Coastal Creationism - Part 2: Complex curves
What are the fundamental shapes of epic reefs?
As we discussed in the last article, creating epic surfing reefs requires a lot of critical factors to come together by chance. However, when they do the result is a rare natural wonder - a perfect surf reef! Surfers know them by sight, and from an entire watery planet only a small selection of truly world class reefs seem to reappear in the photos, movies and webclips, each with very distinctive characteristics: Pipeline, P-Pass, Cloudbreak, Teahupoo, Mavericks, Trestles, The Right, North Point etc.
There are many reefs that can get really good but are not at that ‘world class’ level. They can be fickle, have slow sections, shut down or pinch closed too often. They may have some good waves, but not many “perfect” waves. However it seems that many of these good reef breaks are surrounded or close to other good reef breaks. The North Shore of Oahu is often described as the ‘7 Mile Miracle' because the density of quality reefs lined up one after the other along a short section of coast. Of course it also helps that the coast catches all the good swells from a variety of consistent swell windows and is offshore in the prevailing trade winds. Clearly there is something special that has shaped the undersea contours so that more often than not, swells are tripped up in to steeply plunging waves that peel at excellent angles.
Referring to the North Shore as the 7 Mile Miracle acknowledges the unlikeliness that Pipeline (First, Second,and Third Reefs), Backdoor, Off The Wall all occur in the space of a few hectares. Then of course there are all those other spots, Rocky Point, Sunset, Velzyland, Waimea Bay, Chuns, Jockos, Alligators, Laniakea, Haleiwa.
The 7 Mile Miracle is not just a function of swell and wind, it is also a geological marvel. The basalt lava flows spreading out from the the now extinct Oahu hot spot volcanoes were of just the right thickness and shape to result in convex shaped lozenges of rocky reef oriented at good peel angles to the prevailing swells. Nature has further contributed with coral reef growth and even occasional sand banks.
The lessons we have learned from Artificial Surfing Reef research is that reefs with flat gradients become less interesting with larger swells. That is, a reef or beach that is dipping at the same angle throughout the surf zone could produce good steep waves on smaller swells but it will produce fat, slow, spilling waves on bigger swells. This is because the plunging wave intensity is proportional to the beach steepness, i.e the steeper the gradient of the reef, the steeper the plunging wave.
However, of greater significance is that plunging wave intensity is also inversely proportional to the square root of the wave height (The Irribarren Number). This means that the bigger the wave the less intense (steep and hollow) the plunging wave for a given reef gradient. The period of the swell is also significant but we will leave this discussion for another time.
Based on the relationship between reef gradient and wave height a little bit of applied mathematics reveals that optimal reef shapes are convex. That is, they curve upwards with the steepest face of the reef in the deepest water, and the dip angle of the reef becoming flatter progressively into the shallower water.
Sketch of convex reef profile for an artificial surfing reef. Waves approach from the right (Image courtesy ASR)
As surfers well know, waves will bend around a reef, and as they do they trace the curves of the reef. The wrapping effect is caused by refraction - when waves travel faster in deeper water and slower in shallower water. In deeper water this can ‘lens’ the wave, magnifying wave heights resulting in some spots enjoying bigger and better waves than others (more on this later).
When we look at the section of the reef that is creating the breaking wave, the unbroken wave is still in deeper water and travelling faster than the breaking section, bending (wrapping) the unbroken wave toward the reef. This also has the effect of reducing the peel angle, creating a faster peeling section.
If we look at the ‘orthogonal’ of the wave as it breaks we are looking at the perpendicular to the breaking waves crest. If a swell wraps in toward the reef, then the orthogonal of the wave is actually a curve toward the shallowest part of the reef. So the convex shape of the reef is not based on a straight cross section; it is the water depth as the wave orthogonal curves toward the shallows.
Reef gemoetry is a complex interplay of curved surfaces that intersect with wave energy, and when describing it we run the risk of sounding like a bunch of shapers standing around talking about why their boards work (or don’t). Despite the different disciplines, there are many similarities as complex curves comprise the boards we ride and they also give shape to the waves themselves. In fact, some shapers are actually making the leap from shaping surfboards to actually shaping waves, such as the work of Webber Wave Pools.
(Image above is of the proposed artificial surfing reef for the City of Albany. It appears courtesy of Royal HaskoningDHV)
Coastal Creationsim is an eight part series written by Chris Buykx. Chris Buykx 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 Part 1: Basic Reef Shapes
Comments
super interesting. probably worth mentioning that another great asset the north shore has is lotsa o freshwater runoff. The freshwater kills the reef which makes a great channel and helps explain why there are so many different breaks in such a small area which would otherwise be mostly closeouts. Most famously waimea bay (inside the reef that causes the wave to break) has no reef from the waimea creek (thus giving the bay its size holding ability) and the channel between ehukai and pipe is also due to a freshwater gulch which drains out there.
Macaronis is a pretty flat reef, among others. Not quite sure how the reef angles off outside the take off zone or how the the deep water bathymetry manipulates the swell as it moves into the bay though.
Like many rock/coral reefs, it has those mini channels (Ulu's is a good example) that allows water to draw through the reef without impacting wave quality.
Convex reef forms may be the theoretical ideal but there appears to be many remarkable exceptions to the rule where reefs are inclined and relatively flat.
Anyone dived around Cloudbreak?
Looking forward to No 3.
I have a suspicion, and Chris will be able to correct me if he's in internet range, that the most clearly convex part of the reef occurs beyond the break, out where the ocean swell feels the bottom and begins to take shape. Hence, anywhere where the waves are broken - and where you might dive on a small day - is flatter and more of a platform anyway.
Good point about the channels in tropical reefs. I'd never thought of that before, and yet it's seemingly obvious why they'd help wave shape.
Yes have thought about the channels along certain reefs as well, allowing water to draw out as you say, more than if it were just a totally flat and non-undulating bottom. Very interesting.
I've noticed channels running more or less perpendicular to the waves as well in Samoa and the Maldives (I even got jammed into one in Samoa).
Does anyone know if any artificial reefs have used them?
If you dive off what we'll call 'second reef' cloudy, it basically drops off a vertical wall into the abyss.
I have noticed that you changed the picture from a relatively unknown reef (difficult to access) to a much better known one in the pacific :)
Figured it's probably best not to draw attention to that reef, Fitzy. I think people with boats will agree.
Nice one.
I was going to say something yesterday but didn't want to do exactly that, bring attention to it. Delicately played Stu :)
I am on the edge of range today so can just pick up enough signal to read and respond. I am actually working on the Coral Reefs story now - but there is a lot of other important processes to discuss first. Channels in the reef are indeed very important to wave quality - and almost unique to Coral reefs as they created by living organisms in response to their environment. But more on that in later articles.
Stu is correct in that the most visibly convex section of the reef is usually the deeper part, outside where the biggest waves break. However keep in mind that a reef can appear flattish - but if the wave wraps on to the reef at an acute peel angle, the orthogonal can climb up the reef along a more convex profile than would appear in a straight cross section
Great reading. The corollary (for me at least) is that we are able to 'fill the gaps' or fix the 'shutdown' sections of the multitude of potential coastline in the city areas. May be jumping the gun here but can't we use this knowledge to 'modify' the many possible points or reefs that we have around the populated coastal cities. Hence avoid the strain or demand we get on our 'miracle' breaks ?
Glad to read that you're looking at (and trying to explain) wave mechanics in a 3D way. The amount of times I hear and read about wave formation in a one dimensional way, even from people who should know better, it's infuriating. ASR have got a tonne of info on reef architecture, they understand reefs as well as anyone on the planet, and they STILL couldnt replicate one with any kind of success. That alone should illustrate how complex they are.
And further to the comments above from Chris and Stunet...a large part of wave formation happens beyond the break, not directly under where the wave is cresting. Again it's something rarely understood by people who should know.
I'm looking forward to reading more.
One reef I know can turn 2 ft of windswell into 4 foot powerful barrel. It has a spine of rock that sticks out perhaps 100 metres out to sea that focusses the swell (as pipeline does on a bigger scale). The depths and angles have to be right of course but in my view man made reefs of high quality would not be that hard to make in technical terms with unlimited dollars. Why it is so hard, is that the volume of rock / geotextile sand bags needs to do the job properly is massive. ASR have only been using perhaps a 5% of what is really needed to create the steady rise from way out to focus swells into a peak and then lay down an accurate gradient for long enough. Next time you surf a really good reef try to picture the volume of rock / coral below you that makes up the whole structure -especially if it works on all tides. If you stacked all the geotextile bags need to create say North Point on dry land you would have a small hill.
If the researchers could harness longshore drift and natural rip actions to build sand around a smaller structure, nature could do 90% of the work. This would require a lot of testing but I could imagine an underwater shape would exist for example that would create a permanent rip flow that would sculpt a good wave in certain situations and not cost too much.
Thats what perth needs, a copy of Gallows near Gracetown.
Yes this is super interesting stuff indeed, angles of the dangle, swell directions, channels in a reef, currents this a way currents that a way.
Ok we have all seen study after study and still nothing Ever gets actually built. I come from a construction background why the fuck can't someone get an excavator cut a track down to some shit point or somewhere with potential. Bring in the rock with trucks then do it !!!
You can get an excavator with gps that will be accurate to about 20mm , this is not really scientific stuff we cut batters on the side of highways accurately to maybe within 50mm, even in Rock. I've built coffer dams and had pumps running to keep the water back so we can safely work on the other side. Everyone knows of some spot that just need a bit of rock to fill in that hole or connect that bit to another.
Your not talking millions I'm talking in the hundreds of thousands
Excavator for the day $1500
Truck $900
Rock per load $1200
I'll operate the digger for free I'm sure someone else in Australia will also donate some time.
When can we get started ?
I Know a spot to make 150 mtr left .Already setup just needs rocks to fill the hole & dozer to push shit loads of sand ,rip will do the rest ,mainly a fishos beach as other Known breaks r further south. I,ve been joking about it for ages but when u you put it like that its defiently feasible !!!!Lets get to work....
We can start as soon as the $100k enviro impact studies are completed
That's concervative udo..............I would have put 10x that! Then there would be all the gov departments checks, taxes and levies.............Community consultation processes.......... the list goes on & on. The construction would be the easiest and cheapest part! :)
The cheapest surf spot to create would be carving an angled slice out of a pre-existing cliff face to create a rebounding wedge peak. Technically simple. Drill and blast. Thousands of suitable headlands all around the world where this would work. The slice needs to ceate a surge up onto the cliff and then have an overhang to catch the surging wave and re-direct it sideways to the beach line. Off go the rebounds to create peaks as they intersect incoming waves. Dbah peak perfection. The notch could be at the point of the inner sand bar to create a wave right there. It would also work if done say 100 metres out along a cliff where the waves have built up some height as they near a cliff face and currently just surge up and dissipate their energy randomly. Out there the right shape would project a series of peaks way down the beach.
Environmental impacts statemrnt would be required of course but the impact on the sandflow and environment would be minor. After few years the notch would be weathered and blend in with the coast. Sand flow impacts would be minor and very localised. It would only need to be about 20 metres of cliff affected. Groynes could also be modified to enhance their rebound effects.
Will it happen? Well if a buck could be made out of it yes, but only through some farsighted coastal town who wanted to invest a little to create more surf tourism.
Removing a small slice off a cliff is sooo much easier than building a complex reef with all the issues of sand infill or scouring issues, geotextile bags moving, sinking or breaking up. Do it once and it is done forever.