Sforno on Genesis 6:17/ספורנו על בראשית ו׳:י״ז

1ואני הנני מביא את המבול, G’d is saying, that Noach’s task is to build the ark, whereas He, when the time expires, will bring on the deluge He had threatened as soon as the ark would be completed. The word מבול is a variant of מפלה, referring to something lost as in Deut. 14,21 כל נבלה, a carcass being a life lost. The definitive article ה in המבול refers to the destruction, the nature of which had not previously been spelled out. [my paraphrasing. Ed.]