EM 1110-2-1100 (Part III)
30 Apr 02
(c) Identification of unique minerals within the sand has also been used to deduce transport paths. By
using the heavy mineral augite as a tracer of longshore sand movements, Trask (1952, 1955) demonstrated
that the sand filling the harbor at Santa Barbara, California, originates more than 160 km up the coast. The
augite was derived from volcanic rocks in the Morro Bay area north of Santa Barbara. Likewise, Bowen and
Inman (1966) estimated the directions and magnitudes of the net transport along the California coast by
studying the progressive dilution of beach sand augite by addition of sand from other sources. In other
examples, Meisburger (1989) utilized oolitic carbonate as a natural mineral tracer to deduce transport paths
along Florida's east coast. Johnson (1992) noted the net transport paths along parts of the Lake Michigan
coastline by examining the movement of gravel placed as beach restoration.
(d) Many of the geomorphic and sedimentological indicators of longshore sediment transport directions
are not absolute, and too strong a reliance upon them can lead to misinterpretations. It is best to examine all
potential evidence that might relate to transport direction, and consider the relative reliabilities of the
indicators.
(3) Quantitative indicators of longshore transport magnitude.
(a) Some of the qualitative indicators of longshore transport discussed above can also be used to estimate
the quantities involved in the process. Repeated surveys over a number of years and analyses of aerial
photographs of the longshore growth of sand spits have been used to establish approximate rates of sediment
transport. For the estimates to be reasonable, it is typically necessary that such surveys span a decade or
longer, so the results represent a long-term net sediment transport rate.
(b) The blockage of longshore sediment transport by jetties and breakwaters and the resulting growth
and erosion patterns of the adjacent beaches have yielded reasonable evaluations of the net (and sometimes
gross) transport rates at many coastal sites. Sand bypassing plants have been constructed at some jetties and
breakwaters as a practical measure to reduce the accretion/erosion patterns adjacent to the structures. The
first measurements obtained relating sand transport rates to causative wave conditions were collected by
Watts (1953a) at Lake Worth Inlet, Florida, using measured quantities of sand pumped past the jetties. The
best correlation was obtained using month-long net sand volumes. A number of subsequent studies have
similarly employed sand blockage by jetties and breakwaters to obtain data relating transport rates to wave
conditions. Caldwell (1956) estimated the longshore sand transport from erosion rates of the beach downdrift
of the jetties at Anaheim Bay, California. Bruno and Gable (1976); Bruno, Dean, and Gable (1980); Bruno
et al. (1981); and Walton and Bruno (1989) measured transport rates by repeatedly surveying the
accumulating blocked sand at Channel Islands Harbor, California; Dean et al. (1982) measured sand
accumulations in the spit growing across the breakwater opening at Santa Barbara, California; and Dean et al.
(1987) collected data from the weir jetty at Rudee Inlet, Virginia. All of these studies yielded measurements
of longshore sediment transport rates that are used in correlations with wave energy flux estimates. Errors
are introduced with the use of jetties and breakwaters to measure sediment transport rates, the foremost being
the local effects of the structure on waves and currents, and the long-term nature of the determinations. In
some cases it takes a month or longer of sand accumulation to have a volume that is large enough to be
outside the range of possible survey error, during which time the waves and currents are continuously
changing. Therefore, a correlation between a given wave condition and the resulting sediment transport rate
due solely to that particular wave condition cannot be obtained; rather, wave parameters occurring during the
interval of interest are integrated and then correlated with the total accumulated volume. Nevertheless, rates
determined by impoundment and erosion are very valuable as they are closely related to gross quantities
involved in the design of projects, such as the amount of sediment required to be bypassed at an inlet.
III-2-6
Longshore Sediment Transport
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