Coarse Surface Layer Research Articles

Armor or Pavement

The necessary condition for, and the process of, evolution of a coarse layer over finer sediment underneath are presented. Flow regions for pavement and armor formation are identified. A new definition for armor and pavement is proposed. The relevation of the two flow regions contributes towards an improved understanding of the distinction between armor and pavement. With increasing shear velocity, the surface layer coarsens in the armor region and becomes smoother in the pavement region. The eroded material (EM) in the armor region becomes coarser with increasing shear velocity but remains finer than the parent-bed material (PBM). In the pavement evolution region, the EM is finer than PBM; and in the equilibrium pavement region, the composition of EM is identical to that of the PBM. The available experimental data are in agreement with the proposed hypothesis of evolution of the coarse surface layer.

Bed load transport of mixed size sediment: Fractional transport rates, bed forms, and the development of a coarse bed surface layer

Fractional transport rates, bed surface texture, and bed configuration were measured after a mixed size sediment had reached an equilibrium transport state for seven different flow strengths in a recirculating laboratory flume. Fractional transport rates were also measured at the beginning of each run when the bed was well mixed and planar. The start‐up observations allow us to describe the variation of fractional transport rates with bed shear stress for a constant bed surface texture and bed configuration. The start‐up and equilibrium observations together allow, for the first time, an unambiguous description of the mutual adjustment among the transport, the bed configuration, and the bed surface, as the transport system moves toward equilibrium. We find that a substantial interaction exists among the transport, bed surface, and bed configuration. Bed forms and a coarse surface layer coexist over a range of bed shear stress. Under some flow conditions the size and shape of the bed forms are controlled by the presence of the coarse surface layer. At higher flows the coarse surface layer is eliminated by scour in the lee of the bed forms. If the bed surface is defined as that over which the bed forms move, a coherent relation between the bed surface texture and the transport grain size distribution may be defined. At equilibrium the transport rates of all fractions were not equally mobile, defined as identical transport and bed grain size distributions, although equal mobility was approached for runs in which the bed shear stress was more than twice that for initial motion of the mixture. Under some flow conditions the transport was observed to adjust away from equal mobility as the bed adjusted from a well‐mixed start‐up condition to an equilibrium state. Development of a partial static armor, wherein some individual grains become essentially immobile even though other grains in the same fraction remain in transport, is suggested to explain these adjustments between the transport and bed surface grain size distributions. Constraints on equilibrium mixed size sediment transport are defined. The special conditions for which equal mobility must hold and the relevance to natural conditions of flume results and the equal mobility concept are discussed.

Sediment supply and the development of the coarse surface layer in gravel-bedded rivers

THE bed surface of most gravel rivers is considerably coarser than the sub-surface or the gravel load transported over it, a phenomenon affecting river dynamics, morphology and ecology. The coarse surface layer, often called an armour or pavement, has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1,2; and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3,4. Where bedload supply is cut off below dams, selective erosion causes coarsening5,6. Using a simple quantitative model, we propose that surface coarsening develops in gravel-bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load. We present laboratory results which support this hypothesis, and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse, less active bed. It may therefore be possible to relate the degree of river-bed surface coarsening to sediment supply resulting from land use.

Observations on Debris in the Basal Transport Zone of Mýrdalsjökull, Iceland

Mýrdalsjökull is a temperate ice cap with an area of 596 km2, covering the volcanic Katlamassif in southern Iceland. Since 1900, extensive areas of ground moraine have been exposed during glacier retreat along the northern margin of the ice cap. The ground moraine surface is characteristically covered by a coarse layer of rock particles 10 to 150 mm in size. At the present glacier front, particles of corresponding size can be seen melting out from the lowermost glacier ice. Samples of ice and debris were collected from the basal transport zone. here generally 20 to 50 mm thick. and the volume, grain size, shape, and surface texture were determined. The orientation of rock particles in the englacial position. in the basal transport zone, and in the under lying lodgement till were analysed. The rock particles that dominate the debris content in the basal transport zone and constitute the coarse surface layer beyond the glacier margin are interpreted as a residual, which has escaped subglacial frictional deposition.