MEASUREMENT OF SUSPENDED SEDIMENT IN THE SURF ZONE

Abstract

The sea is an important source of water for cooling and other industrial purposes. From practical considerations, the water is usually drawn from a zone close to the shore, perhaps within one or two kilometres of the shore line through an intake structure such as a tunnel, pipeline or harbour or some comhination of these. If the coast is exposed to strong ware action and the local sea bed is of a sandynature, sediment will he lifted into suspension "by the oscillatory motion induced by the waves, and thus any sea water transported to the shore will contain a sediment load. This sediment content may have an adverse effect on the operation of the intake structure in various ways. If the water velocity is maintained at a high enough value throughout to avoid deposition, then the sand water mixture will tend to abrade the surface with which it is in contact. If however the water velocity is permitted to fall below the deposition value, sediment accumulation will occur which may be intentional as in a settling basin (requiring regular dredging or other removal) or inadvertent as in a pipe conduit (leading to partial blockage of the pipe). The writer has been associated with two sea water intake schemes, one on the western coast of South Africa, being a cooling water supply for a nuclear power station with a water consumption of about 100 m3/s; the other an intake for the recovery plant of a mining company in Namibia, water capacity about 1 m3/s. In both cases, information was needed concerning the variation of sediment concentration with distance above the sea bed, this distribution being measured at selected distances from the shore line. This led to a review of available techniques for determining the sediment content of sea water, and thence to development of new procedures and instruments. The details of these developments are presented in this paper.