Longshore drift is solely determined by wind-driven waves hitting the shore at an angle. This creates a sediment wash, or swash, that carries material down the shore. The swash generally keeps moving until the waves run out of energy, or more commonly on inhabited shores, until it hits an object perpendicular to the shore, like a jetty or seawall. It will then get deposited and continue to build up on that one side of the perpendicular object and form small pieces of growing land, whereas the other side of the object will have no sediment build up at all. This land shows up over time on maps and photos. Your arrow should point down the shore in the same direction that the accretion builds. The base of the arrow would be on the accretion side, and the pointy tip of the arrow should be pointing to the no-accretion side.
