PrintBeach
The beach is located landward of the nearshore zonation and can consist of a wide range of sediment types including pebbles, gravel and even sediment particles that are as large as boulders. The composition of a beach can also be highly variable, and sediment on a beach can be anything from grains of the mineral quartz to fragments of calcium carbonate shell produced by shelled organisms. The size of the sediment as well as the composition of the sediment that is located on a beach is wholly a function of the character of the sediment that is available at that location. Because beaches are constantly being influenced by the impact of waves and by changes in the elevation of the water levels because of tides, they can be extremely dynamic environments, capable of undergoing drastic changes in morphology within a timescale that ranges between nearly instantaneous to seasonal or longer.
During high energy events, such as large storms or tropical cyclones, waves increase in size and therefore are more capable of eroding sediment from a beach and transporting it either farther offshore or along the shoreline to a new location in very short periods of time. During longer time periods, similar results can occur even during fair weather when low energy waves are affecting the beach. In general, however, most beaches undergo the most substantial erosion and loss of sediment during storm events and then experience a period of recovery to regain a cross-sectional profile that is in equilibrium with the wave and tidal conditions.
The slope of beaches can also be highly variable and is a function of the composition of the beach and the characteristics of the waves impacting the beach. Consider what happens as a wave breaks and the swash of the wave runs onto the beach and then returns. As a wave breaks onto a beach, it rushes up the slope of the beach and carries some sediment with it. The energy of the wave is expended due to friction with the surface and the pull of gravity downslope on the beach. As the water of the wave returns downslope, referred to as backwash, it still carries some sediment with it. However, if the beach is very coarse-grained, then the backwash can rapidly percolate into the subsurface and very little water remains to carry sediment back down the beach to the sea. In this case, any sediment carried up the beach by the breaking wave becomes stranded at the landward limit of the breaking wave, and no backwash is available to carry sediment back down to the base of the beach. Alternatively, if the beach is fine-grained and saturated with water, then the backwash cannot percolate into the subsurface, and as the water returns to the sea it carries relatively more sediment with it, leading to a gentle gradient. As a result, coarse-grained beaches with a significant amount of percolation have overall steeper gradients than fine-grained beaches under similar wave conditions.
