Tuesday, September 13, 2016

The Ghyben-Herzberg lens decides how unsinkable your aircraft carrier is

Islands with sufficient flat land have long been termed "unsinkable aircraft carriers". This idea is quite appealing, you can easily dredge out a harbor and use the dredged sand to reclaim land on an atoll. Just as the Chinese, they think that is the best way to create airbases in the Paracel Islands and in the Spratlys.

There is a problem though - something that your average armchair strategist might miss easily.

The problem stems from something called a Ghyben-Herzberg lens. Under every landmass is a body of fresh water. If the landmass is large and the soil has the right characteristics, rain water literally pools under ground forming a bubble of fresh water (density 1gm/cc) over a layer of salt water (density 1.025 gm/cc). The exact shape of this water bubble depends on a lot on soil and size of the land mass but generally speaking the bigger the island the greater is the likelihood of a significant Ghyben-Herzberg lens and the more likely you are to have freshwater sources on the island itself.

Why does that matter? - well unless you have fresh water on the island, you can't support any sizable troop presence on it. It takes a brigade size formation to protect an island airbase, a brigade size formation to run the air operations and another brigade size formation to provide support and basic amenities. If you also have a port (which you must to set up the airbase), you need another brigade to operate and support port operations. The result is a division size formation numbering several thousands to keep the unsinkable aircarft carrier running.

Again - doesn't sound like a real problem until you realize that many people need a lot of water to remain combat effective. They need water to drink, cook and manage solid waste. If there isn't a sufficient supply of fresh water on the island, then you have to bring the water in by ship or you could use desalination techniques- which basically makes the price of water on the island equal to the price of gasoline. That is very uneconomical.

There are two main reasons why the Ghyben-Herzberg lens could be insufficiently large -

1) The reservoir is so small that drawing water from the top causes salt-water to intrude into the reservoir from the bottom.

2) The reservoir is so small that dumping waste into it causes the water in distant portions to become contaminated.

There is no fix for this problem.

Now back in WWII - naval strategists were quick to recognize this fact and permanent military presence was seldom established on islands where there was insufficient fresh water supply. Temporary bases were constructed but a very expensive supply chain provided water to these bases and that eventually counted against long term operations.

That is why even the Allies or Axis took islands in the Spratlys and Paracels - it was more to score political brownie points than to demonstrate any serious levels of military prowess.

In sum - the long term military value of an unsinkable aircraft carrier is directly proportional to the size of its Ghyben-Herzberg lens.

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