EVALUATION OF THE CERC FORMULA USING LARGE-SCALE MODEL DATA
Abstract: Longshore transport experiments were conducted in a large-scale physical model to
evaluate predictions of the CERC formula with measured longshore sediment transport rates. It was
found that the CERC formula produced reasonable estimates if the coefficient K was calibrated and
applied to waves with similar breaker type. The calibrated K values are much smaller than values
that are commonly used, and there appears to be a strong dependency of transport rate on breaker
type. Additional comparisons were made with the formula proposed by Kamphuis (1991). The
Kamphuis equation, which includes wave period, a factor that influences breaking, gave good
estimates. Examination of the cross-shore distribution of longshore sediment transport indicates that
there are three distinct zones of transport: the incipient breaker zone, the inner surf zone, and the
swash zone, with each zone contributing a different fraction to the total transport rate. Transport in
the incipient breaker zone was influenced by breaker type, transport in the inner surf zone was
controlled by depth, and transport in the swash zone showed dependencies on wave height and, in
particular, wave period. Swash zone transport was found to have a significant contribution to the
total transport rate.
INTRODUCTION
Total longshore sediment transport rate and its cross-shore distribution in the surf zone are
essential to many coastal engineering and science studies. Practical engineering applications such as
beach response in the vicinity of coastal structures, beach-fill evolution and re-nourishment
requirements, and sedimentation rates in navigation channels all require accurate predictions of
longshore sediment transport rates. To maintain navigable waterways along U.S. coasts, the U.S.
Army Corps of Engineers regularly applies analytical and numerical models to estimate the total
longshore sediment transport rate and its cross-shore distribution in the surf zone. Present predictive
tools have been developed based primarily on field studies; however, obtaining high-quality data in
the field is difficult (Wang and Kraus, 1999). Arguably the most widely used model for estimating
total longshore sediment transport rate is the "CERC" formula (Shore Protection Manual, 1984).
The model, based on the assumption that the total longshore sediment transport rate is proportional
to longshore energy flux, is given as:
3
5
K
ρg H sin(2θ )
Q=
(1)
2
2
sb
16 γ b
b
in which Q is the submerged total longshore transport rate, K is an empirical coefficient, ρ is density
of water, g is acceleration due to gravity, Hsb is significant wave height at breaking, γb is the breaker
index, and θb is wave angle at breaking. The Shore Protection Manuals recommends a value of K of
0.39, which was derived from the original field study of Komar and Inman (1970) using sediment
1) U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road,
Vicksburg, MS 39180, USA.
2) Department of Geology, University of South Florida, 4202 E. Fowler Avenue, Tampa, FL. 33620, USA.
3) Coastal and Ocean Engineering Division, Texas A&M University, College Station, TX, 77843 USA.
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