Littoral cells are usually an area where changes in the volume of sediment directly affects changes in the coastline, and ideally they are defined to minimise longshore sediment exchange with other littoral cells, for example, a pocket beach surrounded by rocky headlands (which are presumed to exclude sediments). Sub-cells are usually defined to better measure the sediment budget of a coast with varying rates of accretion and erosion. The landward boundary of a littoral cell is usually the foot of a dune or cliff, however, the seaward boundary is difficult to define as mechanisms of sediment transport here are poorly understood. There are three kinds of boundaries between littoral cells: longshore, landward, and seaward; across which sediment may enter the littoral cell or leave it by various processes. It is important to identify which processes operate on a particular littoral cell and also important to identify sediment sources and sinks, as by measuring the sediment gained or lost by these sources and sinks, a sediment budget can be determined.