I have always admired the technical masterpieces of huge shipping canals
like the Panama Canal, but I have one question to which I could not find a
proper answer in encyclopaedias. When they dig a canal, they usually dig the
bed first and keep it dry, and this bed is filled with water only after the
completion of excavation works. But when digging is finished, there is still
one (or two, one at every end of the canal) dike left which separates the bed
from the open sea. This dike has to be cut, but how is this done? They can
hardly dig through it, since if they did, a huge flood would come over the
workers and kill them as soon as the dike is cut through. I thought they
problably blow it off using explosives, but then how did ancient canal
builders like the Romans solve this problem?
One method that I know has been used was to create a dam of wood (or even wicker) on the water side of the dike and pump out the water between it and the dike. The dike was then manually dug away and, when it was gone, gates in the dam were opened to let the water through. Once the water pressure was equal on both sides, the dam was lifted out or dragged out of the water.
I do not know whether this was the common method, but I know it is one method.
Sorry, I didn’t see the first letter in the OP. Sorry for the confusion.
Carry on.
In the case of the Panama Canal, I believe (but I’m not absolutely sure) that the lower courses of the waterway were filled with fresh water flowing down from Gatun Lake, and not salt water flowing in from the oceans. Almost all of the excavation was done when the waterway was dry, but the very last of the excavation was done after the waterway was flooded, using floating dredges. I get this from The Path Between the Seas by historian David McCullough, an excellent book on the building of the Panama Canal.
The sea-level Suez Canal is a different story. I know there was a fresh-water canal dug parallel to the Suez Canal to bring in supplies. The water for it was diverted from the Nile by the Ismailia Canal. I don’t know if the Suez Canal originally was flooded by water from the Nile or not.