People love living near the beach. More than 50% of the U.S. population lives within 50 miles (80 km) of a shoreline. Once developed, communities make an effort to protect their beach homes and coastal businesses. Throughout history, humans have attempted to slow or alter the dynamic coastal zone. The anthropogenic (human-influenced) changes to coastal environments may take many forms: creation or stabilization of inlets, beach nourishment and sediment bypassing, creation of dunes for property protection, dredging of waterways for shipping and commerce, and introduction of hard structures such as jetties, groins, and seawalls. These modifications change coastal features and have far-reaching effects on coastal processes and ecosystems. An understanding of how human changes alter shoreline environments and park resources is vital for the protection and preservation of coastal areas.
Variations in sea levels are natural responses to climate change, geodetic variations, movements of the sea floor, and other Earth processes. Human actions, including drainage of wetlands, withdrawal of groundwater (which eventually flows to the sea), and deforestation (which reduces terrestrial water-storage capacity), may also contribute to global rise in sea level. Additionally, human-induced climate change, primarily through the burning of fossil fuels, is also of importance. Local changes may be caused by large engineering works nearby, such as river channeling or dam construction that influence sediment delivery and deposition in deltaic areas.
The National Park Service allows natural coastal processes to continue without interference. However, when natural processes, including coastal erosion and storm events, interfere with the preservation of cultural resources and park infrastructure, modifications to coastal dynamics may be necessary. How coastal modifications will affect natural park resources (biological and physical) must be investigated thoroughly in order that wise decisions are made. Park managers in coastal parks strive to achieve a balance between preservation of historic landmarks (e.g., forts and lighthouses) and the protection of natural ecosystems. In addition, a history of long-term human alteration, combined with a lack of historical documentation, makes defining a natural coastal system difficult. An understanding of how anthropogenic modifications will alter shoreline environments and park resources is vital for effective coastal management.
Soft Structures
Dredging
Dredging, the removal of sediment, including sand, silt, rock, and other subaqueous materials from our coastal waterways is a hotly debated topic in coastal management. The effects of dredging waterways and ports to benefit shipping, transport, and recreation are not fully understood. Opponents claim that coastal dredging may have detrimental environmental impacts and may interfere with sediment transport and flow dynamics in coastal and marine systems.
Dredged sediments may include harmful contaminants and pollutants. After dredging, these sediments are often redeposited offshore or used for the creation of dredge spoil islands adjacent to the scoured waterways. In addition, dredged sediment may be incorporated into beach nourishment projects; however, the sediment grain size of dredged materials may not be compatible with native beach sediment. Grain-size alterations and contamination may exceed flora and fauna tolerances, negatively impacting native ecosystem functions.
Proponents of dredging cite that this method of removal is necessary for commerce, recreation, and national defense. Interagency partnerships, such as the Nature Conservancy and the U.S. Army Corps of Engineers, have been established to promote a better understanding of how dredging could impact coastal environments.
For more information about dredging, check out the Web sites:
- Information on dredging research, benefits, and resources; Dredging Operations Technical Support (DOTS) provides direct environmental and engineering support to the U.S. Army Corps of Engineers’ Operations and Maintenance (O&M) dredging missions
Beach Nourishment
Beach nourishment is the process of placing additional sediment on a beach. This material is obtained from another source that either lies inland or is dredged offshore. Nourishment entails the removal of sediment from “borrow sites,” and the subsequent transport of the sediment to beach areas. Borrow sites may alter sediment transport, hydrodynamic patterns, marine ecosystems, and sediment transport, such as creating erosional “hot spots” on adjacent shorelines.
Subaqueous nourishment is an alternative form of replenishment. The creation of offshore berms (mounds) may be utilized for the subsequent landward migration of sediments, often leading to sediment accretion on adjacent beaches. Although still a fairly new replenishment method, and not documented as fully effective, subaqueous nourishment may be substituted on account of cost limitations or biotic complications (e.g., migration and preserving endangered species) resulting from direct beach nourishment.
Often, beach nourishment is needed to counteract the effects of the hard-structure stabilization of coastlines. These structures (e.g., jetties, groins, and seawalls) typically increase downdrift erosion rates, promoting a need for continued coastal modifications through nourishment. The need for beach nourishment after human alteration is quite evident at Assateague Island National Seashore (Maryland). In the mid-1930s, the U.S. Army Corps of Engineers established a jetty system to stabilize the Ocean City inlet. While sediment transported by north-to-south longshore currents increased sediment accretion updrift of the jetties at Ocean City, excessive erosion and barrier island migration have occurred on the island to the south of the jetties. That is, Assateague Island has migrated westward more than 1,148 feet (350 m) since 1933! The deterrence of longshore sediment transport to Assateague Island National Park has had numerous detrimental impacts on biological, geologic, and cultural resources. Both short-term and long-term beach nourishment plans are in place to mitigate the destructive effects of jetty placement.
For more information about Assateague Island National Seashore, check out these Web sites:
- Report by the U.S. Geological Survey on coastal conflicts that highlights Ocean City jetty construction and Assateague Island migration
- Article in Maryland Newsline that describes nourishment efforts by the U.S. Army Corps of Engineers on Assateague Island
Beach Scraping
Beach scraping (i.e., grading and bulldozing) is the process of reshaping beach and dune landforms with heavy machinery. Usually a layer of sand from the lower beach is moved to the upper beach. Beach scraping creates dunes, which are used to give property owners some security from beach erosion, severe storms, and winter washover events. During the summers, the created sandbanks may be bulldozed flat, providing water views to property owners. However, the effects of beach scraping on coastal environments are little known, and this procedure may be harmful to coastal biota and habitats. Proponents claim that beach scraping is a time and cost-effective method to ensure shoreline protection, while opponents state that this method may be the most ecologically destructive form of coastal manipulation to date.
Hard Structures
Hard structures are often placed in marine environments to counteract erosion in sediment-deficient areas, or to deter accretion in sediment-rich areas such as inlets. Unfortunately, these anthropogenic modifications may accelerate erosion in adjacent downdrift areas, increasing the need for additional hard structures. The creation of new hard structures is currently banned in many states, or strongly discouraged as coastal management practice.
Groins are shore-perpendicular structures, used to maintain updrift beaches or to restrict longshore sediment transport. Permeable groins are becoming popular, and may negate some of the negative effects of impermeable groins. Another type of shore-perpendicular hard structure are jetties, which are normally placed adjacent to tidal inlets to control inlet migration and to minimize sediment deposition within the inlet.
Shore parallel structures include seawalls, bulkheads, and revetments. These structures are designed to protect coastal property. Development permits are relatively easy to obtain in many states because seawalls may be built above the high-water mark on private property, and they are relatively inexpensive, compared to beach nourishment. Ironically, seawalls usually accelerate erosion on beaches they are intended to protect. Wave energy is reflected off seawalls, increasing erosion in front of them. The placement of a seawall will decrease the sediment supply near seawalls, increasing erosion on adjacent beaches. In many areas, beaches have completely eroded and disappeared on account of seawalls.
Other anthropogenic structures that are used to stop or alter natural coastal changes include breakwaters, headlands, sills, and reefs. These structures are composed of either natural or artificial materials, and are designed to alter the effects of waves and slow coastline erosion and change. Submerged reefs and sills dampen wave energy and may create new habitat, which is significant for local fisheries. However, the long-term effects of these structures, on both physical and biological processes, are not understood and require thorough examination.
For more information concerning these and other anthropogenic modifications, check out these Web sites:
- Wilmington Morning Star article concerning the construction of hard structures on North Carolina shores; contains links to other articles discussing coastal management issues, Dr. Orrin Pilkey, and barrier-island geomorphology
