ERDC/CHL CHETN-II-46
March 2002
wave case. The large difference in LST rate at the incipient breaker line between the plunging
and spilling cases is attributed to the much higher sediment concentrations higher in the water
column that were created by the plunging breakers. In the swash zone, differences seem to be
attributed to the much higher energy of the uprush and downrush associated with the longer-
period waves that characterized the plunging case.
TOTAL RATE OF LST: In the LSTF, the total rate of LST was obtained by simply summing
the sediment flux per unit width measured at a series of traps located at the downdrift end of the
facility. The total rate measured during the spilling case was 2,660 m3/year, substantially less
than the total rate of 7,040 m3/year measured during the plunging case. It is worth noting that the
breaking wave (significant height) was about 13 percent higher during the plunging case, 0.27 m,
versus 0.24 m during the spilling case. However, the 13 percent higher breaking wave height
certainly could not explain the fact that measured LST rates differed by a factor of 2.65.
The measured total transport rates were substantially lower than the predictions from the CERC
(Equation 1) and the Kamphuis-86 formulas (Equation 2) for both the spilling and plunging cases
(see Table 1). The Kamphuis-91 (Equation 3) formula, on the other hand, underpredicted the
measured rates for both cases. The empirical Kl value of 0.39 as recommended by the Shore
Protection Manual was used in the CERC formula.
Table 1
Comparison among Measured and Predicted Total Rates of LST
Spilling Case
Plunging Case
3
Transport Rates (m /year)
3
Measured (m /year)
2,660
7,040
3
CERC formula (m /year)
18,040
23,850
3
Kamphuis-86 (m /year)
8,130
9,100
3
Kamphuis-91 (m /year)
1,870
5,360
3
Kraus-88 (m /year)
2,670
3,150
Percentage Over (+) Or Under (-) Prediction
CERC
+578%
+239%
Kamphuis-86
+206%
+29%
Kamphuis-91
-30%
-24%
3
Kraus-88 (m /year)
0%
-55%
The CERC formula overpredicted the total rate for the spilling condition by nearly 600 percent,
while for the plunging waves, the overprediction was less than 250 percent. This inconsistency
of the CERC formula under different breaker types indicates that a simple reduction of the Kl
value as examined by Bodge and Kraus (1991), Schoonees and Theron (1993), and Wang, Kraus,
and Davis (1998) cannot completely address the discrepancy. In other words, the comprehension
that the total rate of LST is proportional to a measure of the longshore wave-energy flux might
not be complete. The Kamphuis-86 formula also had a similar inconsistency. Using it, the
spilling case was overpredicted by more than 200 percent, while the plunging case was
overpredicted by less than 30 percent.
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