There are many people today who try to say that the flood was not a miraculous event, but rather a description of a typical, local, Mesopotamian flood, such as the floods sometimes seen along the Mississippi, Nile, and other rivers.  The Bible however shows conclusively that the flood was universal, and it does so in numerous ways:  It's purpose, it's preparation, and it's particulars.
    First, God intended, or purposed, for the flood to destroy all things that walked on the earth save Noah and his family and the animals that were placed on the ark (which is proven to have been plenty large to have held all of the kinds of animals).  God saw the state of the people (Genesis 6:5,11-13) and regretted having made man on the earth (Genesis 6:6) and sought to destroy "both man and beast" (6:7).  Verses 6:8; 7:1 show that Noah and his house were allowed to escape the flood since they were of the many people in the world the only righteous ones.  God then destroyed all flesh by the flood (6:17).  Were the flood not universal there would have been some animals, and men as well, who had spread out enough before the flood that they would not be reached by a localized flood.
    Second, the ark was being prepared for many years.  If the flood were to be local, why build an ark?  Why not just move to higher ground?  Furthermore were the flood localized why did God command that all the kinds of animals be on board the ark to "keep seed alive upon the face of all the earth?" (7:3).
    Last, the particulars of the flood included "all the fountains of the great deep broken up" (volcanos and other sources producing water and nuclei of condensation) and "the windows of heaven were opened" (the pre flood canopy/firmament (1:7) condensed and fell as rain).  The rain continued forty days and nights (7:12,17) and the flood waters "prevailed" 150 days.  Furthermore the highest hills of the earth were covered by 15 cubits (22 1/2') of water (7:19,20) so that the ark could just move over them (its height being 30 cubits (45'), and the draft of a fully loaded ship of its form being half the height of the ship).  Considering these facts, this certainly must have been a universal flood!