Surfrider Foundation: Mitigation Through Surf Enhancement

CHAPTER 3
(Page 6 of 7)

DISCUSSION:
As an initial test of the model and to create a control situation, three wave simulations were completed on a planar beach with a slope of 0.03 with the reef absent. Three different wave heights were used in the initial conditions. As expected the waves shoaled as they propagated onshore and broke simultaneously at a constant offshore distance [See figure 3.11]. This confirmed a typical wave response to a shoreface without a reef. It also happens to coincide with the California Coastal Commission's finding that an "oversteepened" beach has resulted "closed out" surf conditions. These waves are characterized by an infinite down-line velocity. Next, a reef was incorporated into the bathymetry and the reef enhanced bathymetries were used for the rest of the model runs.

Plot of wave to a planar beach w/out reef

Figure 3.11: Plot of wave response (height) to a planar beach without a reef. The wave breaks (represented by the peak in wave height) at the same cross-shore distance in the longshore direction.

    The incorporation of the reef affected the wave response in some general ways. Assuming that the reef was in water that was shallow enough to initiate breaking, the reef effectively moved the break point offshore and the pattern of breaking was controlled by the reef. This demonstrates the reef-enhanced surfing potential by creating a wave that had a non-infinite down-line velocity. This implies that down-line velocity can be controlled via the nose angle of the reef. In general the reef increased the maximum height at breaking as compared to the planar shore-face wave response. In addition, in no tested cases did the predicted length of the ride significantly exceed the wing of the reef. At the depths tested, the input direction of wave propagation (up to 30° ) had little effect on the wave response to the reef. These general conclusions indicate that with appropriate reef depth and wave height, REF/DIF simulated enhanced surfing conditions with the installation of the reef. Wave response to variations in each reef and model parameter are summarized next.
    The first five models runs were designed to investigate wave response to shifting the reef increasingly offshore. The first three models runs pushed the break point of the waves beyond offshore breaking distance of the reefless shoreface. [See figure 3.4]. At 175 meters offshore, the ride length began to decrease because the water depth increased beyond y on the backslope of the reef and the wave stopped breaking. To investigate wave response to larger initial wave at this offshore reef location, I increased the input wave amplitude to 1 meter. This created a longer ride at this distance and also satisfactorily broke waves out to a reef location 200 meters offshore. These simulations clearly demonstrate that the reef effectively extends the breaking point farther offshore to a limit that is dependent on reef height and wave height. It also demonstrates that the line of breaking is controlled by the reef and that the predicted ride length is effected by the depth of reef placement.
    Maximum height at breaking was affected most dramatically by offshore reef location, toe angle, wave period, and obviously by increased wave height in the initial conditions. As reef location progressed offshore maximum breaking height increased modestly [See figure 3.12]. Interestingly enough, water depth above the reef seemed to have little effect on maximum wave height up to a threshold depth where breaking shut off. This threshold effect may be result of the critical depth that REF/DIF used to define breaking. Waves larger than 2.3 meters broke beyond the reef at the 175 meter location. The large waves created unsurfable model results based on this offshore location of the reef.

Graph: Wave Ht. vs Offshore position

Figure 3.12: Breaking wave height increased as the reef was positioned farther offshore. Offshore distance is measured from the 0 sea level datum in the bathymetry.

Wave height response to toe angle has some non-intuitive results. Maximum heights were recorded when toe angle measured 65° and 90°. The abrupt reef wall created when toe angle is 90° may explain the increased wave height. However, as the toe angle is gradually reduced the wave height quickly drops off until 65° is reach where wave height increases again. Wave height was not enhanced by a toe angle below 65° [See Figure 3.13]. This may be explained by artifacts created when REF/DIF encountered a abrupt bottom feature such as the reef. These unusual patterns were observed wave height contour maps. There are two possible explanations for this artifact. Wave interference patterns may be produced when REF/DIF propagates a single wave crest over abrupt bathymetry and turns the wave crest into multiple "phase-locked" crests which may interfere with each other. Another possibility is that the unusual pattern is a result of high angle diffraction noise. Because REF/DIF can only propagate waves at ~ 55° to the x-axis, abrupt bathymetry (a reef) can cause REF/DIF to attempt to propagate waves at angle higher than this threshold which may create short waves (patter) which radiate away from the reef (O'Reilly, personal communication). This patter is significantly reduced when the toe angle is reduced below 90°. Perhaps the 65° toe angle represents the position where the interference (pattern) is minimized while the reef effect on wave height is maximized.

Graph: Wave Ht. response to toe angle

Figure 3.13: Wave height response to toe angle. Note the peaks at 65 degrees and at 90 degrees.

The increased wave height response to longer period waves is a predicted result of shoaling. It is know that wave energy flux remains constant as wave propagate and shoal. As celerity decreases and wavelength becomes shorter the height of the wave increases to maintain a constant energy flux. Large period waves also have longer wavelengths and thereby create larger maximum breaking height as the waves crash over the reef.

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