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in which a pump or siphon head is used to draw the fluid-sediment mixture 

 into retaining bottles, can only establish mean concentrations. Streamer traps 

 (Kraus 1987) have also been used to measure the mean suspended load carried 

 in the longshore direction. Arrays of Niskan-type bottles (Kana 1979) provide 

 "snapshot" water-sand samples, taken at a single instant in time. Although 

 physical sampling is especially valuable in that the grain size distribution in 

 each sample can be examined, analysis of the samples is labor-intensive and 

 time-consuming. 



Radiation-scattering detection technology utilizes a source of radiation (i.e., 

 light or ultrasound) and an electronic detector that measures the energy that is 

 scattered from any particulates present in the water. Output is at very high 

 speeds, so temporal resolution is greatly enhanced. Acoustic backscattering 

 devices can measure the entire vertical profile of sand concentration almost 

 instantaneously (see, e.g., Hanes 1988); but unfortunately, they cannot be 

 used in the aerated water of the surf zone. The infrared Optical 

 Backscatterance Sensor (OBS) (Downing, Sternberg, and Lister 1981) can 

 continuously monitor local concentration of suspended solids in aerated water, 

 and has greatly expanded data collection capabilities in the surf zone. 



Although characterizing suspension in the zones of initial wave breaking is 

 arguably the most important facet of the complete description of sand 

 suspension in the surf zone, little is known quantitatively, and only limited 

 predictive capabilities exist. In the field, study of suspension in the zone of 

 initial breaking is complicated by practical considerations. Establishing 

 a priori where initial breaking will take place during the experiment is 

 extremely difficult. In addition to the underlying randomness of the incoming 

 waves, the outer boundary of the surf zone moves as (a) the tide rises and 

 falls, (b) the wave climate and wind conditions change, and (c) the nearshore 

 bar migrates and changes shape. During the field experiments reported in 

 Sternberg, Shi, and Downing (1984), Beach and Sternberg (1988), and Beach 

 (1989), five vertical arrays of five sensors each were installed in various 

 planform configurations. Although there were times at which individual 

 waves broke in the vicinity of an array, spacing of the arrays was too great 

 (2-7 m) to directly resolve the structure of sand clouds and the suspension 

 boundary. 



In a small-scale laboratory wave channel, Shibayama and Horikawa (1982) 

 focussed on the formation of the sand cloud created under plunging breakers. 

 Using high-speed photography, it was confirmed that the large-scale vortex 

 created during initial breaking generated the cloud, which the authors coarsely 

 mapped from the photographs. Because of the control that can be exercised 

 on breaking conditions, the laboratory is an excellent place for basic 

 examination of discrete suspension events under breaking waves. However, in 

 the tests conducted to date, sediment and wave scales have been mismatched, 

 i.e., full-sized sand and small-scale waves were used. 



Chapter 7 Sediment Suspension Measurements from a Platform 



