[samrohan]

You seem to have a very romanticized idea of elves. They might be a special and gifted race but they nonetheless have to eat and sleep like others and they would have had to grow crops, raise animals and so on like other races.

In the case of Rivendell, I am sure that trade was a very important means of gathering food for the inhabitants. During the siege they must have done what other people have done during sieges, which is starve. However Tolkien probably overlooked this issue. The guy was good but certainly not faultless.