tracers. However, the expression is best used if the coefficient is calibrated using data for a particular
site. For design applications with adequate field measurements, the CERC formula can be calibrated
and applied to estimate total longshore sediment transport rates with reasonable confidence (+ 50
percent). However, many sites do not have transport data available to calibrate K, and for design
applications without calibration data the CERC formula provides only order-of-magnitude accuracy
(Fowler et al., 1995; Wang et al., 1998). The recommended value of K = 0.39 has been commonly
used to represent the potential longshore transport rate. However, Miller (1998) found that the
CERC formula sometimes over and sometimes under predicted longshore transport rate for
measurements during storms, indicating the value of K also can be higher than 0.39.
Most of the data available for calibration of empirical longshore sediment transport formulas
were obtained from field measurements. Field measurements in the dynamic surf zone are non-
controllable and non-repeatable, which may lead to large uncertainties (Schoonees and Theron,
1993, 1994; Wang et al., 1998; Wang and Kraus, 1990). In addition, only a limited number of
parameters can be measured in the field with coarse temporal and spatial resolutions.
The few laboratory studies on longshore sediment transport are advantageous in that they are
controllable and repeatable, and therefore, should be more accurate than field data. Laboratory data
have not been broadly used in the calibration of longshore transport formulas, largely because
typically small scales were used. However, Kamphuis (2002) found that experiments conducted with
a relatively small model had little scale effect and uncertainties were less than that of field results.
Kamphuis suggests that it is difficult to improve estimates of longshore sediment transport rate
based solely field data because of large uncertainties associated with measuring basic variables and
the subjectivity of interpreting results. Kamphuis concludes that any improvements to sediment
transport relationships need to be developed from controlled and controllable model tests, despite the
shortcomings of physical models.
Kamphuis (1991) developed a relationship for estimating longshore sediment transport rates
based primarily on physical model experiments. The equation, which Kamphuis (2002) found to be
applicable to both field and model data, is expressed as:
Qu = 2.27 H s2bT p .5 mb .75 d 5-00.25 sin 0.6 (2θ b )
1
0
(2)
in which Qu is the transport rate of underwater mass in kg/s, Tp is the peak wave period, mb is the
beach slope from the breaker line to the shoreline, and d50 is the median grain size. Kamphuis (2002)
redefines the beach slope as the slope that causes breaking, i.e., the slope over one or two
wavelengths offshore of the breaker line. In the present study, the Kamphuis (1991) version was
used because of difficulty in defining the slope offshore of breaking. Equation 2 is appealing
because it includes wave period, which influences wave breaking (Galvin, 1968), and grain size
diameter, a relevant factor in incipient sediment motion.
Research is ongoing in a large-scale physical model to improve present predictive equations for
longshore sediment transport. The purpose of this paper is to compare estimates from Equations 1
and 2 to measurements obtained in the model. The CERC formula, specifically the coefficient K,
will be evaluated based on the comparison.
Smith et al
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