SURFING AND ARTIFICIAL SURF REEFS

To date, Four artificial surfing reefs have been constructed:

•       Narrowneck, Queensland, Australia

•       Cables, Western Australia, Australia

•       Pratte's Reef, El Segundo, California, Los Angeles, USA

•    Bagarra, Queensland, Australia

•    2 more reefs have been commenced

                                              ** Full comparison

Of these, Narrowneck, although primarily a coastal protection structure, has the most consistent swell conditions and surf.   

Figure 1 : Narrowneck Reef

Surf reefs can only enhance natural wave conditions but with huge media promotion, there are expectations of perfect waves at all reefs.  Surf conditions are a product of wave shoaling and breaking, which are complex physical processes in a highly variable environment.  Thus, they are difficult to model accurately and at best only a limited number of combinations of idealised conditions can be modelled and these may occur only rarely.  There has been a lot of hype about designing the perfect surf reef with complex and expensive computer models, but improving surf quality is not difficult, provided the wave conditions [height, direction and period] and wind conditions [direction and strength] at the site are suitable – i.e. if there is only a small, short period wind chop and / or a strong onshore wind, the perfect seabed or artificial reef will have limited benefit.  Secondary variables to be considered include:

• Natural seabed shape and material

• Accessibility of the site

• Available construction equipment, materials and expertise. 

There are numerous cases where surf quality has been improved as a by-product of coastal engineering works such as groynes and nearshore nourishment where the works change the local seabed bathymetry causing waves to break earlier and progressively, for example the Kirra Point groyne and the “superbank” from the Tweed River Entrance Sand Bypassing (Figure 2).  In other cases, the wave height and / or angle have been improved by large structures, for example The Wedge in Southern California where a curved groyne amplifies the wave height, although these tend to result in a dangerous extreme-type surf.

 Figure 2: Dredged and pumped sand “Superbank” at Coolangatta

Surfing is a complex and highly variable activity.  The following parameters determine the surf characteristics and “quality”:-

• Wave height and period
• Breaker type [spilling or plunging]
• Peel angle

Wave height and period both influence the 'quality' of the surf.  Wave transformation during the shoaling process is complex – the wave period remains constant, but the wave speed decreases and the wave height increases as the wave interacts with the seabed.  The higher-speed swell offshore can typically only caught by tow-in surfers, but as shoaling slows the wave down, surfers can generally catch the wave without being towed - although this depends in part on the surf craft and the level of experience of the individual surfer.  An artificial reef, by affecting the shoaling process, can influence wave speed and height at the break point. 

The type of breaker is very important as different breaking types suit different types of surf craft and levels of experience [Table 1], and the “perfect” surf for one group of surfers may not be suitable for another group [Jackson et al., 2001], for example waves at the spilling end of the range are safer for beginners and slower surf craft but may not be suitable for all craft and while a body surfer will have difficulty catching a spilling wave, a surf ski will not.  The breaking type also relates to the difficulty of the take off and thus the ability and experience of the surfer.  This means a steep take-off on a fast plunging breaker is more suitable for expert surfers whereas a flatter slope will provide a slower spilling breaker that means an easier take-off for less experienced surfers and a safer ride.  The skill level of the surfers targeted and the need for safety for all surf users has a large impact on what is the “perfect” surf for any particular location.  The local wave climate will also restrict what is practical - an area with a short period and low wave height will never be able to produce a large hollow plunging “pipeline” type wave.

Table 1: Type of surfing vs. Type of breaker

Peel angle (Figure 3) is related to the wave breaking location, which is determined by water depth and wave characteristics.  Generally, waves start to break when the wave height is approx. 1 to 0.7 times the water depth.  Thus, a 1m wave will break in approx. 1 to 1.4m of water.  If the seabed at this depth has contours which are parallel to the direction of the wave crests, the wave will break almost simultaneously along the crest and the wave will 'close out' [i.e. peel angle is close to zero].   If the wave encounters a variable depth along the length of the crest [as is the case for oblique swells, desirable bar formations and natural or artificial reefs], the wave will break at the shallow point initially and then progressively along the crest as the wave moves onshore (Figure 3), creating ideal conditions for surfing. 

 Figure 3: Peel Angle

 A natural or artificial reef can improve surf quality and the occurrence of surfable waves by:

• Increasing the wave height at the break point.
• Influencing the breaker type.
• Promoting a progressively breaking rideable wave [peel angle].

In the design of any reef, the location and wave characteristics created are important but the final design is more social science than pure physical science as any works need to suit the existing or desired local environment, culture and economy.  For example, in considering the final reef design for Narrowneck, the abundance of good quality point and beach breaks in the area and the popularity of these area with medium level surfers on day trips from the nearby state capital was considered. 

It is interesting to note that the world’s most popular surfing reef is probably Waikiki (Figure 4) where long period spilling waves on the scattered nearshore reefs attract many more surfers than the nearby legendary hollow plunging surf breaks such as Pipeline and Waimea Bay.  Surfing using a variety of surfcraft is definitely an activity associated with Waikiki where casual, inexperienced or learner surfers can participate in the sport safely.   

Figure 4: Spilling reef break at Waikiki

While artificial reefs can serve the dual purpose of beach protection and improved surfing conditions, the importance of each of these objectives (and a number of other factors) will have an impact on the final design for a specific location.  Typical Reef Shapes.  [Note : This diagram presents a number of design options appropriate for a high wave energy coastal environment with a longshore transport and well-developed offshore bar system, as found on Australia's east coast and Southern California.]

Narrowneck Video - Beach, Surf & Dive

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