Your favourite beach might appear as if it has not changed for generations. But sand is mobile, and its shifting alters the outline of the shore, the contours of the beach, the adjoining seabed, and the shape and character of the waves that ebb and flow across its surface.
This changing scene is a consequence of "entrainment" of the sand by water. If you look closely at the shallow waves lapping on the shore you will see that a cloud of sand grains is suspended in, and moving with, the water.
In general, a swiftly-moving, turbulent flow picks up more grains than a relatively quiet and placid sea.
When the waves are small, sand is lifted as a wave moves towards the shore but tends to gravitate to the bottom as the wave returns; this increases the amount of sand upon the beach. With large, turbulent waves, however, the sand remains suspended as a wave recedes and is carried out to sea, often forming an underwater sandbank some distance from the shore.
Since storms and large waves are common in winter, and lazy seas frequent in summer, one might expect a seasonal rhythm to a beach's shape.
One might envisage sand piling up along the shoreline during the summer, and large quantities of it removed in winter to be deposited on an offshore sandbar. And, in fact, for many beaches, this model works quite well.
Sand can also be observed to move "along" the beach. Waves, in general, do not approach the shoreline perpendicularly, but at an angle which depends on the prevailing wind and the topography of the local coast; the water moves obliquely up the beach and then falls perpendicularly back by gravity.
A grain of sand entrained by this water is carried up the beach at an angle, and back down in a straight line; the next wave transports it on a similar journey.
Each individual grain moves along the beach on a kind of saw-tooth track, and the result is a lateral shifting of the sands on this "littoral conveyor belt".
A consequence of the littoral conveyor is that certain parts of a beach will tend to gain or lose sand. Sand can be prevented from leaving a particular zone by a long wall, or groin, built at right angles to the shore, which will accumulate sand on its sides.
With the conveyor interrupted, the far side of this groin will no longer be replenished, and erosion will occur as the sand, that was originally there, is carried off farther along the beach.