Cliff sculpturing to create rebound A frames - a do it yourself how to guide

frog's picture
frog started the topic in Sunday, 8 Jan 2017 at 9:51am

Lots of waves around the world owe their existence to a cliff or groyne creating a ricochet effect (D bah, Sebastians, Whale Beach Wedge, Wedge in California, Knights beach etc). We all know and ride some. Their key benefit is that they can turn semi closeouts into quality peaks and they can create powerful hollow peaks that are double the actual swell size.

From a creating a man made surf spot perspective they are of interest because (if you exclude the pre-existing underlying groyne or cliff) the actual wave creating structure is relatively small and simple. All you need is a strong surging wave to hit a largely vertical surface that is angled correctly to then lead to a slamming effect into the rock wall that re directs the wave energy sideways along the beach. Typically the resulting A frame peak breaks close to the headland / groyne. Sometimes it travels further creating two or even three intersection points with the incoming swell along the beach. If no banks exist to help it you will have to pick your waves and rides will often be short. If any hint of a supporting bank is there is can be an excellent peak that slingshots you down the line.

All around the coast there are headlands, shore platforms and groynes that are almost right for a rebound set up but the angles are not quite right or the wave surges over the rocks but never hits a structure angled to cause a ricochet. Technically all that would be required in a lot of cases is to get out the chalk, draw a nicely angled V shaped notch into the rock profile, or a retaining wall style rock line to create the rebound point on a shore platform and drill and blast out the rock.

This of course is not a quiet little "do it yourself: project and introduces all the complications of approvals, environmental impact statement and controversy (despite the reality that any change to the rock structure would be limited to perhaps 10 metres of coast and weathering would make it blend in very quickly).

Now there is a possible do it yourself option which is low in cost, low in impact on the environment and could be done by a few mates in a day to two:
1. Find the spot - must have substantial wave energy already pounding the rock section and where the waves have built some height already.
2. Sketch out the required slice of rock to remove - best to have an overhang angle on the end wall so wave energy slams into it and is forced strongly sideways.
3. Hire large rock hammer drill to drill holes every 10 cm along chalk line - so your spot needs to be away from populated areas so the drilling is not seen.
4. Use expandable mortar ( a well known low cost means of non-explosive breaking rock / concrete technology) which is mixed and pored into holes late one evening on a small swell day. As it dries, it expands with huge, slow forces and breaks up the rock.
5. Wake up the next day to find the rock has cracked and split. Repeat if needed.
6. Move broken boulders or just wait for the next swell to do it for you.
7. Sit back and admire your handiwork which should provide a non stop source of peaks for you and others to enjoy.

frog's picture
frog's picture
frog commented Sunday, 8 Jan 2017 at 3:06pm

What a wedge can do to a closeout beachbreak:

https://www.youtube.com/watch?v=FSC_vmZzrWo

Frogg

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Sunday, 8 Jan 2017 at 8:23pm

Good topic and it's Interesting idea but I'm skeptical in moat cases more likely just to cause bad backwash.

I guess if it's just a close out there is nothing to lose though.

frog's picture
frog's picture
frog commented Sunday, 8 Jan 2017 at 9:10pm

The whole concept is sidewash, not backwash - the more dramatic the sidewave the better the peak. So the key is an angled refection point (a sort of V shaped notch to compress the swell into with an overhang to slam int and then redirect it sideways) to push the wash energy off to the side of the rock wall so the side wave travels off at right angles to the beach.

The concept is of little interest if you live on the Gold Coast where quality points abound and you just needs swell. Victoria has so many spots it is not really needed.

But areas with swell, headlands and groynes but lots of closeouts such as Victor Harbour, southern Tasmania, some Sydney beaches and other places in Australia and elsewhere all around the world would benefit. Good spots are often like little nuggets of gold in the endless expanse of closeout or semi closeout beaches. Rebound wedge type spots can help increase the number of breaks without the massive engineering and cost required for all other types of artificial reef.

Seriously, the cost be as low as hiring a drill and generator for a day or a few days, buying some bags of the mortar for a few hundred dollars. Under $2000 is possible if the morphology of the cliff just needed a simple slice taken out.

The key thing of interest is that I spotted this expandable mortar stuff which is pretty safe, low cost, no licences is required, is non-explosive and is used from small scale to break up concrete through to large scale quarrying. If you can drill a hole you can break up the rock. Check out the quarrying in this video - quite large scale cuts are possible.

https://www.youtube.com/watch?v=sEpIYQqLAg4

In the rebound spots I have observed the actual reflection point is quite small - just a ten metre or so nicely angled bit of cliff. So we are not talking major work if the general cliff or shore platform is close to being right but just needs a bit of a shave. The end result doesn't have to be perfect - any sidewave will create a peak but with better design the power of the sidewave could be maximised and point of intersection could be optimised.

Frogg

frog's picture
frog's picture
frog commented Sunday, 8 Jan 2017 at 9:14pm

January 4th's wave of the day shot of Knights Beach shows how these spots work and in this case the sidewave would be so much better if the headland had more overhang.

Frogg

thermalben's picture
thermalben's picture
thermalben commented Monday, 9 Jan 2017 at 11:42am

I wouldn't use Knights as a template.. it's somewhat of fickle wave (at its best) and the ride it produces - whilst very intense and sometimes amazing - is pretty short in length.

Brickwalls's picture
Brickwalls's picture
Brickwalls commented Tuesday, 31 Jan 2017 at 10:28am

Northwall Coffs Harbour - had the perfect opportunity in the recent rebuild after the last ECL carnage. But they didn't want to hear about it - $$$$$$ + the fiddle too much factor. Nonetheless, some locals reckon the reef break & suicides faces have changed a little.

http://www.mygc.com.au/wp-content/uploads/2016/06/13321719_1170216189687...

Now where did that shed go?
https://www.youtube.com/watch?v=gm2Pc8DGBOA

gray's picture
gray's picture
gray commented Tuesday, 4 Jul 2017 at 10:05pm

I dunno about this approach (although I assume it is somewhat 'tounge-in-cheek')...surely most of the angled cut would need to be below the water line, or at least the high-tide mark, else the energy is not hitting the angled rock/boulders and refracting at all...?

Plus, the more rock / boulders you have after all the demolition works, the more wavev energy is going to be absorbed by all the gaps and various 'internal' angles in the rocks. Not saying it couldn't be done, but...sounds a bit suss to me. Really, the best bet would be to simply drop big-ass pieces of slab concrete in the right angles, rather than trying to remove rock to get the right shape. For starters, it would be quicker and you wouldn't need to worry about waiting for low tides to do it. Just drop them in. Bang! Job done. Maybe.

frog's picture
frog's picture
frog commented Wednesday, 5 Jul 2017 at 7:30am

Providing there is a surge up the cliff face already, all you need is an angle in the cliff face starting around low water to do the re-directing. If you watch these spots, all the action is above sea level. Concrete could be better but then environmental concerns would stop it ever happening.

Frogg

Clam's picture
Clam's picture
Clam commented Wednesday, 13 Dec 2017 at 7:57pm

Good idea frog,
"The wedge" is a great example but it could be much better if the wall was smooth.
So many missed opportunities with groynes and rockwalls.....

The deadly Glenelg breakwater :
http://www.adelaidenow.com.au/messenger/west-beaches/access-to-glenelg-b...

Clam's picture
Clam's picture
Clam commented Wednesday, 13 Dec 2017 at 8:16pm

There was no swell, and the poor girl drowns .
"Warning signs for strong currents and large waves are already in place at the breakwater after 11-year-olds Frank Ndikuriyo and Thierry Niyomwungere drowned on New Year’s Day 2016."

https://goo.gl/images/4Hh3aP