What, exactly, is ice-transplantation as used to reinforce ice for the Elfstedentocht?

Question

Several weeks ago, I read an article in The Economist about the Elfstedentocht, a 120 mile long Dutch ice-skating race. The race is held only in years when the canal ice is strong enough to safely hold the 15,000 or so participants -- 300 serious competitors and about 15,000 amateurs. It was last held in 1997.

(The Economist article is here, but I do not subscribe to the on-line Economist, and have used up my three freebies. If you have not, you should be able to read the article.)

An in-depth description of the race can also be found in this conference paper from the Project Management Institution. It's free and quite fascinating!

According to Wikipedia, Elfstedentocht,

There are often points along the route where the ice is too thin to allow mass skating; they are called "kluning points" (from West Frisian klúnje meaning to run on skates over a carpet), and the skaters walk on their skates to the next stretch of good ice. In 1997 ice-transplantation was re-introduced to strengthen weak places in the ice, for instance under bridges. (emphasis added)

What is ice-transplantation, and what are its limits?

Answer 1

This is pretty much exactly what it sounds like. A large piece of ice is cut from a place where it isn't needed and moved to another place. This is done with for example chainsaws and axes.

Usually the part right under a bridge is the last part to freeze and those spots are very weak. To transplant ice there means to put some more adequate ice there (at least 15 cm thick) so it can be skated on. If the water under the bridge already froze but isn't good enough/thick, it is removed first.

In other places with for example a lot of wind, ice can be added to reduce water movement. This will make it freeze closed faster too.

Of course you get the benefit of having the actual ice there but it also helps in getting the local temperature lower so natural ice forming is sped up. There is always only a small window where it is actually cold enough for having the Elfstedentocht or ice-skating significant lengths on natural ice in general, so boosting the natural freezing process by artificially reducing the temperature further helps.

I am not exactly sure about its limits. Of course it is quite a massive operation so only suitable for relatively small parts. Once the ice is frozen re-attached to the existing ice, I don't imagine there is a large difference in quality though.