Stephen E. Coleman

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.

Sand wavelets in laminar open-channel flows

The results of experimental investigations indicate that sand wavelets can be generated from plane-bed conditions in open-channel laminar flow, the lengths, shapes and patterns of generation for these wavelets being consistent with observations for alluvial flows. Wavelets are of a preferred wavelength which is relatively insensitive to the characteristics of the applied flow and primarily a function of the size of the sediment, these wavelengths λ for alluvial and laminar open-channel flows over beds of quartz and lightweight sediments of size d = 0.2 mm to d = 1.6 mm being simply described by λ = 175d 0.75, where λ and d are expressed in millimetres. The laminar-flow sand-wavelet data present significant implications to contemporary understanding of bed-form mechanics, with both ripples and dunes being postulated to subsequently develop from these wavelets for alluvial flows. The data also raise significant questions as to whether the generation of ripples and dunes in alluvial flows can be attributed to an organised structure of turbulence within the flow.

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...

Spatially Averaged Flows over Mobile Rough Beds: Definitions, Averaging Theorems, and Conservation Equations

AbstractThis paper reports the double-averaged (in space and in time) hydrodynamic equations for mobile-boundary conditions that are derived based on the refined double-averaging theorems, modified Reynolds decomposition, and improved definitions of the spatial and time bed porosities. The obtained double-averaged conservation equations provide a mathematical framework for studying mobile-boundary flows such as gravel bed rivers during flood events or flows over vegetated beds. These equations will help in designing measurement campaigns for obtaining mobile bed data and their interpretation and parameterization, eventually leading to improved and more robust predictive models.

On the definition of solid discharge in hydro-environment research and applications

Various forms of mass balance equations are commonly used in sediment transport studies and applications. However, the quantities involved in such equations are not always clearly defined, with sediment flux (or solid discharge) as a typical example. Starting with the fundamental definitions, this paper provides a general and consistent framework for integral (Eulerian) mass balances and gives scale-consistent definitions for instantaneous and time-averaged variables. In particular, alternative expressions for the instantaneous and averaged solid discharge are proposed and compared with the existing formulations. Conceptual developments of this study are illustrated using sediment transport data from laboratory experiments.

Scouring at bed sills as a response to flash floods.

The temporal development of clear-water local scour depth at bed sills in uniform gravel beds is considered. Experiments are presented on the development of scour holes under unsteady hydraulic conditions, with the triangular-shaped hydrographs tested being of different durations and different rates of flow variation. Based on the experimental results and a theoretical framework, a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter. A “flash flood” is here defined as an event for which the scour doesn’t attain its potential magnitude, i.e., the equilibrium value for the peak hydrograph flow rate. This flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream. A quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter. Results show that the ratio between the final scour depth and the potential scour depth ...

Bed load transport by bed form migration

A theoretically-based methodology is presented for the determination of bed load transport from high-resolution measurements of bed surface elevations for steady-state or developing dunes. The methodology is based on the general form of the Exner equation for sediment continuity and requires information on the distribution of sediment volume concentration as well as the migration velocity of bed layers. In order to determine layer speeds, a new method based on cross-correlation analysis of elevation slices is proposed. The methodology is tested using artificially-created data as well as data from a physical model and from a flume study of developing bed forms. The analyses show the applicability of the method to determine bed load transport without the need to introduce assumptions about the form of the migrating surface. It is shown that predicted transport rates match measured or theoretical transport rates for steadily moving bed forms of an arbitrary shape. The method can also be used to predict transport rates over deforming bed forms, with the reasons for potential deviations between predicted and measured or theoretical transport rates for deforming bed forms identified and discussed. It is further shown that a simplified bulk-surface approach, that is relatively straightforward to apply and in which it is assumed that bed-layer velocity is constant with depth, gives results that are comparable to analyses based on determined bed-layer velocity variation with depth.

Effect of Flood Recession on Scouring at Bed Sills

The effect of the flood recession time on the local scour depth at bed sills in gravel deposits is examined. Experiments were carried out to study the development of scour holes under time-varying hydraulic conditions with no upstream sediment feed. Triangular-shaped hydrographs, having recession times up to three times the duration of the rising limb, were used. Traditionally, the peak water discharge in any flood event is used as a design value in estimating the final depth of scour formed by a flood. This approach is overly conservative when the flow hydrograph is steep, i.e., during the occurrence of flash floods. The actual reduction of the scour depth from this estimated value is dependent on both the characteristics of the flood event and the characteristics of the stream. The results show that the maximum potential scour depth can be achieved only for hydrographs with long recession times, while the rate of this process can be estimated as a function of the ratio between a characteristic flood time and the steady-state temporal scale of scour development. A method is proposed for the prediction of the scouring process under unsteady flows in terms of two dimensionless temporal parameters. Results obtained for clear-water boundary conditions can be extended to sediment-supply tests if specific supply input conditions hold.

Experimental Investigations of Sandy Riverbed Morphology

Contemporary advances in experimental methods and technologies are facilitating new and valuable insight into research questions of long standing. Following a review of the history of investigations regarding sandy riverbed morphology, this chapter presents a series of recent investigations into outstanding gaps in understanding of fluvial bedforms. Equipment advances are highlighted in terms of use of a viscous-fluid flume, a water tunnel, an induction power transfer (IPT) carriage, a custom-built particle image velocimetry (PIV) system, a 3D laser scanner, and a flying-probe measurement system. Advances in understanding of fluvial bedforms arising from these investigations are also presented.

Local Scour at Complex Piers

A methodology to predict local scour depth at a complex pier is presented that recognises the relative scouring potentials of the components of complex piers, and the transition of scouring processes occurring for varying pile-cap elevation. Scour depths are predicted over the entire range of possible pile-cap elevations using a combination of existing expressions for scouring respectively at uniform piers, caisson-founded piers, pile groups with debris rafts, and pile groups alone. The validity of the method is confirmed using measurements of local scour at complex piers, and a case study is used to highlight application of the methodology. For design purposes, the method highlights respective pile-cap elevations that maximise (i.e. to be avoided over the pier life) and minimise local scour at complex piers. The method reinforces that where the pile-cap elevation relative to the bed can vary with time at a bridge site, potential local-scour depths need to be assessed over the range of possible pile-cap elevations for the pier.