EM 1110-2-1100 (Part III)
30 Apr 02
a. Energy flux method.
(1) Historical background. An extensive discussion of the evolution of energy-based longshore transport
formulae is presented by Sayao (1982), in his dissertation. The following is a summary of Sayao's discussion,
focussing on evolution of the so-called "CERC" formula.
Munch-Peterson, a Danish engineer, first related the rate of littoral sand transport to deepwater wave energy
in conjunction with harbor studies on the Danish coast (Munch-Peterson 1938). Because of a lack of wave
data, Munch-Peterson used wind data in practical applications, which gave preliminary estimations of the
littoral drift direction. In the United States, use of a formula to predict longshore sediment transport based
on wave energy was suggested by the Scripps Institute of Oceanography (1947), and applied by the
U.S. Army Corps of Engineers, Los Angeles District to the California coast (Eaton 1950). Watts (1953a) and
Caldwell (1956) made the earliest documented measurements of longshore sediment transport (at South Lake
Worth, Florida, and Anaheim Bay, California, respectively) and related transport rates to wave energy,
resulting in modifications to the existing formulae. Savage (1962) summarized the available data from field
and laboratory studies and developed an equation which was later adopted by the U.S. Army Corps of
Engineers in a 1966 coastal design manual (U.S. Army Corps of Engineers 1966), which became known as
the "CERC formula." Inman and Bagnold (1963), based on Bagnold's earlier work on wind-blown sand
transport and on sand transport in rivers, suggested use of an immersed weight longshore transport rate, rather
than a volumetric rate. An immersed weight sediment transport equation was calibrated by Komar and Inman
(1970) based on the available field data including their tracer-based measurements at Silver Strand,
California, and El Moreno, Mexico. Based on Komar and Inman's (1970) transport relationship and other
available field data, the CERC formula for littoral sand transport was updated from its 1966 version, and has
been presented as such in the previous editions of the Shore Protection Manual (1977, 1984).
(2) Description.
(a) The potential longshore sediment transport rate, dependent on an available quantity of littoral
material, is most commonly correlated with the so-called longshore component of wave energy flux or power,
PR ' (E Cg)b sinαb cosαb
(III-2-2)
where Eb is the wave energy evaluated at the breaker line,
2
ρ g Hb
(III-2-3)
Eb '
8
and Cgb is the wave group speed at the breaker line,
1
Hb
2
(III-2-4)
Cgb ' g db ' g
κ
where κ is the breaker index Hb / db . The term (ECg)b is the "wave energy flux" evaluated at the breaker
zone, and αb is the wave breaker angle relative to the shoreline. The immersed weight transport rate IR has
the same units as PR (i.e., N/sec or lbf/sec), so that the relationship
IR ' K PR
(III-2-5)
III-2-10
Longshore Sediment Transport
Previous Page
