8
6
4
2
0
-2
-4
-6
-8
0
100
200
300
400
500
600
700
800
Cross-shore Distance (m)
Figure 5. Typical beach profile used for breaking wave energy flux calculation
The longshore sediment transport rate (Q, m3/sec) is calculated (SPM 1984, Equation 4-49)
0.77 Pls
Q=
(7)
g ( ρ s - ρ )(1 - n)
where
= mass density of sediment (2650 kg/m3 for quartz sand)
ρs
n
= sediment porosity (0.4)
The transport rate was calculated for each 3-hr wave condition. The bias and RMS errors for the longshore sediment
transport rate are given in Table 3. The larger errors for 1998 are driven by the substantial overestimate of wave
heights associated with Hurricane Bonnie. The transport rates were integrated over each 3-hour wave condition to
give the cumulative net volume of sediment transport. Figures 6 and 7 shows the cumulative transport for the two
wave model results and calculated from the 8-m array measurements for 1997 and 1998, respectively. Positive
transport is to the south and negative transport is to the north. The cumulative transport plots show that most of the
transport occurs over relatively short events when the wave height exceeds approximately 2 m. These events show
up as step functions in Figures 6 and 7. In general, the spectral results provide improved estimates of the cumulative
sediment transport (e.g., 25% error in the cumulative transport for 1997 for the spectral results versus 48% error for
the parametric results). However, the large overestimate of northward transport associated with Hurricane Bonnie
results in cumulative volumes in 1998 for both the spectral and parametric models that are in the wrong transport
direction (north instead of south). The cumulative volumes from the spectral method are reasonable for 1998 up to
Hurricane Bonnie in late August.
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