Results
Figure 2 shows the transformation of waves from offshore to nearshore. Spilling waves
show a fairly gradual decrease in wave height across the beach. Plunging breakers shoal
farther shoreward and the break point is evident where wave height sharply decreases. The
corresponding equilibrium beach profiles formed by the two wave types are shown in
Figure 3. The profile associated with spilling breakers consists of three near-planar slopes.
The offshore slope of the beach becomes gentle through the surf zone to the swash zone,
and the slope becomes steep in the nearshore. A breakpoint bar and trough formed under
plunging waves, and slight erosion occurred compared to the spilling wave case.
0.30
Spilling
Plunging
0.25
0.20
0.15
0.10
0.05
0.00
0
5
10
15
20
25
Cross-shore Location (m)
Fig 2. Wave transformation of spilling and plunging breakers
Average longshore current measured in the center portion of the beach for both wave
cases is shown in Figure 4. The spilling case shows an upward trend towards the nearshore
with a peak immediately seaward of the swash zone. Longshore currents associated with
the plunging waves also show an increasing trend towards the nearshore. However, the
plunging waves currents have a double peak in velocity; one at the break point and one in
the swash zone. Although, measured velocities are slightly higher with plunging waves, the
two cases have similar cross-shore distributions of longshore velocity.
Sediment flux was calculated from the rate of sand collected in each trap. Although the
wave heights were identical for the two wave cases, and the resulting wave-driven
longshore currents were similar, the sediment flux of the two breaker types is significantly
different (Figure 5). The plunging wave case has peaks in transport at the break point and
in the swash. A single peak occurs in transport with the spilling wave case in the swash
zone.
Smith and Wang
1222
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