How constantly growing and changing coral can create the perfect reef profile, with good wave climate and idyllic weather too!

In the last article we investigated the ways that coral reefs form and adapt to change. We will now extend this to the structure and shapes of the reef and what this means for waves. In the process we'll look at the Pacific’s best known coral reef breaks, Palikir Pass (P Pass) and Cloudbreak. The fundamentals of world class surfing reefs apply as much to coral as they do to rock reefs, sand points, and the full variety of surf. As discussed in Part 3, The Good the Bad and Ugly of Coasts these are:

• Coastline or coastal features aligned at a good angle to the prevailing swells to create an optimum peel angle. Cloudbreak forms a west facing broad arc of reef that meets the predominant SW swell-trains at an optimal peel angle. Similarly P Pass does the same in the Northern hemisphere with NW swells 
• Coastal topography aligned at favourable angles to prevailing winds so as to maximise offshore and minimise onshore winds. Cloudbreak is offshore with east-southeast trade winds. P Pass is the same with east-northeast trades, although both struggle when the wind swings more south or north of east.
• Geology of reef-forming rocks that will form convex lozenges of reef aligned at good peel angles to the prevailing swells. Nothing does convex lozenges better than coral!
• Geology of reefs not vertical, too blocky, or otherwise chunky that would interrupt the formation of good reef gradients. Coral reefs can create random bombies and crags, however their adaptability is better that any other at smoothing the chunks.
• Offshore features that create favourable seafloor contours so as to lens prevailing swells on to the reef at good amplitude and peel angles. Barrier reefs have a habit of growing on the edge of very deep water, allowing the full energy of the oceanic swell to wrap onto the reef unaffected. Other fringing reefs in shallower water can enjoy the focussing of swell if offshore bombes are in the right position.
• Lastly, a coastline slightly oblique to main swell train is also very helpful. P Pass is on the north-west of the Pohnpei barrier reef so the predominantly north swell trains wrap around the reef before they finally detonate on the fabled deepwater pass. Not so for Cloudbreak as it's the first thing a swell hits as it rises out of the Coral Sea.

Looking more closely at how reefs grow and adapt, the growth of hard coral is influenced by two main factors. The first is sunlight - a function of clean, clear water - and the second is wave energy, and this is the bit we really like!

Considering the role of sunlight, if you take it away, coral stops growing and eventually dies. Most surfers are aware of the relationship between fresh water outlets and reef passes. The process is simple: a creek or river brings nutrients and sediment into the surface waters, resulting in algal growth and turbidity, cutting off sunlight to the corals and stunting their growth. The result is a gap in the reef and a chance it may form an epic pass. This is easy to understand in the case of Teahupoo, with the valley and the river flow related directly to the reef pass. But what about P Pass situated five kilometres offshore? I think once a reef pass is formed by freshwater, nutrients, and turbidity, the chances are it will persist for a long time. Tidal currents and wave energy will keep the pass open even as the reef grows and the island subsides. This relates to the second factor influencing coral growth...

Wave energy is significant in the way it controls the type and growth of the hard corals - it shapes the reef. Also, wave energy drives the creative destruction of the coral reef, breaking and tearing off chunks, adding rubble to the pile and creating new space for corals to grow.

A coral reef is a construction of living and dead elements. The carbonate limestone skeletons of the corals pile up over time and provide an excellent shallow water platform for more coral to grow. This happy situation means that coral reefs can adapt to relative changes in sea levels. Hard corals can add anywhere between 2mm and 10cm of carbonate exoskeleton each year. That's a whole lot of CO2 taken out of the ocean (and atmosphere) and locked up in limestone. 

The coral reefs of interest to surfers are fringe reefs, barrier reefs, and atolls. All are formed in the same way and in fact there is a progression from one to the next. The simplified process begins when lump of land appears in tropical waters. Most commonly this will be a volcanic island created when either a hotspot or a subduction zone causes magma to push through the ocean crust and start building up in to an island. However, larger land masses could move into equatorial waters due to tectonic forces, or even ocean currents delivering warm waters to more southerly islands and coasts, for example Lord Howe Island (31’55” south) in the Tasman Sea and Houtman Abrolhos Islands (28’50” south) on the west coast. Both have active coral reefs growing despite being well south of the Tropic of Capricorn.

The shape of the reef at any point is a function of the shape of the substrate, the type and growth of the corals, and what happens to the coral when it dies.

A typical cross section of a coral reef from deepest to shallowest begins at 80-100m depth on the the fore reef or reef slope. This zone may be a near vertical wall or a shallower slope. Moving up the reef into shallower water and higher wave energy of the fore reef typically sees abundant massive corals as well as slopes of coral rubble. If the reef crest collapses in shallow water it will build up in this zone, filling holes and creating additional platform for massive corals to grow up toward the light. This is the most convex part of the reef and will feel the energy of the wave base as it rises out of the deep water. However, the wave doesn't break on the fore reef, that happens on the reef crest which is relatively flat.

The reef crest is built up by massive and platy corals and receives abundant sunlight and clean water. The wave energy will break off corals and push them up on to the reef crest. Occasional these build up in to a cay, or it washes over into the lagoon or back reef. The reef crest is interesting because wave energy also helps the formation of channels and grooves, and these usually follow the orthogonal of the wave as it climbs up on to the reef. Channels and grooves can be very helpful in directing water back out to sea. The limited and directed flow along the channels and grooves into the base of the wave as it climbs up on to the reef enhances drawing water up in to the plunging lip. i.e.: thicker steeper barrels and the ultimate example of this process, Teahupoo!

Not all coral reef waves are formed by passes, some are formed by a bend in the reef, setting up an ideal peel angle as the wave wraps on to the reef. These bends in the reef are of course a natural result of a barrier reef encircling an island. Waves wrapping around a bend in the reef do not end in a deep channel so there is much higher chance of surfers getting pushed into the shallows of the reef crest.

Red Bluff and the Gnarloo reefs are fringing reefs, growing on fossil coral reef formed from an earlier high sea level. The reefs that form the perfect waves at Red Bluff, Turtles, Tombstones etc are hybrid reefs created by both living and fossil corals. The nearly horizontal beds of reef and beach rock limestones are a function of varying sea levels over the last few hundred thousand years, however the combination of ancient coral reef limestone slightly higher than current sea level, a cape with deep water offshore, and the infilling and/or rounding effects of living corals and recently formed limestone beach rock all contribute to these well graded and aligned reefs.

When we consider just how good reefs are, keep in mind that our corner of the planet is blessed with the greatest diversity of corals on earth. The Great Barrier Reef and Papua New Guinea regions average over 300 species of coral. Indonesia and Fiji are almost as diverse. This variety is awesome when compared to, say, the Carribean with just 20 species of reef forming corals.

Coastal Creationsim is an eight part series written by Chris Buykx. Chris is a geologist, traveller and lifelong surfer. Specialising in eco-tourism, his passion is interpreting nature and the environment. Chris is a resident of Sydney’s Northern Beaches though he's currently doing a lap of Australia with his family. Read past articles:
Part 1: Basic reef shapes
Part 2: Complex curves
Part 3: The good, the bad, and the ugly of coasts
Part 4: Sedimentary sequences and superior shapes
Part 5: Sand dunes and limestone
Part 6: Slabs!
Part 7: Coral reefs are even better than you think!