Journal of Hydraulic Engineering

A study on the hydraulic and sediment conditions at the Tacoma Narrows Bridge, in Washington State, was carried out to examine the stability of the bed material around the bridge caissons. Specifically, this was conducted around the east caisson, where the maximum velocities around either of the two caissons are experienced. This was performed for the peak tidal exchange event of May 27th to 28th, 2002. During this max flow event, multibeam surveyed bathymetry and three-dimensional acoustic doppler current profiler velocity data were collected around the east caisson in the course of both the flood and ebb. The surface of the bed material surrounding the east caisson was videotaped during the slack conditions following the yearly maximum flow event, and used to determine the particle size distribution and spatial arrangement of those distributions around the caisson. This was done by lowering a submersible video camera and appropriate lighting to the bottom of the Narrows, a depth of approximately $45\mathrm{m}$ . Flow and sediment observations were coupled to determine the commencement of sediment motion for different size classes of sediment. Two methods were utilized to calculate friction velocity in order to assess the stability of different bed particle size fractions during these high flow conditions. Friction velocity was first calculated from measured velocity profiles at various locations around the east caisson. The second method was based on the concept of a free stream power-law expression for depth-averaged velocity. Stability was then examined using the critical shear stress concept and captured video data of the bed. General results showed the particles $⩽30\mathrm{mm}$ in diameter were in motion during the flood and ebb. The work is here presented as a case study because of the unique large-scale flow conditions that are present around the east caisson of the Tacoma Narrows Bridge.

More than $50\text{years}$ ’ river training practices in the Lower Yellow River provide valuable experience in river management for flood control in rivers having rapid flow changes, silting beds, and active channel migrations and are of importance in understanding the fluvial processes in regulated rivers with high sediment loads. Planned channel alignments for river training in the Lower Yellow River usually consist of a series of consecutive moderate bends representing the natural tendency of flows. Flow guide works, namely spur dikes, were constructed on the concave banks of the planned bends to protect the channel against scouring and migration by deflecting the current away from bends and further guiding the main flow from one bend to the next one. As a result, well-planned flow guide works can play a crucial role in limiting channel shifting and migration and in establishing a relatively stable channel. Enough flow guide works, on both sides together reaching about 80% of the channel length, may change the transitional and braided channel patterns to a confined meandering pattern.

This research intends to clarify the limitations of a local scour equation recently proposed, based on extended laboratory data collected at VAW, Zurich, Switzerland. The present project is concerned with four items: (1) clarification of the minimum laboratory dimensions required to apply Froude similitude; (2) effect of sloping abutments on scour advance; (3) extension of scour formula to spur dikes; and (4) effect of unsteady flow on scour development. These items were investigated mainly from an experimental point of view based on some 150 laboratory experiments and accounted for by a hydraulic approach. It was found that the basic scour equation mentioned may be applied provided additional limitations are specified. These are discussed in the light of the densimetric particle Froude number, the threshold Froude number, and other important parameters that influence the progress of local scour. The results of this study may be applied to practice, provided the limitations of the computational approach are respected.

A power law relating bed-form magnitudes (lengths or heights) to time, with all parameters normalized by equilibrium values, is promoted to describe ripple or dune growth with time from plane-bed to equilibrium conditions. In order to use this relation for prediction purposes, new expressions are presented herein for both the times required for ripples and dunes to achieve equilibrium magnitudes, and also the respective growth exponents for the development of bed-form heights and lengths. These new relations have been determined based on 15 new and 76 previously published laboratory experiments covering ranges of sands and flow strengths, although additional work is warranted to confirm these expressions for field scales. The power-law relation for bed-form growth from a plane bed is also extended in concept herein to describe changes in bed-form size from nonzero initial magnitudes. In preliminary tests, this modified power-law expression was found to provide reasonable descriptions of trends in increasing and decreasing bed-form heights and lengths with changes in flow strength. At this stage, application of this modified expression is principally conceptual in nature, with further work required to facilitate prediction for changes in flow of both times to achieve equilibrium magnitudes and also height and length growth exponents.

This paper describes the design and calibration of a compound sharp-crested weir consisting of two triangular parts with different notch angles. This weir provides accurate measurement for a wide range of flows without discontinuities. The lower triangular part of the weir handles the normal range of discharges at the measurement structure while the upper part measures the occasional higher peak flows. This weir will be installed on the crest of small concrete gravity dams for measuring runoff in experimental catchments. The weir was calibrated in the laboratory using a scaled model. Different geometries (combination of notch angles) were tested in order to validate the proposed theoretical discharge equations.

Uncertainties in hydrodynamic model calibration and boundary conditions can have a significant influence on flood inundation predictions. Uncertainty analysis involves quantification of these uncertainties and their propagation through to inundation predictions. In this paper the inverse problem of sensitivity analysis is tackled, in order to diagnose the influence that model input variables, together and in combination, have on the uncertainty in the inundation model prediction. Variance-based global sensitivity analysis is applied to simulation of a flood on a reach of the River Thames (United Kingdom) for which a synthetic aperture radar image of the extent of flooding was available for model validation. The sensitivity analysis using the method of Sobol’ quantifies the significant influence of variance in the Manning channel roughness coefficient in raster-based flood inundation model predictions of flood outline and flood depth. The spatial influence of the Manning channel roughness coefficient is analyzed by dividing the channel into subreaches and calculating variance-based sensitivity indices for each subreach. Replicated Latin hypercube sampling is used for sensitivity analysis with correlated input variables. The methodology identifies subreaches of channel that have the most influence on variance in the model predictions, demonstrating how far boundary effects propagate into the model and indicating where further data acquisition and nested higher-resolution model studies should be targeted.

Potential flow solutions of a point/line sink are extended to study the velocity field upstream of a finite-size orifice and sluice gate. It is found that, in the “near field” zones, the iso-velocity surfaces appear to be semiellipsoidal; while in the “far field” zones they become hemispheres. The shape and size of the orifice/sluice gate were found to be of no effect on the flow behavior beyond a certain distance. The development of velocity profile away from the orifice and sluice gate is examined, and the effects of water depth are studied. The results of this study compare very well with other numerical and experimental studies, and provide a general understanding of the flow field upstream of orifices and sluice gates.

Free overfall is treated by using two-dimensional steady potential flow theory. Based on the theory of the boundary value problem of analytical function and the substitution of variables we derive the boundary integral equations in the physical plane for solving the free overfall in a rectangular channel. A numerical iterative method has been developed to solve these boundary integral equations. The free water surface profiles, pressure distribution, and the end-depth ratio are calculated for a wide range of bed slopes, bed roughness, and incoming upstream Froude number. The computed results agree well with the available experimental data.

This paper revises current understanding of the flow at the base of a headcut and suggests a new model which employs the concept of a vortex flow. A distinctive feature of the vortex flow is the presence of the upstream-directed jet, which attacks the vertical headcut boundary. Numerical simulations are performed to demonstrate the significance of the effects of the upstream-directed jet for breach development.