Fact: Beaches erode. This has been a problem in many coastal communities. However, erosion is the thing that initially created the beaches. When oceans first covered the surface of the earth millions of years ago, there were no beaches. There were only rocky shores. Over the course of millions of years, these solid stones were broken down into rocks by erosion, which in turn were broken down into pebbles, and then into gravel, and then into sand. Rivers also carried down silt and sand from the mountains and deposited it into the oceans and seas of the world.
When communities sprung up, there were no problems. The beach always shielded people and houses from the raging waters during storms and hurricanes. After a while though, the beach began to wear out. The beaches became narrower and narrower and finally water washed away whole houses. No one is sure why beaches erode only at certain times. Pessimists say that the world's beaches are disappearing. Optimists say that beaches are actually becoming wider in the process. The fact is that both are correct. When beaches erode in one place, they grow in another. It is part of a natural cycle. Everything grows, falls apart, and then grows again. Same is true with beaches. No one knows for sure what determines this cycle, but it does happen. When the area was first settled, the beaches were extremely wide in many places. People always liked the beach. Whether the reason is swimming, fishing, surfing, or any one of the other things one does at a beach, people have settled there. Over 75% of the US population lives within 100 miles of the beach.
There are many causes of beach erosion. Most of them are natural. These are currents, storms, earthquakes, winds, waves, tides, and also the gradual movement of tectonic plates. Some beaches are also destroyed by men when harbors are built, or any other structures that disrupt the fragile balance of nature.
Waves are probably the most important components of beaches. Everyone likes waves. Waves form as the result from deep ocean currents, winds and the rotation of the earth. Although waves look like large volumes of water moving in and out of the shore, they really aren't. Water moves very little in waves, in circular paths perpendicular to the ground. Waves take sand from the depths of the ocean and deposit it on our beaches. Waves also carry little stones and pebbles, and smash them into each other producing sand in the first place. Waves can also be harmful though. When waves get large and are powered by storms, they break much higher on the beach and when they wash back down to the ocean, they take sand with them.
Currents can make and break the shore, depending on the circumstances. There are two main types of currents that shape the beaches: longshore and nearshore currents (also called rip tides). Long shore currents develop when waves hit the beach at non 90 degree angles. The current moves in the direction in which the waves crash parallel to the shore. Longshore currents develop on long and straight beaches. These currents carry loose sand grains down the shore and deposit them in slow moving areas. These currents can sometimes wash away great quantities of sand when the conditions are right. The nearshore current is created when waves hit the beach at 90 degree angles. These currents flow perpendicular to the beach in the seaward direction. They are very strong and narrow. These can also carry away large quantities of sand.
Storms are one of the most dangerous things that can happen to beaches. By storms I refer to storms, blizzards, noreasters, hurricanes, typhoons, tornadoes, cyclones, and anticyclones. These generate high winds which can drift entire beaches out into the sea. They also create huge waves and currents that can wash whole beaches away. Winds fuel waves. The more wind, the more waves, the more wave action. Winds can also move sand by themselves. Grains of sand are blown around. When winds get strong enough, one can get sand storms which can move whole dunes from one place to another.
Earthquakes and other geological disasters can dramatically change beaches over a course of hours, or even minutes. If an earthquake occurs somewhere in the ocean, a huge wave that can be over 30 feet tall can result. One such wave could wash away great amounts of sand and also damage whole towns and coastal communities. Tides can build beaches. When a tide is high, waves will break higher up on a beach and sand will be deposited. Unfortunately also, the nearshore current gets to work on the lower part of the beach.
The above was transferred from: http://www.bergen.org/AAST/Projects/ES/BS/erosion.html