Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p32d0572m&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P32D-0572
Physics
3022 Marine Sediments: Processes And Transport, 4558 Sediment Transport, 4568 Turbulence, Diffusion, And Mixing Processes
Scientific paper
Little is known about the combined spatio-temporal evolution of velocity structure, concentration and grain size stratification within particulate gravity currents. Yet these data are of primary importance for numerical model validation, prior to application to natural flows, such as pyroclastic density currents and turbidity currents. A comprehensive study was carried out on a series of experimental particulate gravity flows of 5% by volume initial concentration. The sediment analogue was polydisperse silica flour (mean grain size ~8 microns). A uniform 30 liter suspension was prepared in an overhead reservoir, then allowed to drain (in about one minute) into an flume 10 m long and 0.3 m wide, water-filled to a depth of 0.3 m. Each flow was siphoned continuously for 52 s at 5 different heights (spaced evenly from 0.6 to 4.6 cm) with samples collected at a frequency of 0.25Hz, generating 325 samples for grain-size and concentration analysis. Simultaneously, six 4-MHz UDVP (Ultrasonic Doppler Velocity Profiling) probes recorded the horizontal component of flow velocity. All but the highest probe were positioned at the same height as the siphons. The sampling location was shifted 1.32m down-current for each of five nominally identical flows, yielding sample locations at 1.32, 2.64, 3.96, 5.28 and 6.60m from the inlet point. These data can be combined to give both the temporal and spatial evolution of a single idealised flow. The concentration data can be used to defined the structure of the flow. The flow first propagated as a jet, then became stratified. The length of the head increased with increasing distance from the reservoir (although the head propagation velocity was uniform). The maximum concentration was located at the base of the flow towards the rear of the head. Grain-size analysis showed that the head was enriched in coarse particles even at the most distal sampling location. Distinct flow stratification developed at a distance between 1.3 m and 2.6 m from the reservoir. In the body of the current, the suspended sediment was normally graded, whereas the tail exhibited inverse grading. This inverse grading may be linked to coarse particles in the head being swept upwards and backwards, then falling back into the body of the current. Alternatively, body turbulence may inhibit the settling of coarse particles. Turbulence may also explain the presence of coarse particles in the flow's head, with turbulence intensity apparently correlated with the flow competence.
Baas Jacob
Choux C.
Haughton P.
McCaffrey W.
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