The waters of Noah-The purpose of the flood was to purify the world:^[1]

The prophet Yeshaya refers to the great waters of the flood in the times of Noah as the waters of Noah, as he writes in the verse, “and I have sworn that I will never allow the waters of Noah to pass again upon the earth.” The reason for this is because the name Noach derives from the word “Naycha” which means pleasantness, and like a person who rests from work, and as it states that G-d rested on the seventh day, and the Targum writes that G-d was “Noach.” Now, for what reason do we refer to the greatest natural disaster to ever hit humanity in the entire history of the world, having wiped out almost all living creatures, as something that is pleasant and nice? The reason for this is because there is in truth an advantageous aspect in the fact that G-d decided to wipe away the evil of the world specifically using a flood of water. As seemingly, if the entire purpose of the flood was simply to kill the evil inhabitants of the world, why the necessity to make such a huge flood with all of its ensuing details. G-d could have simply made them die in one moment without making a flood at all. Rather, the explanation for why G-d chose specifically a flood to eradicate the evil of man is because the flood also contained an independent purpose to purify the earth from its defilement and state of impurity, as the verse states that the land became filled with abomination and became very coarse, hence requiring purification. G-d thus sent specifically a flood in order to purify that which is impure just like a mikvah purifies the impure. Now just like a mikvah requires 40 Seah of water in order to purify the impure, so too the flood remained on the earth for 40 days corresponding to the 40 Seah of a Mikveh, in order to purify the entire earth. It is for this reason that the waters of the flood are called “Meiy Noach,” as these waters contained an aspect of kindness in them to bring refinement.

The waters of the flood of Noah came from the Chesed of Tohu:^[2] While normal waters of rain derive from the attribute of kindness from the world of Tikkun, this is only when the waters serve a productive purpose on earth such as to cause plants to grow. However, when the water ends up flooding the earth then such waters derive from the world of Tohu in which all of the attributes are revealed with tremendous intensity. Nonetheless, the flooding rains come from the attribute of Chesed of Tohu, as although the rains are intense and cause flooding, they served a purpose to purify the world from its many impurities that were drawn to it through the sins of man during the generation of the flood. Being that the world was so impure, waters from the world of Tikkun would not suffice to purify it, and specifically waters from the world of Tohu were necessary. It is for this reason also that all of the inhabitants of earth were killed, as the divine energy released through the flood from the level of Tohu was too intense for them to intake.

[1] Torah Or 8c and 10a

[2] Torah Or 10a