Depth-averaged Flow Velocity Research Articles

Multi-phase simulation for understanding morphodynamics of gravel beaches

This paper presents the use of an Eulerian–Eulerian multi-phase model to examine sediment transport over a gravel beach under regular waves. A previously published full-scale experiment is reproduced numerically; the simulated wave deformation, flow velocity, pore pressure, and morphological evolution are consistent with measurements. The multi-phase model can produce berm formation for smaller waves and beach erosion for larger waves as observation in previous field studies. The sensitivity analysis suggests that the nonlinear component of drag has significant effect on the berm formation through infiltration and exfiltration. The two simulated forces applied on the sediment (drag and buoyancy) are examined. The drag is the main force directly for sediment transport; therefore the sediment transport rate has high correlation with depth-averaged flow velocity. The buoyancy that is caused by plunging flow impacting beach face can push the sediment landward in the bore front, where the flow acceleration has its peak. The flow acceleration is considered in some previous sediment transport models empirically for simulating morphological evolution of gravel beach under waves. The buoyancy links the flow acceleration and sediment transport rate in the bore front.

Modeling of the field of currents and sea level using the Cardinal 
 software package

The article describes the application of the Cardinal software package for modeling currents and sea level on a rectangular and curved grid with the assignment of topographic data. The calculation is made for 1 day for the Red Sea area when the wind is set to 5 m/s and data on tidal fluctuations for 10 days. The simulation results are presented in cartographic form with a representation of the fluctuations of the vectors of the depth-averaged flow velocity at tides in the Red Sea.

The influence of passive wedge-wire screen aperture and flow velocity on juvenile European eel exclusion, impingement and passage

The European eel (Anguilla anguilla) is a critically endangered catadromous fish. The decline has partly been attributed to water management infrastructure that abstract water from rivers for potable and industrial water supply, irrigation, hydroelectric power generation and flood defence; eels can be impinged on weedscreens and trashracks and entrained in pumps and turbines. The Eel Regulations (England and Wales) 2009 stipulates measures are required to provide safe (upstream and downstream) passage of eels past such hazardous intakes. Preventing impingement and entrainment of upstream migrating (glass eel and elver) and river-resident (yellow) juvenile eels at hazardous intakes may require fine-mesh aperture screens and low approach velocities due to eels' small size and relatively poor swimming capacity but quantitative evidence is lacking. Here, passive wedge-wire screen aperture (1, 2, 3 and 5 mm) and depth-averaged flow velocities (0, 0.1, 0.15 and 0.2 m·s−1) both influenced the fate (i.e., impingement or passage) and behaviour (i.e., migratory separation or behavioural avoidance) of two size classes of juvenile eels (60–80 mm glass eels and 100–160 mm elvers) in an experimental flume. One and 2 mm aperture screens were required to physically exclude 60–80 mm and 100–160 mm, respectively. Up to 90% and 100% of the 60–80 mm and 100–160-mm size class eels were impinged at 0.2 m·s−1 depth-averaged flow velocity, which also positively influence number of screen contacts per eel and time to eel fate (from first contact). A small proportion of 60–80 mm eels (9.2%) did not approach the screen due to migratory separation (i.e., positive rheotaxis) and eels narrower than the screen aperture did not always pass through the screen, and thus other biological or hydraulic processes may also influence screen passage. It is hoped that these findings help improve screening guidance for regulators, key stakeholders and water abstraction managers to further improve protective measures required for critically endangered eels.

Numerical analysis of sediment transport and depth averaged flow velocity in non-prismatic compound channels

The flow velocity in compound open channels is greatly affected by the geometry of both the main channel (MC) and floodplain (FP). In this study, a numerical investigation of the joint effect between the FP divergence angle and the size of suspended sediment particles on sediment transport and flow depth-averaged velocity (DAV) in non-prismatic diverging compound channels was conducted. In addition, the influence of relative flow depth on the DAV in different flow directions (i.e., stream-wise and span-wise) was examined. Three divergence angles of 4.0, 6.3 and 11.3°, four sediment particle sizes of 1, 5, 50 and 500 µm and two relative depths of the FP with respect to the MC of 0.20 and 0.40 were considered during simulations. A Computational Fluid Dynamics (CFD) software ANSYS-Fluent was used to perform the required simulation scenarios and the DAV was expressed in terms of a dimensionless velocity (i.e., relative depth-averaged velocity; RDAV) to generalize the use of the obtained results. Results revealed that the span-wise RDAV gradient between the MC and FP at the middle and the end sections of the divergence reach is much larger in the case of higher divergence angle. Moreover, the presence of coarse sediment particles in the sediment-laden flow resulted in an augmentation in the fluctuations of span-wise and stream-wise RDAVs as compared to those obtained in the case of fine sediment particles. Results also showed that the span-wise RDAV gradient between the two subsections (MC and FP) increases with decreasing the relative flow depth. Sediment particle size had a considerable impact on the span-wise RDAV values, especially in the case of lower relative depth. As the sediment particle size increases, the distortion in the span-wise RDAV distribution curves increases. However, the stream-wise RDAV along the MC and FP decreases, as the sediment particle size decreases, especially in the case of higher relative depth. Based on the results, it can be concluded that the span-wise DAV gradient was greatly affected by the FP divergence angle and sediment particle size. The fluctuations in the span-wise DAV gradient were more pronounced in the case of higher divergence angles and coarser sediment particles. The stream-wise DAV distribution along the MC and FP was mainly influenced by the relative depth and sediment particle size. The stream-wise DAV dip along the MC and FP was more explicit in the case of higher relative depths and finer sediment particles.

Numerical Simulation for Flow over A Broad-Crested Weir Using FLOW-3D Program

The free water flow over a broad-crested weir with different down- and upstream inclinations was studied numerically using computational fluid dynamics software (FLOW-3D v11.0.4). Weirs of four various upstream and downstream configurations were investigated to determine the effect of weir shape on the produced coefficient of discharge (C_d), upstream energy grade line (H₁), and depth-averaged flow velocity using five different flow discharges. The obtained numerical results revealed that FLOW-3D program can be depended to simulate the flow over broad-crested weir adequately, as the differences between the numerical and experimental upstream head (h₁) were in the range of 0.0 to 9.6%. Furthermore, reducing upstream slope played an essential role in increasing the discharge coefficient, and reducing the upstream energy grade line (i.e. static pressure above the crest), while downstream inclination showed a negligible influence. Generally, reducing the upstream slope by 50.0 and 70.5% increased discharge coefficient by 9.5 and 13.2%, while decreased the total energy grade line by 4.3 and 8.7%, respectively. According to flow velocity measurement, for the same upstream slope and discharge, the velocity resulted by the weir of inclined upstream and vertical downstream (BRV) was lower than that of the weir of vertical upstream and inclined downstream (VRB) by 28.0%.

Migration of buoyancy-controlled active particles in a laminar open-channel flow

The migration of active particles in a laminar open-channel flow is significant for many ecological and biological processes associated with ambient water, for example, the harmful algae blooms in the long-distance water conveyance project. This paper presents an analytical analysis of the migration of the buoyancy-controlled active particles in a laminar open-channel flow. The analytical solution of concentration distribution has also been derived for the active particle cloud in the laminar open-channel flow under the combined action of ambient flow and self-migration of active particles, by combing Chatwin’s long-time asymptotic expansion and central concentration moments. The analytical solution shows that the active particle cloud accumulates near the surface rather than uniform distribution in the cross-section even for the long-time evolution. The analytical solutions are also been rigorously derived to reflect the holistic characteristics of the active-particle cloud, including the total quantity over each streamline, moving velocity of mass center, longitudinal dispersivity, skewness, and kurtosis have been rigorously derived for an instantaneous point source in a laminar open-channel flow. It is found that the distribution of total active particles over each streamline, centroid motion, and longitudinal dispersion eventually reach a stable status with a necessary time dependent on the relative strength of the vertical migration velocity and the total vertical diffusion. However, the self-migration of active particles results in the deviation of the moving velocity of mass center from the depth-averaged flow velocity. The longitudinal dispersivity of the active particle cloud gradually increases to the peak and then decreases to a constant value, in contrast to the monotonous increase for the passive particle cloud. It is also found that the strong vertical self-migration can result in changes of skewness and kurtosis of concentration distribution: negative skewness and positive kurtosis during the initial stage. However, both of them gradually tend to zero for the long-time evolution of the active particle cloud.

2D vs. 3D Numerical Approaches for Fish Habitat Evaluation of a Large River — Is 2D Modeling Sufficient?

Computational Fluid Dynamics is an effective tool for assessing non-present conditions, thus also in habitat evaluation within ecohydraulics. Deciding whether to apply a one-, two- or three-dimensional numerical approach, is an optimization that needs to be performed by every task, given the capability and the demands of specific approaches. In this paper we compare the utility of two-dimensional (2D) versus three-dimensional (3D) hydrodynamical simulations for ecohydraulic purposes. The basis of the comparation were 1) three simulated abiotic variables: water depth, depth averaged flow velocity and bed material composition, and 2) an overall performance in a meso-scale fish habitat evaluation, based on the simulated three variables. The biotic parameters for the models were the habitat suitability curves of three fish species, the Danube streber (Zingel streber), the round goby (Neogobius melanostomus) and the white bream (Blicca bjoerkna). We found that in terms of ecohydraulic utilization, the 2D approach performed sufficiently to simulate the hydrodynamics of a large river. The errors originating from the 3D-2D simplification yielded negligible differences in habitat evaluation, and the agreement in the habitat suitability indices calculated from the simulated metrics was satisfactory. Henceforth, the theory was turned into an application as we performed habitat mapping on a 100 km long, Hungarian reach of the Danube River, with the abiotic parameters resulting from a 2D hydrodynamical simulation. The possibility of simplifying the approach from 3D to 2D provides a cost-efficient numerical tool at larger scales for ecohydraulic studies, and especially for evaluating habitat suitability of riverine fish.

Surface Flow Velocities From Space: Particle Image Velocimetry of Satellite Video of a Large, Sediment-Laden River

Conventional, field-based streamflow monitoring in remote, inaccessible locations such as Alaska poses logistical challenges. Safety concerns, financial considerations, and a desire to expand water-observing networks make remote sensing an appealing alternative means of collecting hydrologic data. In an ongoing effort to develop non-contact methods for measuring river discharge, we evaluated the potential to estimate surface flow velocities from satellite video of a large, sediment-laden river in Alaska via particle image velocimetry (PIV). In this setting, naturally occurring sediment boil vortices produced distinct water surface features that could be tracked from frame to frame as they were advected by the flow, obviating the need to introduce artificial tracer particles. In this study, we refined an end-to-end workflow that involved stabilization and geo-referencing, image preprocessing, PIV analysis with an ensemble correlation algorithm, and post-processing of PIV output to filter outliers and scale and geo-reference velocity vectors. Applying these procedures to image sequences extracted from satellite video allowed us to produce high resolution surface velocity fields; field measurements of depth-averaged flow velocity were used to assess accuracy. Our results confirmed the importance of preprocessing images to enhance contrast and indicated that lower frame rates (e.g., 0.25 Hz) lead to more reliable velocity estimates because longer capture intervals allow more time for water surface features to translate several pixels between frames, given the relatively coarse spatial resolution of the satellite data. Although agreement between PIV-derived velocity estimates and field measurements was weak (R2 = 0.39) on a point-by-point basis, correspondence improved when the PIV output was aggregated to the cross-sectional scale. For example, the correspondence between cross-sectional maximum velocities inferred via remote sensing and measured in the field was much stronger (R2 = 0.76), suggesting that satellite video could play a role in measuring river discharge. Examining correlation matrices produced as an intermediate output of the PIV algorithm yielded insight on the interactions between image frame rate and sensor spatial resolution, which must be considered in tandem. Although further research and technological development are needed, measuring surface flow velocities from satellite video could become a viable tool for streamflow monitoring in certain fluvial environments.

Wave overtopping flow striking a human body on the crest of an impermeable sloped seawall. Part II: Numerical modelling

The present paper is the second of two companion papers on the investigations of wave overtopping flow striking a cylinder, which is the schematisation of a human body, on the crest of an impermeable sloped seawall with a deep foreshore. This paper numerically examines the detailed characteristics of the overtopping flow and the force on the cylinder in-line with the flow direction by using a volume-of-fluid (VOF) based Reynolds-Averaged Navier-Stokes (RANS) model. The numerical model is successfully validated against the experimental data on the wave overtopping flow depth and the inline force on the cylinder. The characteristics of the overtopping flow velocity are analysed using the validated numerical model. It is observed that the maximum depth-averaged flow velocity usually occurs before the maximum flow depth during an overtopping event, and decay of the overtopping velocity is much slower than the flow depth along the seawall crest. For plunging-breaker cases, the overtopping force on the cylinder usually comprises a cycle of a first impact peak, a main peak and a secondary peak. However, for surging-breaker cases, it is largely dominated by the main peak. The first impact peak is due to the impact of the tip of the overtopping flow on the cylinder, which usually has a higher flow velocity than the main stream. The main peak is generated by the asymmetric pressure distribution around the cylinder, which co-occurs with the maximum local momentum flux of the overtopping flow. As the force decreases after the main peak, there sometimes exists a secondary peak, which is formed by the complex free surface motion locally behind the cylinder. Additional numerical experiments are presented to demonstrate the applicability of a simple maximum force predictor developed in Part I. A prototype-scale simulation of irregular waves overtopping a sloped seawall is conducted to obtain the maximum force. The predictor also gives the maximum force in a wave-by-wave manner using the incident wave condition. These two approaches produce very similar estimates, suggesting that the predictor can be used for engineering applications.

Estimation on afflux of bridge on plain rivers related to blockage ratio and Froude number

The construction of hydraulic structures such as abutments and piers in natural waterways contributes to the contraction of river channels and the upstream water level rise. The maximum increase in the water level (afflux) is considered to be a major flood risk after a new bridge is built, and the accurate estimation of it is always an important work in designing a bridge construction. However, systematic studies on the prediction of the backwater effects of piers considering the blockage ratios within 10% are rarely reported. In this paper, a 2D hydrodynamic model was numerically simulated to investigate the effect of blockage ratios in the range of 2% - 9% on the amount and location of afflux. The distribution of depth-averaged flow velocities and water level changes around bridge piers was also discussed. Results indicate that the development and attenuation of backwater effects along the river channel falls naturally into 4 regions, and as the blockage ratio gets greater, afflux increases with its location moving farther from the zone strongly influenced by the bridge piers. It can be concluded that a blockage ratio of more than 7% should be one of the key concerns to river engineers in evaluating the capacity of river channels in plains to carry flood flows. On this basis, this article presented a parametric optimization formula based on blockage ratio and Froude number to estimate afflux of rectangle bridge piers with relatively low blockage ratios on plan rivers, and to provide effective assistance in hydraulic calculation of bridge design.

Deep learning technique for fast inference of large-scale riverine bathymetry

Riverine bathymetry is of crucial importance for shipping operations and flood management. However, obtaining direct measurements of depth is not always easy. Conversely, with recent advances in sensor technology, indirect measurements can be obtained and used to estimate high-resolution river bed topography. Physics-based inverse modeling techniques have been used to estimate bathymetry using indirect measurements like flow velocity at the surface. However, these methods are computationally expensive for large-scale problems. Recently, deep learning has opened a new door toward knowledge representation and complex pattern identification in many fields; however, these techniques have not been used for high-dimensional riverine bathymetry problems since they require a large amount of data in the training phase to have a good estimation performance that can be generalized for new river profiles. Also, unless one reduces the dimension of the problem, these methods can have a computationally expensive similar to that of physics-based techniques. Here, we develop a new deep learning framework for riverine problems that can be trained using only a few river profiles and in a computationally efficient way that allows finding solutions on personal computers. The proposed method exploits the spatially local connection between the observations and river bed profile and combines a fully connected Deep Neural Network (DNN) with Principal Component Analysis (PCA) to image river bed topography using depth-averaged flow velocity observations. The new method is presented and applied to three riverine bathymetry identification problems. Results show that the proposed method achieves satisfactory performance in bathymetry estimation, providing a powerful data-driven technique for riverine bathymetry in terms of prediction quality, robustness, and computational cost that requires only a relatively small number of training samples.

Numerical study of the flow structure at a swash tip propagating over a rough bed

Understanding the boundary layer flow structure at the tip of a swash flow is important for improving predictions of wave run-up, coastal flooding and sediment transport processes on beaches, but this is difficult to investigate experimentally. Recently, there has been debate regarding the mechanics of boundary layer growth during the uprush flow. Here, an extensively validated RANS model is used to investigate the swash tip dynamics during a dam-break driven swash event. The model enables the analysis of the spatial and temporal variation of important parameters, in particular the water surface gradients, the bed shear stress, and the non-uniformity of the velocity profile. The rate of flow convergence of surface particles toward the tip, and the ratio of the depth-averaged flow velocity and celerity of the swash tip are also obtained. The flow depth varies with distance behind the wave tip approximately as a power law, with a power of 1/2–3/4. The surface elevation dips offshore at distances greater than 0.2 m behind the front. The shear stress decreases quite slowly in the spatial region immediately behind the wave tip, and is in very good agreement with the flat-plate boundary layer model of Barnes and Baldock (2010). The model results indicate that the boundary layer structure is well-developed or depth-limited at the wave tip, and fits a power-law velocity profile with an exponent of order 1/3, consistent with a rough bed. The normalised vertical variation in the velocity profile is very uniform in the region 1 m behind the wave tip, and matches that for a 1/3 power law. Alternate logarithmic fits to the velocity profile yield consistent values of bed roughness at different locations, but corresponding to a grain size that is about twice that used in the experiment. The rate of flow convergence of surface particles toward the wave tip is also consistent with a fully developed boundary layer and a 1/3 power law.

HEM Impoundment—A Numerical Prediction Tool for the Water Framework Directive Assessment of Impounded River Reaches

A novel prediction tool is presented as a component of the Habitat Evaluation Model (HEM), which allows the assessment of the ecological status of impounded water bodies based on environmental factors that were shown in literature to correlate with the abundance of benthic macro-invertebrates. Main model parameters are the observed grain sizes and depth-averaged flow velocities obtained from a hydrodynamic simulation. The tool was tested in three Austrian river reaches. It was found that the river lengths predicted to be ecologically affected by the impoundments were substantially shorter for mean flow conditions than previously assessed when employing a physical mapping approach. The differences disappeared for low discharge conditions. The numerical prediction tool allows us to perform a status assessment for discharge conditions, which are potentially more representative of the annual discharge spectrum than those within the in-situ observable range. This property, thus, bears the potential to facilitate the recommendation of sediment management strategies in impounded river reaches in the future.

Migration of active particles in a surface flow constructed wetland

Presented in this paper is a theoretical analysis of migration of active particles in a surface flow constructed wetland, in which the longitudinal and vertical swimming behaviors are taken into account besides the hydrodynamic effects in the conventional analysis of passive particle dispersion. Theoretical solutions of mass distribution, moving velocity of the mass center, and the longitudinal dispersion coefficient are rigorously derived for a gyrotactic micro-organisms patch, based on Aris’ moment method and the method of separation of variables for linear partial differential equations. It is found that the mass distribution, motion of the mass center, and longitudinal dispersion of the gyrotactic micro-organisms patch can reach a stable status on the time scale 3/(π2+PeV2/4), where PeV is the Péclet number characterized by the total vertical diffusivity and vertical swimming velocity, and the necessary time corresponding to the dispersion of soluble contaminants can be embodied as a case of PeV=0. The total quantity of gyrotactic micro-organisms in each streamline satisfies exponential distribution for large PeV, while it approximately satisfies linear distribution for small PeV. The asymptotic velocity of mass center for gyrotactic micro-organisms in each streamline is, in general, not equal to the depth-averaged flow velocity, due to the longitudinal mean swimming caused by gyrotaxis in the shear flow. The increase of PeV and Ur (dimensionless flow velocity at free water surface relative to swimming speed) can enhance the motion of the mass center and longitudinal dispersion of the gyrotactic micro-organisms patch, while the increase of α (a dimensionless parameter to reflect shape of flow velocity profile) can weaken them.

Modelling of Flow Parameters through Subsurface Drainage Modules for Application in BIOECODS

The flow resistance of the existing modules in the bio-ecological drainage system (BIOECODS) is high and may lead to flood instead of its mitigation. As part of efforts to enhance the performance of the system, the river engineering and urban drainage research center (REDAC) module was developed. This study modelled the hydrodynamics of flow through this module using FLOW-3D and laboratory experiments for two cases of free flow without module (FFWM) and flow with a module (FWM) to understand and visualize the effects of the module. With less than 5% error between the numerical and experimental results, REDAC module altered the flow pattern and created resistance by increasing the Manning’s roughness coefficient at the upstream, depth-averaged flow velocity (43.50 cm/s to about 46.50 cm/s) at the downstream and decreasing water depth (7.75–6.50 cm). These variations can be attributed to the complex nature of the module pattern with further increase across the porous openings. Therefore, the technique used herein can be applied to characterize the behavior of fluids in larger arrangments of modules and under different flow conditions without the need for expensive laboratory experiments.

Experimental investigation into the reduction of erosion of sand at high flow velocities

Significant reduction of the rate of erosion of a sand bed is obtained when sand is mixed with a small amount of bentonite. In previous experiments this behaviour has already been shown for relatively low flow velocities. In this case, the erosion process is dominated by grain-by-grain erosion, which is characterised by low ratios of the erosion velocity and permeability (ve/k < 3). It is unknown whether these reductions in the erosion process also occur at relatively high flow velocities, where dilatancy-reduced erosion dominates (ve/k > 3). Experiments were executed in a tilting flume to investigate the erosion rate of the sand–bentonite mixtures. In 13 different tests, the dry volume percentage of the bentonite additive, the diameter of the sand particles and the depth-averaged flow velocity were varied. The depth-averaged flow velocities ranged from 1 to 2 m/s and all erosion tests were performed under supercritical flow conditions. The experiments show that the bentonite additive did not influence the strength characteristics of the sand however the permeability did decrease significantly. This proves that the significant decrease of the erosion rate was caused by the decrease of the permeability of the sand and that the test conditions were in the dilatancy-reduced regime.

Entrainment, motion, and deposition of coarse particles transported by water over a sloping mobile bed

In gravel-bed rivers, bedload transport exhibits considerable variability in time and space. Recently, stochastic bedload transport theories have been developed to address the mechanisms and effects of bedload transport fluctuations. Stochastic models involve parameters such as particle diffusivity, entrainment and deposition rates. The lack of hard information on how these parameters vary with flow conditions is a clear impediment to their application to real-world scenarios. In this paper, we determined the closure equations for the above parameters from laboratory experiments. We focused on shallow supercritical flow on a sloping mobile bed in straight channels, a setting that was representative of flow conditions in mountain rivers. Experiments were run at low sediment transport rates under steady nonuniform flow conditions (i.e., the water discharge was kept constant, but bedforms developed and migrated upstream, making flow nonuniform). Using image processing, we reconstructed particle paths to deduce the particle velocity and its probability distribution, particle diffusivity, and rates of deposition and entrainment. We found that on average, particle acceleration, velocity and deposition rate were responsive to local flow conditions, whereas entrainment rate depended strongly on local bed activity. Particle diffusivity varied linearly with the depth-averaged flow velocity. The empirical probability distribution of particle velocity was well approximated by a Gaussian distribution when all particle positions were considered together. In contrast, the particles located in close vicinity to the bed had exponentially distributed velocities. Our experimental results provide closure equations for stochastic or deterministic bedload transport models.

Block and boulder transport in Eastern Samar (Philippines) during Supertyphoon Haiyan

Abstract. Fields of dislodged boulders and blocks record catastrophic coastal flooding during strong storms or tsunamis and play a pivotal role in coastal hazard assessment. Along the rocky carbonate coast of Eastern Samar (Philippines) we documented longshore transport of a block of 180 t and boulders (up to 23.5 t) shifted upslope to elevations of up to 10 m above mean lower low water level during Supertyphoon Haiyan on 8 November 2013. Initiation-of-motion approaches indicate that boulder dislocation occurred with flow velocities of 8.9–9.6 m s−1, which significantly exceeds depth-averaged flow velocities of a local coupled hydrodynamic and wave model (Delft3D) of the typhoon with a maximum < 1.5 m s−1. These results, in combination with recently published phase-resolving wave models, support the hypothesis that infragravity waves induced by the typhoon were responsible for the remarkable flooding pattern in Eastern Samar, which are not resolved in phase-averaged storm surge models. Our findings show that tsunamis and hydrodynamic conditions induced by tropical cyclones may shift boulders of similar size and, therefore, demand a careful re-evaluation of storm-related transport where it, based on the boulder's sheer size, has previously been ascribed to tsunamis.

Impact of a large flood on mountain river habitats, channel morphology, and valley infrastructure

The Biała River, Polish Carpathians, was considerably modified by channelization and channel incision in the twentieth century. To restore the Biała, establishing an erodible corridor was proposed in two river sections located in its mountain and foothill course. In these sections, longer, unmanaged channel reaches alternate with short, channelized reaches; and channel narrowing and incision increases in the downstream direction. In June 2010 an 80-year flood occurred on the river; and this study aims at determining its effects on physical habitat conditions for river biota, channel morphology, and valley-floor infrastructure. Surveys of 10 pairs of closely located, unmanaged and channelized cross sections, performed in 2009 and in the late summer 2010, allowed us to assess the flood-induced changes to physical habitat conditions. A comparison of channel planforms determined before (2009) and after (2012) the flood provided information on the degree of channel widening as well as changes in the width of particular elements of the river's active zone in eight stretches of the Biała. The impact of the flood on valley-floor infrastructure was confronted with the degree of river widening in unmanaged and channelized river reaches. Before the flood, unmanaged cross sections were typified by finer bed material and greater lateral variability in depth-averaged and near-bed flow velocity than channelized cross sections. The flood tended to equalize habitat conditions in both types of river cross sections, obliterating differences (in particular physical habitat parameters) between channelized and unmanaged channel reaches. River widening mostly reflected an increase in the area of channel bars, whereas the widening of low-flow channels was less pronounced. A comparison of channel planform from 2009 and 2012 indicated that intense channel incision typical of downstream sections limited river widening by the flood. Active channel width increased by half in the unmanaged cross sections and by one-third in the channelized cross sections. However, damage to the valley-floor infrastructure was practically limited to the channelized river reaches with reinforced channel banks. This indicates incompetent management of riparian areas rather than the degree of river widening as a principal reason for the economic losses during the flood.

Passive acoustic measurement of flow velocity in the Straits of Florida

Group speed of sound in moving fluids depends on the propagation direction, which breaks acoustic reciprocity. Acoustic nonreciprocity provides a means to measure fluid motion. Using nonreciprocity, one can measure fluid velocities that may be small compared to uncertainties in sound speed. Interferometry of diffuse acoustic noise, with receivers replacing the transceivers employed in active techniques, offers a simple, low-cost means of measuring nonreciprocity. Here, the feasibility of using passive measurements of acoustic nonreciprocity to estimate current velocity in the ocean is experimentally demonstrated for the first time. Estimates of depth-averaged flow velocity are retrieved from cross-correlations of low-frequency noise recorded in the Straits of Florida by near-bottom hydrophones separated by 5 and 10 km.