I'm about to begin volunteer tour guiding at a WWI museum, and this bit of information isn't in the training material I was given. I've been googling and reading with no success, and I'd really appreciate someone pointing me in the right direction so that I can give an informed answer if asked by a guest.
Predictions of 30 000 casualties were made, but all the evacuations seem to have gone smoothly. Some animals and supplies couldn't be evacuated and were left behind, but nothing much beyond that.