Poor Knights marine reserve
the mystery of Barren Arch
by Dr J Floor Anthoni (2007)

Barren Arch could well be the worst hell-hole in New Zealand, due to the wave forces inside during hurricane storms. In order to move and throw one tonne stone blocks through the archway and out the other end, grinding them to round marbles, requires astounding water velocities. No wonder the archway is barren inside.
  • Underwater beach: an underwater beach consisting of one tonne 'pebbles', rotated during storms.
    • Evidence: the evidence for the concept of an underwater beach
    • Water speed: how fast must the water flow to lift one tonne blocks?
    • Why here?: why is Barren Arch found only here and nowhere else? The seascape topology explains.
    • What now? Barren Arch deserves further scientific investigation including computer modelling.
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Underwater beach
Barren Arch may well be one of the most unique places in the world, perhaps even more astounding than Rikoriko Cave, for what we have here is a rotating underwater beach made up of one tonne and larger 'pebbles'. Unbelievable as it may seem, the evidence is there for all to see, to investigate and to admire.
boulder movement inside Barren ArchEver since it was first discovered, in the early years of SCUBA diving, divers have recognised it for what it is: a barren arch. Recent attempts to rename it in more gentle terms (Splendid Arch), are doing a disservice to this most unique of all archways. Already many years ago I figured out what is happening here, and since then kept a watchful eye out for changes that could confirm my conclusions. Essentially there is a beach in the sheltered northern cove, consisting of large round boulders of 0.5 to 1.5m diameter. During mild storms, these stones move down the slope outside the cave, being mopped towards its entrance at about 40m depth. During cyclonic storms (hurricanes) coming from the north-east, these stones are swept through the cave to be spit out at the shallow end. Those that don't make it, roll back inside the cave along two hollow grooves (a debris gully) on each side. In the process, wherever they collide with the inside of the cave, all life is ground off. After a large storm, one can find many new round stones on top of the more embedded ones, outside the archway in the shallow northern cove. The diagram shows the basic idea.

Let's first look at the archway with some pictures.

f012536: a yacht shows the commanding walls of the southern, deep cove. Underwater these walls are as steep as above.
f012537: a look at the deep end of the archway. It is close to 40m deep here, and large round blocks are waiting for the next storm.
f012600: looking back through the shallow end of the archway. The sil is breaking at low tide. It's usually rough here. Big rocks are spit out here.
debris gully above boulder bed
f024907: a debris gully is found at the shallow end of the cave, ground deep and hollow by rolling stones. Nowhere else around the Poor Knights such a  feature is found.
carpet sponges on scoured size of archway
f024910: inside Barren Arch, the walls show that sessile life is frequently removed and also to what height. Only carpet sponges survive.
middle berm in Barren Arch
f024918: two deep grooves return the rolling stones to the deep end, but in the berm in between, life is more bearable as one can find sessile life that survived some 10-15 years. 
upper boulder bed of Barren Arch
f012601: round stones in the shallow cove show that these are frequently moved. Life on it is sparse. There was another layer of round stones on top, but these have recently been moved to the deep end.

What evidence exists for the idea of a beach?
On a normal sandy beach, the sand particles are just the right size that they are moved by waves. The deeper the sand, the larger the waves must be before they move, and the finer the sand grains. In fact, the wave length between crests is more important as this determines the depth of the wave, whereas wave height determines its strength. Thus storms move sand from deep areas to the beach, but temporarily remove sand from the top of the beach. During fair-weather waves, the shallow sand returns. For more about this, read oceanography/beach.

The beaches we know are all around the main islands, after waves travelled over the sloping continental shelf, and in the process lost much of their energy. But here at the edge of the continental shelf, storm waves arrive with undiminished strength, enough to move unusually large 'sand grains'.

Here are some facts that point in the right direction:

Where our underwater beach differs from a standard beach, is that the ramp inside the archway conveys the boulders only up and out, whereas on a normal beach the sand moves up and down the same ramp.
How fast must water flow?
sedimentation and erosion diagramTo estimate the water speed necessary for moving such large boulders, we need to extend the work of Heezen and Hollister, as shown on right. These scientists actually measured what water speeds were needed to shift sand particles of a large range of sizes (vertical scale). We've extended the graph upward by extrapolating it, and one can see that a round block of 1m (1000mm) diameter can be moved by a water speed of about 1000cm/s (right-hand red curve) or 10m/s or 10x3600m/h or 36 km/h. The block settles down at about one quarter that speed (leftmost red curve).
36km/h may not seem a lot, but this should be the water's speed at the entrance to the cave, 40m deep. Because water velocity decreases rapidly with depth, the water's speed at the surface must be many times 36km/h, at least 100km/h, and this becomes quite respectable. We've called this arch a hell-hole, and now you may understand why.
You can also understand, that once a boulder moves up the inside ramp, it moves into shallower water, picking up water with higher velocity, until at the exit, it may travel at over 100km/h, enough to be spat far out.

Why only here?
Barren Arch's parabolic wave mirrorWhy do we find this type of arch only here and nowhere else? The answer must be found in the topology of the area. I think that the deep steep wall at the southern cove forms an almost ideal parabolic reflector to throw wave fronts back and concentrate them onto the archway's deep entrance. In doing so it could focus and amplify waves by at most three times. In the picture a black dashed curve shows the idea. It is known that vertical walls reflect waves back without absorbing any energy. Ironically, the life attached to such a deep and steep wall, experiences this as shelter, reason why an exposed steep wall like Macro City can have sensitive life growing on it.
Remember for a moment that the arch must have been formed at some time in the past, as it is not likely formed by under-earth flows of lava. It could well be that the wave reflector also created the archway in a relatively short time (say 1000 years), after which it stabilised.

What now?
Barren Arch is sufficiently interesting to be investigated further in a scientific manner. Oceanographers familiar with computer simulation of waves, could attempt to simulate the conditions here in varying storm intensities. It would be a fun project too.
In the meantime, no doubt more people will come to investigate the ideas and observations explained here. One thing is for sure, a dive in and around Barren Arch will never be boring.