Trevor B. Hoey

FIELD EVIDENCE FOR RAPID DOWNSTREAM FINING OF RIVER GRAVELS THROUGH SELECTIVE TRANSPORT

Doubts have been expressed about the ability of either abrasion or sorting to explain strong downstream fining of river gravels. We describe pronounced fining over a short distance in a Scottish river that has no human disturbance or lateral input of water and sediment. Measured abrasion rates are far too small to explain observed downstream fining, but bed-load trap measurements and the dispersion of magnetic tracer pebbles in six subreaches both show sorting during transport. The observed downstream fining is also simulated well by a numerical sediment routing model using hydraulic and transport laws consistent with our field measurements. The geomorphological cause of the strong fining is slope reduction above a local base-level control. In this common situation the development of downstream fining is part of the river9s tendency to minimize downstream variation in bed-load transport rates, and can proceed far more rapidly than the major aggradation otherwise required for equilibration.

Temporal variations in bedload transport rates and sediment storage in gravel-bed rivers

Temporal variability in bedload transport rates and spatial variability in sediment storage have been reported with increasing frequency in recent years. A spatial and temporal classification for these features is suggested based on the gravel bedform classification of Church and Jones (1982). The identified scales, meso-, macro-, and mega- are each broad, and within each there is a wide range of processes acting to produce bedload fluctuations. Sampling the same data set with different sampling intervals yields a near linear relationship between sampling interval and pulse period. A range of modelling strategies has been applied to bed waves. The most successful have been those which allow for the three-dimensional nature of sediment storage processes, and which allow changes in the width and depth of stored sediment. The existence of bed waves makes equilibrium in gravel-bed rivers necessarily dynamic. Bedload pulses and bed waves can be regarded as equilibrium forms at sufficiently long timescales.

Mobility of river tracer pebbles over different timescales

[1] Tracer pebbles are widely used to learn about gravel transport along rivers. Movement over short times and distances is dominated by factors controlling entrainment: relative particle size and shear stress. Movement at longer scales also involves depositional factors: burial and reexposure and exchange between channels, bars, and other depositional environments. We mapped mixed-size tracers in six reaches of a small Scottish river after 2 and 8 years to investigate differences in relative and absolute mobility and infer the importance of burial and exchange. Patterns of relative mobility according to size and shear stress, both within and between reaches, did not change significantly. Some local bunching of tracers was apparent in both surveys, with redistribution from pools into riffles and bars. The main change was that virtual velocities were ∼50% lower, and estimated gravel fluxes were also lower, in the longer term. This slowdown is attributed to vertical mixing giving decreased mobility as surface-seeded tracers become buried, long-term storage in bars and other less active parts of the system, and in this channel, advection of tracers downstream onto a finer bed giving higher relative size.

Critical shear stress for incipient motion of sand/gravel streambeds

Results of an experimental study of the incipient motion of streambeds composed of sand/gravel sediment mixtures are reported and compared with the earlier findings for uniform sediments. The experiments were conducted in an 8 m long by 0.30 m wide glass-walled tilting flume and an 18 m long by 0.80–1.10 m wide trapezoidal concrete channel. A reference transport method is used to define the beginning of bed material movement. The experiments demonstrate that the incipient motion of individual size fractions within a mixture is controlled by their relative size with respect to median size (intergranular effects), mixture standard deviation (effect of the shape of grain-size distribution), absolute value of median size (absolute size effect), and bed slope (effect of relative depth on overall flow resistance). The shear stress at incipient motion of median-sized grains in mixtures is found to be the same as for uniform sediment of this size. The present findings are consistent with available flume and field data. A technique for calculating the critical shear stress of different grain sizes in coarse uniform sediments and unimodal/weakly bimodal sediment mixtures is proposed.

The Irrawaddy River Sediment Flux to the Indian Ocean: The Original Nineteenth-Century Data Revisited

The Irrawaddy (Ayeyarwady) River of Myanmar is ranked as having the fifth‐largest suspended load and the fourth‐highest total dissolved load of the world’s rivers, and the combined Irrawaddy and Salween (Thanlwin) system is regarded as contributing 20% of the total flux of material from the Himalayan‐Tibetan orogen. The estimates for the Irrawaddy are taken from published quotations of a nineteenth‐century data set, and there are no available published data for the Myanmar reaches of the Salween. Apart from our own field studies in 2005 and 2006, no recent research documenting the sediment load of these important large rivers has been conducted, although their contribution to biogeochemical cycles and ocean geochemistry is clearly significant. We present a reanalysis of the Irrawaddy data from the original 550‐page report of Gordon covering 10 yr of discharge (1869–1879) and 1 yr of sediment concentration measurements (1877–1878). We describe Gordon’s methodologies, evaluate his measurements and calculations and the adjustments he made to his data set, and present our revised interpretation of nineteenth‐century discharge and sediment load with an estimate of uncertainty. The 10‐yr average of annual suspended sediment load currently cited in the literature is assessed as being underestimated by 27% on the basis of our sediment rating curve of the nineteenth‐century data. On the basis of our sampling of suspended load, the nineteenth‐century concentrations are interpreted to be missing about 18% of their total mass, which is the proportion of sediment recovered by a 0.45‐μm filter. The new annual Irrawaddy suspended sediment load is \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Morphological controls on the downstream passage of a sediment wave in a gravel-bed stream

364\pm 60

Surface process models and the links between tectonics and topography

\end{document} MT. Our revised estimate of the annual sediment load from the Irrawaddy‐Salween system for the nineteenth century (600 MT) represents more than half the present‐day Ganges‐Brahmaputra flux to the Indian Ocean. Since major Chinese rivers have reduced their load due to damming, the Irrawaddy is likely the third‐largest contributor of sediment load in the world.

Controls of strength and rate of downstream fining above a river base level

[1] Bedrock rivers exert a critical control over landscape evolution, yet little is known about the sediment transport processes that affect their incision. We present theoretical analyses and field data that demonstrate how grain entrainment, translation and deposition are affected by the degree of sediment cover in a bedrock channel. Theoretical considerations of grain entrainment mechanics and sediment continuity each demonstrate that areas of exposed bedrock and thin sediment depths cause sediment transport to be size-independent, albeit excluding extreme grain sizes. We report gravel and cobble magnetic tracer data from three rivers with contrasting sediment cover: the bedrock River Calder (20% cover), the bedrock South Fork Eel River (80%) and the alluvial Allt Dubhaig (100%). These data sets show that: 1) transport distances in the River Calder are controlled by sediment patch location, whereas in the other rivers transport distances are described by gamma distributions representing local dispersion; 2) River Calder transport distances are size-independent across all recorded shear stresses, whereas the other rivers display size-selectivity; 3) River Calder tracers are entrained at a dimensionless shear stress of 0.038, which is relatively low compared to alluvial rivers; and, 4) virtual grain velocities in the River Calder are higher than in a comparable reach of the Allt Dubhaig. These contrasts result from differences in the thicknesses and spatial distribution of sediment in the three rivers, and support the theoretical analysis. Sediment processes in bedrock rivers systematically vary along a continuum between bedrock and alluvial end-members. Citation: Hodge, R. A., T. B. Hoey, and L. S. Sklar (2011), Bed load transport in bedrock rivers: The role of sediment cover in grain entrainment, translation, and deposition, J. Geophys. Res., 116, F04028, doi:10.1029/2011JF002032.