While the rest of Florida began picking itself up after Hurricane Irma, this city in the far northeastern corner of the state was still experiencing serious flooding and was bracing for more inundation to come. Some floodwaters from the massive tropical storm were expected here, but the scale of the unfolding disaster took Jacksonville and nearby towns by surprise. Driven by tidal flow, an already saturated inland waterway system and Irma’s powerful winds and rains, the swollen and fast-rushing St. Johns River crashed over sea walls and sandbags and left much of the area underwater. Officials called the flooding “epic” and “historic,” with the river through this city of nearly 900,000 hitting levels not seen since 1846 — a year after Florida became a state. On Tuesday the city started to recover, but meteorologists warned that some flooding is likely to return as storm-generated waters rush south from the Carolinas toward the Atlantic Ocean.