Humans use the shore for several purposes, including recreation, waste disposal, shipping, and commercial fishing. The residential and industrial development associated with these uses seriously affects coastal regions.
Perhaps the most direct human effect on the shore is the attempt to control sediment transport in order to prevent erosion in some places and deposition in others. People use artificial barriers known as groins to maintain beaches for recreation or to protect homes. Similarly, they build jetties and breakwaters to protect and stabilize harbor entrances. Like groins, these structures block the longshore drift of sediment and result in deposition updrift and erosion downdrift of the obstruction. People also combat erosion by beach replenishment programs that dredge sand from far offshore and use it to rebuild a beach. Dredging is also used to maintain sufficient water depth in shipping channels and harbors, which are continually filled in by deposition. Sediment dredged from a harbor is often contaminated by urban pollution and shipping fuels, making it unsuitable for beach replenishment. Instead, it must be dumped far offshore in specific underwater garbage sites designated to receive dredged material.
The intense urbanization of some coastal regions is polluting coastal waters with human sewage and other wastes from both homes and industry. This problem is especially severe in some older cities, where storm-water drainage uses the same pipes as sewage drainage. During strong rains, the sewer system may be overloaded, and excess water that contains raw sewage must be released. Such a release can contaminate bathing beaches and shellfish beds, creating problems for both human health and the environment.
Outflows from sewer pipes and industrial waste pipes are considered point sources of pollution because pollution empties out of a pipe from a single point. Point sources of pollution are easier to treat than nonpoint (or dispersed) sources of pollution, such as runoff from fertilized lawns, farms, and golf courses. Runoff from these land areas often carries excessive amounts of nutrients. The extra nutrients disrupt coastal ecosystems by causing eutrophication, an overstimulation of plant growth. Soon the excessive amounts of plant material die off and decompose, a process that uses oxygen. As oxygen levels decline, oxygen-dependent animals are put in jeopardy. Salt marshes can act as a buffer for this problem because they trap much of the excess nutrients. Unfortunately, many salt marshes have been filled to create dry land for development.
Another problem is thermal pollution, or the artificial heating of the water. Thermal pollution results primarily from electric power plants that use large quantities of cooling water and discharge it at temperatures as much as 10 Celsius degrees (18 Fahrenheit degrees) above that of the surrounding water. The increase in water temperature disrupts the life cycle of many marine organisms and encourages invasion by creatures that live in warmer waters. These creatures then displace the original populations.
Urbanization of the coast often causes the land to subside, or drop. As the population near the coast grows, so does the demand for fresh water. Often, groundwater is pumped out to meet the demand. However, the water within the ground helps the ground resist compaction. So, when the water is withdrawn, the ground compresses under the weight of overlying soil layers, and the land subsides. The loss of groundwater is accelerated by the increased areas of pavement in urban areas. Pavement prevents rain from soaking into the ground and replenishing the groundwater. Instead, the rainwater runs off the pavement into storm sewers and then is transported directly to the ocean. As a consequence of subsidence, New Orleans, Louisiana, is currently 4 m (13 ft) below sea level. To keep the city from flooding, extensive dikes were built and water must be pumped out constantly. Otherwise, the shoreline would retreat inland, and New Orleans would become a submarine city.