Terry W. Sturm

Effect of Sediment Size Scaling on Physical Modeling of Bridge Pier Scour

Local pier scour experiments were performed in the laboratory to investigate the effect of relative sediment size on pier scour depth using three uniform sediment sizes and three bridge pier designs at different geometric model scales. When the data from a large number of experimental and field investigations are filtered according to a Froude number criterion, the effect of relative sediment size on dimensionless pier scour depth is brought into focus. The choice of sediment size in the laboratory model distorts the value of the ratio of pier width to sediment size in comparison with the prototype which in turn causes larger values of scour depth in the laboratory than in the field. This model distortion due to sediment size is shown to be related to the scaling of the large-scale unsteadiness of the horseshoe vortex by studying the relevant time scales of its coherent structure upstream of a bridge pier using acoustic Doppler velocimeter measurements. Observations of sediment movement, probability distributions of velocity components, and phase-averaging of velocity measured upstream of a bridge pier reveal properties of coherent motions that are discussed in terms of their contribution to the relationship between dimensionless pier scour depth and the ratio of pier width to sediment size over a large range of physical scales.

Turbulence statistics in compound channels with deep and shallow overbank flows

The results of large eddy simulations (LESs) of turbulent flow in a compound open channel with deep and shallow flood plain depths are presented. These LESs are validated with experimental data, resulting in a good agreement between measured and calculated data. The floodplain-depth-to-main-channel-depth ratio is an important parameter affecting first- and second-order turbulence statistics. The streamwise momentum balance is analysed with respect to the momentum transfer at the interface between the main channel and the floodplain. Depth-averaging of the streamwise momentum equation reveals the dominance of turbulent shear stress terms in the main channel and of secondary currents in the floodplain. Comparisons of the LES results with an analytical solution of the depth-averaged streamwise momentum equation that demonstrate the importance of case-specific calibration of the parameters of the analytical solution. The turbulence anisotropy is quantified and its role in generating secondary currents is illustrated.

Flow dynamics through a submerged bridge opening with overtopping

ABSTRACT This paper reports the results of detailed numerical simulations of the flow over a submerged bridge. The method of Large Eddy Simulation (LES) is applied, which allows elucidation of the instantaneous flow and accurate quantification of its turbulence statistics. The in-house LES code employs a free surface algorithm based on the Level Set Method (LSM) to determine the complex water surface profile over the bridge, which is validated with data from a physical model of the bridge under analogous flow conditions. Numerically predicted water surface profiles show good agreement with measured data. The water surface resembles that of the flow over a broad-crested weir with a plunging nappe and a standing wave downstream of the bridge. The mean flow is characterized by a multitude of complex flow features including horizontal recirculation zones upstream and downstream of the bridge abutment and vertical recirculation zones of the separated plunging flow. The latter influences significantly the near-bed turbulence and the bed shear stress.

A micromechanics approach for attachment and detachment of asymmetric colloidal particles

Abstract In this study, a micromechanics interaction model has been developed for kaolinite particles, and experiments have been performed on attachment and detachment of particles in porous media columns. The experimental results are discussed and interpreted in terms of microscopic forces, in contrast to conventional computations in the colloid literature which are based on energy terms. The force calculations are done using expressions developed for the natural shape (hexagonal platelets) of the kaolinite particles without making use of unrealistic assumptions of particle sphericity. The forces considered are van der Waals forces, electrical double layer forces, hydration forces and Born repulsion. Under constant chemical conditions, the three likely modes of interaction of kaolinite platelets showed significantly different total force curves. The microscopic force analysis was found to be consistent with experimental results.

Free-Surface versus Rigid-Lid LES Computations for Bridge-Abutment Flow

Two separate large eddy simulations (LES) are carried out to investigate the effects of accurate computation of the curvilinear water-surface deformation of the flow through a bridge contraction. One LES employs the rigid-lid boundary condition at the water surface, while the other uses a highly accurate level-set method (LSM), which allows the water surface to adjust itself freely in response to the flow. The simulation with the LSM is validated with data from complementary physical model tests under analogous geometrical and flow conditions. Streamwise velocity, bed-shear stress and second-order turbulence statistics obtained from both simulations are compared, and it is shown that the turbulence structure of this flow is influenced strongly by the water-surface deformation. While bed-shear stresses and first-order statistics are very similar for both cases, the instantaneous turbulence structure and consequently, the second-order statistics, are distinctly different. The correct prediction of the water-surface deformation of such flows is deemed important for the accuracy of their simulation. Read More: http://ascelibrary.org/doi/10.1061/%28ASCE%29HY.1943-7900.0001028

Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction

This report reviews the present state of knowledge regarding bridge-abutment scour and the veracity of the leading methods currently used for estimating design scour depth. It focuses on research information obtained since 1990, which is to be considered in updating the scour estimation methods that are recommended by AASHTO, and used generally by engineering practitioners. Though considerable further progress has been made since 1990, the findings indicate that several important aspects of abutment scour processes remain inadequately understood and therefore, are not included in current methods for scour depth estimation. The state-of-the-art for abutment scour estimation is considerably less advanced than for pier scour. Moreover, there is a need for design practice to consider how abutment design should best take scour into account, as scour typically results in the geotechnical failure of an abutment’s earthfill embankment, possibly before a maximum potential scour depth is attained hydraulically. Abutment scour herein is taken to be scour at the bridge-opening end of an abutment, and directly attributable to the flow field developed by flow passing around an abutment. This definition excludes other flow and channel-erosion processes such as lateral geomorphic shifting of the bridge approach channel but includes contraction and abutment scour as part of the same physical processes that should be treated together rather than separately in their estimation. The review shows that, since 1990, advances have been made in understanding abutment-scour processes, and in (1) estimating scour depth at abutments with erodible compacted earthfill embankments, and at those with solid-body (caisson-like) foundations; (2) identifying the occurrence of at least three distinct abutment scour conditions depending on abutment location and construction; and (3) utilizing the capacity of numerical modeling to reveal the flow field at abutments in ways that laboratory work heretofore has been unable to provide. The review identifies and evaluates leading scour formulas and suggests a framework for developing a unified abutment scour formula that depends on satisfying several targeted future research needs.

Three-dimensional RANS modeling of flow around circular piers using nested grids

Abstract The paper introduces a three-dimensional numerical model that solves the Reynolds Averaged Navier-Stokes (RANS) equations on a curvilinear grid system using a novel nested grid approach. The main benefit of the model is the possibility to model locally complex hydraulic features in large rivers like the flow field at hydraulic structures. The entire study domain in such a case can be discretized with a coarser resolution, whereas a much finer resolution can be applied to a defined zone of the obstructions, where a detailed description of the flow field is needed. The model is tested on a laboratory experiment carried out at the Georgia Institute of Technology, where the flow field around a single and two double circular cylinders in a flatbed flume was studied. Simulated flow velocity, turbulent kinetic energy and bed shear stress distributions are in good agreement with measurements. However, deviations downstream of the piers indicate the limitation of the steady state description of the flow in the unstable wake zone. Nevertheless, the nested grid approach presented herein is a promising step towards the modeling of the local scouring phenomenon due to the relatively low computational demand.

CLEAR-WATER SCOUR AROUND BRIDGE ABUTMENTS UNDER BACKWATER CONDITIONS

An experimental study was conducted of the depth of clear-water scour around the end of a square-edged bridge abutment terminating in the floodplain of a compound channel. The studys purpose was to improve current techniques of abutment scour prediction, which are based primarily on laboratory studies in rectangular channels. It is indicated that a discharge contraction ratio arising from a theoretical contraction scour analysis for equilibrium conditions can be used for explaining the effect of flow distribution on the local abutment scour depth in the case where significant backwater occurs from bridge contraction. The use of reference values of approach flow depth and velocity in the floodplain for undisturbed conditions without the bridge is shown to collapse experimental results for scour depth in both the case of a contraction with negligible backwater, and the case of a contraction with significant backwater in the bridge approach section.

Clear water abutment scour in a compound channel for extreme hydrologic events

AbstractPeak 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...

ONE-DIMENSIONAL AND TWO-DIMENSIONAL ESTIMATES OF ABUTMENT SCOUR PREDICTION VARIABLES

An experimental study of the estimation of hydraulic parameters needed in bridge abutment scour formulas is presented. Two different compound channel geometries are studied with rough floodplains in shallow overbank flow. Vertical-wall and spill-through abutment shapes are included in the study, with the abutment face located on the floodplain for various embankment lengths. Water surface profiles and velocity distributions are measured and compared with predictions made by the one-dimensional (1-D) model WSPRO and a two-dimensional k-ε turbulence model developed in previous research. The results show that a 1-D model can predict scour parameters reasonably well in the bridge approach section but not in the bridge contraction section.