Does alluvial fan forms when sediment gets laid down where a river flows into an ocean?

No, an alluvial fan forms when sediment is deposited at the base of a mountain or hill as a river flows out onto a flatter area, such as a valley or plain. This typically occurs when the river loses energy and slows down, causing it to drop its sediment load. While sediment can also be deposited where a river meets an ocean, this process typically creates a delta rather than an alluvial fan.