Colour fronts in False Bay

Colour front seen from Sunny Cove on 27 November 2015
Colour front seen from Sunny Cove on 27 November 2015

Perhaps you have wondered what causes the patterns of strange coloured water in False Bay during the summer months. Perhaps you have dived in it, and wondered why sometimes you can’t see your hand in front of your face! Wonder no more – I am here to help.

Colour fronts

Frequent visitors to and residents of the shores of False Bay will observe that at certain times of the year, the ocean is marked by bands and arcs of sharply contrasting coloured water. This phenomenon is known as a colour front. In oceanography, a front is the interface or boundary between two separate masses of water. In this case, the water masses are easy to discern, because they are of different colours. There are usually other characteristics of the water on each side of the front that differ, too. Fronts are either convergent (the water masses are moving towards each other) or divergent. The presence of marine debris (like pieces of kelp) at the front boundary suggests that it is convergent.

Causes of colour fronts in False Bay

Prior to 2005, there was much conjecture about the causes of these fronts (including the usual pollution bugbear), but little evidence to support any of the theories. By sampling, the fronts were found not to be caused by pollution, or by plankton blooms in the surf zone. It was known that a colour front was most likely to occur in False Bay after a period of southerly or south easterly wind lasting a few days. October and November seem to be prime months for the phenomenon.

When a large, obvious colour front arose near Simon’s Town in November 2005 with milky green water on one side, and darker blue-green water on the other, researchers from UCT and IMT sprang into action, sampling the water on each side of the boundary so that they could measure its characteristics. Speed is of the essence in these situations; colour fronts can disappear quickly. The one in the picture below is busy decaying – notice the smudged boundary.

Colour front in north western False Bay on 13 November 2014
Colour front in north western False Bay on 13 November 2014

Measurements revealed that the milky green water overlaid the clearer, bluer water, down to a depth of 11-12 metres (this will vary from front to front). The milky water did not extend to the ocean floor.  Scuba divers around the Cape Peninsula will be familiar with the experience of diving through two or more layers of water, with varying turbidity (clarity) and temperature! (Here is picture of Tony and Christo diving near Oudekraal in the Atlantic that shows what the boundary between two layers of water can look like.)

The researchers found that the milky coloured greenish water was full of fine, almost neutrally buoyant particles of calcium-rich sediment. The green-blue water contained much less calcium, but relatively more silicon, which would suggest the presence of diatoms (a kind of phytoplankton – you can think of them as teeny tiny plant-like organisms) or sand in the water. The origins of the calcium-enriched sediment in the milky water are interesting: one source is from the shallows (less than 30 metres deep) of north western corner of False Bay, where the ocean floor is made up of rocks that are rich in calcium carbonate (such calcrete and limestone), some areas covered by a thin layer of sand.

Milky-white water near Swartklip on 29 November 2014
Milky-white water near Swartklip on 29 November 2014

The second probable origin for the particles of calcium-rich material is the interface between the sea and the land at the northern end of False Bay. The cliffs at Wolfgat/Swartklip at the head of the bay are made of calcrete, and at Swartklip the beach narrows to the extent that the cliffs erode directly into the water when the sea is high. Strong southerly winds create a wide (of the order of one kilometre) surf zone at Muizenberg and Strandfontein; a spring tide also adds to ideal conditions for the generation of a colour front.

The temperature of the milky water was found to be slightly (0.4 degrees Celcius) higher than the green-blue water. This measurement will also vary from front to front. The researchers speculate that the temperature difference could be because the milky water originated in the surf zone, which is shallower and therefore warmer, or because the high concentration of suspended particles in the milky water caused greater absorption of heat from the sun.

Colour front at Smitswinkel Bay on 24 October 2014
Colour front at Smitswinkel Bay on 24 October 2014

Summary

Here’s the tl;dr: strong southerly and/or south easterly winds, perhaps coupled with spring tide conditions, set up a very wide surf zone along the northern end of False Bay, which disturbs the sediment on the ocean bottom and drives the waves further up the beach than usual. Particles of buoyant calcium carbonate from the sea floor and eroded from the cliffs at Swartklip are lifted up into the water column, changing its colour to a milky-green shade. Wind-driven circulation patterns in the bay push the front from its original location in a southerly direction, towards Simon’s Town.

What to do?

Contrary to what your friends on social media may claim, not all colour changes in the ocean around Cape Town can be attributed to a giant sewerage plume. Hardly any of them can, in fact. In summer, the reason for the ocean looking green, red or even brown is likely to do with a plankton bloom of some description, or related to suspended sediments (as in this case) or other naturally arising material in the water. Instead of using this as an opportunity to become hysterical on the internet, how about celebrating the incredibly dynamic system that we can observe, living near the ocean? Drive up a mountain next to the ocean and take in the spectacle from on high. Dip your face in the water and see what it does to the viz. Take some pictures for posterity. And – if you don’t know what’s causing it – try to find and question someone who does know, like a scientist, or consult a good non-fiction book, to find out some facts.

If you’d like to read more about colour fronts in False Bay, take a look at this scientific paper (pdf), which I used as source material for most of this post. The paper is called A Prominent Colour Front in False Bay: Cross-frontal structure, composition and origin

The new Cape Point lighthouse

View of the new Cape Point lighthouse from the sea
View of the new Cape Point lighthouse from the sea

To me it would seem totally logical to build a lighthouse as high above sea level as possible. As we saw with the old Cape Point lighthouse, there is such a thing as too high, particularly when you’re building in an area that is prone to heavy mist and fog. The most notable shipping casualty that occurred after construction of the old Cape Point light was that of the Lusitania, wrecked on Bellows Rock in 1911. A new lighthouse was planned, along with a light at Slangkoppunt in Kommetjie, to replace the old light at Cape Point.

View of the new lighthouse at Cape Point
View of the new lighthouse at Cape Point

The new lighthouse was built on a 15 metre high pinnacle of rock called Diaz Point, which was dynamited to form a flat platform upon which the lighthouse would be built. Building materials were hauled by oxen from Simon’s Town, and transported by tram down a track on the cliffs. Most of the way the gradient of this tram track was 1 in 4; for a short stretch it was 1 in 2. This is incredibly steep. At the end of the tram track, the building materials were lowered by crane onto a ledge. Building sand was excavated from a cave at the bottom of the cliff, and carried up to the platform on which construction took place. Water was brought close to the building site by trolley, and piped down onto the location.

 

Like the old lighthouse, the new lighthouse is nine metres high, but instead of cast iron, it is constructed from masonry and the tower is square. The lantern house on top is white. The new lighthouse’s elevation is 87 metres above sea level, giving it a range of visibility of 32 nautical miles. The fully automatic light flashes three times every 30 seconds, and there is a subsidiary red light in the base of the lighthouse facing towards Anvil and Bellows Rock. This light is only visible from the sea, if you go around Cape Point to the western side of Cape Point.

On 11 March 1919 the new lighthouse was commissioned (put into service). The view from this lighthouse covers a full 353 degrees, with seven degrees obscured by Da Gama Peak behind it. It was manned for a time, but is now automated.

Getting closer to the new lighthouse
Getting closer to the new lighthouse

The public is not allowed to visit the new lighthouse, or even to get particularly close to it. It can be viewed from a viewpoint at the end of the Lighthouse Keeper’s Trail, a highly recommended short walk from the old lighthouse at Cape Point. Along the way you will see the remains of World War II bunkers and a radar station, and you will traverse the most fantastically narrow ridge of rock (in perfect safety). The wind is likely to be extremely strong, whatever time of year you go – dress accordingly. Also note that the walk does not take nearly as long as is suggested by the signage at the start. It is approximately one kilometre each way.

View from the old lighthouse towards the new one
View from the old lighthouse towards the new one

As usual, everything I know about this lighthouse that I didn’t learn by looking at it (i.e. most everything), is thanks to Gerald Hoberman’s wonderful Lighthouses of South Africa book.

Cape Town’s visible shipwrecks: MFV Harvest Capella

The rocky peninsula at the northern end of Maori Bay, on the opposite side of the bay to the MV BOS 400 crane barge wreck, is called Oude Schip. It can be reached by walking and bouldering from Llandudno, or, as we (predictably) prefer, on a boat ride out of Hout Bay. We are usually in the area with the aim of diving the wrecks of the Maori, the Oakburn or the BOS 400.

High and dry at Oude Schip
High and dry at Oude Schip

On the rocks at Oude Schip are the remains of a Sea Harvest fishing vessel called MFV Harvest Capella. This 44 metre long diesel trawler ran aground in early October 1987, apparently during a south easterly gale. There are some pictures of her aground here and here.

MFV Harvest Capella at Oude Schip
MFV Harvest Capella at Oude Schip

Over the years, part of her bow has been pushed right up onto the rocks by the force of the waves. At the same time it has been breaking up, and perhaps in a few years will be almost indiscernible. The wreckage is quite unstable, and not really suitable for clambering about in any more.

MFV Harvest Capella on the rocks behind our boat
MFV Harvest Capella on the rocks behind our boat

Next time you’re in the area, ask your boat skipper to take you towards the rocks on the Sandy Bay side of Oude Schip to see how the Harvest Capella is looking these days!

If you’re interested in visible shipwrecks, check out my ebook Cape Town’s Visible Shipwrecks: A Guide for Explorers!

Cape Town’s visible shipwrecks: Winton and Hermes

Being (in midlife) a creature of the south peninsula, I tend to focus my attentions on False Bay and the Atlantic coast from Hout Bay southwards. But there are rewards for the shipwreck hunter who ventures further north, and even for the shipwreck hunter who doesn’t necessarily want to get their feet wet. A visit to Milnerton beach, and a walk north from Milnerton lighthouse, reveals two shipwrecks in the surf zone. Milnerton beach is surpassingly filthy, but while I was there a beach cleanup was making some headway on the mounds of rubbish tossed off ships in Table Bay that ends up on the beach. The view of the lighthouse from the beach is also far more fetching than the view from the car park, if you can overlook the garbage.

Milnerton lighthouse
Milnerton lighthouse

About one kilometre north of the lighthouse, where the beach is cleaner and pebbles roll euphoniously in the waves, you will come across the massive boilers of the Hermes in the surf. The NSRI gets calls every year from concerned locals worried that a whale is stranded near the beach; the sea spray sometimes pushes through holes in the top of the wreck creating an illusion of a whale’s blow. The Hermes was a liner, built in 1899, on her way to Cape Town with a large cargo of livestock, forage and a few passengers. When she arrived in May 1901 the harbour was full, and she was forced to drop anchor for the night. A north westerly gale came up, she dragged her anchors, and when the captain ordered her engines started, they failed.

Hermes (front) and Winton (back)
Hermes (front) and Winton (back)

Seawards and to the north of Hermes, the engine block of the Winton is visible, in much the same way as the SS Clan Stuart can be seen at Glencairn in False Bay. The Winton came aground in July 1934, carrying a cargo of wheat from Port Lincoln in Australia to Liverpool, England. Her captain was unfamiliar with Table Bay and had mistook the red lights on top of the radio mast at the Klipheuwel Wireless Telegraph Station near Milnerton for the harbour lights. Attempts were made to pull her off the beach and some of her cargo was salvaged, but the wheat ignited and efforts to refloat her were to no avail.

On a calm day, an aerial view of the site reveals the full outline of both vessels surrounding the parts that protrude from the water. When I visited, it was rough after a large swell, but the tide was low. At high tide the view will be considerably less impressive.

The boilers of the Hermes in front of Table Mountain
The boilers of the Hermes in front of Table Mountain

It is possible to scuba dive this site, and Underwater Explorers dives the Winton every year during their summer Table Bay wreck diving jamboree. Obviously very calm, low swell conditions are required because the wreck is so shallow and so close to the beach.

If you’re interested in visible shipwrecks, check out my ebook Cape Town’s Visible Shipwrecks: A Guide for Explorers!

Newsletter: Surf, then dive

Hi divers

Weekend dive plans

Sunday: Boat dives – if the weather forecast turns out to be wrong!

We had a good dives last weekend, and our shore dive at Fisherman’s Beach was pretty pleasant. The site is very dependent on the size of the shore break – if it’s too big you end up barrel rolled on the sand before you even get started! Fortunately is was not that bad and we had a good 6 metre viz. A couple of photos here, on facebook.

Briefing at Fisherman's Beach
Briefing at Fisherman’s Beach

Thanks to Jan de Bruyn who took this picture of the boat last Friday, when I was out in False Bay!

Boating in False Bay, pic by Jan de Bruyn
Boating in False Bay, pic by Jan de Bruyn

Surfing

I am sure the best option for tomorrow will be watching some of the best big wave surfers give Dungeons or Sunset Reef a ride, we will launch from Hout Bay at 10.00 am for this.

The dive conditions for the weekend look a little poor. There is strong north north westerly wind forecast for Saturday, too much for pleasant boating, and a fair amount of south easter for Sunday. The water should be very clean given the last few days of wind so I am open to the idea of launching on Sunday if the wind turns out to be a little less than it is currently forecast to be.

To provisionally book or put your name on the list in case we launch on Sunday, you can SMS, Whatsapp, call, email, call on a landline, send a carrier pigeon, or (preferred method) pop in with cake. You decide.

Date to diarise

As part of the South African Shark and Ray Symposium happening next month, there’s a fantastic speaker evening being held on Monday 7 September. Check out the details on facebook here. There are some riveting speakers who will take you through False Bay’s fauna from the microscopic to the massive (orcas, anyone?)!

regards

Tony Lindeque
076 817 1099
www.learntodivetoday.co.za
www.learntodivetoday.co.za/blog/

Diving is addictive!

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Article(s): The New York Times on the Christmas 2004 tsunami

Yesterday marked the 10th anniversary of the massive Sumatra-Andaman earthquake in the Indian Ocean. The quake’s epicentre was off the coast of the island of Sumatra in Indonesia, and triggered a massive tsunami with waves over 30 metres high. 230,000 people in coastal communities around the Indian Ocean lost their lives, with most of the fatalities occuring in Indonesia, Sri Lanka, Thailand and India. The scale of the devastation is hard to explain; there are plenty of videos to be found online, showing the sea receding and then rushing back, far inland, with unimaginable force. The coastline was reshaped, modified, and entire towns were razed to the ground.

A year after the event, the New York Times published a series of articles about the earthquake and subsequent tsunami, and they are well worth reading, today especially. Their author, Barry Bearak, spoke to survivors of the tsunami, and wrote down their stories. He outlines the hard-to-understand calculus of natural disasters (and life itself) in the first article in the series:

But the disposition of who lived and who died was more than a matter of distance from the sea. Indeed, some people lived for the very reasons others died. They were in one part of the city when they ordinarily would have been in another. Some were fortuitously late, others disastrously early. Survival was decided by which road taken, which stairs climbed, which hand held. Once in the grip of the waves, hurled and churned through the malign darkness, some made it through the gantlet of deadly debris. And some did not.

You can find the series of four articles here:

Part 1: A Ghost in the Water

Part 2: The Sea is Coming

Part 3: Living Among the Dead

Part 4: A Special Burden

 

Friday photo: In between

Rocky shore between Muizenberg and St James
Rocky shore between Muizenberg and St James

On the rocky shore between Muizenberg and St James (accessible via this walk), there are many nooks and crannies, bursting with life that a young marine explorer would derive great enjoyment from discovering. We did the walk close to high tide, on a day with a large swell and the corresponding big waves. I wouldn’t recommend rockpooling in these conditions.

Friday photo: More Muizenberg

Muizenberg beach looking towards Simon's Town
Muizenberg beach looking towards Simon’s Town

Taken on the same day as this photo, this picture of Surfers Corner clearly shows the large swell that was pounding Muizenberg at the time. This corner of False Bay – close to its northernmost reaches – often has very murky water.

Friday photo: Muizenberg waves

Muizenberg beach
Muizenberg beach

I took this photo while a large swell was pushing into False Bay. The sea at Muizenberg was an appetising (Appletising?) green colour and despite the large waves, there were not many surfers in the water – too messy?