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Many of the empirical formulas used for the prediction of the expected scour depth at piers are excessively conservative, providing substantial overestimations. On the other hand, the recently proposed neural networks methods generally issue accurate predictions but also high percentages of underpredictions, due to the use of a symmetric error function for their parameterization. A novel error function is proposed in this paper for optimizing neural networks, giving more weight to underestimation than to overestimation discrepancies, in order to obtain safer design predictions. The performances of the proposed model on independent field records are compared with those of a conventionally trained neural network and with those of a set of widely used formulas. The asymmetric error function (that might be applied to parameterize any other model or equation, as a proficient alternative to least-square errors or envelope curves) allows researchers to obtain predictions closer to the measurements than those issued by traditional formulas, substantially reducing the extent of unnecessary overdesign and at the same time the percentage of severe underestimations is comparable with those of the safest formulas. [ABSTRACT FROM AUTHOR]

Peak discharges during large floods can often result in submerged orifice flow (also called "pressure flow"), or embankment and bridge overtopping flow, in which the embankment and abutment foundation of a bridge are subjected to severe scour and possible failure. In this study, abutment scour experiments were carried out in a wide, laboratory compound channel to investigate the characteristics of abutment scour for the three flow types of free, submerged orifice, and weir (overtopping) for an erodible but riprap-protected embankment and abutment. Detailed bed contours and three-dimensional (3D) velocities and turbulence quantities were measured with acoustic Doppler velocimeters. The results show that the contracted flow around an abutment (due to lateral or vertical flow contraction) and the local turbulent structures near the downstream face of the bridge are the main features of the flow field responsible for the maximum scour depth near the abutment. Experimental results for combined abutment and contraction scour depth are presented in terms of the theoretical long contraction scour for all three types of flow. The experimental results for maximum scour depth are compared with those of other investigators, and it is shown that the erosional strength of the embankment defines an upper and lower bound for maximum scour depth for a solid abutment versus a riprap-protected embankment and abutment. [ABSTRACT FROM AUTHOR]

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 antenna) on the received VIN. It is found that VIN exhibits a universal behavior as a function of d as it decays inversely with the cube of the distance (∼d-3) not only for air but also for water, gravel, and sand. More importantly, it is shown that the orientation angle between the excitation and the transponder miniature antennas as well as the sediment-bed composition/texture (e.g., sand, clay, gravel) surrounding the transponder can have a significant effect on the magnitude of the VIN and on the decay rate of the VIN versus d curve. A change in the transponder orientation from perpendicular to parallel reduces the VIN by one order of magnitude and the maximum detection d by 26-76% for the different media. The greatest reduction in VIN occurs for sand, likely due to its smaller void ratio that causes excessive defragmentation of the RF waves. These promising findings allow a relatively accurate, remote estimation of d using the inexpensive LF, passive RFID technology and warrant future research and development on this topic. [ABSTRACT FROM AUTHOR]

In this paper we experimentally investigate the final scour depth around cylindrical piers under flood waves. Only clear-water conditions are considered. An approach to estimate the final scour depth under a flood wave is proposed, based on the local scour depth calculated with steady flow equations under peak flow conditions. The new experimental results are also used to investigate the performance of existing expressions or approaches to predict time evolution under steady and unsteady flow conditions. [ABSTRACT FROM AUTHOR]

An analysis is presented of the 309 known laboratory data for local scour depth developed by two-dimensional (2D) horizontal jets, which leads to development of a new comprehensive prediction equation. The new equation comprises a number of mutually independent multiplying factors, which account for the effects on local scour depth of the characteristics of the applied jet flow and the bed sediment, as well as the extent of a downstream protective apron. When unitized in terms of the jet thickness, the equation shows that the maximum possible dimensionless local scour depth is equal to three times the jet Froude number expressed in terms of jet velocity and jet thickness. The equation is compared to most of the existing predictive equations by applying each equation to the existing data, the comparison indicating that the new method has superior performance. Indeed, many of the existing predictive equations are shown to perform poorly when judged against the known laboratory data. The known existing equations are listed in the paper. [ABSTRACT FROM AUTHOR]

The sediment erosion rate flume (SERF) device was computationally modeled using CD-adapco's Star-CCM+ at varying flow rates and sample roughnesses so that wall shear stresses could be evaluated during a piston-style erosion test. Shear stress data were matched between the model and data from previous physical tests. Pressure differential upstream and downstream from an eroding specimen displayed similar behavior during both physical and modeled tests in that as eroding sample roughness increased, pressure differential did not appear to change. A series of complicated bed configurations were added to the computational model to quantify the effects of blocking, chunking, or sample overadvancement during an erosion test. Results appeared to indicate that small deviations in sample geometry may have large effects on localized shear stresses. Another series of models was run to provide an explanation for the beginning of blocking or chunking as a result of shear stress development over a rough sample. Results showed that relatively large localized shear stresses may develop simply because of the introduction of roughness. Finally, results also indicated that the most conservative method for future testing is to keep the bottom edge of an eroding sample flush with a flume bottom. [ABSTRACT FROM AUTHOR]

Studies on scour in the wake region of piers in cohesive sediments containing gravel were not reported in literature, to the best of our knowledge. Results from the laboratory experiments based study on the process of wake scour are presented here. The sediment used for the experiment were of two types: (1) fine gravel mixed with clay in proportions varying from 20% to 60% by weight, and (2) fine gravel and fine sand in equal proportion by weight mixed with clay in proportions varying from 20% to 60% by weight. The experiments revealed that the process of scour as well as depth, shape, and geometry of scour hole developed in such cohesive sediment were significantly different from that of cohesionless sediments. Proportion of clay fraction and unconfined compressive strength of cohesive sediment mixture were found to be the most significant variables controlling the depth of scour in the wake of piers. On the basis of dimensional considerations, relationships have been proposed for the estimation of the depth of scour in the wake region of piers in clay-gravel and clay-sand-gravel sediment mixtures. [ABSTRACT FROM AUTHOR]

Scouring downstream of submerged weirs is a common problem resulting from the interaction of the three-dimensional turbulent flow field around the structures and the mobile channel bed. This paper presents the distributions of flow patterns, bed shear stresses, and turbulence structures in the approach flow and the scour hole downstream of a submerged weir. The experiments were conducted under the clear-water scour condition for an equilibrium scour hole. The experimental results show that the flow structures are considerably changed by the presence of the structure. A large recirculation zone and a flow reattachment region are formed downstream of the submerged weir. Strongly paired cellular secondary flows are observed in the scour hole. The turbulence structures ahead of the recirculation zone govern the dimensions of the scour hole. [ABSTRACT FROM AUTHOR]

Twenty-three of the more recent and commonly used equilibrium local scour equations for cohesionless sediments were evaluated using compiled laboratory and field databases. This investigation assembled 569 laboratory and 928 field data. A method for assessing the quality of the data was developed and applied to the data set. This procedure reduced the laboratory and field data to 441 and 791 values, respectively. Because the maturity of the scour hole at the time of measurement for the field data was unknown, they were only used to evaluate underprediction by the equations. A preliminary quality control screening of the equilibrium scour methods/equations reduced the number of equations from the initial 23 to 17. For this screening procedure the equations were used to compute scour depths for a wide, but practical, range of structure, flow, and sediment parameters. Those methods/equations yielding unreasonable (negative or extremely large) scour depths were eliminated from further consideration. The remaining 17 methods/equations were then analyzed using both laboratory and field data. Plots of underprediction error versus total error for the laboratory data and underprediction error for field data versus total error for laboratory data along with error statistics calculations assisted in the ranking of the equations. Equations from previous publications were melded and slightly modified to provide the best performing equation in that it yields the least total error and close to the least under-prediction error of those tested. The new equation is termed the Sheppard/Melville (S/M) equation in this paper. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) depth-averaged river model Finite-Element Surface-Water Modeling System (FESWMS) was used to simulate the hydraulic conditions at a contracted bridge site. The site studied was the James River bridges near Mitchell, South Dakota. The parallel bridges are located in a crossing between the two bends of a meander. The floodplain alignment relative to the channel and skewed bridges produces complex 2D flow patterns that cannot be predicted accurately by using a one-dimensional (1D) river model. The 2D model was validated by using flow measurements collected by the USGS during three high-flow events with return periods ranging from 25 to 100 years. The validated model was used to examine the site characteristics that influence the concentrated flow on the right side of the main channel and the exchange of flow between the main channel and floodplains. The rating curves derived from the 2D model and the results of soil erosion tests were used to evaluate live-bed and clear-water contraction scour at the bridge site. The scour analysis was conducted by using the equations in Hydraulic Engineering Circular No. 18 (HEC-18) and a method that accounts for the soil erodibility by using a curve of measured erosion rate versus shear stress. The study found that channel meandering, the no-flow boundary condition imposed by the walls of the river valley and skewed roadway embankment, and the dense trees along the left bank are the three main factors that create the unique hydraulic conditions at the bridge site. It is shown that using a 2D flow model could improve the estimation of contraction scour by providing more accurate information on the hydraulic parameters. The predicted scour depth was very sensitive to the critical shear stress and slope of the curve of erosion rate versus shear stress. Therefore, design should incorporate uncertainty in soil properties. It is also shown that an unsteady-flow approach to scour would produce a more realistic curve of predicted scour depth versus time. However, the cumulative effects of multiple flood events must be evaluated if time-dependent scour is used in design. [ABSTRACT FROM AUTHOR]

New long-duration clear-water scour data were collected at single cylindrical piers with the objective of investigating the effect of sediment coarseness, ( = pier diameter; = median grain size) on the equilibrium scour depth and improving the scour depth time evolution modeling by making use of the exponential function suggested in the literature. Experiments were carried out for the flow intensity close to the threshold condition of initiation of sediment motion, imposing wide changes of sediment coarseness and flow shallowness, ( = approach flow depth). The effect of sediment coarseness on the equilibrium scour depth was identified; existing predictors were modified to incorporate this effect for ; and ; the complete characterization of a known scour depth time evolution model was achieved for , and . [ABSTRACT FROM AUTHOR]

Groups of piles are frequently used as bridge foundations. Different group configurations, characterized by different pile spacing, skew-angle, number, and arrangement of pile group columns interact differently with the flow field and lead to different scour patterns and equilibrium scour depth. There have been a number of past studies on the characterization of scouring at pile groups, but most of them report short duration scour experiments. A priori, such short durations may be postulated to inherently carry important uncertainties into existing scour predictors. In this study, 75 long-duration laboratory tests were run under steady, clear-water flow close to the threshold for initiation of sediment motion, to address the effect of time, pile spacing, skew-angle and number of pile group columns on the equilibrium scour depth. Pile groups consisted of matrical arrangements of one, two, or three columns of four rows, with spacings of 1, 2, 3, 4.5, and 6 pile diameters; the tested skew-angles were 0, 15, 30, 45, and 90°. Important contributions were achieved on (1) the impact of the duration of tests on the shape of the scour hole as well as on the precision of coefficients involved in current predictors, (2) the most unfavorable skew angle, (3) the behavior of collapsed pile groups, (4) the maximum scour depth at pile groups composed of a single alignment, and (5) the performance of two current predictors of scour depth at pile groups. Two formulations of a predictor for the calculation of an aggregated pile group factor are suggested. [ABSTRACT FROM AUTHOR]

The scour hole induced by a propeller wash is experimentally investigated with noncohesive sediments. The associated scour profile at the asymptotic condition, which can be expressed by a combination of three polynomials, comprises (1) a small scour hole beneath the propeller, (2) a primary scour hole downstream of the small scour hole, and (3) a deposition mound farther downstream of the primary scour hole. The time-dependent maximum scour depth induced by propeller jets is closely related to the densimetric Froude number, a reference time scale, offset height relative to the propeller diameter, and sediment size. A semiempirical formula, which shows good agreement with the experimental data, is proposed to describe evolution of the maximum scour depth. An equation for the determination of the time to initiate scour is also proposed. Moreover, empirical equations for the maximum asymptotic scour depth induced by a propeller and three-dimensional submerged offset jets agree well with the experimental data. The critical condition for the initiation of scour caused by propeller and offset jets is also presented. [ABSTRACT FROM AUTHOR]

Scour at piers embedded in a bed of cohesive sediment mixture is investigated to quantify the effects of bed compaction. Experiments were conducted in a large rectangular laboratory flume with natural clay sediment collected from a stream located at the Chilean coastal range. Nonintrusive and high-resolution topographic measurements of scoured bed in the vicinity of piers were performed during the experimental runs with an installation of a laser distance sensor (LDS) that was traversed by the precision step motors. Measurements were taken by the LDS in different azimuthal half-planes to study the spatiotemporal variation of the topography of evolving scour holes. Experiments were conducted over 40 h, until an advanced stage of scour was reached, with the approaching flow having a bed shear stress equaling 95% of the Shields critical condition for the initiation of motion of sand in a sand-clay mixture. Results show that the bed is successively scoured in three different forms, chunks of aggregates, aggregate by aggregate, and particle by particle, when increasing the ratio of actual to Proctor's optimum molding water content, . Maximum scour depth was mainly observed at the wake region (downstream of the pier) and ranged from 10 to 58% of that in the sand alone case for mixtures with the same size of the sand fraction. For a given compaction energy, scour depths first decrease with , up to a value of , as fine clay particles are flocculated and increasingly dispersed with water content. For values of , the sediment matrix behaves as a fluid mud. In the latter case, scour depths increase with water content as the sediment bed becomes increasingly liquid. [ABSTRACT FROM AUTHOR]

Piers with back-to-back stems or columns and piers for which part of the foundation becomes exposed as a result of the development of scour over large periods of time or because of severe flood events are fairly common at bridge waterways. The present paper uses eddy-resolving numerical simulations to study flow and turbulence structure at piers of complex shape and/or with multiple components. In particular, the study considers cases with one and two back-to-back pier columns for which the section of the main column is neither circular nor rectangular. In addition to a design case for which the foundation of each pier column is submerged, the study analyzes a case when scour exposes part of the foundation of the main column. The results show that the shape and size of the pier column have a significant effect on the spatial and temporal distributions of the bed friction velocity induced by the horseshoe vortex system. The large-scale shedding behind the main column greatly influences flow structure and increases bed friction velocity around the downstream column for piers with two back-to-back columns that are aligned with the incoming flow direction. The present study shows that the presence of large-scale unsteady coherent structures in the vicinity of the bed around piers of complex shapes results in very complex distributions of the bed friction velocity and in large-scale temporal oscillations of the bed friction velocity. The results of eddy-resolving simulations strongly suggest the need to account for the effect of these large-scale oscillations around the mean value when bed friction velocity distributions are used to estimate the flux of entrained sediment in movable bed simulations that do not resolve the large-scale turbulent flow structures. [ABSTRACT FROM AUTHOR]

Using riprap stones is a common method to prevent scouring around bridge piers. Many parameters affect stable riprap size. In the present work, stability of riprap stones around rectangular bridge piers are studied experimentally under clear-water conditions. Round-nose rectangular piers with three different aspect ratios of 3, 5, and 7 were tested. Furthermore, each pier was tested aligned with the flow and skewed at 5, 10, and 20° to the flow. Failure of the riprap layer occurred either because of high shear forces at the upstream face of the pier or under a combination of forces attributable to wake vortices and shear stresses downstream of the pier. Analysis of experimental data showed that relative flow depth, relative stone size, and effective pier width are the important factors affecting the stability of the riprap layer. At different stone sizes and pier skew angles, the region at which the riprap layer fails varies. Based on more than 140 sets of experiments, a design equation is presented for stable riprap size. [ABSTRACT FROM AUTHOR]

The experimental study investigates how the application of point suction at different bed locations affects local scour around a cylindrical bridge pier under clear-water conditions. The results show that suction has a profound effect on pier-scour development, and the extent of the influence is related to the location of the suction source. When the suction source is located at or upstream of the pier, the equilibrium scour depth is reduced by up to 50%, with , where and = suction flow rate and undisturbed approach flow rate, respectively. The data also show that for the suction source that is located downstream of the pier, the equilibrium scour depth is reduced by up to 30% with the same suction flow rate. The extent of pier-scour depth reduction at different locations from the suction source is closely associated with changes to the local effective shear and critical shear velocity. Results of this study show that bed suction may be used as an effective pier-scour countermeasure. [ABSTRACT FROM AUTHOR]

An experimental campaign of 21 long-duration tests on scouring at complex piers of different configurations was performed to investigate the effect of pile cap thickness on the temporal evolution of the maximum scour depth and the development of equilibrium conditions. Two test series were performed with two significantly different pile cap thicknesses over a wide range of pile cap elevations with respect to the expected scour hole and the approach flow depth. In addition, the accuracy of existing methods to calculate the maximum scour depth was evaluated and discussed. The results show that, in general, the thicker the pile cap, the deeper the corresponding scour hole. The increase in scour depth that is attributable to the pile cap thickness depends on the distance between the pile cap and the original bed level. The observed maximum scour depth was greater with the thicker pile cap (with respect to the thinner pile cap) when it was entirely immerged into the flow, partially buried, or totally buried at a small depth from the initial bed level. Depending on the distance between the pile cap and initial bed level, the pile cap thickness may also influence the temporal evolution of the maximum scour depth by increasing or reducing the scouring rate. [ABSTRACT FROM AUTHOR]

The flow characteristics around two partially buried objects, a short cylinder (SC) and a truncated cone (TC) were examined experimentally and numerically. Experiments were conducted in a large tilting flume with vertical walls made of acrylic plastic and steel floor covered by a 24-m long sand pit. For each experiment, the object was placed on a flat sandy bed, the flume was filled to a water depth of 41 cm, and a steady unidirectional current was established at high Reynolds numbers while sinking of the object was tracked over time. Velocity records were taken with an acoustic Doppler velocimeter (ADV) probe at the final stage of burial and scour hole evolution. The flow structure around each object was further investigated using a model that solves the Reynolds-averaged Navier-Stokes (RANS) equations coupled with a turbulence closure model, and results were compared to the experimental measurements. Measurements and numerical simulations allowed identification of regions of high and low velocity and turbulent kinetic energy accounting for zones of potential sediment erosion and deposition that dictate the burial of the object. The turbulence closure model was shown to reproduce the experimental mean flow velocity and turbulent kinetic energy with acceptable accuracy. [ABSTRACT FROM AUTHOR]

Laboratory flume experiments were conducted to study the flow field and sediment dynamics in a mobile-bed channel bend with and without the presence of stream barbs (upstream-angled submerged groynes). This paper presents spatial distributions of the vorticity field and the turbulent Reynolds shear stresses, kinetic energy, and integral scales. The objectives of the present paper were to (1) advance understanding of the role that vorticity and turbulence play in the erosion process in channel bends with and without barbs, and (2) optimize barb design for reducing erosion along the outer bank regions of channel bends through consideration of flow-field turbulence and vorticity. Results indicate that (1) increased -vorticity occurred in the outer bank scour zone for all runs with and without barbs; (2) for the runs without barbs and with moderately sized barbs, increased streamwise-cross-stream Reynolds stress () was found to coincide with locations of scour; and (3) for the run with large barbs, all three Reynolds shear stresses increased, but was most consistently associated with scour locations. In general, local scour near the barbs was associated with increased -vorticity, tke, and nonprincipal Reynolds stresses ( and ). Furthermore, it was observed that the outer bank region (particularly between adjacent barbs) may be susceptible to increased erosion if stream barbs, due to their size and layout, generate excessive turbulence and secondary velocities. [ABSTRACT FROM AUTHOR]

A series of laboratory flume experiments were performed to study the effect of stream barbs on flow field dynamics and sediment erosion in a 135° mobile-bed channel bend. Stream barbs (also known as spur dikes or submerged groynes) are low-profile linear rock features that redirect high velocity flow away from the outer bank of channel bends. Unlike emergent groynes, the submerged nature of these structures creates a unique combination of horizontal shear (plunging type flow) and vertical shear (at the groyne tip). Spatially dense, high frequency velocity data were collected and analyzed to describe the pattern and magnitude of three-dimensional (3D) velocity throughout the bend and in the vicinity of the stream barbs. This paper demonstrates that the outer bank region (particularly between barbs) may still be at risk of erosion (or even increased erosion greater than the same case without barbs) if stream barbs generate excessive secondary velocities (because of their size and layout) that are opposing the primary secondary flow naturally occurring in channel bends. Characterizing the role of flow field dynamics on the pattern of deposition and erosion through experimental measurements provided valuable data about how such flow features contribute to local scour, and about the performance of these structures. [ABSTRACT FROM AUTHOR]

The sediment erosion rate flume (SERF) is used to evaluate various methods to estimate shear stresses in a flume-style erosion rate testing device. Results show that direct shear stress measurements most closely correspond to shear stress estimates obtained from the Colebrook equation using a roughness height of approximately 77% of the sediment particle diameter. Using pressure drops to estimate shear stress appears to underestimate rough sample (grain sizes ) stresses. Likewise, smooth-wall assumptions and analytical methods also appear to produce lower than expected results for similar heights. Colebrook equation results should be interpreted as 'instrument specific' because of the different devices' hydraulic diameters. [ABSTRACT FROM AUTHOR]

Bridge piers having a nonuniform cross section over their height are termed as compound bridge piers. Mostly circular piers resting on large diameter wells or caissons (termed here as a circular compound bridge pier) are adopted for use in the Indian subcontinent for the road and railway bridges. In the present study, a new mathematical model is proposed for computation of temporal variation of scour depth at circular compound bridge piers. Two series of laboratory experiments were conducted to collect data for model application. In the first series of experiments, the data on temporal variation of scour depth were collected, albeit up to the transient (developing) stage of scour, around the circular uniform piers and circular compound piers for the clear-water scour condition. The second series of experiments was carried out to measure the size and hence the area of the principal vortex of the horseshoe vortex system and to determine the bed shear stress at the upstream face of the scour hole around the circular uniform piers and the circular compound piers. The mathematical model proposed in this paper has the capability to compute the temporal variation of scour depth around circular compound piers for all possible cases of footing position with respect to bed level, i.e., footing above the bed level, footing at the bed level, and footing below the bed level. [ABSTRACT FROM AUTHOR]

This paper presents results of an experimental study on three-dimensional scour at submarine pipelines with uniform sediment under a unidirectional current in clear-water conditions. The data show that propagation of the scour hole in the transverse direction of flow may be divided into a rapid and a slack phase of development. The former is characterized by a higher and constant velocity, whereas the latter a lower and reducing propagation velocity. Four nondimensional parameters are identified and their effects examined experimentally. Pipeline embedment and water depth to pipeline diameter ratios, which represent the stability force, inhibit the scouring process, resulting in a reduced propagation velocity and a dominant slack phase of development. Conversely, Froude number and Shields parameters represent the environment force; they enhance the scouring process, causing a high propagation velocity and a dominant rapid phase of development. The experimental results reveal that the scour process is not sensitive to Shields parameter under clear-water conditions but is closely related to the other three parameters. The effect of all the parameters can be viewed in a balance between the environment and stability forces. [ABSTRACT FROM AUTHOR]

When a sediment beach covered by stones or an armor layer is exposed to breaking waves, the turbulence generated by the breaking waves can cause mobilization and removal of the sediment underneath the stones. In two earlier studies by the Technical University of Denmark (DTU)-group, the suction removal of sediments from between armor blocks has dealt with the case of steady current and nonbreaking waves. The present study is an extension of these studies and the results will be presented in a similar way. The critical conditions for removal of sediment are determined. It is found that the onset of removal of sediment is governed by three parameters: (1) the sediment mobility (based on the sediment size, wave height, and wave period), (2) the ratio between the sediment size and the stone size, , and (3) surf similarity parameter, . The variation of the critical mobility number for removal of sediment as function of and is determined for the range and . The experiments were made on , , and slopes. Spilling, plunging, and surging breakers were used in the experiments. Both one and two layers of armor stones and rectangular blocks were studied. [ABSTRACT FROM AUTHOR]

The paper proposes a semiempirical model to estimate the temporal development of scour depth at cylindrical piers with unexposed foundations. A cylindrical pier with a foundation is considered as nonuniform pier. The concept of primary vortex and the principle of volumetric rate of sediment transport are used to develop a methodology to characterize the rate of evolution of the scour hole at nonuniform cylindrical piers. The model also simulates the entire scouring process at nonuniform cylindrical piers having the discontinuous surface located below the initial bed level. The scouring process includes three zones; viz Zone 1 having the scouring phenomenon similar to that of a uniform pier, Zone 2 in which the scour depth remains unchanged with its value equal to the depth of the top level of foundation below the initial bed level while the dimensions of the scour hole increase, and in Zone 3 the geometry pier foundation influences the scouring process. A concept of superposition using an effective pier diameter is proposed to simulate the scouring process in Zone 3. In addition, the laboratory experiments were conducted to utilize the laboratory results for the validation of the model. The simulated results obtained from the proposed model are in good agreement with the present experimental results and also other experimental data. Also, the effect of unsteadiness of flow is incorporated in the model and the results of the model are compared with the experimental data. The model agrees satisfactorily with the experimental data. [ABSTRACT FROM AUTHOR]

Kootenai River white sturgeon spawn in an 18-km reach of the Kootenai River, Id. Since completion of Libby Dam upstream from the spawning reach in 1972, 1974 is the only year with documented significant recruitment of juvenile fish. Where successful in other rivers, white sturgeon spawn over clean coarse material of gravel size or larger. The channel substrate in the current (2008) 18-km spawning reach is composed primarily of sand and some buried gravel; within a few kilometers upstream there is an extended reach of clean gravel, cobble, and bedrock. We used a quasi-three-dimensional flow and sediment-transport model along with the locations of collected sturgeon eggs as a proxy for spawning location from 1994 to 2002 to gain insight into spawning-habitat selection in a reach which is currently unsuitable due to the lack of coarse substrate. Spatial correlations between spawning locations and simulated velocity and depth indicate fish select regions of higher velocity and greater depth within any river cross section to spawn. These regions of high velocity and depth occur in the same locations regardless of the discharge magnitude as modeled over a range of pre- and postdam flow conditions. A flow and sediment-transport simulation shows high discharge, and relatively long-duration flow associated with predam flow events is sufficient to scour the fine sediment overburden, periodically exposing existing lenses of gravel and cobble as lag deposits in the current spawning reach. This is corroborated by video observations of bed surface material following a significant flood event in 2006, which show gravel and cobble present in many locations in the current spawning reach. Thus, both modeling and observations suggest that the relative rarity of extremely high flows in the current regulated flow regime is at least partly responsible for the lack of successful spawning; in the predam flow regime, frequent high flows removed the fine sediment overburden, unveiling coarse material and providing suitable substrate in the current spawning reach. [ABSTRACT FROM AUTHOR]

The effect of a single-peaked flood wave on pier scour is investigated both theoretically and experimentally. The conditions considered involve clear-water scour of a cohesionless material of given median sediment size and sediment nonuniformity, an approach flow characterized by a flow depth and velocity, a circular-shaped cylindrical bridge pier, and a flood hydrograph defined by its time to peak and peak discharge. A previously proposed formula for scour advance under a constant discharge was applied to the unsteady approach flow. The generalized temporal scour development along with the end scour depth are presented in terms of mainly the densimetric particle Froude number based on the maximum approach flow velocity and the median sediment size. The effect of the remaining parameters on the end scour depth is discussed and predictions are demonstrated to be essentially in agreement with model observations. [ABSTRACT FROM AUTHOR]

The effectiveness of a novel approach of using vanes, installed at a low angle and attached to the bank, for bank protection and for the restoration of river meanders has been investigated in a laboratory study. Experiments were carried out in a large-scale meandering mobile-bed channel with graded sediment. The bed topography, three-dimensional flow pattern, and turbulence characteristics in the meandering channel with or without structures are analyzed. When a single or an array of such vanes is installed, the scour hole at the base of the outer bank is infilled and the thalweg is relocated toward the center of the river. The structures induce a secondary flow cell near the outer bank which counteracts the main spiral flow in the bend. In contrast to common spurs and bendway weirs, large-scale horizontal vortices are not generated behind the structures. Vanes which grade to the bed from bankfull level at the bank show better performance than low level ones, whereas multiple structures show positive effects as far downstream as the crossover section. [ABSTRACT FROM AUTHOR]

Submerged vanes are submerged foils of low height and larger length, constructed in a river at an angle of attack α to the flow to modify the near-bed flow pattern and redistribute flow and sediment transport within the channel cross section. At a Froude number (F) of 0.13, the local scour development around the submerged vane without a collar was not enough to dislodge the vane whereas at F=0.25, there was a significant local scour hole around the vane and the vane was dislodged. With the introduction of a collar at the leading edge of a submerged vane, the scour depth at the leading edge of the vane was reduced to zero. A collar of circular shape was found more suitable for a rectangular vane. Recommendations for sizing collars at two values of F are given. The optimal α for a rectangular vane with a collar was found close to 40°. The study clearly indicates the advantages of using collars in case of submerged vanes and provides insight into selection of appropriate collar shapes. [ABSTRACT FROM AUTHOR]