Much of the northeast United States coast shoreline consists of sand islands separated by tidal channels. In the 19th century, land developers filled these areas with housing. While suitable as public beaches, they made poor sites for high rise resorts. These islands absorb the punishment of 7.5 m waves and buffer the mainland from hurricanes and winter storms. This natural buffer is forever retreating and advancing in concert with wave action. When suffering losses after hurricanes, these beaches had been replenished naturally with sand from the dunes; but the dunes were leveled for subdivisions. Today some residents are buffeted by sand drifts as the offshore winds are still blowing the sand that remains, while others suffer a surfeit of beach because not enough sand is left for replenishment. Krone, 1979
Breakwaters are fairly simple structures that are constructed to reduce wave energy by creating a shadow zone behind the breakwater. Depending on the nature of the areas that are being protected, breakwaters are classified into two categories: harbor protection and shore protection structures. Breakwaters made to protect harbors are usually connected to the shore, while those designed to protect shores are usually detached offshore structures. Breakwaters create calm waters, which promote accretion of protected beaches, but if they are not well planned, they can do more damage than good. A small and permeable breakwater (made of rocks and/or rubble) permits some wave energy to pass behind and allows materials to drift behind it so that the problems are reduced. An over-designed breakwater may be too effective and turn a beach into a mud flat , negating the value of expensive construction.
Groins are structures (usually made of rocks or wood) that run perpendicular to the beach and into the sea, and they may be multiple in number in the form of a groin field . Like breakwaters, they have multiple purposes, which can be summarized as: 1) widen beaches by retaining littoral drift, and 2) control erosion by reducing the rate of loss of materials. Groins trap materials moving parallel to the shore to maintain a sand beach. They are usually perpendicular to the shore and extend from a point landward of possible shoreline recession into the water a sufficient distance to stabilize the shoreline. A groin creates a total or partial barrier to littoral drift. In order to minimize possible adverse effects due to the presence of groins, a simple solution has been proposed which consists in putting replenishment materials from another location on the updrift end of the groins so that they pass to nearby downdrift beaches due to the action of the current.
Jetties can be confused with groins. They are, in many respects, similar: both go from the shoreline straight into the sea, both accrete materials on their updrift side, and both can cause erosion on downdrift areas. However, they differ in one major aspect, their location and purpose. Jetties are constructed at the entrance of harbors, small bays and inlets, not through a whole stretch of shoreline. Harbor channels require jetties and a dredging program to maintain required depths. The quantity of sediment available to shoal a harbor entrance is a function of the gross sediment transport and the hydraulics of flow into the inlet.
A jetty interposes a total littoral barrier between the seaward end of the structure and the limit of wave uprush on the beach. Accretion occurs updrift at a rate proportional to the longshore transport rate, and erosion occurs downdrift at about the same rate. Planning for jetties should include some method of bypassing the littoral drift to eliminate or reduce channel shoaling and erosion of the downdrift shore. Design of the jetty to minimize sedimentation problems is the greatest concern. Extending the jetty into deep water will avoid shoaling, but besides being costly, this introduces permanent loss of sand. It is often best to accept some shoaling and try to direct the accumulation to where it is least troublesome to navigation and most easily removed and bypassed downdrift.
Seawalls include revetments and bulkheads constructed parallel to the beach and onshore. Revetments are usually constructed with boulders at the foot of the first dune, while bulkheads are usually vertical walls constructed on the upper beach. Inclusion of a ramped face and rock boulders at the base reduces some of the problems associated with wave reflection. Their intended purpose is to stabilize the area behind them by absorbing or reflecting waves. Usually seawalls are constructed with impermeable materials (such as concrete) to reflect waves and to protect valued areas from storms. By constructing them this way, seawalls can bring changes to the beach, which are, for the most part harmful and they may contribute to beach erosion. Sea wall construction is as controversial as groins. If the beach erodes to rapidly, the seawall may be eventually doomed to early collapse. In order to alleviate beach responses to seawalls, some engineers think that the best way to construct these structures is by putting them as far back as possible. If a seawall is placed too far outward onto the beach, the interaction between the structure and waves would probably be continuous, and problems due to erosion may arise. Krause, 1987
Instead of a seawall or breakwater, a shoreline may be reinforced with rip-rap , which can range from car bodies, torn up highway concrete, and kitchen sinks to blocks of rock emplaced behind a wire screen. Rip-rap refers to any large (usually 1 to 6 ton) rocks used for coastal protection. These are often just dumped over seacliffs or placed on top of the sand in front of an endangered coastal property. Engineered rip-rap (also called gabons) is carefully placed layers of different sizes of rock with excavated foundations or keyways, and/or filter cloth. If these are cemented in place, the life span is increased. The success rate of rip-rap walls is often marred by relatively high maintenance requirements, and by the fact that significant property damage often occurs when these suffer even partial failure. Griggs and Fulton-Bennett, 1988, Griggs et al., 1994
Breakwaters, groins, jetties and seawalls are structural approaches to deal with erosion and shoreline stabilization. Before expending money and energy in planning, some questions should be considered:
We would like to think of replenishment as a long term and relatively inexpensive solution to beach erosion. This is in fact the case as described by consultants and the U.S. Army Corps of Engineers when they develop a plan for beach nourishment. However, several geologists have examined the record of survival of replenished beaches and presented very different numbers for longevity. A survey of the history of beach replenishment along the U.S. Atlantic barrier coast suggests that the methods used to estimate longevity and volume requirements for replenished beaches are inadequate. According to Pilkey 1992 the amount of sand required to maintain a beach close to its design dimensions is consistently underestimated. Dixon and Pilkey 1989 examined the record for 35 beach replenishment projects in the Gulf of Mexico. In general, they claimed that beach replenishment along the Gulf shorelines had been sporadic in both application and maintenance. The primary reasons for this appear to be availability of funds and the unwillingness of communities to meet federal requirements. The engineering calculations seem to satisfy grain size, volume, and length requirements, but fail to account for storm conditions, current patterns and other variables that affect a beach transport system. If the beach system was being eroded, then logic prescribes that the new sand will be eroded, and must be replaced on a regular basis, consistent with the original beach losses. Houston 1991 maintained that Pilkey's data was inadequate and cites cases where the design criteria have been met. A series of articles by both sides has not resolved the question.
Structural and non-structural approaches to stabilize shorelines are really meant to protect human lives and properties. For that reason, it should be borne in mind, while discussing these approaches, they are not meant to protect nature (that is, coastlines). In fact, there is no erosion problem until someone builds next to coastlines. Morelock 1978 suggested that a reasonable approach for controlling coastal development in Puerto Rico would be to cease using public money to support the cost of construction too close to the shore and subsequent damage that these structures might receive. This approach has been expanded upon by Pilkey and others in advocating greater setbacks for coastal construction.
Pollution is not a new problem, but rather an old one. By medieval times, garbage and sewage in city streets were evidence of the human species' general lack of concern for the removal of wastes. Ingmanson and Wallace, 1979 Early in the history of America and Europe, urban and industrial wastes were dumped into natural bodies of water. As population increased and demand for material goods increased, the amount of waste pumped into the rivers and wetlands grew until by the early 1900's few rivers in the eastern United States near a population center were unpolluted. With few exceptions, this process has been continuing into the present. We have had a population explosion that makes the problem critical.
Historically, people depended on access to salt water for trade and transport and to fresh water for their living requirements. Intracoastal waters are not only of great value in their contribution to the natural ecosystems that evolved within them, but they also enhance the efficiency of economic activities that are not directly dependent on the health of the estuary. Shipping, logging, manufacturing, waste disposal and other activities can potentially damage the estuarine environment. In the U.S. today, over one-half of the population lives within 50 miles of the coasts. This population with its necessary industries, energy generating facilities and waste treatment plants has created a tremendous burden on the fringing areas of the ocean and the rivers entering these areas. In the past, these natural waters were assumed to be infinite in ability to absorb and remove the products of human populations. However, too much use and disposal into too small an area, at too rapid a pace have produced problems that cannot be overlooked any longer. Duxbury and Duxbury, 1997
The 1970's saw a rise in environmental legislation that produced laws and agencies that presently monitor and control the uses of wetlands. Although millions have been spent, the problems are far from solved. A large residual remains and cleanup efforts of designated superfund sites has been slow. Although the indiscriminate industrial dumping of materials in estuaries and coastal areas has been substantially reduced in response to the legislation, there are other sources of pollution. Pesticides, long lived toxic organic compounds, and heavy metals are still making their way through the environment, although many of these materials are closely controlled or no longer used or manufactured. Surface runoff from agricultural lands finds its way through fresh water systems to the estuaries, marshes and lagoons. This runoff supplies pesticides and nutrients, which can poison or overfertilize the waters.
Metropolitan areas have surface runoff via storm sewers that add a wide mix of materials including hydrocarbons from oil, lead from gasoline, residues from industry, pesticides and fertilizers from residential areas, and coliform bacterial from animal fecal waste. Cities also add primary treated sewage effluent. At best, primary waste water treatment is supposed to decompose organic wastes converting them to substances that are not harmful to people and other living things. But with overloaded treatment plants, the best is not always achieved. And primary treatment does nothing to rid the water of such dangerous inorganic wastes as heavy metals and nutrients. Environmental overload is the rule rather than the exception. Natural resources and the environment itself are beginning to give out.
The most damaging solid material dumped into the intracoastal environment is fill. The early settlers placed their farms, villages and towns along the coasts. Salt marshes, estuaries and river banks were easiest to settle. Present day cities such as Boston exist on marshes that have long since been filled. In the name of progress, bays, marshes, lagoons and estuaries have been and are being filled to provide more land area near the water for marinas, housing developments and industrial sites.
Over the past 50 years, the changes in San Francisco Bay have been dramatic. Originally there were 1,131 km2 of bay, now slightly more than 463 km2 remain; the rest has been filled. Young, 1975 As the fill increases, the total tidal prism decreases and the flushing of the bay by tides decreases, and pollution increases.
Construction activities have crowded close to the shoreline because of limited available land areas and reduced construction costs. This has not only aggravated the erosional process, but has put valuable new property in areas of natural erosion . This has in turn created the need to institute urgent and expensive protective techniques to protect the investment. These remedies may have adverse effects that are not immediately recognized.
The coastal environments described can be put into two groups for discussing the effects of human activity. Coastlines, beaches and dunes fit together in a discussion of modification of the coastline features. This includes these features on the barrier element of barrier systems. These have not only been encroached upon by human activities and structures, removed for use of the sand deposits, but also modified to fit the environment to particular shapes and changes in natural cycles. Removal of sand from river mouths includes the collection of material that would be part of the littoral drift sand and results in severe erosion such as that seen near the mouths of many rivers. Removal of sand dunes and beach sand for use as fill and aggregate have certainly contributed to the depletion of many beach systems. In general, efforts have been made to preserve beaches despite human activities that have promoted loss.
Deltas, marshes, estuaries and lagoons are part of the inland waters and wetlands, which have been modified, but in different ways than the coastline. Generally, these areas have been dredged to promote drainage, and filled to provide solid land for farming or urbanization. In contrast to beaches, wetlands have been destroyed to provide new land. Those that remain have been polluted via direct discharge of sewage and chemicals, and discharge into the rivers that feed into them. National inventories in the United States have estimated that the total acreage of marshes has declined by half since the late 19th century with many of the losses attributed directly or indirectly to human activities.