Settling and pick-up of sediment from a highly concentrated flow is encountered in nature as well as in dredging practice (sedimentation in a Trailing Suction Hopper Dredger and hydraulic transport in a pipeline near the deposition limit). Very often an over-loaded situation is encountered: a high sediment concentration in combination with a relative low bed shear stress. To study the sedimentation process during this typical situation, tests were carried out in a slurry circuit. The basic approach of the test was to pump a concentrated sandwater mixture at a velocity higher than the deposition velocity through the circuit. The deposition velocity is the velocity at which a stationary (settled) bed is formed in a pipeline. The circuit contained a section for measurements with a rectangular cross section. Parallel to this section a by-pass pipe was mounted. The flow velocity in the test section could be lowered in a relative short time by (partly) closing the butterfly valve upstream of this section. At the same time the velocity increased in the by-pass section. Using this arrangement the sedimentation process could be measured for different bed shear stress, concentration and grain size. The concentration profiles and the sedimentation velocity were measured using electric conductivity probes mounted in the side-wall of the test section. The discharge was measured using an electromagnetic flow meter. From the measured concentration development as a function of time the sedimentation velocity could be derived. A relation between sediment pick-up and bed shear stress was measured which differed from the empirical pick-up functions as derived from low concentration tests. For all tested sands a very clear relationship is observed between the flow velocity at which sedimentation takes place and the porosity of the settled bed. The loosest state of the bed is reached when the grains settle at zero velocity. The porosity difference between the loosest and densest state is approximately 10% for all sands tested. From the test a relationship between the sedimentation velocity and the bed shear stress was observed. This relationship was used to determine the pick-up rate for flow velocities below the deposition limit.
Sediment settling and pick-up at a flow velocity below the deposition limit
Sedimentabsetzung und -aufnahme bei einer Strömungsgeschwindigkeit unter der Ablagerungsgrenze
2004
12 Seiten, 11 Bilder, 5 Quellen
Conference paper
English
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