Zunami is an event from "Z Nation", first encountered in Season 1 Episode 6 "Welcome to the Fu-Bar", and culminating in Season 1 Episode 7 "Zunami". A second zunami appears in Season 2 Episode 10 "We Were Nowhere Near the Grand Canyon".
As a result of a refugee camp in Alberta, Canada failing to provide food or water, over a million people turned zombie in a week.
The zombies head south in a mass. And everything they overrun adds to the crowd. It's miles wide and there is no way to go around it.
A second event occurs in the tenth episode of season 2, one large enough to destroy a casino.
The rest of the group, however, flee right along the path of the Zunami. It catches up to them in a Nebraska Small Town.
The group takes shelter inside the town's morgue, eventually hiding in the corpse drawers.
Murphy can move freely within the horde and the zombies don't hurt him. Some of them even stop and look at him until he yells at them. They eventually move on.
It's believed that the zombies are moving south for the winter since they don't like the cold.