Sand migrates in wind-driven currents, similar to a river, along the shore, as well as inshore and offshore with each wave. This effect is called longshore current. These currents can switch back and forth with wind but usually have predominant direction. This migration of sand keeps each beach along the way nourished with new sand as it sweeps the beaches' sand along to another, maintaining a sand balance.

Natural Causes of Erosion
Sometimes natural features can effect this sand balance. Impediments along the way, such as an inlet, can push the sand balance in one direction, since sand can not migrate across their strong cross-shore currents easily. Sand accretes or gets trapped in shoals and on the beach "updrift" of the inlet, while the beach "downdrift" of the inlet erodes or is sand starved for new sand. Erosion is a net effect over time when there is less new sand entering a coastal system than is leaving it. Erosion by itself is not harmful, as long as the shape of the shoreline can change and allow for such losses.

A barrier island is a dynamic ecosystem that tends to migration towards land over geologic time. This is due to the effect of severe storms which break through or breach the dunes and deposit the sand they erode from the oceanside onto the bayside in overwash fans. This normally builds up the landward side of the island as the ocean side narrows. This erosion/accretion is a slow, natural movement of sand.

But what happens to coastlines when sea levels rise? Many coastal geologists believe that sea level is rising at a rate of one foot per century, depending on the coastline. This seemingly small amount of change to water elevation can cause extensive changes to the shoreline over time as a low sloping, flatter beach is inundated. The most dramatic effects are seen during extreme storms, when tidal range swings to even greater extremes. Increases in levels of bay waters as well as ocean waters will narrow barrier islands from both sides, causing flooding and erosion.

Another natural cause of localized erosion is hydrodynamic conditions. The bathymetry or sea bottom contours of a particular area may focus wave energy at one point along a coast. Material with little structure such as softer marsh sediments will often subside, lowering the shoreline. A shoreline which lacks a source of fresh sand, such as little direct flow of sediments from inland rivers, will experience erosion. Storms such as hurricanes and northeasters often are the most obvious precipitator of severe erosion, but much of the sand that doesn't travel too far off shore in the storm will return to the beach with favorable weather in the following weeks.

Human-made Causes
Some coastal experts argue that erosion is not a problem at all unless people build on or near the beach. However, it is impractical to impossible in many developed areas to return the beach to a pristine condition. New development must be undertaken in a well planned, responsible manner in order not to make erosion a worse problem than it has to be. For example, if a beach builds outward during an atypical accretional phase, the newly created land should be considered temporary in its existence and not viewed as a new development opportunity.

Sometimes the very act of trying to prevent erosion will make the problem worse. Certain types of hard armoring that draw a line on erosion, such as sea walls and bulkheads, may protect the land behind them at the eventual expense of the beach in front of them. This is because wave energy bouncing off of these structures can scour or pull sand away at their base. Groin fields, when constructed improperly or in the wrong order, may act as a block to littoral drift, similarly to the inlet program described above. Hardened inlets where large jetties have been built and deep navigational channels are maintained may be especially troublesome. Flood control structures such as levies in river systems may block inland sand's usual migration to the sea. Removal or alteration of the natural beach system, such as bulldozing or building on dunes, will hamper the beach's ability to act as a buffer to erosion and flooding.

Solutions
There are a continuum of philosophies in dealing with beach erosion. The following approaches are examples of the ends and middle of the spectrum of ideas:

1. Allow erosion to continue unchecked, not building or rebuilding within a considerable buffer distance of the beach. This is often referred to as retreat strategy.

2. Draw a hard line in the sand, using engineering techniques to protect structures or natural features behind that line but sacrificing the dynamically shifting nature of the coastal beach and dune system.

3. A moderate approach which considers the infrastructure and natural resources at stake in making a decision about how to protect a specific beach location. This middle road may include a combination of the 'soft' engineering solution - beach renourishment, sand holding structures, and planning to prevent overdevelopment.

Beach renourishment is the engineering solution most popular in The U.S.A. today. This is when sand is brought from another source to restore the beach. Sand from maintenance dredging of navigation channels can sometimes be utilized for beach renoursihment purposes. More often an offshore sand borrow area must be identified for the sole purpose of beach renourishment and the sand is pumped to shore from this location. Smaller beach maintenance projects sometimes involve trucking sand in from an inland sand and gravel pit. When engineers design these projects, they overdesign the height and width of the beach berm so that the desired beach equilibrium will be reached after a period of time. This is because the artificially filled beaches will erode more rapidly at first until they reach a more natural (but still improved) state of slope and height. After this adjustment, if the project was well-designed, the beach will still be much improved and will not continue to erode at an accelerated rate.

Sometimes the best design includes the use of structures such as groins or breakwaters along with the placement of sand. The use of structures in combination with renourishment may better hold the new beach sand and slow the rate of erosion. While rock groins sit shore-perpendicular and inhibit the littoral drift of sand, breakwaters are positioned near the shore and parallel to the beach.

As their name implies, they breakwaters break the waves and create a shadow of calmer water in their lee which allows suspended sand to settle out of the water and deposit in the near shore area and on the emergent beach. Breakwaters are usually built to a height above high tide and are made of stone or concrete armoring units. These emergent breakwaters, if not properly designed, can break too much wave energy and form a sand spit or tombolo out into the water, obstructing littoral flows. Breakwaters International Inc. has developed a new type of submerged breakwater called the Beachsaver reef that works more effectively with the natural beach system and improves a beach's equilibrium.