It is a model to sharpen our understanding. Scientists use such models to study the transport of sediment near beaches. The size of the boxes could be a part of the beach or the entire beach, and it is called a 'beach cell'. Sand moves towards the beach by wave action and away from the beach by gravity down its gradual slope. In the lower left corner the origin of the sand is shown.
Sand arises from coastal erosion and is transported by rivers towards the sea.
As rivers deposit sand nearshore and offshore, it enters the corresponding compartments. Following the boxes from right to left, the sand in the deep sand compartment is stirred only by large waves that may pull sand towards the beach or allow it to drift deeper down by the force of gravity. Sand moving out of the deep sand compartment towards the beach, enters the sand bar or nearshore compartment which is subject to much more wave action and corresponding sand movement. From there, the sand is moved to the intertidal zone, the actual beach. Sea winds move it further onto the dunes but large waves can do so too by washing over the dry beach and dunes.