Bruce W. Melville

FLOW CHARACTERISTICS IN LOCAL SCOUR AT BRIDGE PIERS

The paper summarises the major results of an investigation of How patterns, turbulence intensity distributions and boundary shear stress distribution in the scour zone of a circular pier under clear water scour conditions.

Local scour around bridge piers

Local scour around cylindrical bridge piers in uniform, cohesionless sediment was investigated experimentally. Three empirical functions which relate equilibrium scour depth with approach velocity, sediment size and How depth were obtained. The variations of equilibrium scour depth with approach velocity show that the depth of scour decreases at velocities just above the threshold until it reaches a minimum at about twice the threshold velocity. Thereafter, it increases again until a maximum is reached at the transition flat bed condition. At still higher velocity, the equilibrium scour depth decreases due to the formation of antidunes. Both the effects of sediment size and flow depth were found to be independent of the stage of bed particle motion and the approach velocity. Adjustment factors, for the effects of sediment size and flow depth were defined and evaluated.

Clear-water scour development at bridge abutments

Development of local scour depths at vertical-wall bridge abutments of varying lengths was investigated in several series of experiments of ranges of uniform sediments and clear-water flow intensities. For each flow-abutment-sediment combination tested, the change with time in maximum local scour depth from plane-bed to equilibrium conditions was recorded and analysed. The results of similar earlier abutment-scour experimental studies were also incorporated in analyses of scour development. Variations in scour rates and depths with flow and sediment parameters are found to be different for short (flow depth/abutment length = y/L > 1) and long (y/L ≤ 1) abutments. In order to provide a consistent framework for assessment of the development of scour depth with time, a revised definition of the time to achieve equilibrium conditions is developed. Based on this definition, the dimensionless time to equilibrium for scour development from plane-bed conditions can be expressed as a function of relative flow intensity and relative abutment length. This expression can be adopted into existing methodologies to predict scour magnitudes occurring at bridge abutments. Utilising extensive data sets, expressions enabling the determination of scour depths at abutments, and also piers, are reassessed for the present definition of time to achieve equilibrium conditions. Application of the developed expressions is highlighted in an example.

Local scour and flow measurements at bridge abutments

The three dimensional flow field for local scour at bridge abutments was measured in the laboratory using the hydrogen bubble technique. The results show that the flow structures are dominated by a large primary vortex and its associated downflow and that they are confined predominantly within the scour hole beneath the original bed level. The value of this paper is also in the large amount of data collected, which would be useful for the future development and calibration of numerical models of flow and scour at bridge abutments.

Evaluation of Existing Equations for Local Scour at Bridge Piers

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.

Bayesian neural networks for prediction of equilibrium and time-dependent scour depth around bridge piers

Abstract The physical process of scour around bridge piers is complicated. Despite various models presented to predict the equilibrium scour depth and its time variation from the characteristics of the current and sediment, scope exists to improve the existing models or to provide alternatives to them. In this paper, a neural network technique within a Bayesian framework, is presented for the prediction of equilibrium scour depth around a bridge pier and the time variation of scour depth. The equilibrium scour depth was modeled as a function of five variables; flow depth and mean velocity, critical flow velocity, median grain diameter and pier diameter. The time variation of scour depth was also modeled in terms of equilibrium scour depth, equilibrium scour time, scour time, mean flow velocity and critical flow velocity. The Bayesian network predicted equilibrium and time-dependent scour depth much better when it was trained with the original (dimensional) scour data, rather than using a non-dimensional form of the data. The selection of water, sediment and time variables used in the models was based on conventional scour depth data analysis. The new models estimate equilibrium and time-dependent scour depth more accurately than the existing expressions. A committee model, developed by averaging the predictions of a number of individual neural network models, increased the reliability and accuracy of the predictions. A sensitivity analysis showed that pier diameter has a greater influence on equilibrium scour depth than the other independent parameters.

Statistical downscaling of watershed precipitation using Gene Expression Programming (GEP)

Investigation of hydrological impacts of climate change at the regional scale requires the use of a downscaling technique. Significant progress has already been made in the development of new statistical downscaling techniques. Statistical downscaling techniques involve the development of relationships between the large scale climatic parameters and local variables. When the local parameter is precipitation, these relationships are often very complex and may not be handled efficiently using linear regression. For this reason, a number of non-linear regression techniques and the use of Artificial Neural Networks (ANNs) was introduced. But due to the complexity and issues related to finding a global solution using ANN-based techniques, the Genetic Programming (GP) based techniques have surfaced as a potential better alternative. Compared to ANNs, GP based techniques can provide simpler and more efficient solutions but they have been rarely used for precipitation downscaling. This paper presents the results of statistical downscaling of precipitation data from the Clutha Watershed in New Zealand using a non-linear regression model developed by the authors using Gene Expression Programming (GEP), a variant of GP. The results show that GEP-based downscaling models can offer very simple and efficient solutions in the case of precipitation downscaling.

Hydrodynamic Forces Generated on a Spherical Sediment Particle during Entrainment

The objective of this research is to study the relationship between the coherent flow structures and the hydrodynamic forces leading to entrainment of a spherical bed sediment particle for a rough bed uniform turbulent flow. Two types of experiments, namely, movable and fixed balls, were conducted using spherical roughness-element beds with particle image velocimetry to measure the instantaneous flow-velocity field. Miniature piezoelectric pressure sensors were used to capture the instantaneous pressure on the surface of the sphere. Movable ball experiments reveal the predominance of large sweep structures at the instant of entrainment. Fixed ball experiments carried out at entrainment conditions show the importance of both vertical and horizontal pressure gradients on the ball leading to entrainment. Probability distribution function plots of pressures based on quadrant analysis of velocities also reveal the higher probability of occurrence of high magnitude force induced by sweep ( Q4 ) events.

Equilibrium hydrodynamics concept for developing dunes

Experiments utilizing two-dimensional fixed dune profiles and varying flow depth (dune regime flows) highlight the equilibrium (self-similar) nature of the near-bed boundary layer over developing dunes with flow separation in the dune lee. The negligible variation in roughness layer (comprising the interfacial and form-induced layers) flow structure for developing dunes was confirmed in terms of spatial fields of time-averaged velocities and stresses; and vertical distributions of: (a) double-averaged (in time and space) longitudinal velocity, (b) double-averaged normal stresses, and (c) the components of the momentum balance for the flow. The finding of an equilibrium nature for the near-bed flow over developing dunes is significant in its centrality to understanding the feedback loop between flow, bed morphology, and sediment transport that controls erodible-bed development. Further research is required into the form of the distribution of double-averaged velocity in the form-induced layer above roughne...

Clear-Water Local Scour around Pile Groups in Shallow-Water Flow

The results of an experimental study on clear-water scour at pile groups under steady flows at threshold velocity are presented. A wide range of pile group arrangements, spacing, and submergence ratios are studied. Uniform bed material in a 46-m long, 1.52-m wide, and 1.9-m deep flume was used to obtain comprehensive data. Empirical relations to demonstrate the effects of the studied variables are presented. On the basis of the findings of this study, a new method to predict the effect of pile group arrangements, pile spacing, and submergence ratio on local scour depth is proposed. The experimental data obtained in this study and data available from the literature are used to evaluate predictions of existing methods and the accuracy of the proposed method. The proposed method gives reasonable scour depth predictions.