More to the point, Floods weren't caused on any shards except the First and Thirteenth - Floods are counterproductive to the Ascians' goals.

The trick is to tip the elemental balance toward an element without causing a Flood, so that the excess elemental aether from the shard will trickle into the Source. After this has gone on long enough, it cracks the barriers between the shard and the Source, after which a Calamity can be used to collapse the shard and return all its aether to the Source.

Floods occur when the barrier isn't cracked, but the elemental aether has built up to catastrophic levels on a shard.