Luca Solari

Near-Bed Turbulence Characteristics at the Entrainment Threshold of Sediment Beds

This experimental study is devoted to quantification of the near-bed turbulence characteristics at an entrainment threshold of noncohesive sediments. Near the bed, the departure in the distributions of the observed time-averaged streamwise velocity from the logarithmic law is more for immobile beds than for entrainment-threshold beds. In the Reynolds shear stress distributions, a damping that occurs near the bed for sediment entrainment is higher than that for immobile beds. Quadrant analysis reveals that in the near-bed flow zone, ejections and sweeps on immobile beds cancel each other, giving rise to the outward interactions, whereas sweeps are the dominant mechanism toward sediment entrainment. The bursting duration for entrainment-threshold beds is smaller than that for immobile beds. On the other hand, the bursting frequency for entrainment-threshold beds is larger than that for immobile beds. The third-order correlations indicate that during sediment entrainment, a streamwise acceleration associated...

FINITE AMPLITUDE BED DEFORMATIONS IN TOTALLY AND PARTIALLY TRANSPORTING WIDE CHANNEL BENDS

We develop an analytic approach able to predict flow and bed topography in curved cohesionless wide channels. The novel feature of the present theory, compared with previous analytic approaches, is its ability to treat bottom perturbations of finite amplitude and situations such that sediment transport does not occur within the whole cross section. The theory is presently applied to the case of constant curvature channels though it can be extended, in principle, to channels with variable curvature. Results show that the dominant mechanism controlling the establishment of bed profile is the topographic feedback of bottom deformations on the flow field, while the role of the dispersive transverse transport of longitudinal momentum and of the metrically induced transverse variations of longitudinal slope is usually relatively small. The theory extends previous linear analyses which are shown to underpredict deeping of the cross section close to the outer bank. Also, unlike linear theories, the present approach predicts a transverse slope of the bed profile increasing towards the outer bend in agreement with experimental observations. The maximum depth is found to depend on the friction coefficient Cu of the undisturbed uniform stream and on the parameter v(θu)1/2/Cu where v is curvature ratio, that is, the ratio between channel half width and radius of curvature of the centerline, while θu is the Shields stress of the undisturbed uniform stream. Comparison with experimental observations is fairly satisfactory. The shape of the cross section is then determined also for values of θu close enough to the critical values not to allow bedload transport within the whole cross section. The threshold value of θu separating the total transport from the partial transport regime is finally determined as a function of the curvature ratio v.

Gravitational effects on bed load transport at low Shields stress: Experimental observations

[1] In the present study we investigate the bed load transport mechanism due to the action of a free surface water flow when the transversal and longitudinal slopes of the bed are not negligible and in the case of low values of the applied Shields stress. Such a problem, which has been recently tackled by Parker et al. (2003) by means of a theoretical formulation, does not appear to be fully addressed in the literature from an experimental point of view. To this aim, average motion characteristics and transport dynamics of bed particles over an arbitrarily inclined nonerodible bed are studied by means of an experimental activity based on different techniques. A laser Doppler and an image-based technique are developed here and employed to study the average properties of saltating particles, while volumetric sampling is applied to estimate the average bed load transport intensity at equilibrium. Results show the intensity and direction of average particle velocity and average bed load transport as a function of the applied Shields stress for various bed inclinations. It appears that the intensity of bed load transport increases with the applied Shields stress much faster than predicted by the linear formulations on horizontal beds, while its direction may largely deviate from the direction of the applied shear stress at the bed when the bed is transversally tilted. Experimental results are compared with the theoretical results of Parker et al. (2003) and show a good agreement.

On salt marshes retreat: Experiments and modeling toppling failures induced by wind waves

Salt marshes are delicate ecosystems which are disappearing in many areas of the world, mainly due to increasing rates of sea level rise, subsidence, and anthropic pressure. The lateral erosion of the edge of salt marshes is one of the most important processes in determining changes in morphology, and wind waves have a key role in this retreat. Lateral retreat occurs by means of several types of mass failure processes, typically cantilever, sliding, and toppling. In the literature, no mechanistic models for the description of toppling failure are available. In this study, we performed a set of experiments to quantify the pressure field and the hydrodynamic forcing induced by wind waves during toppling failures of unstable blocks on a salt marsh edge. We propose a model interpreting toppling failure based on the experimental evidence as well as on the physics of the system. We model the system as a dynamic rigid block of cohesive soil, identified by the presence of a tension crack, subjected to hydrodynamic forces caused by impact of waves and resistive forces due to the blocks weight and soil cohesion. The failure of the blocks occurs when the soil tensile strength is exceeded along the failure surface located at the base of the block. The model is able to reproduce failure processes observed in the laboratory. Moreover, the model reveals that the most critical conditions for marsh bank instability due to toppling failure are associated to the presence of water inside the tension crack and low water levels in front of the bank.

Applicability of the De Marchi Hypothesis for Side Weir Flow in the Case of Movable Beds

Side weirs are widely used hydraulic structures typically designed and studied in the case of fixed bed conditions. In the case of subcritical flows, the hydraulics of side weirs can be modelled by using the classical De Marchi hypothesis. In the present work, a generalization of this hypothesis is developed for the case of side weirs over movable beds. Experiments showing the effects and feedbacks between the spilling discharge and the bed morphodynamics are presented. The application of the experimental observations to the generalized De Marchi hypothesis clearly show that the functioning of side weirs on a movable bed can be modelled by using this hypothesis. These findings could be instrumental for the design and verification of these structures. DOI: 10.1061/(ASCE)HY.1943-7900.0000566.

Estuarine Patterns: An Introduction to Their Morphology and Mechanics

The aim of the present review is to expose the average reader (assumed to have little previous knowledge of morphodynamics) to an overview of some aspects of estuarine morphodynamics tackled from a mechanical perspective. It will appear that the tidal analogue of several phenomena which are extensivelydiscussed and fairlyw ell understood in the fluvial literature, still await to be fully explored. When writing reviews of this kind, authors are usuallyconfron ted with a dilemma: either treating superficiallya large number of aspects of the problem or discussing fewer topics in greater depth. In the present case we have chosen the former alternative, which seems appropriate to the non specialistic audience assumed above, though space limitation will not allow a systematic treatment of the subject. The reader interested in achieving a more advanced understanding is referred to the extensive literature quoted in the paper.

On the estimation of the bed-material transport and budget along a river segment: application to the Middle Loire River, France

Sediment load and budgets are a fundamental component of the process-based hydromorphological framework developed by the REFORM project, and are needed to accurately assess the current condition of a river, its sensitivity to change, and its likely future evolutionary trajectory. This paper presents an evaluation of three different methods for estimating both bedload sediment transport and bed-material budget within river channels, using the Middle Loire River as a case study. The first method is based on the stream power concept and does not need any hydraulic calculations. It yields estimates of the sediment transport in the same order of magnitude as measurements but poor results for the bed-material budget in terms of magnitude and tendency. For the second method, hydraulic parameters are computed using the Manning–Strickler equation (or a 1D hydraulic model for steady flow). It provides useful indicators for understanding river dynamics but does not yield significant improvements compared to the first method. The third method uses 1D numerical software for water flow and river bed evolution. It yields the most accurate results for both sediment transport and bed evolution but requires more data and overall more work to construct the model. Guidance is provided on the amount of data required, the competence needed to build the models, and the predictive capability of each of the methods.

On the Morphological Impacts of Gravel Mining: The Case of the Orco River

Gravel mining may induce deep changes in river morphology including bank instabilities, downstream as well upstream bed incision. In the present work, morphological changes due to a large sediment mining in the Orco river (a tributary of the Po river, located in the Piemonte Region) are documented and modeled. The river mining occurred in 2003–2004, a volume of about 130,000 m3 of sediment was extracted over a surface of about 70,000 m2 at about 25 km upstream the confluence with the Po river. The mining and the response of the river to this intervention is well-documented through an experimental methodology developed by the CNR-IRPI and based on four distinct LIDAR surveys realized in the years 2003, 2004, 2006 and 2007. The observed morphological changes that occurred immediately after the gravel extraction in the time period 2004–2006 were then reproduced using the hydro-morphological 1D model BASEMENT developed by the ETH (www.basement.ethz.ch). Results indicate the model is able to capture not only the depositional and erosional areas, but it also provides a good estimate of the volume of sediment mobilized.

On the vulnerability of river levees induced by seepage

Seepage through earthen levees is a mechanism often observed during floods and can lead to levee failure if combined with internal erosion. This notwithstanding, hazard maps for levee failure are often elaborated solely considering the mechanism of overtopping. In this work, the assessment of levee vulnerability relative to seepage is investigated by considering the characteristic time scales of the phenomena and the main factors upon which they depend. In particular, the persistence of hydrological load is compared with the critical time of seepage associated to the emergence of the phreatic line along the landside of levees. The results can easily be applied to long river reaches, elaborating of through seepage-hazard maps and locating the levees that are more vulnerable to seepage failure.

Side Weir Flow on a Movable Bed

AbstractSide weirs are hydraulic structures widely used for flow control in rivers and canals. Whenever the water level rises above the side weir crest elevation, a fraction of the main flow is diverted so that the water discharge flowing downstream in the main channel is reduced. In movable bed channels, the lateral outflow may have significant interactions with the sediment transport processes affecting the side weir flow. The spilled discharge creates a reduction of the downstream sediment transport capacity with a consequent deposition in the side weir proximity. In addition, sediment in the main channel can be diverted into the lateral branch. To investigate these interactions, experimental data at the laboratory scale have been collected and analyzed. The increase of the spilled discharge induced by the local sediment deposition is described according to the classical De Marchi hypothesis. Furthermore, the sediment transport leaving the main channel through the lateral structure is modeled in terms ...