Matteo Tregnaghi

Aquatic interfaces: a hydrodynamic and ecological perspective

ABSTRACT Ecologically-appropriate management of natural and constructed surface water bodies has become increasingly important given the growing anthropogenic pressures, statutory regulations, and climate-change impacts on environmental quality. The development of management strategies requires that a number of knowledge gaps be addressed through interdisciplinary research efforts particularly focusing on the water-biota and water-sediment interfaces where most critical biophysical processes occur. This paper discusses the current state of affairs in this field and highlights potential paths to resolve critical issues, such as hydrodynamically-driven mass transport processes at interfaces and associated responses of organisms through the development of traits. The roles of experimental methods, theoretical modelling, statistical tools, and conceptual upscaling methods in future research are discussed from both engineering and ecological perspectives. The aim is to attract the attention of experienced and emerging hydraulic and environmental researchers to this research area, which is likely to bring new and exciting discoveries at the discipline borders.

Probabilistic description of grain resistance from simultaneous flow field and grain motion measurements

Experiments were carried out using a mobile gravel bed placed in a tilting flume with a modified particle image velocimetry (PIV) system. Individual grain movements were surveyed using data from time series of images. Near-bed velocity flow field measurements were made simultaneously above the same area of the sediment surface by applying cross-correlation techniques to the collected plan view images. Statistics of grain motions were collected through a semiautomatic procedure. Significant changes in the flow field were observed in the proximity of the entrained or deposited particles. A strong correlation is shown between the changes in the local streamwise and lateral velocity and the movement of the grains. The theory of Grass is revisited and developed based on the experimental results. The probability distribution of individual grain resistance has been derived from the statistics of the near-bed velocity field and of the entrainment risk.

Scouring at bed sills as a response to flash floods.

The temporal development of clear-water local scour depth at bed sills in uniform gravel beds is considered. Experiments are presented on the development of scour holes under unsteady hydraulic conditions, with the triangular-shaped hydrographs tested being of different durations and different rates of flow variation. Based on the experimental results and a theoretical framework, a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter. A “flash flood” is here defined as an event for which the scour doesn’t attain its potential magnitude, i.e., the equilibrium value for the peak hydrograph flow rate. This flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream. A quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter. Results show that the ratio between the final scour depth and the potential scour depth ...

Stochastic determination of entrainment risk in uniformly sized sediment beds at low transport stages: 2. Experiments

Fluvial sediment transport is caused by a complex interaction of interdependent grain and fluid processes many of which are stochastic in nature and cannot be adequately represented by deterministic equations. Random variable analysis has been used previously but limited data are available to describe the variability of grain resistance combined with particle arrangements, and thus validate such analysis. In this study low to medium bed load transport tests were carried out in a flume where sediment movement was monitored using a three-camera 3D PIV system. Simultaneous grain motion and flow velocity measurements were made on a plane located slightly above and parallel to the sediment bed. Detailed statistical velocity information was acquired to model the velocity distribution at the bed level. This was combined with the joint probabilistic distribution of particle exposures and grain resistance to motion, which were obtained from discrete particle modeling (DPM) simulations. DPM simulations were used to provide a stochastic mathematical description of the risk that a stationary particle is entrained by the flow. Predictions from the stochastic model equations replicated the observed pulsation in sediment transport. This demonstrates that it is possible to simulate sediment entrainment and transport at a high resolution by adequately modeling all the sub-processes. A number of flow patterns were identified that caused large fluctuations of the entrainment rate. These all exhibit high velocity flow structures, but they selectively cause the dislodgement of individual particles located at different positions. This selective behavior follows from the variability of the interaction between the near-bed flow and the particles having different exposure.

Effect of Flood Recession on Scouring at Bed Sills

The effect of the flood recession time on the local scour depth at bed sills in gravel deposits is examined. Experiments were carried out to study the development of scour holes under time-varying hydraulic conditions with no upstream sediment feed. Triangular-shaped hydrographs, having recession times up to three times the duration of the rising limb, were used. Traditionally, the peak water discharge in any flood event is used as a design value in estimating the final depth of scour formed by a flood. This approach is overly conservative when the flow hydrograph is steep, i.e., during the occurrence of flash floods. The actual reduction of the scour depth from this estimated value is dependent on both the characteristics of the flood event and the characteristics of the stream. The results show that the maximum potential scour depth can be achieved only for hydrographs with long recession times, while the rate of this process can be estimated as a function of the ratio between a characteristic flood time and the steady-state temporal scale of scour development. A method is proposed for the prediction of the scouring process under unsteady flows in terms of two dimensionless temporal parameters. Results obtained for clear-water boundary conditions can be extended to sediment-supply tests if specific supply input conditions hold.

A New Theoretical Framework to Model Incipient Motion of Sediment Grains and Implications for the Use of Modern Experimental Techniques

The entrainment of sediments in rivers is recognized to exhibit an intermittent nature, hence incipient motion is inherently a random process that requires an appropriate stochastic description. The effect of near-bed turbulence on grain entrainment and the variation in stability of randomly configured bed particles due to local surface heterogeneity are included into a probabilistic framework based on a concept first proposed by Grass. Bedload transport tests were carried out in a flume where sediment movement was monitored using a three-camera 3D PIV system. Simultaneous grain motion and flow velocity measurements were made on a plane located slightly above and parallel to the sediment bed. Detailed statistical velocity information was acquired to model the velocity distribution at the bed level accounting for the probabilistic distribution of particle exposures. This was combined with the probabilistic distribution of grain resistance to motion, which was obtained from discrete particle modeling (DPM) simulations. The analysis provides detailed insight, in terms of grain dynamics, into the physical aspects that determine the initiation of movement, and the stochastic equations of incipient motion are derived. The key feature of the proposed analysis is the potential of including into the model as much statistical information as one can obtain from experimental observations based on state-of-the-art flow measurement techniques and from the use of numerical simulations performed with discrete particle models.

Statistical Description on the Role of Turbulence and Grain Interference on Particle Entrainment from Gravel Beds

AbstractA complete understanding of the role of grain-scale particle-flow interaction in sediment entrainment and transport has still not been achieved in spite of recent technological advancement in measurement capabilities. In this study, the initial motion of natural sediment particles in a gravel deposit was detected and combined with simultaneous local measurements of the velocities on a horizontal plane located above the bed surface using a three-component stereoscopic PIV. A series of experimental tests with increasing low values of boundary shear stress were conducted. The acquisition system allowed coupling between streamwise and vertical near-bed velocity and the entrainment of more than 900 individual grains. Initial analysis agreed with previous observations on the predominance of sweeps (Quadrant IV), and to a lesser extent, of outward interactions (Quadrant I) in entraining gravel particles. However, the latter were found to move sediments just as efficiently as sweeps impacting on particles...

Exchange of Pollutants Between Rivers and the Surrounding Environment: Physical Processes, Modelling Approaches and Experimental Methods

The fate of solute and pollutants is controlled by a broad number of different transport and storage mechanisms, ranging from simple processes (i.e. molecular diffusion, advection etc.) to more complex phenomena (i.e. evapotranspiration, groundwater flows, etc.). Different mathematical models, accounting for different exchange processes, have been developed and applied to specific experimental studies to assess transport and storage parameters. Experimental research focused on transport and retention processes induced by the transient storage in the dead zones, by the river bed topography and vegetation, by evapotranspiration. The analysis of these physical processes is generally conducted observing the behavior of solutes in field environments or in scaled laboratory models, using artificial or environmental tracers to track the fate of transported substances and assess transport and retention parameters. To improve the knowledge of pollutant exchange mechanism between a river and the surrounding environment, new experimental techniques focusing on long timescale retention and investigating the link between river biology and hydrodynamics are required. The development of new protocols for tracer tests design and the use of new specific tracers will open future research perspectives.A “one-and-a-half”-dimensional model of a river is developed. It is actually one-dimensional but allows for horizontal curvature using natural curvilinear co-ordinates. The governing long wave equations can be developed with very few limiting approximations, especially using momentum rather than energy. The curvature is then shown to be rarely important and is subsequently ignored. Wave periods, imposed by boundary conditions, are asserted to be fundamental. Long waves have speeds and propagation properties that depend on period, and there is no such thing as a single long wave speed. Examination of dimensionless equations and solution of linearised equations using wave period shows a novel interpretation of terms in the momentum equation: the “kinematic” approximation and wave are misnomers: the approximation lies not in the neglect of inertial terms but is actually a very long period one. The outstanding problem of river modelling, however, is that of resistance to the flow. A large data set from stream-gauging is considered and it is shown that the state of the bed, namely the arrangement of bed grains by previous flows, is more important than actual grain size. A formula for resistance is proposed which contains a parameter representing bed state. As that state is usually changing with flow, one can not be sure what the resistance actually will be. This uncertainty may have important implications for modelling. The momentum principle is then applied also to obstacles such as bridge piers, and a simple approximation gives greater understanding and a practical method for incorporation in river models. Finally, river junctions are considered, and the momentum approach with the very long period approximation shows that they can be modelled simply.

Characterizing retention processes in streams using retention metrics

The temporal retention in storage zones (SZs) has a strong influence on mass transport processes in natural streams. It has been shown that solute retention affects solute breakthrough curves (BTCs) by producing longer tails and thereby increasing their skewness. In terms of ecological effects, this retention increases the contact time of solute with aquatic interfaces and living species, which can lead to degradation of eco-systems when the transported substances are pollutants. An important question that arises is whether the currently available metrics can adequately represent complex retention processes. In this study, we examine the performance of two existing metrics: the hydrological retention factor (R H ) and the fraction of median travel time due to transient storage (F med ). The results presented are based on two conservative tracer tests. The tracer tests were performed in streams with distinct morphological, sediment composition, vegetation and hydraulic characteristics. The recorded concentration-time series were used to derive storage zone parameters such as storage zone area, exchange coefficient and mean residence time. The storage zone parameters were computed using a multiple storage zone model STIR with two separate exponential residence time models for transient storage, representing short timescale (STS) and long timescale storage (LTS) processes. The retention metrics were estimated separately for short and long timescale retention, and for the combined retention. The cross-correlation between the retention metrics and the storage parameters was analyzed using Pearson’s R- and significance p-values. In general, the results reveal a poor correlation between retention metrics and storage zone parameters, except for the exchange rate associated with long timescale storage, α 2. A strong cross-correlation is instead found between the retention metrics.

Numerical Study of Sedimentation in Uniformly Vegetated Wetlands

Constructed wetlands for wastewater treatment are increasingly recognized as a valid alternative to conventional water treatment methods with a high ecological value. Sediment transport and deposition processes play a key role in determining the treatment performance and the morphological evolution of a wetland, and must be carefully considered both in the design and the maintenance phase. This work presents a 2-D numerical study of the effect of vegetation density on sedimentation in wetlands. A depth-averaged hydrodynamic and mass transport model was applied to a rectangular wetland with uniform vegetation density and flat topography. Sediment settling and resuspension are represented in the model by a first-order source/sink term that depends on grain size and shear velocity. Results show that, for the same inflow discharge, the removal efficiency for relatively small grain sizes is lower in wetlands with higher vegetation density. This is a consequence of the more uniform flow distribution found in more densely vegetated wetlands. However, the condition of total removal of suspended sediment is achieved for higher grain sizes in more sparsely vegetated wetlands, meaning there is a range of relatively large grain sizes for which the removal efficiency is higher for higher vegetation densities.