work investigates the effects of navigation on bank erosion along a reach of the regulated Meuse River with recently renaturalized banks. We apply UAV-SfM photogrammetry, RTK-GPS, acoustic Doppler velocimetry, aerial and terrestrial photography, soil tests, and multibeam echosounding to analyze the progression of bank retreat after riprap removal. After having analyzed the effects of ship-generated waves and currents, floods, and vegetation dynamics, a process-based model is proposed to estimate the long-term bank retreat. The results show that a terrace evolves in length and depth across the bank according to local lithology, which we clustered in three types. Floods contribute to upper-bank erosion-inducing mass failures, while near-bank flow appears increasingly ineffective to remove the failed material due to terrace elongation. Vegetation growth at the upper-bank toe reduces bank failure and delays erosion, but its permanence is limited by terrace stability and efficiency to dissipate waves. The results also indicate that long-term bank retreat is controlled by deep primary waves acting like bores over the terrace. Understanding the underlying drivers of bank evolution can support process-based management to optimize the benefits of structural and functional diversity in navigable rivers.

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Distinct bankline patterns appeared after the removal of protection works along a navigable reach of the Meuse River. A series of oblique embayments now dominate the riverine landscape after ten years of bank erosion, but their location and asymmetry cannot be explained yet. This work analyses and integrates field measurements of flow, ship waves, bank composition, bed topography and historical maps to explain the observed patterns along two reaches of the river. An extraordinary low-water-level event generated by a ship accident provided the unique opportunity to also analyse the subaqueous bank topography. The results indicate that the formation of oblique embayments arises from the combination of floodplain heterogeneity, structured by scroll-bar deposits, and the regulation of water levels, resulting in ship-wave attack at a narrow range of bank elevation for 70% of the time. Substrate erodibility acts on the effectiveness of trees to slow down local bank erosion rates, which is possibly enhanced by a positive feedback between woody roots and cohesive soil. The strong regulation of water levels and the waves generated by the intense ship traffic produce an increasingly long mildly-sloping terrace at the bank toe and progressively dominate the bank erosion process. This study demonstrates the important role of floodplain and scroll bar formation in shaping later bank erosion, which has implications for predictive numerical models, restoration strategies, and understanding the role of vegetation in bank erosion processes.

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While the scientific community has long recognized that alluvial rivers are the product of interactions between flowing water and bed material transport, it is increasingly evident that vegetation mediates these interactions and influences the stream channel characteristics. In a novel set of mobile bed laboratory experiments with variable discharge, we demonstrate that vegetation colonization affects bank erosion rates, channel shape, channel sinuosity, and bar pattern. Our analyses compared the morphological evolution of channels with initially steady bars considering the following three scenarios: (1) channel without vegetation, (2) channel with vegetation added to the floodplains, and (3) channel with vegetation added to both the floodplains and the bar surfaces that emerge at low flows. Absence of vegetation produced the widest and shallowest channel with the lowest sinuosity. Floodplain vegetation in the second scenario reduced bank erosion and resulted in a deeper and more sinuous channel with shorter bars. In the third scenario, vegetation establishment on emerging bar surfaces intensified erosion on the opposing bank, enlarging the amplitude of bends. Enhanced sedimentation on vegetated bar areas increased both bar elevation and bar length compared to the second scenario. The results show that the colonization of bar surfaces by plants creates the conditions for new floodplain and island formation, fostering channel meandering and anabranching. Finally, our experiments emphasize the role of alternating high and low flows on the morphological development of streams mediated by vegetation.

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In recent years, many riverbanks in Europe had their protections removed to reactivate natural erosion processes and improve riparian habitats. Yet, other river functions may be affected, such as navigation and flood conveyance. The quantification and prediction of erosion rates and volumes is then relevant to manage and control the integrity of all river functions. This work studies the morphological evolution of riverbanks along two restored reaches of the Meuse River in the Netherlands, which are taken as case studies. This river is an important navigation route and for this its water level is strongly regulated with weirs. Through aerial photographs and two airborne LIDAR surveys, we analysed the evolution over nine years of restoration and reconstructed the topography along 2.2 km. of banks. An extraordinary low-water level after a ship accident provided the opportunity to observe and measure the bank toe. The banks show a terrace of erosion close to the normally regulated water level, highly irregular erosion rates up to 7 m/year, embayments evolving with upstream and downstream shifts, and sub-reaches with uniform erosion. Probable causes of erosion include ship-waves, high water flows and water level fluctuations. Distinct patterns might be explained by the presence of riparian trees and soil strata of different compositions. These intriguing case studies will continue to be studied to disentangle the role of different erosion drivers, predict erosion magnitudes and establish whether bank erosion will stop or continue in the future.@en

We apply structure from motion (SfM) photogrammetry with imagery from an unmanned aerial vehicle (UAV) to measure bank erosion processes along a mid-sized river reach. This technique offers a unique set of characteristics compared to previously used methods to monitor banks, such as high resolution and relatively fast deployment in the field. We analyse the retreat of a 1.2 km restored bank of the Meuse River which has complex vertical scarps laying on a straight reach, features that present specific challenges to the UAV-SfM application. We surveyed eight times within a year with a simple approach, combining different photograph perspectives and overlaps to identify an effective UAV flight. The accuracy of the digital surface models (DSMs) was evaluated with real-time kinematic (RTK) GPS points and airborne laser scanning of the whole reach. An oblique perspective with eight photo overlaps and 20 m of cross-sectional ground-control point distribution was sufficient to achieve the relative precision to observation distance of ∼ 1 : 1400 and 3 cm root mean square error (RMSE), complying with the required accuracy. A complementary nadiral view increased coverage behind bank toe vegetation. Sequential DSMs captured signatures of the erosion cycle such as mass failures, slump-block deposition, and bank undermining. Although UAV-SfM requires low water levels and banks without dense vegetation as many other techniques, it is a fast-in-the-field alternative to survey reach-scale riverbanks in sufficient resolution and accuracy to quantify bank retreat and identify morphological features of the bank failure and erosion processes. Improvements to the adopted approach are recommended to achieve higher accuracies.

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Waterways serve for several functions besides  transporting goods and people. The ecological  importance of navigable rivers has taken much  attention during recent decades bringing efforts  to improve these natural corridors for fauna and  flora (Boeters et al., 1997).  Following the policy of the European Water  Framework Directive (WFD), many Dutch river  reaches have been recently restored through the  removal of bank protections in search for better  riparian habitats (Florsheim et al., 2009), but they  also result exposed to erosive forces. Large  uncertainties generally surround the prediction of  erosion rates (e.g. Samadi et al., 2009) due to  complex flow characteristics in the near-bank  region, variable soil properties, etc. A better  understanding of bank erosion processes is then  of interest to predict erosion rates and improve  the design of future interventions.  @en

Nature manifests itself in sometimes surprisingly simple patterns, even though we know that the underlying coupled equations are complex and highly non-linear. Alluvial estuaries, thNew floodplain formation starts with the development of near-bank sediment deposits such as alternate bars and point bars (e.g. Hickin, 1984). An important step of this process is the colonization of the areas emerging during low flow by plants. Vegetation protects local soil from erosion during subsequent high flows and enhances local sedimentation, increasing the vertical growth of colonized areas. The morphological effects of bar colonization by plants has been studied using numerical models (e.g. Crosato and Samir Saleh, 2011), but laboratory experiments have so far focused on the effects of floodplain vegetation (e.g. Tal and Paola, 2010). This work describes the effects of alternate bar colonization by plants on channel morphology in a large-scale laboratory setting with variable discharge and sediment recirculation. Three situations are analysed and compared: without vegetation (a), with vegetated floodplains only (b) and with vegetation colonizing also the areas emerging during low flows (c). at are the result of interacting forces of nature within a mobile sedimentary medium, are a clear manifestation of such patterns. But what constrains the formation of shape? Which physical laws are behind it? The author does not have the answers, but raises some pertinent questions.@en

The use of photogrammetry based on UAVs allows to survey long distances of riverbanks with a resolution capable of identifying detailed bank erosion features and quantifying retreat rates with an accuracy of centimetres. The identification of these features would be missed in slower and more expensive GPS profiling. Moreover, moving terrestrial laser scanning would require significant greater times and costs for comparable results. @en

Over the past centuries natural river banks have been transformed into banks with artificial revetments or sheet piles to protect them from erosion. Important river features for flora and fauna have disappeared and the ecological quality of the river reduced dramatically. Recently, the importance of the ecological function of rivers has been getting more attention. One river restoration measure is the removal of man-made bank protections to increase habitat diversity and biodiversity of riparian areas and the river basin. The river morphology may change due to the freely eroding banks in the restored section. Reference projects show that the removal of bank protection along rivers may lead to the formation of bars (e.g. Schirmer et al., 2014). Bars increase morphological diversity, providing specific habitats for flora and fauna (Kurth and Schirmer, 2014). There is a lack of knowledge about the formation of bars related to the length and location of the removal of bank protection. The length of river bank protection removal is usually limited, due to human activities along the riversides. Therefore, a guideline is needed for the design of bank protection removal to enhance habitat diversity through bar formation to make this a feasible river restoration method. @en

Managing river bar formation in alluvial channels remains a challenging issue related to the need to free intakes, improve navigation and optimise river restoration works. This work studies the effects of locally varying the channel width on bar formation to see whether channel widening and narrowing could be feasible bar control measures. The investigation focuses on steady (hybrid) bars, the most common type of bars in lowland rivers. Several numerical experiments are performed using a two-dimensional physics-based finite-difference code. Model simplifications include capacity-limited sediment transport, uniform grain size and constant discharge. Previous tests on field and experimental data show that the simulations of the relevant processes are realistic. The results indicate that the formation of steady alternate bars downstream of lateral structures occurs at a distance that depends on the local width reduction and that narrowing the channel for a distance of 10 times the original width appears sufficient to locally suppress alternate bars. A symmetric inflow forces the formation of symmetric bed topography, as for instance a flat bed or central bars. Similarly, an asymmetric inflow forces asymmetric bed topography, as alternate bars. Upstream flow asymmetries disrupt the symmetry of central bars leading to a compound bed configuration characterised by a dominant wandering channel, a common feature in wide lowland rivers. Central and alternate bars are found to coexist even if bar stability theories predict the development of alternate bars only. These results are promising and raise fundamental questions, but need experimental and field confirmation.

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