Bars are key morphological units in river systems, fashioning the sediment regime and bedload transport processes within a reach. Reworking of these features underpins channel adjustment at larger scales, thereby acting as a key determinant of channel stability. Despite their importance to channel evolution, few investigations have acquired spatially continuous data on bar morphology and sediment‐size to investigate bar reworking. To this end, four bars along a 10 km reach of a wandering gravel‐bed river were surveyed with terrestrial laser scanning (TLS), comparing downstream changes in slope, bed material size and channel planform. Detrended standard deviations (σz) were extracted from TLS point clouds and correlated to underlying physically measured median grain‐size (D50), across a greater range of σz values than have hitherto been reported. The resulting linear regression model was used to create a 1 m resolution median grain‐size map. A fusion of airborne LiDAR and optical‐empirical bathymetric mapping was used to develop reach‐scale digital elevation models (DEMs) for rapid two‐dimensional hydraulic modelling using JFlow® software. The ratio of dimensionless shear stress over critical shear stress was calculated for each raster cell to calculate the effectiveness of a range of flood events (2.33–100 year recurrence intervals) to entrain sediment and rework bar units. Results show that multiple bar forming discharges exist, whereby frequent flood flows rework tail and back channel areas, while much larger, less frequent floods are required to mobilise the coarser sediment fraction on bar heads. Valley confinement is shown to exert a primary influence on patterns of bar reworking. Historical aerial photography, hyperscale DEMs and hydraulic modelling are used to explain channel adjustment at the reach scale. The proportion of the bar comprised of more frequently entrained units (tail, back channel, supra‐platform) relative to more static units (bar head) exerts a direct influence upon geomorphic sensitivity. © 2018 John Wiley & Sons, Ltd.

The Tongariro Power Development Scheme ( TPDS) is used to regulate flow in the headwaters of the largest catchment on the North Island of New Zealand ( the Waikato). Two small dams, the Rangipo Dam and the Poutu Intake Dam, were constructed in 1973 and 1983. The flow regime of the river is managed to divert freshes into the power scheme, but allows flows larger than 100 m(3) s(-1) to be released, to rework and transport sediment through the catchment. Analysis of aerial photos and maps spanning 1928 to 2007, alongside field measurements, show that there have been few hydrogeomorphic adjustments since dam construction. This includes limited changes to channel geometry, channel planform and bed material organization immediately downstream of the dams. In addition, offsite effects are minimal, both 500 m downstream of each dam, and in the more sensitive, less confined reaches in the lower catchment (11 km downstream of the Poutu Intake dam). The limited changes can be attributed to the locations of the dams within reaches characterised by bedrock gorges and confined within terraces. These locations act to flush sediments and impose margins that allow minimal adjustment of the channel. Bed material within this reach is characterised by the presence of a boulder lag. This is sourced from long-term incision into lahar deposits, and acts to limit the rate of incision, creating a steep and stable base upon which active fractions are transported. Just as importantly, significant storage in the low-relief volcanic plateau located in the upper catchment acts to disconnect and store the high sediment yields generated by active volcanic cones in the western sub-catchment upstream of the dams. This limits the rate of sediment supply to regulated reaches. Findings from this study show that analysis of reach-scale controls is essential in framing dam site locations in relation to the distribution of reaches and landscape units across the catchment. In this instance, tributary inputs downstream of the dams do not replenish the sediment and flow removed at the dam locations, as has been observed in other regulated systems. Rather, the river itself is resilient to change and flow variability is well managed allowing geomorphically effective floods to occur. Landscape setting is a key consideration in determining the hydrogeomorphic impact of flow regulation.

Research into fluvial dunes spans disciplines, studies at grain to reach scales, and methodological approaches that include theoretical, experimental, numerical and field investigations. Despite significant research efforts to date, it remains difficult to provide definitive answers to some fundamental questions regarding dunes. This paper reviews three notable challenges that remain regarding fluvial dunes, namely scale-consistent linking of bed morphologies with turbulent flow fields, the intriguing question of what causes trains of highly-ordered sediment waves to form in beds of river sediments, and how to define the important characteristics of a dune-covered bed, including lengths, shapes, and their statistical nature. In each case, the particular challenge is discussed and then recent research and ways forward are presented. Copyright (C) 2010 John Wiley & Sons, Ltd.

A series of experiments was carried out to investigate the bed shear stress distribution downstream of a backward-facing step. A viscous-fluid flume and a custom-made PIV system were used for the experiments of low Reynolds number flows to enable improved sampling resolutions for the near-bed flow. Reattachment lengths were found to match the indications of previous studies. If the hypothesis that the same process acts to generate sediment waves in laminar and turbulent flows is correct, then the present tests indicate that the bed shear stress distribution downstream of a bed perturbation does not lead to sediment-wave formation. Alternative aspects of boundary-layer redevelopment downstream of a bed perturbation are conjectured to lead to sediment-wave formation in nature.

In contrast to using a standard control volume approach, general statements of sediment mass balance are derived herein from spatial averaging of the sub‐particle‐scale differential equation of solid mass conservation. The general form of the Exner equation for sediment continuity that is obtained enables analyses in terms of size fractions and also in terms of individual or successive layers, where layer interfaces (e.g., for the bed surface and for bed and suspended loads) are defined on the basis of isosurfaces of sediment concentration (volume fraction) or other sediment properties (e.g., densities or transport rates) within regions of constant concentration. The presented expressions highlight the averaged nature of variables and also the effects of the scales of consideration on definition and interpretation of the macroscopic (mixture‐scale) sediment and layer properties (e.g., averaged densities, volume concentrations or fractions, velocities, transport modes and rates, interfaces, and bed layers). For appropriate simplifications, the general form of the Exner equation is shown to reduce to give more specific conventional expressions, revealing the assumptions implicit in these equations.

New PIV‐based experiments show that the nascent seed waves from which both ripples and dunes develop are generated on planar mobile sediment beds in a two‐stage process. The first stage comprises the motion of random sediment patches that reflect the passage of sediment‐transport events caused by attached eddies. These eddy‐transport events propagate at speeds that are proportional to their size and less than overhead eddy convection velocities, but potentially larger than local average fluid and sediment velocities. In the second stage, interactions of the moving patches result in a bed disturbance that exceeds a critical height and interrupts the bed‐load layer. Quasi‐regular seed waves are then generated successively downstream of this stabilised growing disturbance via a scour‐deposition wave that arises from the requirement of sediment mass conservation and the sediment‐transport and bed‐stress distributions downstream of a bed perturbation. Seed waves are thereby of preferred lengths that scale with the grain size, i.e. length = O(130) grain diameters, agreeing with compiled measurements. This two‐stage generation mechanism is valid for fully‐turbulent hydraulically‐smooth and rough‐bed flows of small to large sediment transport rates. It is furthermore valid for laminar flows, although the critical disturbances leading to seed‐wave generation arise through bed discontinuities, and not eddy‐based sediment‐transport events. The identified generation mechanism, which accounts for turbulence effects, explains the observed similar scaling of alluvial, closed‐conduit and lightweight‐sediment seed waves. The present measurements highlight further aspects of the flow dynamics preceding seed‐wave generation, including: decreases in von Kármán's constant due to bed mobility, near‐bed eddy convection speeds in excess of local double‐averaged (in time and space) streamwise velocities, and the validity of the four‐range spectral scaling model for open‐channel flows proposed earlier by Nikora.

A new framework is presented that describes the hydrodynamic entrainment of sediments. The framework is derived from the general equations of particle stability and fluid motion and covers a wide range of scales, from that of the individual particle to en masse entrainment at the scale of the stream reach. Particle entrainment is explicitly shown to be driven by the combination of instantaneous fluid stress and pressure gradients, particle surface contact forces, particle submerged weight, and the weight and motion of particle‐associated fluid. Of these factors, bed shear stresses, across‐particle differences in pressures and fluxes of momentum, and sediment bed characteristics are found to be key factors in particle entrainment. The conventional Shields entrainment parameter is shown to be applicable to averaged en masse entrainment by steady uniform 2‐D flows. This nondimensionalized bed shear stress at threshold is further demonstrated to be a function of interparticle forces and bed normal hydrodynamic forces, and thereby sediment size and other fluid, sediment and bed characteristics. Observed Shields parameter dependencies are discussed. The presented framework, which includes all relevant particle and fluid characteristics and flow dynamics, may aid understanding of entrainment mechanics, the design and analysis of further studies of entrainment, the design of parameterizations that lead to the solution of entrainment problems, and numerical modeling of combined fluid and sediment dynamics.

Spatial averaging of the Reynolds-averaged Navier-Stokes equations gives the double-averaged Navier-Stokes equations, for which boundary drag appears naturally and explicitly in momentum Conservation equations. Increasing use of the double-averaged equations, e.g., for relating flows to three-dimensional bed toughness, for evaluation of profiles of flow stresses and velocities in ecologically significant regions below roughness tops, and for modeling purposes, requires parameterization of boundary drag at subelement scales. Based on seven flows over repeated square-rib roughness and ten flows over repeated fixed simulated sand waves, with measured velocities and bed pressures, expressions for form-drag coefficient C-D = f (elevation below roughness top, relative roughness submergence, roughness steepness) are obtained for each of the two-dimensional roughness types. Using these equations, form drag variation with elevation below roughness can be calculated using either the double average of the square of local velocity (preferred based Oil conceptual considerations, trends in coefficient prediction, and also overall drag prediction) or the squared local double-averaged velocity, the roughness area being normal to the flow in each case. Integration of subelement drag given by these expressions is shown to Give form-drag coefficient magnitudes and trends for complete individual elements comparable to those obtained by other authors based on measurements or numerical simulations. The ranges Of roughness steepness and relative roughness submergence upon which the present equations have been derived need to be noted in consideration of application of the equations. In addition, effective application of the expressions is limited in regions of strongly negative double-averaged velocity. Further work remains to determine drag parameterization for alternative roughness geometries.

A series of experiments was undertaken to assess fully rough turbulent subcritical flow over two-dimensional transverse repeated-rib roughness of varying spacing lambda/h = I - 16 (roughness spacing/height ratio), with h/H = 0.09 (roughness height/flow depth). Each of the 11 experiments involved centerline measurements of three-dimensional velocity vectors, water surface profiles, and bed pressures and forces. The structure of flow over rib roughness consists of a pair of principal vortices of opposite sign set up by the ribs. These vortices are superimposed on an overall double- (time and space)-averaged velocity profile that is (quasi-) logarithmic above roughness tops, and that below roughness tops changes with increasing rib spacing from exponential (lambda/h < 10) to linear (lambda/h >= 10) to logarithmic (lambda/h >> 10). The measured double-averaged velocity profiles are parameterized herein, and double-averaged Reynolds and form-induced stress profiles and trends are also identified. Maximum drag due to wall roughness is found to occur for lambda/h approximate to 8. The experimentally determined momentum balance, including effects of secondary currents and flow acceleration, is found to agree well with theoretical expectations. It is shown that the knowledge of the effects of form-induced stresses, secondary currents, and flow nonuniformity may be particularly important for describing and modeling flows over roughness elements.

A cost-effective Particle Image Velocimetry (PIV) system has been developed that is capable of resolving flow fields at frequencies of up to 200 Hz (covering the typical scales of interest for hydraulic researchers), with recording durations of over 8 min. The system uses a highspeed camera to image flow fields illuminated by a scanning-beam lightsheet, where the lightsheet is generated by a galvanometer-driven mirror (computer controlled) together with a parabolic mirror. The system has several advantages over rotating-polygon type scanning systems, including: that the mirror is always positioned at the focal point of the parabola, and that the lightsheet generation is extremely versatile, with the lightsheet width (for a single parabolic mirror) and beam scan velocities easily and independently adjustable. Additionally, the beam scan velocity, which is typically nonlinear in rotating-polygon systems due to inherent properties of parabolic reflectors, can be constant in a galvanometer-based system (giving a uniform intensity lightsheet) by driving the galvanometer at an unsteady angular velocity. Integrated synchronisation options for the system permit frame-straddling techniques to be used in order to reduce interframe times to below I /(camera frame rate). The system also offers additional benefits over equivalent double-pulsed or twin-laser setups that rely on beam expansion by lens systems, and that typically only allow measurement at frequencies up to 50 Hz. Manipulation of the beam diameter using lens systems is outlined. The system has been implemented and used to obtain PIV measurements in flows of water and oil. (c) 2007 Elsevier Ltd. All rights reserved.

A comprehensive data set of 3-D sand waves developing from a flat bed to equilibrium stage (known as the research program SWAT.nz - Sand Waves and Turbulence New Zealand) was obtained during investigations in a 5-ft-wide flume at The University of Auckland. The recorded sand bed is treated as a continuous field of sand-bed elevations, rather than subdivided into discrete sand waves. Aiming at 3-D interpretation of the random field of sand-bed elevations, 2-D autocorrelation analyses are introduced. It is shown that the 2-D autocorrelation function approach is a useful tool to depict 3-D characteristics of sand waves and their further potential is outlined.

The Water of Leith is a steep mountain stream which has been heavily modified in the past to facilitate intensive urban development of an alluvial floodplain at the lower end. An extreme flood in 1929 resulted in major channel breakout of floodwaters and extensive floodplain inundation. A 1999 review of the flood hazard for the stream identified serious channel capacity and other deficiencies at several locations across the floodplain. Following this review, a comprehensive scheme was developed to address these deficiencies. This paper provides a brief over-view of the catchment and the flood hazard issues associated with the stream, and describes three innovative aspects of the scheme developed to improve flood security.

Along the lower reaches of the Waipaoa River, New Zealand, cross‐section survey data indicate there was a 23 per cent decrease in bankfull width and a 22 per cent reduction in channel cross‐section area between 1948 and 2000, as the channel responded to increased inputs of fine (suspended) sediment following deforestation of the headwaters in late C19 and early C20. We determined the bankfull discharge within a ∼39 km long reach by routing known discharges through the one‐dimensional MIKE 11 flow model. The model runs suggest that the bankfull discharge varies between ∼800 and ∼2300 m3 s−1 and that the average recurrence interval is 4 ± 2 years on the annual maximum series; by contrast, the effective flow (360 m3 s−1) is equaled or exceeded three times a year. The variability in bankfull discharge arises because the banks tend to be lower in places where flood flows are constricted than in reaches where overbank flow is dispersed over a wide area, and because scour has counteracted aggradation in some locations. There is no downstream variation in Shields stress, or in relative shear stress, within the study reach. Bankfull shear stress is, on average, five times greater than the shear stress required to initiate motion. At the effective discharge it is more than twice the threshold value. The effective discharge probably has more relevance than the bankfull discharge to the overall picture of sediment movement in the lower reaches of the Waipaoa River but, because width is constrained by the stability and resistance of the bank material to erosion during high flows that also scour the bed, the overall channel geometry is likely determined by discharges at or near bankfull. Copyright © 2006 John Wiley & Sons, Ltd.

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 roughness tops, and also to complete generalization for varying dune steepness of the universal expression for double-averaged longitudinal velocity (varying linearly with elevation) determined herein for the interfacial layer (below roughness tops). Work is presently focusing on the additional effects on flow structure due to sediment transport and three-dimensional flow and bed morphology, although it is expected that the equilibrium boundary layer flow structure patterns identified herein will still be evident for these more complex systems.

An oil flume in combination with a 100 Hz Particle Image Velocimetry system has been used to study entrainment of 11 turn diameter spherical Nylon particles. The flow field near the bed and around sediment particles holds essential information for studying entrainment mechanics, but measurements in water are often hampered by a lack of adequate resolution, particularly for smaller particle sizes. The higher viscosity of oil compared to water increases length scales, allowing water flows to be modelled with much larger sediment sizes and deeper near-bed flow layers. The 11 mm diameter particles used in the present oil flow experiments are dimensionally equivalent to 0.74 mm diameter quartz particles in water flow. Analysis of the flow field at the instant of bed particle movement indicates that sweep structures (higher than averaged streamwise velocity combined with bed-directed vertical velocity) are the dominant mechanism of entraining particles for the bed geometry and flow regimes studied.

A new methodology to predict local scour depth at a complex pier is presented herein that combines existing expressions for scouring respectively at uniform piers, caisson-founded piers, pile groups with debris rafts, and pile groups alone. The method recognises the relative scouring potentials of the components of complex piers and the transition of scouring processes occurring for varying pile-cap elevation. The validity of the method is confirmed herein using the present and also historical measurements of local scour at complex piers. The proposed methodology has the advantages of being conceptually consistent with observed scour behaviours, relatively simple to apply, applicable to wide ranges of flow and sediment conditions (through incorporation into a more general analysis framework), and applicable over the entire range of possible pile-cap elevations. For design purposes, the present method highlights respective pile-cap elevations that maximize (i.e., to be avoided over the pier life) and minimize local scour at complex piers. The present 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.

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.

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 less than or equal to 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.

The flow fields around spill-through abutments in compound channels were measured using a surface particle tracking velocimetry technique. The flow fields were seeded with 25mm fluorescent-yellow, cube shaped wooden blocks that were scattered on the surface of the water, and illuminated under UV light so that the particles could be easily identified in the image processing. Surface plots of scalar velocity, vorticity, and normalised shear stress were produced from the measurements. By systematically altering the floodplain width and the abutment length, the velocity, vorticity, and normalised shear stress fields were determined for several different configurations. The results show that as the abutment length increases, the velocity and vorticity strength at the abutment tip also increase. However, when the abutment geometry is fixed and the compound channel geometry is altered, the velocity and vorticity strengths at the abutment tip decrease as the distance between the abutment tip and the main channel decreases. Values of normalised shear stress exceeding one are indicative of regions where scour will occur due to increased local velocities. Similarly, regions of high vorticity identify the area where scour is initiated.