Jamieson, Fausset and Brown on Nahum 1:8

8. with an overrunning flood--that is, with irresistible might which overruns every barrier like a flood. This image is often applied to overwhelming armies of invaders. Also of calamity in general (Psa 32:6; 42:7; 90:5). There is, perhaps, a special allusion to the mode of Nineveh's capture by the Medo-Babylonian army; namely, through a flood in the river which broke down the wall twenty furlongs (see on Na 2:6; Is 8:8; Da 9:26; 11:10, 22, 40).

end of the place thereof--Nineveh is personified as a queen; and "her place" of residence (the Hebrew for "thereof" is feminine) is the city itself (Na 2:8), [Maurer]. Or, He shall so utterly destroy Nineveh that its place cannot be found; Na 3:17 confirms this (compare Psa 37:36; Da 2:35; Re 12:8; 20:11).

darkness--the severest calamities.