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Large eddy simulations (LES) of flow over wall-mounted boulders placed on a rough, permeable bed, composed of one layer of densely-packed spheres, are performed. This configuration is based on the experimental configuration reported in [A.N. Papanicolaou, C. Kramer, A. Tsakiris, T. Stoesser, S. Bomminayuni, Z. Chen, Effects of a fully submerged boulder within a boulder array on the mean and turbulent flow fields: implications to bedload transport, Acta Geophysica 60(6), 2012, pp. 1502–1546.] the data of which are used to validate the LES. A fairly good agreement between calculated and measured time-averaged streamwise velocity and turbulence intensity profiles is found confirming the accuracy of the simulation. Details of the time-averaged and instantaneous flow are revealed using various visualisation methods. The time-averaged flow is characterised by a symmetric pair of funnel vortices comprising the recirculation zone in the lee of the boulder. The funnel vortex is bounded by significant shear layers caused by high-momentum fluid filling the low momentum wake zone behind the boulder. Bed roughness and permeability appears to play an important role in this flow, it affects the near-bed velocity profiles and subsurface fluid is entrained into the low pressure zone behind the boulder, specifically into the funnel vortex, moreover surface-subsurface flow exchange in particular in the vicinity of the boulders disrupts the secondary flow in the wake. The instantaneous flow features various turbulence structures which are educed by means of isosurfaces of the Q-criterion. Shear layer roll-up leads to high-frequency shedding of roller vortices, which are subsequently stretched by the significant velocity gradients and deformed into a hairpin or horseshoe form. Vertical and horizontal shear layer flapping is revealed and quantified via vorticity contour plots and velocity spectra.

A generalized probabilistic model is developed in this study to predict sediment entrainment under the incipient motion, rolling, and pickup modes. A novelty of the proposed model is that it incorporates in its formulation the probability density function of the bed shear stress, instead of the near-bed velocity fluctuations, to account for the effects of both flow turbulence and bed surface irregularity on sediment entrainment. The proposed model incorporates in its formulation the collective effects of three parameters describing bed surface irregularity, namely the relative roughness, the volumetric fraction and relative position of sediment particles within the active layer. Another key feature of the model is that it provides a criterion for estimating the lift and drag coefficients jointly based on the recognition that lift and drag forces acting on sediment particles are interdependent and vary with particle protrusion and packing density. The model was validated using laboratory data of both fine and coarse sediment and was compared with previously published models. The study results show that all the examined models perform adequately for the fine sediment data, where the sediment particles have more uniform gradation and relative roughness is not a factor. The proposed model was particularly suited for the coarse sediment data, where the increased bed irregularity was captured by the new parameters introduced in the model formulation. As a result, the proposed model yielded smaller prediction errors and physically acceptable values for the lift coefficient compared to the other models in case of the coarse sediment data.

Arrays of large immobile boulders, which are often encountered in steep mountain streams, affect the timing and magnitude of sediment transport events through their interactions with the approach flow. Despite their importance in the quantification of the bedload rate, the collective influence of a boulder array on the approach time-averaged and turbulent flow field has to date been overlooked. The overarching objective is, thus, to assess the collective effects of a boulder array on the time-averaged and turbulent flow fields surrounding an individual boulder within the array, placing particular emphasis on highlighting the bed shear stress spatial variability. The objective of this study is pursued by resolving and comparing the time-averaged and turbulent flow fields developing around a boulder, with and without an array of isolated boulders being present. The results show that the effects of an individual boulder on the time-averaged streamwise velocity and turbulence intensity were limited to the boulder’s immediate vicinity in the streamwise (x/dc < 2-3) and vertical (z/dc < 1) directions. Outside of the boulder’s immediate vicinity, the time-averaged streamwise velocity was found to be globally decelerated. This global deceleration was attributed to the form drag generated collectively by the boulder array. More importantly, the boulder array reduced the applied shear stress exerted on the individual boulders found within the array, by absorbing a portion of the total applied shear. Furthermore, the array was found to have a “homogenizing” effect on the near-bed turbulence thus significantly reducing the turbulence intensity in the near-bed region. The findings of this study suggest that the collective boulder array bears a portion of the total applied bed shear stress as form drag, hence reducing the available bed shear stress for transporting incoming mobile sediment. Thus, the effects of the boulder array should not be ignored in sediment transport predictions. These effects are encapsulated in this study by Equation (6).

The purpose of this study is to develop a relationship for predicting the bedload rate from the acoustic signal of a geophone, under the rolling and saltating bedload transport modes. Controlled experiments corresponding to near-incipient, marginal, and general bedload motion were conducted. The geophone recorded impacts and resulting acoustic signal impulses at the lower “Frequency 1” (100–200 kHz) and the higher “Frequency 4” (380–480 kHz) ranges. Rolling particles produced stronger responses in Frequency 1 and weaker in Frequency 4 due to rotational vibrations. Instead, saltating particles generated stronger responses in Frequency 4 and weaker in Frequency 1 because of spheroid vibrations. The impulse in both Frequency 1 and 4 was related to bedload transport, as it accounts for the number and magnitude of the signal spikes due to the particle impacts. This study complements efforts for quantifying bedload transport using geophones by relating bedload to impulse for rolling and saltation.

A reliable depth-integrated model that is suited for capturing the complex flow found with an array of submerged boulders over a permeable rough bed can be an attractive tool for river management. This paper presents a new nonhydrostatic quasi-three-dimensional (3D) calculation method called the general bottom velocity computation of the fourth-degree polynomial equation for vertical distribution of velocity coupled with the dynamic wall law (GBVC4-DWL). The computational domain of the GBVC4-DWL method considers the presence of the boulders, a vortex layer on the bed surface, and a roughness layer under the bed surface. The novelty of this study is that it introduces a dynamic rough wall law for the bottom boundary condition that includes the continuity and momentum equations for vortex and roughness layers to evaluate the nonequilibrium flow induced by the boulders. Comparisons of the results of the present method with previous experimental results and results of previous calculation methods indi...

The objective of this coupled experimental and numerical study is to provide insight into the mean and turbulent flow fields within an array of fully submerged, isolated, immobile boulders. Our study showed that the velocity defect law performed well for describing the mean flow around the boulder within the array. A prerequisite, however, was to accurately estimate the spatial variability of u* around the boulder, which was achieved via the boundary characteristics method. The u* exhibited considerable spatial variability within the array and form roughness was shown to be up to 2 times larger than the skin roughness in the boulder near-wake region. Because the boulders bear a significant amount of the flow shear, the available bed shear stress for entrainment of the mobile sediment, τ ols , near the boulders was roughly 50% lower than the ambient τ ols . The τ ols variability induced by the boulders could lead to a threefold overestimation of the sediment transport rate.

The emphasis of this paper is on the fractal-analysis of irregular objects like particle clusters that fill the two-dimensional Euclidean space R2. A common fractal descriptor of such objects is DL (1 5, pseudo-triangle, and -rhomboid. The analysis for this study is performed by applying the box-counting method for analyzing cluster planview images from four groups of cluster morphologies derived from synthetic, laboratory, and field data sets. The DAT is determined via Richardson plots by estimating the slope of the best-fit line over the entire range of box sizes. Doing so, provides a single aggregate dimension that includes the embedded effects from the objects projected area and perimeter. The results show that the average DAT¯ and its standard deviation can effectively discriminate the four cluster morphologic groups. The DAT¯ provides a clear demarcation between the nonlinear morphologies and the in-line morphology with ER > 5. The analysis also shows that the in-line clusters with ER

Radio frequency identification (RFID) technology enables bidirectional, remote communication between a reader and a transponder (or tag) with a unique ID via an excitation antenna. This study provides a framework for estimating the burial depth of a transponder from a fixed location as a means of assessing scour depth or for other riverine applications. In doing so, a low frequency (LF), passive RFID system is used, which operates at a resonance frequency of 134.2 kHz and can penetrate through saturated sediments. The objective of this research is the development and validation of a semitheoretical expression that relates the decay of the received input voltage (VIN) at the reader with the transponder burial depth or detection distance, d, through a series of well-controlled experiments. This study examines the correspondence between VIN and d in air, as well as in water, gravel, and sand, and assesses the role of transponder orientation (perpendicular and parallel with respect to the excitation a...

The mean wake structures and turbulent flow fields of a wall-mounted spherical obstacle placed in a thin laminar boundary layer, of thickness 14 % of the sphere diameter, were investigated at a Reynolds number of 17,800. Digital particle image velocimetry was used to interrogate the flow in the vicinity of the obstacle, and thermal anemometry measurements were performed to characterize unsteadiness in the wake. Streamwise features observed in the mean wake flow included counter-rotating tip vortices inducing downwash, horseshoe vortices, weak vortices inducing upwash at the top of the near wake, and counter-rotating ‘lobe’ vortices formed by the somewhat unique convex geometry of the sphere near the base. Under the present flow conditions, the sphere base geometry also prevented the roll-up of a horseshoe system upstream of the obstacle. The wake flow field, including lobe structures, is consistent with a vortex skeleton model developed to describe the simpler wakes of low-aspect-ratio wall-mounted semi-ellipsoidal obstacles, with modifications due to the unique junction geometry. Point velocity measurements in the wake identified a weak dominant frequency close to the bed. Cross-spectral analysis of these data at symmetrically located points revealed that, on average, flow oscillations were in phase. The turbulent stress distribution in the wake of the sphere showed a region of high magnitude near the bed not observed for other geometries, and spatially consistent with the lobe structures.

Scour around bridge piers can undermine the bridge integrity and cause catastrophic bridge failures. The goal of this coupled experimental and theoretical study is to set a framework for applying Radio Frequency IDentification (RFID) technology to develop a bridge scour remote monitoring system. RFID involves the wireless exchange of information between a base station (reader) and a transponder via an antenna. The proposed bridge scour monitoring system utilizes the Return Signal Strength Indicator (RSSI) of a transponder buried in the vicinity of the pier to determine the transponder distance from the antenna and thus the scour depth. In this study we present a novel methodology for experimentally determining the RSSI voltage for a low frequency (134.2 kHz), passive RFID system.

Geophones are passive acoustic instruments, which can provide continuous, unattended bedload transport rate measurements, based on the acoustic energy generated by the impacts of transported bedload particles onto a metal plate mounted on the geophone. The purpose of the present study is to investigate the correspondence of the geophone acoustic signal recording frequency with the mode of bedload particle transport, i.e., rolling and saltation. Controlled experiments corresponding to near incipient, marginal, and general bedload motion were conducted. The geophone was configured to record impacts the lower “Frequency 1” (100-200 kHz) and the higher “Frequency 4” (380-480 kHz) bands. Rolling particles produced responses predominantly in the lower Frequency 1, but also weaker responses in the higher Frequency 4, which were attributed to the particle rotational vibrations. Instead, saltating particles generated responses mainly in Frequency 4 and only weaker responses in Frequency 1, due to particle spheroid vibrations. This study complements efforts for improving the accuracy of geophone calibration curves and hence in quantifying bedload rates under the rolling and saltating transport modes.