Settling velocity – a new approach to assessing soil and sediment properties

Abstract

Listen

Translate article

Settling velocity measurements have not been widely used in soil research, probably largely because of a lack of convenient methods of making such measurements. In the past, the measurement of settling velocity has been a relatively unattractive laboratory technique, being usually time-consuming and difficult to achieve a useful throughput of samples. An automated settling column is described that provides reliable and efficient measurement of settling velocity. The column is computer controlled, with up to 10 user-selected settling velocity classes able to be selected. Electric motors raise and lower the column, and turn a sampling tray as required to separate the various settling velocity classes. Depending on the smallest particle diameter selected, a pair of columns linked to a computer can process up to four samples per hour. Prototypes have been used in erosion/sediment movement studies by the Queensland Department of Natural Resources for approximately 6 years, and have shown the value of this technique for a range of soil aggregation and erosion applications. The most obvious application of settling velocity is for studies of erosion and sediment movement, where sediment transport equations may either consider settling velocity directly, or indirectly via relationships between sediment transport and sediment size and density. Wet densities of the full range of aggregate size classes, down to at least 0.125 mm can be estimated from settling velocity measurements by comparing equivalent sizes (based on settling velocity) with measured size distributions. Accurate measured data on sediment size and density distributions are essential if erosion modelling is to successfully consider sediment movements through a varied landscape. Information on the wet density of aggregates is also useful for assessing changes in soil aggregation. In non-swelling soils, it gives a measure of aggregate porosity and hence, of its internal organisation. In swelling soils, it can be used to assess both aggregate porosity and effects of management on shrink/swell characteristics.