It won't work.

But, what if they used bigger rocks or waited until slack tide?

It still won't work!

Why not?

As a general rule, rock, coral, or whatever you want to call what you see in the picture has a density of about 90 to 100 pounds per cubic foot. Now, even if you are a burly Tikopian, and your job is to pack a rock around on your back for several meters in the tropical heat, a 100-pound one is probably the last one you would choose to pick up.

But, let's pick up our 100-pound rock (by definition, 1 cubic foot of volume) and pitch it into the breach. Suddenly, our 100-pound rock doesn't weigh 100 pounds anymore. As the density of sea water is around 64 pounds per cubic feet, our submerged rock now only weighs (100-64) = 36 pounds. This is due to the buoyancy provided by the sea water, known to boating folks as "displacement".

Now our weight-challenged rock is sitting on the bottom, attached to nothing. He can probably stay there for a while. But, as more of his friends are added to the breach, the opening begins to get narrower. Neither the lake or sea care if the opening is getting narrower, their job is just to push a fixed-volume of water thru there in about 12 hours (tide cycle). To push a fixed amount of water thru an ever-decreasing opening in 12 hours means that the water has to continuously move faster. Soon, our little rock and most of his friends can't stand up to the tidal current, so they're outta' there. Goodbye!

The foregoing is a bit of over-simplification which does not consider other factors, but it does explain the fundamental problem with this approach.

OK, so what's the fix?

The present focus is on the use of "gabions" to plug the breach. A gabion is essentially a wicker or steel basket filled with stone, rip-rap, rubble, broken masonry, or other material. Gabions are used to stabilized soil and prevent erosion. A gabion is a way to build little rocks into quasi-monolithic BIG rocks. Here is an example of the use of gabions in waterway construction:

As you can see, a bunch of little rocks has been transformed into a semi-cohesive mass; a weighty, formidable and immoveable structure. Kinda' like Government.

But, you still have to deal with the "displacement" problem.

True but, with gabions, we've got an edge. For example, a 4'x4'x4' wire gabion filled with stone will weigh around 6,400 pounds in air. Submerged, it will weigh around 2,300 pounds. The tidal stream will impose a force only on one 4'x4' (upstream) side of the gabion. Sparing the math, it would take a current in excess of 12 feet per second (7 knots) to move the gabion. If two gabions are placed back-to-back in the stream and parallel to the flow so that the upstream area exposed to the stream is the same 4'x4', it would require more than 14 knots of current to dislodge them both. Our little 100-pound rock, on the other hand, would have already bailed out at around 3 knots.

That seems reasonable, but where do the Tipokians get the gabions?

But New Zealand is a long way away and there is no shipping.

True, again. The only realistic solution is to charter a dedicated vessel at a reasonable cost.

David Martin is now searching for an economical charter vessel like "Alvei", shown below. David is personally familiar with Alvei, and is now checking on her rates and availability for the project. He feels that this ship, or a similar one, would be very suitable.

"Alvei"

Assuming that we get the gabions on the island. Then, what's the plan?

Closure of the breach would begin. Generally, the gabions would be placed across the breach, arranged and stacked in such a way that a cross-section would resemble a pyramid, wide at the bottom and narrowing toward the surface. Ideally, and contrary to first assumption, construction should proceed from the middle of the breach and progress shoreward from either end. The reason for this is driven by certain principles of fluid mechanics, and it is the preferred method in order to minimize adverse forces due to the ever-increasing velocity of the stream as the breach is gradually closed.

What's all of this going to cost?

Our plan is not does not require a million-dollar effort. Material and equipment costs are minimimal. Aside from any paid advisory assistance that may be required on-site, labor costs will be zero. The big-ticket item is going to be transport. Presently, the cost estimate for the project, as proposed, is US$50,000.00.

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