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Meandering planforms are commonly observed in fluvial systems. A meander consists of a series of two alternate bends connected at the points of inflexion by relatively short, almost straight crossings. The presence of single-thread meandering rivers exhibiting a continuous sequence of such curves is widespread in alluvial floodplains. The study of river meanders has thus fascinated the scientific community, which, for a long time, has tried not only to classify them but also to quantify the complexity of meandering planforms and model their morphodynamic evolution.The idea of classifying meandering rivers has a long history. It has produced a series of non-dimensional parameters to identify a meander (i.e., half-meander amplitude, asymmetry index, half-meander sinuosity). Nevertheless, two main problems arise from the existing methodologies. They are too complicated to encompass as many shapes as possible or lack physical insight into hydraulic and sedimentological parameters.We propose a data-driven approach to address this classification issue, mixing physics-based information and machine-learning algorithms. In our approach, we consider the spatial distribution of meander curvatures and analyse it using different continuous wavelet transforms, getting the energy spectrum for each meander. This physics-based information is then firstly processed as an unsupervised visual classification problem using a neural-network autoencoder mix with cluster algorithms. The output of this first step analysis consists of two pre-trained algorithms that can classify the energy spectrum of pictures of planform curvatures and, therefore, the meander planform shape.The algorithms will be trained with a series of dimensionless, synthetically generated meanders and t subsequently tested with both natural and simulated meanders. The final aim is to identify automatically which type of meanders characterise a given river reach at a certain time. This methodology also has the potential to be extended to Spatiotemporal distributions of channel-axis curvature, thus unravelling key aspects of meandering dynamics, as well as identifying similarities between reaches of different rivers or between observed and synthetically generated river planforms.

Many present-day alluvial floodplains display traces of abandoned meandering channel belts developed during the past millennia (i.e. mid- to late Holocene). Deposits of these ancient rivers represent preferential pathways for groundwater flows and related environmental issues, such as contaminant propagation or saltwater intrusion in coastal areas. However, since formative bankfull flows in such old and abandoned routes are hard to estimate, fossil meanders have not been commonly addressed by morphodynamic numerical models, and most of them have been investigated following classical sedimentological approaches based mainly on punctual data derived from sedimentary cores. This study aims at investigating the sediment distribution within different fossil bends on the southern Venetian Plain (NE Italy), and relating such distribution to numerically-modelled bed shear stresses used herein as a proxy of sediment sorting patterns. For this purpose, formative flows in the studied paleomeanders are first inferred from measured sediment grain size and estimates of bend widths. Then, shear stress distributions are computed along the studied paleobends using a 2D linearised model. Model results are finally compared with conductivity distributions gauged directly in the field through electromagnetic induction investigations in the frequency domain. Our results suggest significant correlations between shear stress distributions and sediment sorting estimated from conductivity data. We deem that the integration between sedimentological reconstructions and state-of-the-art numerical modelling can provide a solid contribution to predicting the spatial distribution of sediment properties within ancient meandering channel belts, with relevant implications for the understanding of shallow aquifer dynamics and soil management.

We survey the problem of the response of coastal wetlands to sea level rise. Two opposite views have traditionally been confronted. According to the former, on the geological time scale, coastal lagoons would be ‘ephemeral’ features. The latter view maintains that marshes would keep pace with relative sea level rise as, increasing the rate of the latter, the sedimentation rate would also increase. In any case, the timescale of morphodynamic evolution is of the order of centuries, which makes it not easily perceived. For example, in Venice, the diversion of the rivers debouching into the lagoon undertaken in the Renaissance has taken centuries to display its consequences (shift from depositional to erosional environment). This process accelerated in the last two centuries due to effects of the industrial revolution and of an enhanced sea level rise. Recent research has employed powerful computational techniques and advanced models of marsh vegetation. Zero-order modeling suggests that marsh equilibrium is possible, provided the rate of relative sea level rise does not exceed a threshold depending on the availability of minerogenic sediments, quantified through a loosely defined ambient sediment concentration. Analysis of the morphological interaction between adjacent morphological units suggests that the ‘equilibrium states’ identified by zero-order modeling correspond to marshes which either prograde or retreat, i.e., are not in equilibrium. Results suggest that available techniques, e.g., artificial replenishment of salt marshes or search for more productive halophytic species, will hardly allow Venice wetlands to keep up with a strong acceleration of sea level rise.

1_Data -> The subfolder "Planforms_data" contains the planform data and physical and sedimentological parameters for the morphodynamic model. The subfolder "Conductivity_and_shear_stress_data" contains the data of electrical conductivity in each rectangular segment andthe corresponding shear stress value (Figures 9,10,11 in the paper). 2_Formative_Flow -> the script "main.mat" computes the formative flow features for the paleomeander given the estimates for granulometric data and the width. The formative flow features are used as input for the meander morphodynamic model. 3_Morphodynamic model -> Meander morphodynamics is addressed by using the bidimensional linearised modelling framework developed byFrascati & Lanzoni (2013).The code was used to compute the equilibrium configuration and shear stress distribution in a paleomeander bend.

The repository contains two folders: 1_Formative_Flow-> the script "main.mat" computes the formative flow features for the paleomeander given the estimates for granulometric data and the width. The formative flow features are used as input for the meander morphodynamic model. 2_Morphodynamic model -> Meander morphodynamics is addressed by using the bidimensional linearised modelling framework proposed by Frascati & Lanzoni (2013).The code was used to compute equilibrium configuration and shear stress distribution in a paleomeander bend.

Understanding and predicting the geomorphological response of fluvial and tidal channels to bank retreat underpins the robust management of water courses and the protection of wetlands.

The geometry of alluvial river channels both controls and adjusts to the flow of water and sediment within them. This feedback between flow and form modulates flood risk, and the impacts of climate and land-use change. Considering widely varying hydro-climates, sediment supply, geology and vegetation, it is surprising that rivers follow remarkably consistent hydraulic geometry scaling relations. In this Perspective, we explore the factors governing river channel geometry, specifically how the threshold of sediment motion constrains the size and shape of channels. We highlight the utility of the near-threshold channel model as a suitable framework to explain the average size and stability of river channels, and show how deviations relate to complex higher-order behaviours. Further characterization of the sediment transport threshold and channel adjustment timescales, coupled with probabilistic descriptions of river geometry, promise the development of future models capable of capturing rivers’ natural complexity.

Confined meandering rivers are bounded between valleys and they are limited in their lateral migration compared to free meandering ones. In this study, we designed a model-based analysis to investi...

The formation and development of tidal channels and salt marshes are controlled by complex interactions between hydrodynamics, sediment transport, and vegetation dynamics. Tidal channels affect and...

The presence of lateral planimetric constraints preventing free migration and avulsion has a significant influence on the planform dynamics of river meanders. This particular confined kind of meandering rivers is definitely understudied, especially in comparison to freely migrating ones. Through a semi-analytical meander model, here we attempt to investigate the effect of the confined floodplain width through a morphodynamic modeling approach. The confined floodplain width is defined as the width between symmetric lateral confinement where the river is free to migrate, on the planform pattern and dynamics. Model results illustrate that weak confinement (i.e. loose floodplain boundaries) increases planform irregularity, with the river centerline preferentially lying close to floodplain boundaries, while strong confinement (tight floodplain boundaries) leads to a remarkable planform regularity, constituted by periodic sequences of sawtooth-shaped meanders. Bend orientation is reminiscent of the sub/super-resonant regime regardless of the confinement width. Model results are supported by good agreement with available field and remote sensing observation on selected case studies of confined meandering rivers in Canada previously studied by Nicoll and Hickin (2014).

Meandering channels extensively dissect fluvial and tidal landscapes, critically controlling their morphodynamic evolution and sedimentary architecture. In spite of an apparently striking dissimilarity of the governing processes, planform dimensions of tidal and fluvial meanders consistently scale to local channel width, and previous studies were unable to identify quantitative planimetric differences between these landforms. Here we use satellite imagery, measurements of meandering patterns, and different statistical analyses applied to about 10,000 tidal and fluvial meanders worldwide to objectively disclose fingerprints of the different physical processes they are shaped by. We find that fluvial and tidal meanders can be distinguished on the exclusive basis of their remotely-sensed planforms. Moreover, we show that tidal meanders are less morphologically complex and display more spatially homogeneous characteristics compared to fluvial meanders. Based on existing theoretical, numerical, and field studies, we suggest that our empirical observations can be explained by the more regular processes carving tidal meanders, as well as by the higher lithological homogeneity of the substrates they typically cut through. Allowing one to effectively infer processes from landforms, a fundamental inverse problem in geomorphology, our results have relevant implications for the conservation and restoration of tidal environments, as well as from planetary exploration perspectives.

In the Dolomitic region, abundant coarse hillslope sediment is commonly found at the toe of rocky cliffs. Ephemeral channels originate where lower permeability bedrock surfaces concentrate surface runoff. Debris flows initiate along such channels following intense rainfall and determine the progressive erosion and deepening of the channels. Sediment recharge mechanisms include rock fall, dry ravel processes and channel-bank failures. Here we document debris flow activity that took place in an active debris flow basin during the year 2015. The Cancia basin is located on the southwestern slope of Mount Antelao (3264 m a.s.l.) in the dolomitic region of the eastern Italian Alps. The 2.5 km2 basin is incised in dolomitic limestone rocks. The data consist of repeated topographic surveys, distributed rainfall measurements, time-lapse (2 s) videos of two events and pore pressure measurements in the channel bed. During July and August 2015, two debris flow events occurred, following similarly intense rainstorms. We compared rainfall data to existing rainfall triggering thresholds and simulated the hydrological response of the headwater catchment with a distributed model in order to estimate the total and peak water discharge. Our data clearly illustrate how debris entrainment along the channel is the main contributor to the overall mobilized volume and that erosion is dominant when the channel slope exceeds 16°. Further downstream, sediment accumulation and depletion occurred alternately for the two successive events, indicating that sediment availability along the channel also influences the flow behaviour along the prevailing-transport reach. The comparison between monitoring data, topographical analysis and hydrological simulation allows the estimation of the average solid concentration of the two events and suggests that debris availability has a significant influence on the debris flow volume. © 2020 John Wiley & Sons, Ltd.

Geng, L.; Gong, Z.; Lanzoni, S., and D'Alpaos, A., 2018. A New Method for Automatic Definition of Tidal Creek Networks. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 156–160. Coconut Creek (Florida), ISSN 0749-0208.Tidal creek delineation is the first step in the analysis of the structure and dynamics of tidal creek networks. This study proposed a new creek definition method, in which we determined the critical elevation of creek delineation by the local elevation distribution. This method can be applied to intertidal zones with inclined bed surfaces, showing a better applicability than existing methods. By comparing creek characteristics within a certain small tidal flat using different delineation methods, we evaluated the accuracy of this new method. The size measured by the new method nicely matches results obtained on the basis of the method proposed by Fagherazzi et al. (1999).

The fate and transport of soluble contaminants released in natural streams are strongly dependent on the spatial variations of the flow field and of the bed topography. These variations are essentially related to the presence of the channel banks and to the planform configuration of the channel. Large velocity gradients arise near to the channel banks, where the flow depth decreases to zero. Moreover, single thread alluvial rivers are seldom straight, and usually exhibit meandering planforms and a bed topography that deviates from the plane configuration. Channel axis curvature and movable bed deformations drive secondary helical currents which enhance both cross sectional velocity gradients and transverse mixing, thus crucially influencing longitudinal dispersion. The present contribution sets up a rational framework which, assuming mild sloping banks and taking advantage of the weakly meandering character often exhibited by natural streams, leads to an analytical estimate of the contribution to longitudinal dispersion associated with spatial non-uniformities of the flow field. The resulting relationship stems from a physics-based modeling of the flow in natural rivers, and expresses the bend averaged longitudinal dispersion coefficient as a function of the relevant hydraulic and morphologic parameters. The treatment of the problem is river specific, since it relies on an explicit spatial description, although linearized, of the flow field that establishes in the investigated river. Comparison with field data available from tracer tests supports the robustness of the proposed framework, given also the complexity of the processes that affect dispersion dynamics in real streams.

Significance Meandering tidal channel networks play a central role in the ecomorphodynamic evolution of the landscapes they cut through. Despite their ubiquitous presence and relevance to sedimentary and landscape features, few observations of tidal-meander evolution exist, and we lack a full understanding of the governing processes. Field analyses show that tidal meanders, traditionally viewed as stable landscape features, display modes of migration and migration rates per unit width quite similar to those characterizing their fluvial counterparts, with important implications for the characterization of the related sedimentary products. The results presented here contribute to our understanding of the morphological evolution of tidal landscapes., The majority of tidal channels display marked meandering features. Despite their importance in oil-reservoir formation and tidal landscape morphology, questions remain on whether tidal-meander dynamics could be understood in terms of fluvial processes and theory. Key differences suggest otherwise, like the periodic reversal of landscape-forming tidal flows and the widely accepted empirical notion that tidal meanders are stable landscape features, in stark contrast with their migrating fluvial counterparts. On the contrary, here we show that, once properly normalized, observed migration rates of tidal and fluvial meanders are remarkably similar. Key to normalization is the role of tidal channel width that responds to the strong spatial gradients of landscape-forming flow rates and tidal prisms. We find that migration dynamics of tidal meanders agree with nonlinear theories for river meander evolution. Our results challenge the conventional view of tidal channels as stable landscape features and suggest that meandering tidal channels recapitulate many fluvial counterparts owing to large gradients of tidal prisms across meander wavelengths.

Annual mean sea-level records over nearly 130 yr in the northern Adriatic Sea indicate a combined influence of lunar precessions and solar activity in causing their interannual variability. Sea-level indices of variability at Venice and Trieste were obtained by removing trends and filtering variations shorter than 5 yr. Inspired by reconstructions with Fourier coefficients, the sea-level indices were restored with harmonics related to solar activity, lunar precessions and their interference. Harmonics were fit to indices, explaining more than 75% of the indices’ variance with skills greater than 0.93, at both sites. Fits also allowed projections of interannual variability of mean sea level in the northern Adriatic for the rest of the 21st century. Elevated mean sea levels are expected to be centered around 2022–23, 2049–50, 2066–67, and 2090–91. It was also found that mean sea levels have been anomalously high in the northern Adriatic during at least 75% of the last 12 highs in the solar cycles.

This work addresses the signatures embedded in the planform geometry of meandering rivers consequent to the formation of floodplain heterogeneities as the river bends migrate. Two geomorphic features are specifically considered: scroll bars produced by lateral accretion of point bars at convex banks and oxbow lake fills consequent to neck cutoffs. The sedimentary architecture of these geomorphic units depends on the type and amount of sediment, and controls bank erodibility as the river impinges on them, favoring or contrasting the river migration. The geometry of numerically generated planforms obtained for different scenarios of floodplain heterogeneity is compared to that of natural meandering paths. Half meander metrics and spatial distribution of channel curvatures are used to disclose the complexity embedded in meandering geometry. Fourier Analysis, Principal Component Analysis, Singular Spectrum Analysis and Multivariate Singular Spectrum Analysis are used to emphasize the subtle but crucial differences which may emerge between apparently similar configurations. A closer similarity between observed and simulated planforms is attained when fully coupling flow and sediment dynamics (fully-coupled models) and when considering self-formed heterogeneities that are less erodible than the surrounding floodplain.

A storm, composed of two delayed cells, hit the Monte Antelao slopes (Dolomites, North Eastern Italy) in the early morning of July 18th, 2009. The resulting runoff triggered two consecutive debris flows along the Rovina di Cancia channel. The detailed topographic data collected before and just after this event allowed an accurate reconstruction of the morphological changes experienced by the channel bed. These data are here used as benchmark to test the ability of numerical models to reproduce the dynamics of a real event, taking into account the morphology changes of the channel bed. The aim is to provide an efficient model for engineering applications on large scales, such as those required by debris flow hazard assessment. A rainfall-runoff transformation is applied to reconstruct the solid-liquid hydrograph needed for computing the debris flow propagation. Two routing models are used: a GIS-based movable bed model, and a widely used fixed bed model (FLO-2D). Although similar results are obtained in terms of areas subjected to deposition, significant differences emerge in terms of mobilized volumes. Only the simulation of both the deposition and entrainment processes allows to reliably reproduce the sediment volumes estimated from the pre- and post-event topographic data. This information is fundamental in any hazard assessment because the volume of sediment mobilized by debris flow events exerts a fundamental control on the extension of areas subjected to inundation and on the thickness of sediment deposits. The capability to reproduce correctly the mobilized volumes also entails a more reliable simulation of the evolution of the peak and volume of the solid-liquid hydrograph as the debris flow propagates downstream, allowing the identification of the channel reach where banks could be overflowed. Conversely, adopting a fixed bed model leads mainly to an underestimation of the both the transported sediments volumes and the area subjected to deposition. As a consequence, the maximum debris flow depth in the portion of the channel subjected to erosion is underestimated and that in the portion of the channel subjected to deposition is overestimated. All these types of information are of great importance for an effective hazard assessment.

The overall dynamics of intertidal environments are governed by complex interactions between their ecological and geomorphological components. A more comprehensive understanding of such dynamics and their predictive modeling can only be achieved through the description of the feedback coupling hydrodynamics and sediment transport on one hand and ecological processes on the other. Toward the goal of a comprehensive theoretical framework describing the large-scale, long-term evolution of the intertidal landscape, we describe an ecomorphodynamic model which allows the investigation of the long-term evolution of tidal networks, the adjacent marsh platforms, and the effects of vegetation competition. On the basis of observational indications regarding the different time scales governing the various landscape-forming processes, the model decouples the initial rapid network incision from its subsequent slower elaboration and from the ecomorphological evolution of intertidal platforms. The planimetric development of tidal channel networks coupled to the vertical accretion of the adjacent marsh platform, explicitly accounting for the vegetation distribution and dynamics, in the face of changing sea level rise and sediment supply is modeled and analyzed. The resulting ecomorphodynamic model reproduces the ecogeomorphic patterns typical of intertidal environments and is used to explore the response of tidal ecogeomorphologies to different scenarios of changing sea level, incoming sediment concentrations, and halophytic vegetation competition. Complex network structures and tidal patterns meeting distinctive statistical properties of actual network structures and several observed characteristics of ecological and geomorphic relevance are reproduced by the model.

In dolomitic headwater catchments, intense rainstorms of short duration produce runoff discharges that often trigger debris flows on the scree slopes at the base of rock cliffs. In order to measure these discharges, we placed a measuring facility at the outlet (elevation 1770 m a.s.l.) of a small, rocky headwater catchment (area ∼ 0.032 km2, average slope ∼ 320%) located in the Venetian Dolomites (North Eastern Italian Alps). The facility consists of an approximately rectangular basin, ending with a sharp-crested weir. Six runoff events were recorded in the period 2011-2014, providing a unique opportunity for characterizing the hydrological response of the catchment. The measured hydrographs display impulsive shapes, with an abrupt raise up to the peak, followed by a rapidly decreasing tail, until a nearly constant plateau is eventually reached. This behavior can be simulated by means of a distributed hydrological model if the excess rainfall is determined accurately. We show that using the Soil Conservation Service Curve-Number (SCS-CN) method and assuming a constant routing velocity invariably results in an underestimated peak flow and a delayed peak time. A satisfactory prediction of the impulsive hydrograph shape including peak value and timing is obtained only by combining the SCS-CN procedure with a simplified version of the Horton equation, and simulating runoff routing along the channel network through a matched diffusivity kinematic wave model. The robustness of the proposed methodology is tested through a comparison between simulated and observed timings of runoff or debris flow occurrence in two neighboring alpine basins. This article is protected by copyright. All rights reserved.

Macroalgal mats of Ulva intestinalis are becoming increasingly common in many coastal and estuarine intertidal habitats, thus it is important to determine whether they increase flow resistance, promote bed stability and therefore reduce the risk of erosion favoring tidal flooding or degradation of coastal lagoons. Venier et al. (2012) [6] studied the impact of macroalgal mats of Ulva intestinalis on flow dynamics and sediment stability for uniform flow. Here we extend their experimental work to the case of vegetation under the combined action of waves and currents. These hydrodynamic conditions are very common in many shallow coastal environments and lagoons. The experimental facility employed in the present study and the series of flow runs are the same as that used by Venier et al. (2012)[6]. However, waves have been superposed to uniform current flowing firstly over a mobile sediment bed covered with U. intestinalis, then over a bare sediment surface. For the depth, wave and current conditions considered in the experiments, the time-averaged vertical profile of horizontal velocity for the case of coexisting waves and current turns out to be very close to that observed for a pure current, both with and without vegetation. However, contrary to what was observed in the case of a unidirectional current, in the presence of waves the time averaged velocity profile is only weakly influenced by the vegetation, whose main effect is to attenuate velocity oscillations induced by waves and to slightly increase the overall bed roughness.

Bendway weirs are low stone sills designed to control erosion at the outer bank of river bends, redirecting the current from the outer to the inner bank. Laboratory experiments were conducted in a movable bed channel with three consecutive meandering bends (relative curvature equal to 2) and continuous sediment supply from upstream. The aim of these tests was to study the effects of weir crest slope on maximum scour depth at the weir nose, on point-bar height and on cross-sectional eroded surface area in a meander live-bed. Concrete trapezoidal bendway weirs were constructed at the outer bank of the central bend. The investigated configurations consisted of series (3–5) of weirs characterized by prescribed: inclination angles (60°, 75° and 90°), length ratio (0.2, 0.3 and 0.4), crest slope (0%, 5%, 10% and 20%) discharge ratio (0.8, 1 and 1.2), the same height and the same distance between weir axes. Results showed that the maximum and minimum scour depths at the nose of sloping crest weirs were a...

Many models have been proposed to simulate and understand the long-term evolution of meandering rivers. These models analyze the hydraulics of the in-channel flow and the river bank movement (erosion – accretion) process in different ways, but some gap still remain, e.g. the stability of long-term simulations when width variations are accounted for. Here we proposed a physics-statistical based approach to simulate the river bank evolution, that erosion and deposition processes act independently, with a specific shear stress threshold for each of them. In addition, we link the width evolution with a parametric probability distribution (PPD) based on a mean characteristic channel width. We are thus able to obtaining stable long-term simulations with realistic and reasonable spatio-temporal distribution of the along channel width.

The fluid dynamics of stony debris flows generated in two small tributaries adjacent to each other and flowing into a main receiving channel was analyzed experimentally at a laboratory scale. The analysis on the propagation along the tributaries and deposition in the main channel provide information about sediment-water mobility, dangerous damming, and potential hazard. Debris flows were generated by releasing a preset water discharge over an erodible layer of saturated gravels material. As a consequence, the debris flow sediment concentration varied accordingly to the entrainment rate which, in turn, was strongly controlled by the tributary slope. The data collected by acoustic level sensors, pore fluid pressure transducers, and a load cell were used to characterize the evolution of bulk density and solid concentration of the sediment-water mixture. These two parameters were relevant to assess the stony debris flow mobility which contributes to determine the shape of sediment deposits in the main channel. The detailed bed topography surveys carried out in the main channel at the end of each experiment provided information on the morphology of these deposits and on the interplay of adjacent confluences. The influences of confluence angle, tributary slopes, and triggering conditions have been investigated, for a total of 18 different configurations. Within the investigated range of parameters, the slope angle was the parameter that mainly influences the stony debris flow mobility while, for adjacent confluences, the degree of obstruction within the receiving channel was strongly influenced by the triggering scenario.

The saltwater intrusion in the estuary of the Adige River has been investigated by a two-dimensional finite volume shock-capturing model. Owing to the relative small tide range characterizing the river mouth, a sharply stratified salt wedge tends to form during low discharge periods (e.g. in summer). Suitable hydraulic relations have been introduced to model the action of a submerged barrage, located close to the estuary mouth and built to hinder seawater intrusion. Field measurements of salinity profiles have been used to calibrate the model. The numerical results suggest that, as a consequence of increased water withdrawal that occurred in the last years, the barrage does not prevent efficiently the intrusion of the salt wedge any more.

We investigate the morphodynamic equilibrium of tidally dominated alluvial estuaries, extending previous works concerning the purely tidal case and the combined tidal-fluvial case with a small tidal forcing. We relax the latter assumption and seek the equilibrium bed profile of the estuary, for a given planform configuration with various degrees of funneling, solving numerically the 1-D governing equation. The results show that with steady fluvial and tidal forcings, an equilibrium bed profile of estuaries exists. In the case of constant width estuaries, a concave down equilibrium profile develops through most of the estuary. Increasing the amplitude of the tidal oscillation, progressively higher bed slopes are experienced at the mouth while the river-dominated portion of the estuary experiences an increasing bed degradation. The fluvial-marine transition is identified by a “tidal length” that increases monotonically as the river discharge and the corresponding sediment supply are increased while the river attains a new morphological equilibrium configuration. Tidal length also increases if, for a fixed river discharge and tidal amplitude, the sediment flux is progressively reduced with respect to the transport capacity. In the case of funnel-shaped estuaries the tidal length strongly decreases, aggradation is triggered by channel widening, and tidal effects are such to enhance the slope at the inlet and the net degradation of the river bed. Finally, results suggest that alluvial estuaries in morphological equilibrium cannot experience any amplification of the tidal wave propagating landward. Hence, hypersynchronous alluvial estuaries cannot be in equilibrium.

Morphodynamic equilibrium is a widely adopted yet elusive concept in the field of geomorphology of coasts, rivers and estuaries. Based on the Exner equation, an expression of mass conservation of sediment, we distinguish three types of equilibrium defined as static and dynamic, of which two different types exist. Other expressions such as statistical and quasi-equilibrium which do not strictly satisfy the Exner conditions are also acknowledged for their practical use. The choice of a temporal scale is imperative to analyse the type of equilibrium. We discuss the difference between morphodynamic equilibrium in the “real world” (nature) and the “virtual world” (model). Modelling studies rely on simplifications of the real world and lead to understanding of process interactions. A variety of factors affect the use of virtual-world predictions in the real world (e.g., variability in environmental drivers and variability in the setting) so that the concept of morphodynamic equilibrium should be mathematically unequivocal in the virtual world and interpreted over the appropriate spatial and temporal scale in the real world. We draw examples from estuarine settings which are subject to various governing factors which broadly include hydrodynamics, sedimentology and landscape setting. Following the traditional “tide-wave-river” ternary diagram, we summarize studies to date that explore the “virtual world”, discuss the type of equilibrium reached and how it relates to the real world.

A systematic set of flume experiments is used to investigate the features of velocity profiles within the body of coarse-grained debris flows and the dependence of the transport sediment concentration on the relevant parameters (runoff discharge, bed slope, grain size, and form). The flows are generated in a 10 m long laboratory flume, initially filled with a layer consisting of loose debris. After saturation, a prescribed water discharge is suddenly supplied over the granular bed, and the runoff triggers a debris flow wave that reaches nearly steady conditions. Three types of material have been used in the tests: gravel with mean grain size of 3 and 5 mm, and 3 mm glass spheres. Measured parameters included: triggering water discharge, volumetric sediment discharge, sediment concentration, flow depth, and velocity profiles. The dynamic similarity with full-sized debris flows is discussed on the basis of the relevant dimensionless parameters. Concentration data highlight the dependence on the slope angle and the importance of the quasi-static friction angle. The effects of flow rheology on the shape of velocity profiles are analyzed with attention to the role of different stress-generating mechanisms. A remarkable collapse of the dimensionless profiles is obtained by scaling the debris flow velocity with the runoff velocity, and a power law characterization is proposed following a heuristic approach. The shape of the profiles suggests a smooth transition between the different rheological regimes (collisional and frictional) that establish in the upper and lower regions of the flow and is compatible with the presence of multiple length scales dictated by the type of contacts (instantaneous or long lasting) between grains.

A formulation of the two-dimensional shallow water equations adapted to general and complex terrains is proposed. Its derivation starts from the observation that the typical approach of depth integrating the Navier–Stokes equations along the direction of gravity forces is not exact in the general case of a tilted curved bottom. We claim that an integration path that better adapts to the shallow water hypotheses follows the “cross-flow” surface, i.e., a surface that is normal to the velocity field at any point of the domain. Because of the implicitness of this definition, we approximate this “cross-flow” path by performing depth integration along a local direction normal to the bottom surface, and propose a rigorous derivation of this approximation and its numerical solution as an essential step for the future development of the full “cross-flow” integration procedure. We start by defining a local coordinate system, anchored on the bottom surface to derive a covariant form of the Navier–Stokes equations. Depth integration along the local normals yields a covariant version of the shallow water equations, which is characterized by flux functions and source terms that vary in space because of the surface metric coefficients and related derivatives. The proposed model is numerically discretized with a first order FORCE-type Godunov Finite Volume scheme that allows straight forward implementation of spatially variable fluxes. We investigate the validity of our SW model and the effects of the geometrical characteristics of the bottom surface by means of three synthetic test cases that exhibit non negligible slopes and surface curvatures. The results show the importance of taking into consideration bottom geometry even for relatively mild and slowly varying curvatures. By comparison with the numerical solution of vertically integrated models, we observe differences of almost 20%, in particular for the peak values and the shape of the hydrographs calculated at given sections of the fluid domain.

Stony debris flows are investigated at the laboratory scale. A set of 11 experiments are carried out, to simulate the stony debris flow propagation in a typical mountain confluence with a downstream water channel. The dynamic of the tributary flow just upstream of the junction with the river, surrogated through the volumetric mixture discharge, is assessed through the analysis of velocity profiles, while observations on deposit morphology provide insight into the condition which cause the blockage of the main stream. Energy dissipation within the solid–liquid mixture is mainly controlled by collisions between grains, and a dilatant fluid model is found to approximately reproduce the velocity profile. The degree of river blockage caused by the delivered material is classified according to three configurations: (i) full blockage, whereby the entire transversal river section is dammed (100% river obstruction); (ii) no blockage when less than 60% of the transversal river section is interested by the deposit; (iii) partial blockage for intermediate damming configurations (>60% and