ERDC/CHL CHETN-II-46
March 2002
By incorporating wave period to a power of 1.5, the Kamphuis-91 formula produced much more
consistent predictions for the different breaker types, relative to the measured values. Wave
period, which is linked to the wavelength through the dispersion relation, has significant
influence on wave steepness and hence, breaker type. The Kamphuis-91 formula underpredicted
the spilling and plunging cases by 30 percent and 24 percent, respectively. The consistent
underprediction, if proven to be true with more data, could be resolved by adjusting the empirical
coefficient.
A different formulation and parameterization were used in the Kraus, Gingerich, and Rosati
(1988) formula. The threshold value Rc of 3.9 m3/sec, which was determined from an Atlantic
Ocean surf zone, is too large for application to the LSTF conditions. For purposes of
comparison, the Rc parameter is ignored. The recommended Kd value of 2.7 is still used. The
longshore discharge was measured directly in the LSTF. Predictions from the Kraus-88 formula
are also compared in Table 1. The predicted value compared well for the spilling case, but
underpredicted the plunging case by 55 percent. As discussed in Kraus, Gingerich, and Rosati
(1988), the coefficient Kd is related to sediment suspension. Sediment suspension in the vicinity
of the spilling and plunging breaker lines was substantially different. The inconsistency in the
prediction using the method of Kraus, Gingerich, and Rosati (1988) was caused by neglecting the
to use a greater Kd value for plunging breakers due to the much more active sediment suspension.
Similar inconsistencies in using the CERC and Kamphuis-86 formulas probably arise for the
same reason. The significantly improved consistency of the Kamphuis-91 formula is attributed
to incorporation of wave period, which has significant influence on the breaker type.
SUMMARY AND CONCLUSIONS: The total rate and cross-shore distribution pattern of
LST were significantly different during the plunging and spilling cases. Nearly 170 percent
more longshore sediment transport was measured for the plunging breaker than for the spilling
breaker, although the plunging breaker height was only 13 percent higher than the spilling
breaker height. The cross-shore distribution patterns of LST were far from being uniform.
During the spilling-breaker case, peak longshore transport was measured in the swash zone.
During the plunging-breaker case, two transport peaks were measured, one in the swash zone and
one in the vicinity of the breaker line. Substantial amounts of longshore sediment transport were
measured in the swash zone during both cases. Interestingly, in the mid-surf zone which is
dominated by surf-bore motions, the measured transport rates were quite similar for both the
spilling and plunging cases. In other words, the much greater rate of total longshore transport
measured for the plunging case was mainly due to the much more active sediment suspension
and transport in the breaker zone and more transport in the wider and more energetic swash zone.
The commonly used CERC formula predicted inconsistent total longshore sediment transport
rate under the spilling and plunging breakers. By including wave period, which has significant
influence on breaker type, the Kamphius-91 formula produced consistent predictions for both the
spilling and plunging cases. Results from the present study suggest that breaker type has a
significant influence on the total rate of longshore sediment transport and its cross-shore
distribution pattern. Parameterization of predictive formulas should include factors that reflect
the breaker type; however, additional data and research are needed to derive the relationship
between LST and breaker type.
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