EM 1110-2-1100 (Part III)
30 Apr 02
(c) As discussed above, sand tracer has been used to make short-term estimates of longshore sand
transport. Other techniques that have been used to measure sediment movements on beaches include various
sand traps, pumps, and optical devices. However, such sampling schemes are also short-term measures and
may be related to specific modes of transport, either the bed load or suspended load, rather than yielding total
quantities of the long-term littoral sediment transport. Generally, it is the latter quantity which is of
importance to engineering design.
(4) Longshore sediment transport estimation in the United States.
(a) Early attempts to estimate the direction and magnitude of net longshore sediment transport along the
U.S. coastline centered chiefly upon examination of sand impoundment and bypassing volumes at jetties and
breakwaters. Johnson (1956, 1957) compiled data of this type for many shorelines and found net transport
rates ranging up to approximately 1 million m3 (MCM) of sand per year in some locations. Based on
Johnson's work, estimated patterns of littoral drift for a portion of the east coast of the United States are
shown in Figure III-2-3. Table III-2-1 lists representative net longshore transport rates for selected
U.S. coasts (SPM 1984). Transport rate magnitudes are clearly related to the general wave climate as
energetic (west coast), intermediate (east coast and parts of the Gulf of Mexico shoreline), and low (Great
Lakes and parts of the Gulf of Mexico shoreline).
(b) Since these early estimates of the transport rates, numerous investigations have yielded refined values
at specific sites along the U.S. coastlines. These values are not generally archived in a single source, and
must therefore be found through a literature search of various site-specific reports and articles. Estimates of
both the net and gross transport rates along any particular portion of the coastline may also be developed
using hindcast wave data (see Part III-2-2.d.(3) of this chapter below).
III-2-3. Predicting Potential Longshore Sediment Transport
In engineering applications, the longshore sediment transport rate is expressed as the volume transport rate
QR having units such as cubic meters per day or cubic yards per year. This is the total volume as would be
measured by survey of an impoundment at a jetty and includes about 40 percent void space between the
particles as well as the 60-percent solid grains. Another representation of the longshore sediment transport
rate is an immersed weight transport rate IR related to the volume transport rate by
IR ' (ρs & ρ) g (1 & n) QR
(III-2-1a)
or
IR
QR '
(III-2-1b)
(ρs & ρ) g (1 & n)
where
ρs = mass density of the sediment grains
ρ = mass density of water
Longshore Sediment Transport
III-2-7