Holiness and the The Flood

In his commentary on p. 198 of Creation and Blessing: A Guide to the exposition of Genesis, Allen P. Ross quotes Marcus Dods on why God chose to blot out completely life upon the earth in the Genesis flood.

This event [The Flood] then gives us some measure by which we can know how much God will do to maintain holiness upon the earth. In this catastrophe every one who strives after godliness may find encouragement, seeing in it the Divine earnestness of God for good and against evil. There is only one other event in history that so conspicuously shows that holiness among men is the object for which God will sacrifice everything else. There is no need now of any further demonstration of God's purpose in this world and His zeal for carrying it out.... The Flood has not been forgotten by almost any people under heaven, but its moral result is nil. But he whose memory is haunted by a dying Redeemer, by the thought of One Whose love found its most appropriate and practical result in dying for him, is prevented from much sin, and finds in that love the spring of eternal hope. [Book of Genesis, pp. 66-67.

We must not think Noah escaped this judgment because of his righteousness. Noah received grace "But Noah found favor in the eyes of the LORD."(Genesis 6:8 ESV). Allen comments earlier in his commentary about Genesis 6:8.

A close study of the word for grace will support that it signifies unmerited favor. If the word is given its proper meaning, it means that the recipients of grace actually deserved the judgment too. The discussion of Noah's righteousness comes in the next section. Here, Noah received sovereign grace and escaped the catastrophe. No one escapes divine judgment apart from grace.(Ross, p. 185)