Alluvial rivers are shaped by sequences of water flows excavating their channels. Observations show that besides the magnitude, also the frequency and duration of streamflow oscillations might be important for the river channel formation. In addition, the river morphology appears influenced also by both size and degree of uniformity of the sediment. Nevertheless, many morphodynamic studies still represent the flow regime with a single value of the discharge, often corresponding to the bankfull condition, and the sediment with its median grain size. This work investigates the effects of streamflow variability and sediment characteristics on channel width formation, analysing the evolution of experimental streams with different sediments and discharge hydrographs. Results show that the formative condition of the channel width is not the geometric bankfull flow but a rather frequent peak flow. Remarkably different channel configurations arise from different sediment characteristics in the laboratory, where sediment non-uniformity produces more stable banks.@en

Alluvial rivers are shaped by sequences of water flows excavating their channels. Observations show that besides the magnitude, the frequency and duration of streamflow oscillations might also be important for the river channel formation. In addition, the river morphology appears influenced by both size and degree of uniformity of the sediment. Nevertheless, many morphodynamic studies still represent the flow regime with a single value of the discharge, often corresponding to the bankfull condition, and the sediment with its median grain size. This work investigates the effects of streamflow variability and sediment characteristics on channel width formation, analysing the evolution of experimental streams with different sediments and discharge hydrographs. Results show that the formative condition of the channel width is not the geometric bankfull flow but a rather frequent peak flow. Remarkably different channel configurations arise from different sediment characteristics in the laboratory, where sediment non-uniformity produces more stable banks.

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The prediction of the morphological evolution of renaturalized streams is important for the success of restoration projects. Riparian vegetation is a key component of the riverine landscape and is therefore essential for the natural rehabilitation of rivers. This complicates the design of morphological interventions, since riparian vegetation is influenced by and influences the river dynamics. Morphodynamic models, useful tools for project planning, should therefore include the interaction between vegetation, water flow and sediment processes. Most restoration projects are carried out in USA and Europe, where rivers are highly intervened and where the climate is temperate and vegetation shows a clear seasonal cycle. Taking into account seasonal variations might therefore be relevant for the prediction of the river morphological adaptation. This study investigates the morphodynamic effects of riparian vegetation on a re-meandered lowland stream in the Netherlands, the Lunterse Beek. The work includes the analysis of field data covering 5years and numerical modelling. The results allow assessment of the performance of a modelling tool in predicting the morphological evolution of the stream and the relevance of including the seasonal variations of vegetation in the computations. After the establishment of herbaceous plants on its banks, the Lunterse Beek did not show any further changes in channel alignment. This is here attributed to the stabilizing effects of plant roots together with the small size of the stream. It is expected that the morphological restoration of similarly small streams may result in important initial morphological adaptation followed by negligible changes after full vegetation establishment.

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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

River restoration is a core component of Building with Nature, but it is hard to predict how a river evolves after restoration efforts. Andres Vargas Luna performed fundamental research into the effects of vegetation on river behaviour. To study this almost unexplored field, he used a combination of field observations and conducted experiments in a unique 45 meter long test basin@en

This paper presents the morphodynamic effects of riparian vegetation growth in a lowland restored stream. Hydrological series, high-resolution bathymetric data and aerial photographs are combined in the study. The vegetation root system was found to assert a strong control on soil stabilization, even during the winter season when plants are dead or degraded. Seasonal variations in plant biomass appear important only during the first stages of establishment, when vegetation has a low density and, more importantly, a root system that is not fully developed yet.@en

Vegetation processes are widely recognized as a key component on the ecological and morphological development of river channels. Moreover, plants reduce flow velocities and bed-shear stresses by increasing the local hydraulic roughness and thus increasing water levels. Therefore, monitoring the vegetation development is an important activity in river management not only for protecting ecological services, but also in flood risk reduction; especially in times of a changing climate. This paper presents the analysis the effects of riparian vegetation growth on the morphology of a lowland restored stream located in The Netherlands, the Lunterse beek. An Unmanned Aerial Vehicle (UAV) was used to obtain aerial imagery at different time steps which was the basis for generating land cover maps with semi-automated image classification. In addition hydrological series and multi-temporal highresolution bathymetric data allowed analysing river bed morphology and the relevance of seasonality. The UAV campaigns were found a crucial step to ease the vegetation mapping and monitoring. The morphological change observed in this stream, represented by the channel-width adjustment and the cross sectional evolution, is slowed down once vegetation is stablished on the stream. Results of this work show that the vegetation root system assert a strong control on soil stabilization, even during the winter season when the plants biomass is highly reduced. Seasonal variations in plant development appear important only during the first stages of establishment, when vegetation has a low density and, more importantly, a root system that is not fully developed yet. @en

Simulating the morphological adaptation of water systems often requires including the effects of plants on water and sediment dynamics. Physical and numerical models need representing vegetation in a schematic easily-quantifiable way despite the variety of sizes, shapes and flexibility of real plants. Common approaches represent plants as rigid cylinders, but the ability of these schematizations to reproduce the effects of vegetation on morphodynamic processes has never been analyzed systematically. This work focuses on the consequences of representing plants as rigid cylinders in laboratory tests and numerical simulations. New experiments show that the flow resistance decreases for increasing element Reynolds numbers for both plants and rigid cylinders. Cylinders on river banks can qualitatively reproduce vegetation effects on channel width and bank-related processes. A comparative review of numerical simulations shows that Baptist's method that sums the contribution of bed shear stress and vegetation drag, underestimates bed erosion within sparse vegetation in real rivers and overestimates the mean flow velocity in laboratory experiments. This is due to assuming uniform flow among plants and to an overestimation of the role of the submergence ratio@en

There is rising awareness of the need to include the effects of vegetation in studies dealing with the morphological response of rivers. Vegetation growth on river banks and floodplains alters the river bed topography, reduces the bank erosion rates and enhances the development of new floodplains through river bank accretion. The role of riparian vegetation on river morphology is examined in this thesis, with particular attention to its effects on bank accretion, focusing on lowland streams in temperate climates. The work is based on the combination of extensive literature review, small- and large-scale laboratory experiments, field observations and numerical simulations in order to overcome the shortcomings of single approaches. The results of the study demonstrated that vegetation is essential for the accretion of river banks in non-clay dominated environments, highlighting the role of the colonization of new deposits by plants, which is strongly influenced by the hydrologic regime. Vegetation establishment plays a key role on the stabilization of the channel-width and on the vertical accretion of both levees and floodplains. The vertical accretion and channel incision induced by colonizing plants showed that vegetation colonization increases the amplitude and length of the bars in the main channel, affecting the final river planform. The outcomes of this research emphasize the relevance of considering the effects of vegetation on the river management and on the designing, planning and maintenance programs of restoration projects. To advance in the understanding of the dynamics of river banks, future research is also recommended on quantifying of the role of root systems and fine sediments on the reinforcement and consolidation processes of soils.@en

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