The Independent carries a dramatic image of the flood-hit village of Moorlands in Somerset on its front page and reports that misplaced subsidies, cost-cutting, official complacency are being blamed for exacerbating the problem. Read more >>
Unnatural has two basic meanings:
1. different from what is normal or expected, or from what is generally accepted as being right. • It seems unnatural for a child to spend so much time alone.
2. different from anything in nature. • an unnatural death (= one not from natural causes)
However, The Independent's headline also plays on the usual expression "natural disaster" (= one due to a sudden and violent event in nature such as an earthquake, hurricane, or flood). The recent floods have been caused in part by human action (or inaction), hence the use of "unnatural".