Why did the allies suffer heavy casualities during the d-day landing

Because there were many obstacles on the beach and in the near sea.

Explanation:

The D-Day was a very ambitious operation with about 155,000 American, British and Canadian forces landing on Normandy beaches. The main reason there was high casualty was the heavy existence of naval mines that exploding on boats trying to land, minefields on the beach and soldiers in bunkers waiting on the cliffs right above the beach shores. It was a 2.400-mile long fortification made by Germany.