Walter Bertoldi

Modeling vegetation controls on fluvial morphological trajectories

The role of riparian vegetation in shaping river morphology is widely recognized. The interaction between vegetation growth and riverbed evolution is characterized by complex nonlinear feedbacks, which hinder direct estimates of the role of key elements on the morphological evolutionary trajectories of gravel bed rivers. Adopting a simple theoretical framework, we develop a numerical model which couples hydromorphodynamics with biomass dynamics. We perform a sensitivity analysis considering several parameters as flood intensity, type of vegetation, and groundwater level. We find that the inclusion of vegetation determines a threshold behavior, identifying two possible equilibrium configurations: unvegetated versus vegetated bars. Stable vegetation patterns can establish only under specific conditions, which depend on the different environmental and species-related characteristics. From a management point of view, model results show that relatively small changes in water availability or species composition may determine a sudden shift between dynamic unvegetated conditions to more stable, vegetated rivers.

Implications of channel processes for juvenile fish habitats in Alpine rivers.

Abstract.We adopt a multidisciplinary approach toward the quantitative assessment of juvenile fish habitats in Alpine rivers using analytical modeling. The study focuses on braided and single-thread channel configurations together with their associated hydrodynamic patterns. A distinct difference between flows in these channels is the number and spatial arrangement of recirculation zones. These are due to the separation of flow from the river banks and result in a higher retention of flow in braided channels. Braided channels were also shown to provide more favourable shelter and nursing conditions for fish larvae and juveniles by mitigating high velocities during floods, by maintaining relatively shallow areas of flow, and by significant adjustments in the thermal regime. A historical analysis revealed a significant reduction of braided reaches along Alpine rivers that have most likely led to a significant degradation of the fish fauna.

The effect of lateral confinement on gravel bed river morphology

In this paper, we use a physical modeling approach to explore the effect of lateral confinement on gravel bed river planform style, bed morphology, and sediment transport processes. A set of 27 runs was performed in a large flume (25 m long, 2.9 m wide), with constant longitudinal slope (0.01) and uniform grain size (1 mm), changing the water discharge (1.5–2.5 L/s) and the channel width (0.15–1.5 m) to model a wide range of channel configurations, from narrow, straight, embanked channels to wide braided networks. The outcomes of each run were characterized by a detailed digital elevation model describing channel morphology, a map of dry areas and areas actively transporting sediment within the channel, and continuous monitoring of the amount of sediment transported through the flume outlet. Analysis reveals strong relationships between unit stream power and parameters describing the channel morphology. In particular, a smooth transition is observed between narrow channels with an almost rectangular cross-section profile (with sediment transport occurring across the entire channel width) and complex braided networks where only a limited proportion (30%) of the bed is active. This transition is captured by descriptors of the bed elevation frequency distribution, e.g., standard deviation, skewness, and kurtosis. These summary statistics represent potentially useful indicators of bed morphology that are compared with other commonly used summary indicators such as the braiding index and the type and number of bars.

Analysis of reach‐scale elevation distribution in braided rivers: Definition of a new morphologic indicator and estimation of mean quantities

This work has been carried out within the SMART Joint Doctorate (Science forthe MAnagement of Rivers and theirTidal systems) funded with the support of the Erasmus Mundus programme of the European Union. Data of the Rees River were derived as part of UKNatural Environment Research Council grant (NE/G005427/1) awarded to PI Brasington, along with further support from the NERC Geophysical Equipmen tFacility (Loan 892) and Leverhulme Trust IAF2014-038

Multidecadal dynamics of alternate bars in the Alpine Rhine River

We report on a multi-decadal analysis of alternate bar dynamics in a 41.7 km reach of the Alpine Rhine River, which represents an almost unique example of a regulated river with fixed levees, straight reaches and regular bends in which alternate gravel bars spontaneously formed and migrated for more than a century. The analysis is based on freely available Landsat imagery, which provided an accurate and frequent survey of the dynamics of the alternate bar configuration since 1984. Bars were characterized in terms of wavelength, migration, and height. Longitudinal and temporal patterns are investigated as a function of flood occurrence and magnitude and in relation to the presence of local planform discontinuities (bends and ramps) that may affect their dynamics. Bars in the upper part of the reach are mostly steady and relatively long (about 13 channel widths); bars in the lower part of the reach are migrating and shorter (about 9 channel widths). Bar height is rather uniform along the reach, ranging between 3 to 4 m. The temporally long hydrological dataset allowed the investigation of bar migration during flood events, showing that bars migrate faster for intermediate floods. The observed relationship between bar migration and wavelength was consistent with linear theories for free migrating and steady forced bars in straight channels. The comparison of theories with observations highlights the key role of theories to support interpretation of observations, for a better understanding of the morphodynamic processes controlling bar formation and dynamics. This article is protected by copyright. All rights reserved.

Capturing the spatiotemporal variability of bedload transport: A time-lapse imagery technique

Monitoring sediment transport in morphologically complex and labile channels remains a difficult task, even at the laboratory scale. To address this challenge, a fully automated imagery technique for continuously mapping the spatial and temporal variability of bedload transport is proposed. This method uses differentiated time-lapse imagery taken from a fixed camera to detect bed variations induced by grain displacement. The technique is not based on tracking the individual particles; rather, it evaluates macroscopic colour changes within a region that contains several grains, which depend on the occurrence and intensity of the bedload transport. Image-derived data were compared with the sediment flux measured during four flume experiments, and produced good correspondence. The method provides continuous tracking of the location of the transporting channels, and enables estimation of local variations in the magnitude of the bedload flux. Moreover, the spatial extent of the monitoring area offers an unprecedented opportunity to aggregate spatially dense and continuous data at the reach scale, as needed to properly capture the full range of variability of morphologically complex and rapidly evolving gravel-bed rivers. Despite being limited to laboratory-scale physical experiments, the method provides useful data to investigate fundamental morphodynamic processes such as bar migration, bank erosion, anabranches opening/closure, and the associate spatial and temporal scales. Further, the data obtained have the potential to enhance numerical model calibration and improve our understanding of the complex dynamics of real-world settings. Copyright

Channelization of a large Alpine river: what is left of its original morphodynamics?: The effect of channelization on the morphology of the Adige River

The Adige River drains 12 200 km of the Eastern Alps and flows for 213 km within this mountain range. Similar to other large rivers in Central Europe, the Adige River was subject to massive channelization works during the 19th century. Thanks to the availability of several historical maps, this river represents a very valuable case study to document the extent to which the morphology of the river changed due to channelization and to understand how much is left of its original morphodynamics. The study was based on the analysis of seven sets of historical maps dating from 1803–1805 to 1915–1927, on geomorphological analysis, on the application of mathematical morphodynamic theories and on the application of bar and channel pattern prediction models. The study concerns 115 km of the main stem and 29 km of its tributaries. In the pre-channelization conditions, the Adige River presented a prevalence of single-thread channel planforms. Multi-thread patterns developed only immediately downstream of the main confluences. During the 19th century, the Adige underwent considerable channel adjustment, consisting of channel narrowing, straightening, and reduction of bars and islands. Multi-thread and single-thread reaches evolved through different evolutionary trajectories, considering both the channel width and the bar/vegetation interaction. Bar and channel pattern predictors showed good correspondence with the observed patterns, including the development of multi-thread morphologies downstream of the confluences. Application of the free-bar predictor helped to interpret the strong reduction – almost complete loss – of exposed sediment bars after the channelization works, quantifying the riverbed inclination to form alternate bars. This morphological evolution can be observed in other Alpine rivers of similar size and similar massive channelization, therefore, a simplified conceptual model for large rivers subjected to channelization is proposed, showing that a relatively small difference in the engineered channel width may have a strong impact on the river dynamics, specifically on bar formation. Copyright

River bank burrowing by invasive crayfish: spatial distribution, biophysical controls and biogeomorphic significance

Invasive species generate significant global environmental and economic costs and represent a particularly potent threat to freshwater systems. The biogeomorphic impacts of invasive aquatic and riparian species on river processes and landforms remain largely unquantified, but have the potential to generate significant sediment management issues within invaded catchments. Several species of invasive (non-native) crayfish are known to burrow into river banks and visual evidence of river bank damage is generating public concern and media attention. Despite this, there is a paucity of understanding of burrow distribution, biophysical controls and the potential significance of this problem beyond a small number of local studies at heavily impacted sites. This paper presents the first multi-catchment analysis of this phenomenon, combining existing data on biophysical river properties and invasive crayfish observations with purpose-designed field surveys across 103 river reaches to derive key trends. Crayfish burrows were observed on the majority of reaches, but burrowing tended to be patchy in spatial distribution, concentrated in a small proportion (<10%) of the length of rivers surveyed. Burrow distribution was better explained by local bank biophysical properties than by reach-scale properties, and burrowed banks were more likely to be characterised by cohesive bank material, steeper bank profiles with large areas of bare bank face, often on outer bend locations. Burrow excavation alone has delivered a considerable amount of sediment to invaded river systems in the surveyed sites (3tkm(-1) impacted bank) and this represents a minimum contribution and certainly an underestimate of the absolute yield (submerged burrows were not recorded). Furthermore, burrowing was associated with bank profiles that were either actively eroding or exposed to fluvial action and/or mass failure processes, providing the first quantitative evidence that invasive crayfish may cause or accelerate river bank instability and erosion in invaded catchments beyond the scale of individual burrows.

Braiding Rivers: State of the Art and Future Challenges

Eleven years after the last Braided Rivers Conference in Birmingham, England, a group of 47 scientists from 11 countries met at le Domaine de Sainte Croix, France, to discuss the current state of knowledge and future research directions for braided rivers. Several core themes and research challenges arose from the 5-day workshop.

6 Bifurcations in gravel-bed streams

Abstract In the present paper we provide an overview on some recent experimental and theoretical works, which have been specifically designed to analyze the behaviour of bifurcations in gravel-bed streams. We first investigate the occurrence of a bifurcation starting from an initially straight channel: the interaction between bed and banks evolution determines flow bifurcation when channel width oscillations reach a maximum amplitude; the process is strongly dependent on the migration speed of bars that form in the channel. Experimental evidence on the equilibrium configurations and stability of a simple Y-shaped bifurcation, both in the case of fixed and erodible-bank channels, shows that bifurcations are likely to display unbalanced configurations, characterized by uneven partition of flow discharge and different values of free surface width of downstream branches. Moreover bed levels of downstream branches differ, in accordance with field observations on gravel-bed rivers. Theoretical predictors based on 1D schemes supplemented by suitable 2D information at the node satisfactorily replicate most of the observed features. When the channels joining at the node are free to evolve, the stability of bifurcations is controlled by the occurrence of migrating bars and by the adaptation processes of channel width and planform, as well by the morphodynamic influence of the bifurcation. The resulting evolution may also depend on the initial mechanism triggering flow bifurcation.