Topic: Sand - Follow-ups
PLEASE NOTE: Dates recorded AFTER 18.9.2010 will be FOLLOW UPS ONLY as the Tongaporutu Project is now completed.
The follow ups will mostly be limited to the dune on the Three Sisters Beach. However, should anything major occur at any of the other locations, then they will also be recorded.
FOLLOW UPS TO THE MAIN TONGAPORUTU PROJECT
THE THREE SISTERS BEACH
6.11.2010 PHO2012-0588, 0590
Though the sand is building up, the dune still appeared to be losing ground at the gap. On the seaward side it appeared to be relatively stable.
Despite an extended period of mostly calm weather thanks to La Nina, the dune at the gap continues to lose ground and more flaxes are toppling onto the beach. As observed on the 6th November, the dune wall on the seaward side is relatively stable. Being composed of sand, it is still losing ground when exposed to high, high tides, but the rate of loss is currently much reduced.
The weather was exceptionally calm and the sand cover was very good. The dune around at the gap appeared to have stabilized. I even noticed some weeds flourishing in some of the sand hollows at the base of the dune wall. Though the dune here appeared to have stopped retreating, equally it was showing no signs of building back.
The beach between the gap and Mammoth Rock had increased quite markedly in height, but it sloped away sharply down on the Tonga River side. Even though it was a very low high tide of 2.6m, it was still high enough to keep a permanent channel carved around the back of Mammoth Rock. A small number of flotsam trees and logs were present. One tree was one that I had documented being being thrashed in the surf on the beach proper during the mega-storm of 18.9.2010.
I took a standard photo looking through the gap towards the Middle Sister. I also included two kayaks and a woman. The woman knew of my project and said that she had been visiting for the past 48 years. She said that as well as the sand/land bridge that connected the dune to Mammoth Rock, there had also been a high ‘hill’ of sand on the landward side of Mammoth Rock that she and others used to slide down. She also mentioned that though she had seen many changes, the changes had gotten worse during the last five years. I concurred with this.
On the beach proper, the dune wall, like that around at the gap, appeared to have stabilized. However, like at the gap, it showed no signs of building back, despite the beach level being very good.
20.3.2011 PHO2012-0514, 0518
Currently we are in the midst of a king high tide cycle. Today the tides are 0.0m and 3.8m respectively. Tomorrow they will be 0.0m and 3.9m, the lowest and highest tides currently possible.
Down at the beach there was an energetic two metre swell running and after a period of relative stability, the dune, particularly at the gap, was once more being carved out. There was a high drop down to the beach at the gap. This forced me to access the beach from the beach proper side of the dune. Though this was still being carved back as evidenced by some plants languishing at the base of the dune, the carving action wasn’t as severe as around at the gap.
Some logs were on the beach. The tree coughed up during the 18.9.2010 mega-storm still resided near the small stream at the gap. The beach state was good. I did notice that at least some of the medium sized roundish stones that had been washed up on 26.8.2009 remained in place. However, due to the high sand cover, particularly at the base of the dune, only a few stones were visible.
The weather had been fine and calm for some time and this was reflected in the beach state. The dune was still retreating, but the rate of retreat had slowed. All three zones on the Three Sisters Beach were exceptionally well endowed with sand. So much so that you could walk right out to and up to the Little Sister, despite it being a quite high 0.8m low tide. I could even have accessed the Four Brothers Beach, but due to time constraints and being with Josefin Carlsson and Anders Fridfeldt from the Stockholm University, Sweden, I didn’t.
Most of the rock platforms and all of the low lying reefs, particularly in Zone Two, were buried under a mantle of sand. Of the few platform tops that were visible, some desperate mussels clung on for dear life.
18.5.2011 PHO2012-0526, 0620-0621
Though it was a very low 0.2m tide, due to storm surge conditions, access beyond the Point was not possible. The sand cover in all three zones was good. However, in Zone Two, more of the rock platforms had been uncovered. I was surprised to see that they all retained their stocks of mussels, and that they were all healthy, despite being buried. I have commented further on perhaps why this is so in Section Nine on Flora and Fauna.
At the dune, more material was being lost, particularly at the gap up to the corner where it turns around to the beach proper. Due to the big tides combined with big seas, a substantial amount of vegetation and dune wall has been cut out. Flaxes, big clumps of grass and a couple of large shrubby plants had tipped down onto the beach. The 50 odd year old large pohutukawa tree that is near the corner is now in serious danger of being lost sooner rather than later. This could well be its last winter. The other larger pohutukawa tree is not safe either, but it will take longer to be dislodged.
The main dune wall on the beach proper is fairly stable by comparison to the gap, but it does continue to lose some material. It slopes to landward and this could be helping to counter more significant loss.
Overall, the dune appears to be in the process of being preferentially carved out from north its northern side. This is probably due to it being subjected to three different wave actions. (Waves coming from three different directions).
The large tree deposited from the 18.9.2010 mega-storm is still there. Presumably in kinder times this was how the sand/land bridge that originally connected the dune proper to Mammoth Rock was built. Over time logs would accumulate and eventually get mostly covered with sand. Then marram grass (now locally extinct), would bind everything together and a new ecosystem would be born. However, with sea levels rising, this is now sadly a feature of the past.
3.7.2011 PHO2012-0625, 0627-0631, 0633
Before I even entered the beach via the Gibbs’ track (the southern one down to the beach, not the northern one above Mammoth Rock, I could see that there had been substantial changes to the seaward side of the dune in particular. Down at the massive log where I used to take out my last roll of film from the camera for the day, the sand dune wall was now only a few feet away from it. In the past, you could have driven a tractor on the track that used to be on the seaward side of it. (Another tractor track also exists on the landward side of it).
The sand wall here was quite high, being around 20 feet. Plant material was evident at the base of this wall. The next surprise came when I tried to get to, then around the first of the two large pohutukawas located on the gap side of the dune. I couldn’t! For the first time ever, I had to track through some lovely karakas to get to the rear of the largest pohutukawa, the one closest to the stream. Then it was a very short walk before dropping down onto the beach.
The other thing I noticed was that the large tree that had been reposing at the gap since the 18.9.2010 mega-storm, had been booted out at long last. It now resided closer to the Tonga River. It will mostly likely eventually end up on the Pilot Point side of the river.
Since May there had been huge changes. The corner that separated the gap from the main beach side of the dune had been completely cut out. This had effectively ‘straightened out’ the dune. This now had the effect of increasing the damage to the seaward side of the dune. Up until this point the gap side of the dune was suffering more severe erosion or carving action than the seaward side of the dune. Now that they are more or less aligned, rather than being at right angles to each other, the entire dune wall, was now subject to highly erosive carving action.
Of the pohutukawa trees, the largest one nearest the stream is about 30 feet away from the dune wall, while the neighbouring one is about 10 feet from the dune wall.
At the gap the dune wall is relatively low in height, being around seven feet. On the main beach or seaward side it climbs in a roughly steady line until it is about 40 feet in height at the point where it meets up with the ‘hard’ cliff face. It’s average height on the seaward side is around 20 feet and for the most part it is a sheer sand wall. Tree roots are quite prominent down at the lower part of the dune on the seaward side. (Heading back towards the gap).
At the gap I also noticed old tree logs sticking out of the newly eroded sand wall – it being in a constant state of retreat. These logs could have been deposited there decades ago, or even longer.
Zones One and Two. The beach state in these two zones was good with high sand coverage.
Zone Three. This zone has good sand coverage at the cliffs but lower down, rock platforms are very prominent. From roughly the middle of the beach onwards, there has been heavy sand scouring in a from what I could see, wide V formation. Sand appeared to be built up further out, but due to scouring in the middle and at the platforms out from the Point, access to the Four Brothers Beach wasn’t possible.
17.7.2011 PHO2012-0527, 0531, 0637, 0639-0640
Another mega-storm stampeded across the country during the past week. Unlike last September’s mega-storm, this beast has had a catastrophic affect on the coastal dune on the Three Sisters Beach. Also, the beach proper in ZONE oNE (I didn’t access Zones Two or Three), had in many places been scoured right down to bedrock.
Normally, I access the Three Sisters Beach via one of the two of the Gibbs’ farm tracks. Pirani had warned that both of the tracks lower down had gone, but I might just be able to get down to the beach using the barn track.
Before going down this track, I headed over to the cliff-top that overlooked Mammoth Rock. This was where the other track was. I was gobsmacked by what I saw. The two large, 50 year old pohutukawas were languishing on the beach at the gap. I estimated that about 50 to 60 feet of the dune here had gone since my last visit a fortnight ago. Further around on the main beach side, this too had been cut back by around 20 to 30 feet. I found it hard to believe what I was seeing. I thought the nearest pohutukawa would be lost this year, but also thought that the other one which was around 30 feet away from the dune wall would survive to see another winter. Wrong!
I then went along to the barn track. About three quarters of the way down, the rest of the track had gone! In its place was a sheer drop down a 40 foot steep wall of sand. The dune here had cut right back to scrubby shrubs. There was no way I could access the beach from this track. I returned to the other track and carefully made my way down.
The bottom ladder had gone. It had ended up over at the Pilot Point dune. At the bottom the only access to the beach was down a slippery, wet, muddy bank that was about 12 feet in height. After a big struggle I made it down to much cursing as my tripod and hands were dirtied up. Fortunately, the stream which had now been cut right back, provided washing water.
As well as my big camera I also had my 10 megapixel digital camera to take some quickie images.
Now, back to observations. At the gap, so much of the dune had disappeared that a small horseshoe type bay had formed. Presumably the visible part of the cliff here was originally a sand free promontory that curved in to landward. This in turn acted as a natural driftwood trap that enabled the dune to be built in the first place.
As for the level of destruction, the gap part of the dune continues to be more severely affected by wave action, but the dune wall on the beach side is catching up. The stream which mostly exited past the Tonga River side of Mammoth Rock, now mostly exits into the sea on the Three Sisters side of Mammoth Rock. This is due to its entry point having being changed.
At the gap side of the dune, the dune forest all sits atop a sand base, (as does presumably the rest of the forest). Several streams are cutting through this, adding to the sea’s wave action. Before being exposed, these water sources entered the beach either at beach level or wicked up from beneath it. (Not sure on this). Now they appear to be flowing down from different points in the newly opened up ‘sand walls’. I see this as a feedback mechanism that will accelerate the rate of loss. Thus, the more mass that is lost, the greater and faster the rate of loss.
Further around, the sand wall has risen in height to around 40 feet at its maximum height where it meets up with the cliff wall proper, this being near the first arch.
In conclusion, the whole of this dune, along with its coastal forest and shrubs, will be totally destroyed. Nothing can stop it now. Moreover, the direction of destruction will continue to be ‘handed’ in that it will track in a north to south direction.
One might think that the loss of the Three Sisters Beach dune would be the Pilot Point dune’s gain. No. Though there is currently a lot of driftwood at the Tonga baches end of the Pilot Point dune, this dune itself, though it appears to have suffered little damage, equally, it is not growing at the Three Sisters Beach dune’s expense. If anything, the part of the dune that supports marram grass appears to be shrinking, while the bare high beach part, resplendent with beached logs, seems to be expanding.
Returning to the Three Sisters Beach, the beach proper in Zone One was deeply scoured out in the middle, V section. This has revealed the petrified remains of 5,000 year old totara logs.
By the time I had finished, I was totally buggered. And I still had to try and clamber back up the twelve foot slipperly bank that I had slid down on my bum. I remembered one of Bear Gryll’s ‘Man Versus the Wild’ television programmes where he used tree roots to climb up out of a steep river gorge. I tested out a long trailing tree root for strength and used it to haul myself up onto a overhanging wedge of wet soil that I hoped would support my weight. Fortunately it did, but in future I will have to access the beach via the Reserve.
This is something that I need to add based on my observations to date. Just as sea caves are created/carved out from a right to left, (south to north) direction, and destroyed in a left to right (north to south) direction, so too apparently are sand dunes. At least this appears to be the case with regards to the Three Sisters Beach dune.
It is easy to say that rock stacks are created when the roofs of sea caves collapse, or that storms can destroy sand dunes. These statements say nothing about the mechanics involved. My observations, both written and photographic, do I feel, go some way to showing what actually occurs.
29.7.2011 PHO2012-0532-0534, 0537-0539, 0542-0543, 0545, 0547-0549, 0551-0552
Normally I wouldn’t have returned to Tonga so soon, but I particularly wanted Gary Bastin and Glyn Church to see and record this, what I believe to be a once in a lifetime event. Specifically, the impending complete destruction of the Three Sisters Beach dune and its coastal forest.
At the dune, little appeared to have changed since my last visit. It was still losing material, but at a much reduced rate after that mega-hit.
The beaches in all three zones had been scoured out down to bedrock. However, the beach at Zone One had been scoured out to a level I have never seen before. Lots of small stones were present on the parts of the beach that did have some sand cover.
Large boulders were present in the middle or V section of the beach as well as bedrock. The ancient log that I saw on the 17th, was still there, but because the beach had lost more material in the intervening two weeks, more fragments of the original forest had been revealed, and in such amazing detail for the first time. Also, where the high part of the dune sand wall meets up with the eroding cliff, ancient tree trunks and smaller wood fragments were also revealed.
14.8.2011 PHO2012-0556-0557, 0559, 0562, 0566-0569, 0572-0574, 0576, 0579-0580
Though I had come up specifically to photograph the dune forest from the cliff-top for a photo competion being run by the New Zealand Geographic, I left myself open to other photographic opportunities.
Despite the awful conditions (wind-blown sand and freezing temperatures), I could see that the sea was still carving chunks out of the dune. Down at the bottom of the track, as I only had my digital camera on me and no cumbersome tripod, I managed to clamber down to the beach.
Zone One. (I didn’t access the other zones). The sand level appeared to be building up where the dune met the beach. Also, at the gap, the sand level was noticeably higher on the dune side than on the Mammoth Rock side of the gap. In the middle V section, sand islands were present as sand was beginning to reclaim its territory. The ancient log and other tree remnants and rocks were still mostly visible though. On the seaward side of the V section, sand level was good.
The two pohutukawa trees were still on the beach and more plants continued to tumble down the sand walls. The sand walls themselves were obviously highly unstable, but particularly so where they were very high.
I haven’t visited Tonga for eight months due to a hip injury. Also, as the main project is finished, I only plan to visit intermittently to document several key elements. One of which is the Three Sisters Beach dune.
The two pohutukawas are still on the beach, but are long dead. They’ve also been more smashed up. The dune appears to have been eaten right back to the cliff proper, whereas it is still in the process of being eroded away further around and continuing to its southern cut-off point. This is because the sand dune height/coverage is much higher at the dune’s southern end, it being quite low in the vicinity of the gap where the sand/land bridge was.
It was interesting to note that at the northern end where the stream feeds down onto the beach, a ‘forest’ of water-cress is now thriving in the water enriched, sodden soil/cliff material environment. Plant material, particularly the further south along the dune you go, continues to be lost. The surviving shrubbery has for the most part, severe salt/wind burn.
The dune shows no sign of recovery at the gap and I don’t believe the sand bridge will ever build back, despite current high sand levels, due to rising sea levels.
This is another time frame on the continuing story of the dune, specifically, the gap separating the dune from Mammoth Rock.
While working on the Tongaporutu Project, I was also working on a spacetime hypothesis. One of the things it seemed to imply was that if ‘something’ lost a third of its mass, then providing the causal agent(s) don’t radically change, the remaining two thirds is unstoppable. Specifically, the third is a tipping point (The opposite equally applies).
Of course, I don’t know if this is true or not. I needed a real world test. The Three Sisters dune fitted the bill. During the Super-Storm Event of July/August 2008, it lost at least a third of its mass (sand/land bridge). I postulated at the time that due to sea level rise/climate change, the remaining two thirds would be unstoppable. Moreover, the rate of loss would accelerate. I believed that the Three Sisters Beach dune would be mostly destroyed within five years. So far as of December 2012, this is on track.
Of course this one third tipping point doesn’t just apply to sand dunes. It applies to everything, including us.