TOE ANGLE: EL SEGUNDO WAVE-BREAKING ARTIFICIAL REEF

TOE ANGLE:

The toe angle of the wave-breaking artificial reef determines how quickly the wave breaks over the

reef. This is important since the quicker the wave breaks, the higher the wave will “fly” off the top of

the reef, and come crashing down on the back side. This can lead to durability problems if the angle

is not calculated correctly.

The engineers at the Surfrider Foundation tested many different toe angles. The results of their

research are found below:

Various tests were conducted with a variety of parameters.

This graph shows wave height vs. toe angle. Note the

Peaks in wave height at 65 degrees and 90 degrees.

As the toe angle increases from 0 to 60 degrees, we can see a fairly linear increase in wave height.

However, as the toe angle increases from 60 to 65 degrees, there is an abrupt jump in wave height.

This jump can possibly by relating the jump to the wave analysis program the Surfrider foundation

used. The program (REF/DIF) may produce interference. As the Surfrider Foundation’s report

states, “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” [Surfrider, 12/12/03]. As the program creates a single wave to go

over the reef, it most likely starts far offshore. Once the wave approaches the reef, the change in the

bottom topography (bathymetry) becomes sharp and abrupt, and the program did not know how to

deal with this sudden change. It possibly changed the breaking wave into several waves, and added

those waves up (since they are in phase with each other being the same wave). This addition of

waves could have produced an unusually large wave height at 65 degrees, where the bathymetry

change must have just been enough to confuse the program.

As the toe angle increases past 65 degrees, the program begins to recognize that the wave is

approaching a vertical wall. It knows how to deal with such changes in bathymetry with respect to

walls, and so we see an increase in wave height as the toe angle approaches a 90 degree wall. The

Surfrider Foundation will most likely chose an angle between 70 and 75 degrees.

FRONT PAGE