A beach is the area of land next to a stretch of water, usually made of sand or small stones. It is a common place to relax and play, especially for children. It is also a great spot to build sandcastles or other structures. Beaches are always changing and moving, with new material arriving during periods of accretion and old materials being washed away during erosion.
The composition of a beach depends on the type and quantity of sediments deposited upstream of the beach, the speed and turbidity of water or wind, and the ability of particles to settle and compact. In calm conditions, fine sediment tends to compact and resist erosion. Established vegetation slows fluid flow at the surface, and may help to stabilize sandy dune fronts.
During storms, the nature of a beach changes, depending on whether waves are constructive or destructive. Constructive waves have long periods between wave crests and allow time for the sand to settle between them. However, waves with short period and high energy force sand into suspension. This sand is then more likely to be carried downstream by longshore currents and out to sea during receding tides.
Beach sands vary in composition and grain size, with the sands of temperate latitudes generally consisting of quartz and some feldspar. In contrast, tropical beaches contain less feldspar and more silica. In general, sands from tropical coastlines have a higher percentage of finer particles and are more friable.
The structure of a beach is determined by its geology, wave climate, and human activity. Coastal structures such as groynes, jettys, and breakwaters protect beaches from the full force of the waves, preventing them from being washed away. Beach nourishment projects are often undertaken to replenish sands lost during erosion. However, it is important to add sand in an appropriate location so that it can settle and compact before it is exposed by aggressive wave action.
An interesting feature of a beach is the presence of sandbars, which are submerged ridges of sand and coarse sediment that are built offshore from the beach. These are created by the deposition of sediments from waves that have swept over the bars during high wave activity. The sandbars are then gouged out by rips during low wave activity, and the ridges become partially or fully exposed during low or high tide. This ongoing movement of sand across the beach is called longshore drift. Beaches on both the Pacific and Atlantic coasts of the United States tend to move sand from north to south. This is because most of the large waves on these coastlines come from the North Pacific on the West Coast and the North Atlantic on the East Coast.