Short version:
Comet strike 100 years long predominately on the North American continent. Much evidence of this - Carolina Bays, Nebraska Freshwater Ponds, etc 10800BC. All mega fauna died off; Clovis peoples wiped out. Firestorms raged for over 100 years continent wide. Massive ice shards from the strikes on the glacier fell to the ground shattering and slicing everything for many miles.
Began the Younger Dryas Age - the coldest period on record; melting glaciers reformed. Ended 1000 years later with another strike; sea levels rose 400-700 feet wiping out everything - your Flood. Remains now buried under 100s of feet of muck and silt on continental shelves