Sediment Sorting Processes Research Articles

Monitoring River Channel and Flume Surfaces with Digital Photogrammetry

This paper describes and illustrates a technique for high resolution monitoring of the surface morphology of water-worked sediments. The monitoring uses close-range digital photogrammetry. While photogrammetry is a long-established technique, more recent developments in digital photogrammetry allow application in fluvial research to be highly cost effective in both flume and natural river channel studies. Results are presented that involve two scales of laboratory flume: a smaller-scale application associated with sediment sorting processes in a straight channel; and a larger-scale application involving sediment transport and bed material feedbacks in a meandering channel subject to overbank flows. A preliminary assessment of data quality is undertaken with encouraging results. The precision of elevation estimates corresponds to the scale of the imagery acquired and hence may be controlled by design of the image acquisition process.

Modelling alluvial fan morphology

A mathematical model which estimates the scale-independent sediment surface profile of alluvial fans has been developed. This model utilizes a diffusive sediment transport model and an unsteady, radial flow, conservation relationship. These equations are approximately solved assuming a quasi-steady-state closure with appropriate modelling assumptions for two end member fan types: (1) fans where most of the fan surface is depositionally active (denoted here as ‘homogeneous’) and (2) fans characterized by channelling and sediment sorting processes. The fundamental result for these two fan types is a dimensionless sediment profile relationship which approximates most fan surfaces. The model suggests that the overall dimensionless morphology of alluvial fans is governed more by fundamental diffusion principles in sediment deposition than by individual environmental or basin characteristics. Additionally, this work potentially can be extended to model temporal variation in fan development. Preliminary comparison with alluvial fan profiles is reasonable, indicating that this model provides useful qualitative and quantitative information relating to alluvial fan process and morphology. Copyright © 1999 John Wiley & Sons, Ltd.

A REVISED VELOCITY-REVERSAL AND SEDIMENT-SORTING MODEL FOR A HIGH-GRADIENT, POOL-RIFFLE STREAM

Sediment-sorting processes related to varying channel-bed morphology were investigated from April to November 1993 along a 1-km pool-riffle and step-pool reach of North Saint Vrain Creek, a small mountain stream in the Rocky Mountains of northern Colorado. Measured cross-sectional areas of flow were used to suggest higher velocities in pools than in riffles at high flow. Three hundred and sixteen tracer particles, ranging in size from 16 mm to 256 mm, were placed in two separate pool-riffle-pool sequences and used to assess sediment-sorting patterns and sediment-transport competence variations. Tracer-particle depositional evidence indicated higher sediment-transport competence in pools than in riffles at high flow. Pool-riffle sediment sorting may be created by velocity reversals, and more localized sorting results from gravitational forces along the upstream sloping portion of the channel bed located at the downstream end of pools. [Key words: channel morphology, fluvial geomorphology, velocity reversal...