8.4: Observing Sediment Transport
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What are the main things about sediment transport you might want to observe? Here are four of them, and some comments on the ways those observations might be carried out.
Particle Movement. The eye is an excellent instrument for perceiving the details of movement of sediment particles, but you need techniques like cinematography (high-speed or slow-motion) or video for a record, and also for speeding up or slowing down the process so that the eye can better perceive the nature of the movement. Various other optical methods for tracing particle movement have also been developed.
Bed Geometry. Visual observation and photography are the main ways of dealing with the bed configuration in still life. Time-lapse cinematography is especially useful for gaining information on the kinematics of individual bed forms: they generally move too slowly for you to see what they are doing in real time. When the bed is obscured by moving sediment, as is so often the case, you have to resort to mechanical profiling or sonic profiling.
Sediment Load. The sediment load can be sampled with traps or samplers of various kinds that purport to catch a representative volume sample of the sediment–water mixture. Indirect methods that involve the effect of the sediment–water mixture on a beam of light or sound are common: these can be as simple as attenuation along a line, or by focusing separate beams they can examine the sediment load in small regions that approximate points.
Sediment Transport Rate. It is notoriously, and frustratingly, difficult to make good measurements of the sediment transport rate without serious disruption of the flow. Local volume sampling of the load along with simultaneous measurements of local time-average velocity, integrated over the cross section of the flow, is the most common way. For bed load, traps of a great many kinds have been devised, and if they are calibrated well they can be quite reliable. I will have more to say on the intricacies of measuring the sediment load and the sediment transport rate in a later chapter.
Here is a list of most of the aspects of sediment transport you might want to observe, measure, or think about, along with some brief comments.
- Entrainment. The flow exerts sufficient force on a bed particle to set it into motion.
- Traction. Particles are moved in contact with or close to the bed by fluid forces.
- Saltation. Particle undergo near-bed ballistic movement, largely unaffected by turbulence.
- Suspension. The flow lifts particles away from the bed after entrainment.
- Turbulence. The particles distort the local fluid velocity field, alter the structure of the turbulence, and themselves undergo turbulent accelerations.
- Settling. Particles settle toward the bed through the surrounding fluid.Hindered settling. The proximity of other settling particles hinders the settling of each particle.
- Collisions. Differing velocities plus inertia leads to collisions (or close encounters) between particles.
- Diffusion. Suspended sediment undergoes upward turbulent diffusion against the concentration gradient.
- Distrainment. Particles come to rest on the bed.
- Liquefaction. Rearrangement of packing in the bed leads to reduction in particle contact, partial or total support of particles by pore fluid, then refreezing of the texture by dewatering.
- Bed configuration. Bed-load transport plus complex dynamic bed instabilities lead to various characteristic bed geometries.