In situ determination of fine-grained sediment erodibility

Cowgill, Christopher Michael (1994) In situ determination of fine-grained sediment erodibility. University of Southampton, Doctoral Thesis.

Abstract

This project has two distinct aims: the design, construction and operation of a device to examine in situ the erodibility of fine-grained sediment; and the examination of the various factors influencing the erodibility of fine-grained sediment, over a 12 month period.

To fulfil the first of these two aims a simple flow-through flume was developed. This flume, constructed of marine bonded-plywood, and of length 2.5 m, of width 0.2 m, and of depth 0.3 m, was built at the Department of Oceanography, University of Southampton. A reservoir section, continually supplied by a simple petrol-driven pump, acted as a head of water. This water passed through two foam baffles and down the 2 m long flow section. An adjustable weir allowed for varying depths of water in the flow section. In the centre of the flow section, an opening 0.25 m by 0.15 m, allowed access to the mud bed on which the flume was positioned. The flow in the flume was allowed to build up until a shear sufficient to cause erosion of the surface layer was achieved.

Flow rates in the flume were monitored and recorded using a miniature impeller current meter, the Nixon Streamflo. Turbidity in the flume was recorded using three separate optical backscatter (OBS) sensors; one placed upstream of the test section, the other two placed downstream. The upstream OBS sensor acted as an indicator of ambient turbidity of the source water. The two downstream OBS sensors were used to indicate an increase in turbidity, suggesting erosion of the mud bed. It is, at present, not possible to run these experiments in real time. Much of the analysis of threshold was undertaken at a later date.

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