Problems with Predicting the Transport of Pumiceous Sediments in the Coastal Environmentby Willem P. de Lange, Univ of Waikato, Hamilton, New Zealand,
Vicki G. Moon, Univ of Waikato, Hamilton, New Zealand,
Terry R. Healy, Univ of Waikato, Hamilton, New Zealand,
Abstract: Pumice is often a major component of coastal sediments associated with subduction zone volcanic settings. Several recent studies in New Zealand have highlighted the difficulties of predicting the transport rates and patterns of coastal sediments when significant (>10%) quantities of pumice are present. Although pumice clasts consist of an interlocking network of glass shards which individually have a density similar to quartz (typically 2330-2413 kg.m-3 c.f. 2650 kg.m-3 for quartz), the clasts themselves exhibit a wide range of densities which are consistently less than that of quartz. The densities of pumice clasts vary with clast size and the degree of saturation; the density generally decreasing with increasing size or decreasing saturation. Calculations of sediment transport in aqueous environments are often based on the assumption of the sediment being reasonably approximated by quartz spheres. Corrections are available to permit calculations for sediment of differing density, such as carbonate, or heavy minerals. These corrections do not exist for pumice, nor would they be appropriate given the variation density with size and degree of saturation of the clasts. The saturated densities measured for a variety of New Zealand pumices range from 1120 to 2370 kg.m-3, and the dry densities are often less than 1000 kg.m-3. Pumice exhibits a wide range of hardnesses, with the hardness decreasing with increasing saturation. Slake durability tests on pumice reveals a wide range of durabilities. These results indicate that pumice clasts may soften considerably under a regime of wetting/drying cycles, and they have little resistance to mechanical abrasion. The sediment entrainment thresholds for pumice clasts are in agreement with those predicted by Shield's curve. The velocities required are lower than necessary for a quartz sphere of the same nominal diameter, particularly if the clast is not fully saturated, and once entrained, pumice clasts tend to be transported for large distances downstream.
Subject Headings: Sediment transport | Coastal environment | Erosion | Spheres | Entrainment | Glass | Hardness (material) | New Zealand
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