Surfrider Foundation: Mitigation Through Surf Enhancement

CHAPTER 3
(Page 7 of 7)

The two calculated wave shape parameters demonstrate the ineffectiveness wave shape parameters for calculating computer simulated wave response to reef designs used in this study [See Table 4]. The K1 values for the gradually sloping shoreface had values ranging from approximately 0.05 to 0.07. These values bound the critical plunging versus spilling value of 0.068. This suggests that on a planar beach with out a reef smaller waves would plunge while large waves would tend to spill. These results demonstrate that Galvin's (1972) wave shape parameter operates as expected for low angle slopes. The K2 values are all below 0.003 which defines a surging wave in Galvin's (1972) model. Empirical evidence suggests that many of the world's most renown surf breaks (i.e. Pipeline) are formed by a reef with a nearly vertical toe angle. This suggests that although Galvin's (1972) model works for gradually sloping beaches it is ineffective for predicting wave shape on steeply sloped structures. Although the Iribarren number was used effectively to predict wave shape in a CERC study (Smith and Kraus, 1990), the maximum slope tested was 40°. This steep slope created what Smith and Kraus (1990) called a confused wave as a result of strong backflow over the artificial bar. All reefs modeled in this study had a toe angle greater than 40°. Although the Smith and Kraus (1990) study was performed in a wave tank with enclosed sides that may have enhance the backflow problem, this presents a potential concern for the designs modeled by REF/DIF.

The down-line velocity is used as an indicator of how quickly the wave will thrust the surfer forward. There are two distinct wave riding styles that are related to this speed. "Tube-riding" waves are waves with a plunging or "hollow" shape and very rapid down-line velocities. These waves are ridden by surfing into the "pocket", or vortex, under the curl with little or no turning. These waves are ridden straight in an attempt to stay just under the curl without allowing the broken part of the wave overcome the surfer. Waves with a slower down-line velocity allow the surfer to perform more turns or tricks. These waves are known as "workable" waves. The down-line velocity is often dictated by daily conditions, so that one location may provide variable down-line velocities. The model results suggest that down-line velocity is well controlled by nose angle. Down-line velocity was also affected by and extremely shallow toe angle (45°) and larger waves both of which lead to an increased wave speed [See Figure 14]. Several expert surfers who were interviewed suggested down-line velocities ranging from approximately 10 mph to 20 mph.

Graph: Down-line Velocity vs. Toe Angle

Figure 3.14: Down-line velocity is insensitive to moderate toe angles. Toe angle smaller than 45 degrees increased down-line velocity because wave celerity is not dramatically effected by the reef.

The length of the ride was maximized when model conditions created a wave that broke along the entire extent of the reef. The ride length never exceeded the length of the wing of the reef. The ride length decreased if the wave conditions resulted in a wave that did not break along the entire wing of the reef. These resulted because the reef was in a water depth such that breaking only occurred along the highest portion of the reef [See Figure 3.15]

Graph: Ride Length vs. Reef Depth

Figure 3.15: Ride length decreased with increasing water depth above the reef. This resulted because the wave did not break over the extend of the reef. This could be analogous to the wave response during an upcoming tide.

CONCLUSION:
    The use of REF/DIF 1 to model wave response to reef design and wave climate parameters marks a significant advance over traditional wave ray modeling techniques. Although REF/DIF 1 represents one of the most current monochromatic water waves models to date, one must remember that model predictions are still only qualitative and do not provide direct predictive results. Model shortcomings are of three categories. One group of shortcomings are limitations in the current physical understanding of wave behavior, especially when interacting with a bar or reef and during breaking. The second group of shortcomings are those specific to the limitations of the model, an example is the exclusion of wind on wave behavior. The third category of potential error is the proper application and interpretation of model results. Despite these shortcomings, the modeling of wave response to artificial reef designs has provided some valuable information that may aid in the design and construction of the reef.
    The modeling results suggest that the installation of an artificial reef will effectively extend the surf zone offshore and create waves that break with a defined pattern. These factors will enhance the surfing potential in the area. In addition, modeling results suggest that the down-line velocity can be controlled by the nose angle and that a threshold depth exists for breaking to occur. For the conditions simulated this depth threshold is almost equal to the maximum tidal range in the El Segundo area. This suggests that the artificial reef will not create "surfable" waves at all tidal ranges. In order to reduce aerial exposure of the reef (one of the design goals) the reef should be designed to break at lower tides.
    Wave shape proved to be a difficult response to predict. The two parameters used to predict wave shape predicted a surging wave for all situations tested. In fact, the parameters measured for this modeling exercise were often outside the given wave shape parameter ranges by more than an order of magnitude. It is difficult to say whether this difference reflects a true potential wave response or suggests that the wave shape models are limited to predicting wave response for low angle structures and natural beaches (where they were developed). It is commonly known that many surfing spots have waves breaking by near vertical reef structures, however it appears no physical model has been developed to predict wave response to such structures.
    Despite the shortcomings of wave shape prediction, the REF/DIF 1 model simulation results are in accordance with anticipated physical response to a reef structure. These data suggest that the artificial reef enhance potential surfing conditions by extending the surf zone offshore and producing waves that break in a manner similar to other "surfable" locations. The physical subtleties that determine a quality surfing wave may be lost in the modeling exercise and are potentially beyond the scope of current understanding of wave breaking. Actual wave response will not be understood until the reef is constructed. This will provide an excellent opportunity to investigate the strengths and shortcomings of the above model predictions.

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