EM 1110-2-1100 (Part III)
30 Apr 02
(b) Measurement of sand tracer transport rates involves tagging the natural beach sediment with a coating
of fluorescent dye or low-level radioactivity. Tracers are injected into the surf zone, and the beach material
is sampled on a grid to determine the subsequent tracer distribution. The longshore displacement of the center
of mass of the tracer on the beach between injection and sampling provides a measure of the mean transport
distance. The sand advection velocity is obtained by dividing this distance by the elapsed time. The time
between tracer injection and sampling is usually an hour to a few hours, so the measurement is basically the
instantaneous longshore sand transport under a fixed set of wave conditions. The technique, therefore,
provides measurements that are particularly suitable for correlations with causative waves and longshore
currents, as in time-dependent numerical models of longshore transport rates and beach change, but is not
particularly useful in determining long-term net transport rates or directions. Identification of the appropriate
sediment mixing depth also limits the technique's quantitative accuracy. Numerous studies have used sand
tracers to determine sand transport rates; examples include Komar and Inman (1970), Knoth and Nummedal
(1977), Duane and James (1980), Inman et al. (1980), Kraus et al. (1982), White (1987), and many others.
Tracer techniques can also be used in conjunction with geologically distinct materials, such as naturally
occurring mineral sediments. These methods, which generally involve larger quantities of material and larger
scales of measurement, are potentially more useful for determining longer-term rates and directions and are
discussed below.
(c) Measurements of suspended load transport have been the focus of numerous studies. One approach
has been to pump water containing suspended sand from the surf zone, a technique which has the advantage
that large quantities can be processed, leading to some confidence that the samples are representative of
sediment concentrations found in the surf (Watts 1953b; Fairchild 1972, 1977; Coakley and Skafel 1982).
These measurements, combined with simultaneous measurements of alongshore current velocity, yield
estimates of the longshore sediment transport suspended load. The disadvantages of the approach are that
one cannot investigate time variations in sediment concentrations at different phases of wave motions, and
the sampling has often been undertaken from piers which may disturb the water and sediment motion. Recent
studies of suspended load transport have employed optical and acoustic techniques (Brenninkmeyer 1976;
Thornton and Morris 1977; Katoh, Tanaka, and Irie 1984; Downing 1984; Sternberg, Shi, and Downing 1984;
Beach and Sternberg 1987; Hanes et al. 1988). These approaches yield continuous measurements of the
instantaneous suspended sediment concentrations. Arrays of instruments can be employed to document
variations across the surf zone and vertically through the water column.
(d) Another method for measuring suspension concentrations is with traps, usually consisting of a
vertical array of sample bins which collect sediment but allow water to pass, and so can be used to examine
the vertical distribution of suspended sediments and can be positioned at any location across the surf zone
(e.g., Hom-ma, Horikawa, and Kajima 1965; Kana 1977, 1978; Inman et al. 1980; Kraus, Gingerich, and
Rosati 1988). Figure III-2-2 shows vertical distributions of the longshore sediment flux (transport rate per
unit area) through the water column obtained in a 5-min sampling interval by traps arranged across the surf
zone at Duck, North Carolina (Kraus, Gingerich, and Rosati 1989). The steep decrease in transport with
elevation above the bed is apparent; such considerations are important in groin and weir design. The sharp
decrease in transport at the trap located seaward of the breaker line indicates that the main portion of
longshore sediment transport takes place in the surf and swash zones.
(e) The only method presently suitable for distinct measurement of bed load transport is bed-load traps.
These are bins which are open-ended or dug into the seabed into which the bed-load transport is to settle.
There are questions as to sampling efficiency when used in the nearshore because of the potential for scour
(Thornton 1972; Walton, Thomas, and Dickey 1985; Rosati and Kraus 1989).
III-2-4
Longshore Sediment Transport
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