Unit 2: Public goods: Flood defences

This BBC article says that after the recent floods, much more needs to be spent on flood defences

http://news.bbc.co.uk/1/hi/uk/6283660.stm

a) What are the 2 conditions needed for a public good?
b) Do either or both of these conditions apply to flood defences?
c) How could the problem of free riders be applied to flood defences if they were provided privately?
d) Evaluate the extent to which flood defences are a public good