Surfrider Foundation: Mitigation Through Surf Enhancement

CHAPTER 3
(Page 4 of 7)

RESULTS:
For each set of reef parameters (nose angle, reef height or depth, toe angle, and offshore distance) and REF/DIF parameters (period, amplitude, and direction) several model simulations were run to test the sensitivity of wave response to a reasonable range of values. Although the only explicit output from REF/DIF 1 is wave height, I have defined several additional criteria to determine whether or not the waves have been enhanced. These criteria are maximum height, distance offshore, wave shape, down-line velocity, and length of ride.

(1) Maximum height is defined as the largest wave height anywhere in the model domain. The distribution of wave heights along the edge of the reef can be further examined through a series of cross-sectional plots [See Figure 3.7].

Graph of wave as it progresses along the reef

Graph of wave as it progresses along the reef

Figure 3.7: Plot of wave height at successive shoreward steps along the edge of the reef. This view demonstrates the waves response to the reef as the crest progresses down the edge of the reef. Note that wave height increases, this is due to increased shoaling as the water depth decreases.

(2) Distance offshore is determined by the distance of the nose of the reef in the cross-shore direction to the zero contour on the depth grid.

(3) Wave shape as defined by Galvin ( 1972) is:

where Hb is maximum height at breaking, g is the acceleration of gravity, m is the beach slope (reef slope) and T is the wave period. According to Galvin (1972) , a value of K > 0.068 represents a spilling or mushy wave. A value of 0.003 < K < 0.068 represents a plunging or "tubing" wave and a K < 0.003 represents a surging wave [See Figure 3.8].

Wave types as defined by Galvin

Figure 3.8: Wave types as defined by Galvin (1972). These wave types also correlate with the Iribarren number.

This parameter was calculated two ways for each model run. One calculation was made using the slope of the shoreface which is 0.03 for all model simulations. Another calculation was made using the toe slope of the reef. These are labeled K1 and K2 in Table 4.

In attempt to further explore predicting wave shape from model results, I also calculated the Iribarren number, or the surf similarity parameters, which is known to describe breaker type (Battjes, 1975). The Iribarren number is calculated as:

where m is beach slope, Hb is wave height at breaking, and Lo is the deep water wavelength as calculated by linear wave theory. An Iribarren number > 2.0 predicts a surging wave. An Iribarren number value between 0. 4 and 2.0 predicts a plunging wave and an Iribarren number < 0.4 predicts a spilling wave (Battjes, 1975). Both wave shape parameters are relatively simplistic descriptors which omit many obviously important variables that effect wave shape, probably the most important being wind. I decided wave shape was criterion worth investigation, despite these obvious limitations, because of the importance of wave shape to the quality of surfing waves.

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