A Decade Of Shape Shifting - Skeleton Bay

Craig Brokensha picture
Craig Brokensha (Craig)
Swellnet Analysis

Ten years ago, in the winter of 2012, Craig Anderson made his first visit to Namibia's Skeleton Bay. During the trip he rode what was then considered one of the best waves yet ridden at the recently-discovered break. The wave was filmed by Alan van Gysen from a helicopter and was the highlight of Ando's biopic Slow Dance.

Ando, back knee folded in, Quiksilver sticker on the nose of his 5'4" Hypto Krypto, trimming deep in the surfing world's latest mysto wave.

A vivid represention of a time and a place.

Yet the place where that wave broke is now dry sand, packed a few feet high, beyond the reach of waves and tide. What was the ocean bottom then, is part of the Namib Desert now.

Craig Anderson in 'Slow Dance' (Marine Layer/Alan van Gysen)

This week is exactly a decade since we published an article, the first of its kind, analysing the shifting sands of Skeleton Bay, a wave that didn't exist in its current form until 2002 and has migrated two football fields north since our first observation.

Introduced to the surfing world by Brian Gable in 2008, winner of Surfing Magazine's 'Google Earth Challenge 2', the wave on the outskirts of Namibia's Walvis Bay was in its prime during the noughties and twenty-tens, grinding perfectly for two kilometres along a hard-packed, ruler-edged sand bank.

Skeleton Bay perfection (Google Earth)

Historical satellite imagery dating back to 1973 has captured the constant evolution of the sand spit harbouring the wave, with its northern most tip growing northward at a rate of 20m/year.

As well as the northern extent of the spit, the elbow that holds the wave - the geographic name of which is Donkey Bay - has also migrated north, but the progression has been more sporadic. At times it's been cut in half, with the Atlantic Ocean breaching an inland lagoon running adjacent to the wave as shown below in April 2000.

Satellite imagery of Donkey Bay in April 2000. Note the breached lagoon (Google Earth)

Located in one of the driest locations in the world, Walvis Bay has an average annual rainfall total just under 20mm. The extremely dry weather and persistent southerly winds deliver a constant supply of airborne sand to the coastline, producing transient 'slugs' - mobile sandbanks in the nearshore zone - that are transported northwards by the prevailing south-southwest swell energy.

As mentioned in our original article, in 1973 the shoreline at Skeleton Bay was an incredible 1km further south than it currently sits. The sand spit was also much shorter and more obliquely-angled to the incoming swells (read: slower peeling).

Since then, however, the constant northward flow of sand has both filled out the bay and shifted it further north, producing a longer wave, that's more acutely-angled to incoming swells. These changes have made it one of the wonders of the surfing world: a backless barrel that breaks metres from the shoreline and continues unchanged for two kilometres.

This shift in shoreline orientation is in reaction to a change to the incoming mean swell direction. When angled swells strike a bend in a sandy coastline, the coastline will slowly change to maximise the sand load along the shore. The sand transport is maximised when the coast is aligned at 45° to the incoming swell. This means, coastal anomalies get 'smoothed out' over time if the swell incidence angle is less than 45° (more straight on), while swells coming in at greater angles than 45° exaggerate coastline anomalies until the coastal alignment comes back to that 45° sweet spot. 

The latter scenario has occured at Skeleton Bay, with an anti-clockwise shift in coastal orientation resulting from the mean incoming swell angle shifting more southerly over the past half-century.

Fascinatingly, the past wave climates can be seen in the ancient sand dunes running south from Donkey Bay - see image below. At times, the shoreline ran more south-west to north-east, signalling the swell climate then favoured more westerly swells, while at other times the shoreline orientation was more south to north, correlating with swell climates of a more southerly bias.

Past coastline alignments are clearly visible in the ancient dune system south of Donkey Bay (Google Earth)

Since our article in 2012, the shoreline at Skeleton Bay has migrated north approximately 200m. That's about 20m a year, which is in line with the evolution of the shoreline since 1973.

All things are not equal along the sand spit though, with some areas eroding and some accreting, and at various rates. The very northern tip of the spit - known as Pelican Point - shows even greater growth northward but it's at the expense of the shoreline between there and Donkey Bay, which has retreated east at a rate of 600m over the past half century.

The beach abutting Pelican Point Lodge, which sits on the northern neck of the spit, is now in serious retreat, with the sand in front of it it receding 170m over the past 20 years. There's only 100m of beach left before the ocean breaches the lighthouse, and if the current rate of erosion continues it'll be underwater in just over a decade.

The image to the left is taken from a 2018 paper which used remote sensing techniques over the thirty year period from 1984-2014 to identify areas of erosion (blue) and accretion (red) along the spit. 

It's clear that the shoreline at Donkey Bay is growing, though not to the pace of the northern tip, while erosion is being observed along the western flank of the neck.

Two questions beckon.

Firstly, why is the neck thinning as the rest of the spit grows?

And secondly, will the northern tip create a long and well-aligned spit much like Donkey Bay?

First things first. The reason the northern spit is growing faster than Donkey Bay is due to a deficit in sediment transport at the neck. There's not enough sand being transported down the point at Skeleton Bay to feed the rate of growth of the spit to the north, so the sand has to come from somewhere.

Town planners have been aware of this problem for decades, with much research being put into various mitigation options to prevent the spit from breaching. Walvis Bay is Namibia's second largest city and its biggest deep water port. On a wild, sandy coastline, the port is a vital cargo hub for Namibia and surrounding countries.

The Walvis Bay Climate Resilience Handbook states: 

"As Pelican Point is a natural barrier limiting the effect of sea storm surges and protects Walvis Bay from these effects, it is essential to monitor and maintain this sand spit that is being eroded through wave action. Bear in mind that the best method of doing this is an ongoing removal of sand from the ever-growing tip of the point and moving it to reinforce where the spit is being compromised. Concrete structures will be eroded around the edges by wave action and will compound the overall problem." 

Sydney's Northern Beaches Council - who recently approved a breakwall at Collaroy - could heed the wisdom in that final sentence.

No mitigation measures have yet been put in place, though there'll need to be some form of intervention taken within the decade, before the neck breaches north of Skeleton Bay. However, if sand is taken from the northern spit to shore up the neck, then that goes some way to answering the second question. The chance of a new mysto wave, just north of the old one, is reduced.

Pelican Point lighthouse, located on the neck north of Donkey Bay (Mark Meddows)

As for Skeleton Bay, recent satellite imagery along with this instagram post shows that an outer bar setup has formed up the tip of the point. This year, Shellharbour surfer Mark Meddows chased a swell to Skeleton Bay. It was the third time he'd been there since his first trip in 2017.

In a phone conversation, Mark explained that the main issue with the outer bar setup is getting out and into the lineup when it's big and consistent. Back in 2017, you could stand on the point, identify a set coming and jump straight in and paddle into the lineup, but this isn't possibly now with the extra dead water and current pushing down off the outer bank.

Satellite capture from July 2020, showing the outer bar setup at the top of the point. Note the 4WD's parked near the end of the point where there's no gutter between the impact zone and the sand (Google Earth)

The wave is still as heavy and hollow as ever, and even a little more perfect in sections thanks to a reduction in backwash, however it's become more sectiony as the dynamics that shape it change from the top of the point to the bottom.

The outer bar ends about halfway down the point, so the wave again breaks close to shore as it did during Ando's visit in 2012 and all those great swells since. During Mark's most recent visit, he twice saw big swells breach the point and wash into the adjacent lagoon. The sand continually piles up, but on the shore it's rarely settled.

Thus the coastline at Skeleton Bay continues to respond to the South Atlantic storms, evolving year after year. Perhaps the quality of 2012 isn't quite there now, however on a coastline where the only constant is change, the prospect of perfection is ever-present.

Comments

DudeSweetDudeSweet's picture
DudeSweetDudeSweet's picture
DudeSweetDudeSweet Wednesday, 28 Sep 2022 at 3:52pm

Though not particularly wanting to quibble with a fellow meteorologist, would it be considered churlish to request you utilise the correct terminology when describing the extremely low precipitation rate of the Namibian desert? Of course I’m referring to the infamous “Paradox of the Nun’s Nasty” of which I’d greatly appreciate you employing in future articles, if only to maintain a modicum of respect for the nuanced language our profession.

Thanks in advance.

Sheep go to heaven's picture
Sheep go to heaven's picture
Sheep go to heaven Thursday, 29 Sep 2022 at 6:49am

..... alternatively, the ballad of the Dead Dingos Donger could be engaged in this situation , only if the Nuns Nasty was considered inappropriate for the kiddies out there

Chris T's picture
Chris T's picture
Chris T Thursday, 29 Sep 2022 at 8:21am

May l ask what is a nuns nasty, does it have something to do with things being dry. l spent 3 weeks in namibia in april truly amazing country but very dry didnt see a nun though.

Crab Nebula's picture
Crab Nebula's picture
Crab Nebula Thursday, 29 Sep 2022 at 9:20am

Oh, I do love a good paradox. Why is it one?

t-diddy's picture
t-diddy's picture
t-diddy Wednesday, 28 Sep 2022 at 6:20pm

super interesting ! thanks for sharing...I remember the original article and can't believe its been 10 years!

Sprout's picture
Sprout's picture
Sprout Wednesday, 28 Sep 2022 at 6:31pm

Great article Craig!

mattlock's picture
mattlock's picture
mattlock Wednesday, 28 Sep 2022 at 6:41pm

Excellant content.
Cheers

john.callahan's picture
john.callahan's picture
john.callahan Wednesday, 28 Sep 2022 at 6:49pm

Thanks Craig, great stuff - the wave at Skelton Bay has been a marvel of the surfing world, let's hope for a southerly bias in swell angles to keep and perhaps emphasise the bend in the sandbar rather than a westerly, which would eventually flatten the top of the bend?

Denyer's picture
Denyer's picture
Denyer Wednesday, 28 Sep 2022 at 8:59pm

Thanks for reminding me I've aged ten years in what feels like two.

groundswell's picture
groundswell's picture
groundswell Wednesday, 28 Sep 2022 at 10:07pm

I guess you are right John Callahan.

Great read and well researched information awesome work Craig!

Crab Nebula's picture
Crab Nebula's picture
Crab Nebula Thursday, 29 Sep 2022 at 10:14am

Intriguing stuff.
While its obvious swell directions mould coastlines both from an erosion and migration sense, I must admit in my ignorant/ novice/ rudimentary understandings of dune systems, I always thought that beyond the shoreline (defined by no direct contact with swell), the "ancient" dunes were more like back dunes that would eventually be stablised by vegetation, would push up against the land, and mostly be shaped by wind. By this, I never discounted how swell gouged the dune systems, though.

Great read

Craig's picture
Craig's picture
Craig Thursday, 29 Sep 2022 at 11:26am

Ancient dunes yes, but those lines shown in the previous coastline stretch back approximately 5km. So at a rate of growth averaging 20m/year they only go back 250 years or so. Young in relative terms.

Crab Nebula's picture
Crab Nebula's picture
Crab Nebula Thursday, 29 Sep 2022 at 12:05pm

Yep, understand your "ancient" reference in the context of its relative youthfulness.

I guess I never really thought about it too much. I figured there were two systems of sand migration: wet and dry, swell and wind respectively. I figured the rear dunes were subject to the prevaling winds more so than an indication of swell climate. This is most likely wrong given my lack education on this and slightly dualistic.
Learn something everyday, eh?

Craig's picture
Craig's picture
Craig Thursday, 29 Sep 2022 at 12:12pm

Here's an amazing before and after showing how quickly a swell can move sand along the point.

These images are only a month apart, with the first taken in July, and the second August 2018.

Crab Nebula's picture
Crab Nebula's picture
Crab Nebula Thursday, 29 Sep 2022 at 12:31pm

I hate them but you've made a good case for getting a drone, haha. Yep. A few months back when the swell on the east coast NSW was uncharacterictically relentless and the rivermouths spewing out sediment, we had holes everywhere in the bank partnered by big lumps of sand migrating northward and peeling off them were point style waves on a long stretch of beach. They left as quick as they came.

goofyfoot's picture
goofyfoot's picture
goofyfoot Thursday, 29 Sep 2022 at 8:36pm

Can’t believe it’s been 10 years since that article, it’s still fresh in the brain.
But where’d those 10 years go!? Fark

boneroni's picture
boneroni's picture
boneroni Friday, 30 Sep 2022 at 9:22am

Wow, I still think of that article. Blown away it was 10 years ago.
Really nice update here. I'm fascinated by this stuff and you write about it very well Craig

PabloM's picture
PabloM's picture
PabloM Monday, 3 Oct 2022 at 10:24pm

Best wave in the world

mibs-oner's picture
mibs-oner's picture
mibs-oner Sunday, 9 Oct 2022 at 6:12pm

The original article got me there for the 1st trip. 3 later and I’m hooked ;)

Craig's picture
Craig's picture
Craig Monday, 10 Oct 2022 at 10:06am

Hell yeah Mibs!