90+ percent of the population of Egypt lives within relatively few miles of the Nile, and the countryside where they live is just a few feet above the Nile water level. Even if they got notice of the impending flood, they’d jam the roads and get nowhere fast, give or take fighting one another to get by first.
I think nuking the Aswan dam would flood Cairo and other populous places along the Nile a foot or two then recede quickly. 10% of Egypt electricity is generated at the Aswan. No Aswan would hurt food production a lot