Prediction of Scour Downstream of Grade-Control Structures Using Neural Networks

A new approach for predicting local scour downstream of grade-control structures based on neural networks is presented. An explicit neural networks formulation (ENNF) is developed using a transfer function (sigmoid) and optimal weights obtained from a training process. A genetic algorithm was used to optimize the neural network architecture and the optimal weights for input and output parameters were obtained using the Levenberg–Marquardt back-propagation algorithm. Experimental data available in the literature, including large-scale results were used for training and validation of the proposed model. The predictive performance of the ENNF was found superior to other regression-based equations and the robustness of ENNF was evaluated using field data.     

Genetic Programming Approach for Prediction of Local Scour Downstream of Hydraulic Structures

This is a pioneer study that presents genetic programming (GP) as a new tool for prediction of local scour downstream of grade-control structures. The objective of this study is to provide an alternative formulation to conventional regression based equations and verify the superiority of GP over regression analysis. The training and testing patterns of the proposed GP formulation are based on well established and widely dispersed experimental results from the literature. Linear and nonlinear regression-based equations were derived throughout regression analysis on dimensionless parameters obtained from dimensional analysis. The GP-based formulation results are compared with experimental results and other equations and found to be more accurate.     

Local Scour at Sloped-Wall Spur-Dike-Like Structures in Alluvial Rivers

The effect of side slope on the prediction of the maximum local scour depth around sloped-wall spur-dike-like structures has been investigated using two existing prediction methods where slope factors were considered. The first one was developed from experimental data and is empirical in nature while the second one was developed using an analytical treatment by considering the flow concentration in the restricted region of scour hole. Both of the above approaches are applicable for clear-water scour. With the aid of the analytical model, the slope factors for the empirical formulas have been extrapolated for milder side slopes. The results of the above two methods together with the maximum measured local scour depth around a spur-dike-like structure in an alluvial river have been compared and discussed with particular emphasis on the effect of side slope on estimates of the maximum local scour depth.     

Control of Scour at Bridge Piers by a Downstream Bed Sill

Results are presented from laboratory experiments to investigate the effectiveness of bed sills as countermeasures against local scouring at a smooth circular bridge pier, for flow conditions near the threshold of uniform sediment motion. The bed sill was located downstream of the pier, and its effectiveness with the distance between pier and sill was evaluated. The dependence of the scour depth on different dimensionless groups was defined. The results showed that a bed sill placed at a short distance downstream of the pier reduces the scour depth, area, and volume. In particular, the smaller the distance between the two structures, the larger the effectiveness of the countermeasure. The bed sill seems to take effect some time after the beginning of the test, as the scour hole downstream of the bridge pier develops sufficiently and interacts with the countermeasure.     

Influence of Tailwater Depth and Pile Position on Scour Downstream of Block Ramps

Scour control downstream of hydraulic structures is an important topic in hydraulic engineering. Block ramps or rock chutes are often used to control scour downstream of hydraulic structures and have the peculiarity to be ecofriendly. Although these structures assure great energy dissipation, the rapid passage from supercritical to subcritical flow at the toe results in a scour hole with geometric parameters that have to be evaluated in order to avoid foundation problems. For this reason, the analysis of the scour process and the comprehension of the hydrodynamic mechanisms on which it is based are extremely important. In this paper, the results of systematic experimental tests are shown that analyze both the influence of the stilling basin tailwater depth and the ramp toe stabilizing structures, for both uniform and nonuniform channel bed materials. In fact, block ramps are generally stabilized by inserting piles or micropiles at the toe. The upper edge level of piles or micropiles was found a relevant parameter for the scour hole geometry. Simple novel relationships that account for tailwater depth, pile position, and bed material gradation are developed to evaluate the main lengths of the scour hole, in the case in which a free hydraulic jump in a mobile bed occurs. These simple relationships give engineers helpful instruments in block ramp design.     

Scour Control Downstream of Block Ramps

Scour control downstream of a hydraulic structure is an important topic in river engineering. A block ramp is a structure that can be built with a low-environmental impact stilling basin realized by means of a sill that controls the scour hole. The scour that occurs downstream of block ramps is analyzed and the effect of rock sills on scour geometry is evaluated. Experiments have been carried out with ramp slopes varying between 1V:4H to 1V:12H. Three different sill types were tested for a total of 300 experiments among which about 140 tests involved rock sills. Useful equations for the design of the stilling basin are proposed.     

Downstream Hydraulic Geometry of Clay-Dominated Cohesive Bed Rivers

Empirical downstream hydraulic geometry equations for consolidated clay-dominated cohesive bed (nonalluvial) natural streams are presented using data from six rivers in eastern Ontario, Canada and four rivers from other regions. The width exponent (0.57) was comparable to the exponents reported for previous studies; however, the depth exponent (0.52) was greater for clay-dominated cohesive bed than for typical alluvial gravel-bed and sand-bed rivers. The width to depth ratio of smaller channels (Qbf<20?m3/s) was greater for consolidated clay bed than for either sand-bed or gravel-bed channels. This study suggests that the concept of hydraulic geometry and bankfull (channel forming) discharge can be extended to nonalluvial consolidated clay-bed channels.     

Influence of Sediment Gradation on Scour Downstream of Block Ramps

The scour process downstream of block ramps is an important research topic of value to engineering practice. The object of the present experimental study is to examine the scour mechanism in order to predict the main geometrical parameters of the scour hole downstream of block ramps. The investigations have been conducted using two physical models located in the hydraulic laboratory of the Department of Civil Engineering in Pisa, Italy. Sediments with different values of the nonuniformity parameter were used as channel bed material. Moreover, different ramp slopes, ranging from 1/4 to 1/12, were tested. Equations and graphs demonstrate that the results can be interpreted by means of simple relationships in the case in which a ridge is present downstream of the scour hole.     

Local Scour Downstream of Positive-Step Stilling Basins

This note focuses on the temporal and spatial evolution of local scour below low-head spillways. Steady-flow experiments were carried out in a 1-m wide and 20-m long rectangular straight channel. The jet was generated by an ogee-crest spillway followed by a positive-step stilling basin. Nearly uniform sandy beds were generally tested, but additional tests were also performed with a special bed of lead spheres. To circumvent the combination of local and general scour phenomena, tailwater depths were set such that tailwater flow intensities were below the threshold of sediment motion. As a consequence, for each run a submerged hydraulic jump formed. Tests were of long durations (of order of days) mainly to achieve conditions of quasi-equilibrium. Based on the data collected, literature approaches are discussed. Then, empirical models are proposed to estimate: (1) the maximum scour depth at the quasi-equilibrium stage and its horizontal distance from edge of stilling basin; (2) the time variation of scour depth; and (3) the axial scour profiles. The proposed equations agree well with experimental data. Findings also highlight that affinity rather than similarity may be the typical property of low-angle eroding jets.     

Flow Intensity Parameter in Pier Scour Experiments

Technical note discusses a detailed approach to dimensional analysis for the bridge pier scour phenomenon and the introduction of flow intensity. It demonstrates the dependence of critical upstream velocity on the rest of the parameters describing the process and its implications on dimensional analysis. Assuming that the viscous effects are negligible in the local scour phenomenon, it is concluded that the flow intensity of the approaching undisturbed flow is not an adequate parameter to describe the process in usual laboratory conditions. A new proposal is established.     

Neural Networks for Estimation of Scour Downstream of a Ski-Jump Bucket

The estimation of scour downstream of a ski-jump bucket has remained inconclusive, despite analysis of numerous prototypes as well as hydraulic model studies in the past. It is partly due to the complexity of the phenomenon involved and partly because of limitations of the traditional analytical tool of statistical regression. This paper addresses the latter part and presents an alternative to the regression in the form of neural networks. The depth of the scour hole developed along with its width and length is predicted using neural network models. A network architecture complete with trained values of connection weight and bias and requiring input of grouped parameters pertaining to discharge head, tail water channel depth, bucket radius, lip angle, and median sediment size is recommended in order to predict the depth, the location of maximum scour, as well as the width of scour hole. The neural network predictions have been compared with traditional statistical schemes. Although the common and simple feed forward back propagation network took a very long time to train as compared to some advanced schemes, it was found to impart equally reliable training as the latter. Use of causative variables in grouped forms was found to be more rewarding than that of their raw forms probably due to lesser scaling effect.     

Three-Dimensional Numerical Model for Flow and Bed Deformation around River Hydraulic Structures

This paper describes a numerical model developed to simulate flow and bed deformation around river hydraulic structures. The model solved the fully three-dimensional, Reynolds-averaged Navier–Stokes equation expressed in a moving boundary-fitted coordinate system to calculate the flow field with water and bed surfaces varying in time. A nonlinear k-ε turbulence model was employed in order to predict flow near the structure where three-dimensional flow is dominant. The temporal change in bed topography was calculated by coupling a stochastic model for sediment pickup and deposition using a momentum equation of sediment particles in order to account for the effect of nonequilibrium sediment transport. In validating the numerical model, a spur dike and a bridge pier, which are considered to be typical river-engineering structures, were selected. By comparing the numerical results with observed laboratory experimental data, the model was found to reproduce flow and scour geometry around these structures with sufficient accuracy.     

Similitude of Large-Scale Turbulence in Experiments on Local Scour at Cylinders

The writers’ experiments on local scour at vertical cylinders placed in a sand bed show that similitude of large-scale turbulence is an important consideration influencing equilibrium depth of local scour. For the range of cylinder diameters used in their experiments, the writers identify a direct trend between equilibrium scour depth (normalized with cylinder diameter) and the intensity and frequency of large-scale turbulence shed from each cylinder; values of normalized scour depth increased when cylinder diameter decreased. The writers offer a scour-depth adjustment factor to account for this trend, which essentially is a scale effect incurred with experiments involving three independent length scales: cylinder diameter, bed-particle diameter, and flow depth. The consequent similitude consideration, or scale effect, has general significance for laboratory studies of local scour associated with hydraulic structures in sediment beds.     

Scour Downstream of an Apron Due to Submerged Horizontal Jets

The results of an experimental investigation on scour of noncohesive sediment beds (uniform and nonuniform sediments) downstream of an apron due to a submerged horizontal jet issuing from a sluice opening are presented. Attempts are made to explain the similarity existing in the scour process and profiles (including dune in the downstream of the scour hole). The scour profiles at different times follow a particular geometrical similarity and can be expressed by the combination of two polynomials. Using experimental scour depth at different times, the time variation of scour depth is scaled by an exponential law, where time scale increases linearly with densimetric Froude number. The equilibrium scour depth, related to the sediment size relative to the sluice opening, decreases with increase in sediment size and sluice opening. On the other hand, the equilibrium scour depth increases with increase in densimetric Froude number. The variation of equilibrium scour depth with tailwater depth indicates a critical tailwater depth corresponding to a minimum equilibrium scour depth. The effect of sediment gradation on scour depth is pronounced for nonuniform sediments, which reduce scour depth significantly due to formation of an armor layer, and therefore prompted study of the reduction of scour depth by a launching apron placed downstream of the rigid apron. The results show that the average reduction of scour depth by placing a launching apron was 39%, having a maximum of 57.3% and a minimum of 16.2%. The characteristic parameters affecting maximum equilibrium scour depth are identified based on the physical reasoning and dimensional analysis. Equation of maximum equilibrium scour depth obtained empirically agrees well with the experimental data.     

Hydraulics of Plane Plunge Pool Scour

Plunge pool scour involves a significant risk with trajectory spillways because of structural undermining at a dam foot or destabilization of adjacent valley slopes. An experimental program towards the understanding of plane plunge pool scour of a completely disintegrated rock surface was conducted, in which the following items received attention: jet shape, jet velocity, jet air content, tailwater elevation, granulometry, upstream flow to the scour hole, and the end scour profile in terms of the basic scour features. These effects were experimentally investigated based on a systematic variation of the governing scour parameters. The results of this paper allow answering questions that have so far not been addressed. Design equations were proposed to sketch the main tendency of the data sets. The significant effect of the densimetric particle Froude number was substantiated. This research may be used to estimate the prominent scour features for nearly two-dimensional jet arrangements involving a pre-aerated high-speed flow.     

Effect of Stilling Basin Geometry on Clear Water Scour Morphology Downstream of a Block Ramp

The scour mechanism downstream of a block ramp in clear water conditions is quite a complex phenomenon that depends on several parameters. Majors role are played by ramp configuration, hydraulic conditions (downstream tailwater level and discharge), material granulometry, and stilling basin geometry. Previous studies analyzed both the scour phenomenon and the effects of all the parameters involved except the last one (i.e., the case in which the ramp has the same width as the downstream stilling basin) and the case of symmetrically expanding stilling basins. This last basin configuration represents an optimal equilibrium between the necessity to dissipate energy and to create a natural pool for the biological species, thus it has a prominent ecological value. The present paper aims to assess the effect of both the width and length of the downstream stilling basin on scour features and flow pattern in clear water conditions. Different scour morphology types are distinguished and classified according to hydraulic and geometric conditions. Simple novel relationships are proposed to evaluate the scour depth and length and the maximum water level in the stilling basin. The results are valid for unsubmerged block ramps.     

Two-Dimensional Scour Hole Problem: Role of Fluid Structures

An experimental program was carried out to understand scour caused by a plane wall jet. A two-dimensional laser Doppler anemometer was used to characterize the velocity field at various locations in the scour hole region. Observations indicate that different types of flow structures influence scour at different time periods. Based on the present tests, the entire test duration is divided into five time zones. Following vigorous scour caused principally by jet shear forces and impingement at the start of the test and during early time periods, the flow was characterized by the presence of longitudinal axial vortices, turbulent bursts, and movement of the jet impingement point during the later stages. Attempts were made to distinguish the fluid structures at asymptotic conditions. The scour hole region was characterized by the presence of randomly forming and disappearing streaks, laterally located concave shaped depressions, rolling and ejection of the bed material. Through analysis of higher order moments and quadrant decomposition, sweep and ejection type events were observed, which can potentially contribute to scour.     

Temporal Evolution of Plunge Pool Scour

The temporal development of plunge pool scour was investigated using a novel experimental approach. Longitudinal profiles along the scour hole were recorded with an optical method to allow its definition at any time, from the initiation of scour to nearly the end-scour condition. The characteristics of the scour hole geometry were investigated, namely the maximum scour hole depth, the maximum ridge height, and their locations relative to the scour hole origin. It is demonstrated that the evolution is logarithmic, similar to that found for bridge pier and abutment scour. A distinction is further made between the developing and the developed scour hole phases. The main issue of the present research was to define the developed scour hole characteristics because the developing scour phase is influenced by turbulence features that may be difficult to assess. This work therefore allows for an appreciation of the temporal evolution of a scour process of engineering interest.     

Local Scour and Riprap Stability at an Abutment in a Degrading Bed

The paper reports on an experimental investigation concerning two important issues: (1) local scour and (2) riprap stability at a 45° wing-wall abutment in a degrading river bed of noncohesive sediment. The abutment considered was short (that is abutment length/flow depth <1). From the experimental observations, no influence of abutment inclusion on bed degradation was evident, as bed profiles with and without abutment were quite identical apart from the immediate vicinity of the abutment. Total scour depth at an abutment is found to be the maximum abutment scour depth in addition to the reduction of bed elevation due to bed degradation. The maximum abutment scour depth can be estimated from the equation given by Kandasamy and Melville in their 1998 paper. For scouring time beyond 24?h, the local abutment scour depth remains independent of time. In a degrading bed, the bed forms cause edge failure of the riprap at an abutment when the dunes propagate over the riprap layer. Initially, the dune height is significant causing the maximum damage of riprap layer. As the flow velocity reduces, the resulting bed-shear stress diminishes with the degrading bed and gradually the formation of dunes ceases. An additional experiment reveals that the damaged riprap layer is significantly vulnerable against a subsequent flood accompanied by large dunes.     

Characteristics of Submerged Jets in Evolving Scour Hole Downstream of an Apron

This paper reports an experimental investigation on the velocity and turbulence characteristics in an evolving scour hole downstream of an apron due to submerged jets issuing from a sluice opening detected by an acoustic Doppler velocimeter. Experiments were carried out for the conditions of submerged jets, having submergence factors from 0.96 to 1.85 and jet Froude numbers from 2.58 to 4.87, over sediment beds downstream of a rigid apron. The distributions of time-averaged velocity vectors, turbulence intensities, and Reynolds stress at different streamwise distances are plotted for the conditions of initial flat bed, intermediate scour holes, and equilibrium scour hole downstream of an apron. Vector plots of the flow field show that the rate of decay of the submerged jet velocity increases with an increase in scour hole dimension. The bed-shear stresses are determined from the Reynolds stress distributions. The flow characteristics in evolving scour holes are analyzed in the context of self-preservation, growth of the length scale, and decay of the velocity and turbulence characteristics scales. The most significant observation is that the flow in the scour holes (intermediate and equilibrium) is found to be plausibly self-preserving.