The possible reason why that portion of the earth flooded and drowned so many people, is not just because of rain but most like the moon went out of its normal orbit , came much closer to earth and gravity pulled the earth oceans to that part of the globe. The reason for the moon going out of it's normal orbit was probably the impact of an asteroid.
* See Twenty-one Reasons Noah’s Worldwide Flood Never Happened by LORENCE G. COLLINS Here’s a geologist’s critical analysis of false perceptions held by many creationists about the Worldwide Flood and the age of the Earth.