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Governments Plan for Development of Land Vulnerable to Rising Sea Level: Delaware
Daniel Hudgents et al. 2010. "Delaware". In The Likelihood of Shore Protection along the Atlantic Coast of the United States. Edited by James G. Titus and Daniel Hudgens. Report to the U.S. Environmental Protection Agency.
The summary is on this page. You can also download a printer quality version of this Delaware sea level rise planning study (pdf 2.8 MB, 63 pp.).
Sea level is rising 12-16 inches per century (3-4 millimeters per year) along the coast of Delaware. Ocean beaches are eroding, prompting beach nourishment projects along most of the developed beaches. Along the shores of Delaware Bay, beaches and marshes are eroding, and some low-lying farmland is converting to marsh. These effects could become more widespread if rising global temperatures cause the rate of sea level rise to accelerate. The Intergovernmental Panel on Climate Change, for example, estimates that by the end of the next century, sea level is likely to be rising 0-3 inches per decade more rapidly than today (excluding the possible impacts of increased ice discharges from the Greenland and Antarctic ice sheets).
Rising sea level erodes beaches, drowns wetlands, submerges low-lying lands, exacerbates coastal flooding, and increases the salinity of estuaries and aquifers. Coastal communities must ultimately choose between one of three general responses:
---Armor the shore with seawalls, dikes, revetments, bulkheads, and other structures. This approach preserves existing land uses, but wetlands and beaches are squeezed between the development and the rising sea.
---Elevate the land and perhaps the wetlands and beaches as well. This approach can preserve both the natural shores and existing land uses, but costs more than other options.
---Retreat by allowing the wetlands and beaches to take over land that is dry today. This approach can preserve natural shores, but existing land uses are lost.
Each of these approaches are being pursued somewhere in Delaware. People have built dikes along the Delaware River since the late 1600s, and stone revetments are common along developed portions of the Delaware River. The federal or state government has placed sand onto the eroding recreational beaches along the Atlantic Ocean, and several communities along Delaware Bay. Few homes have been lost to erosion recently, but farms and forests are converting to marsh in low areas along Delaware Bay.
Nevertheless, there is no explicit plan for the fate of most low-lying coastal lands as sea level rises. Environmental planners do not know whether to assume that the coastal wetlands will be lost or simply migrate inland. Those who plan coastal infrastructure do not know whether to assume that a given area will be submerged by rising waters or protected from the sea. And even in developed low lands that will presumably be protected, public works departments do not know whether to assume that the land surfaces will gradually be elevated or that the area will be protected with a dike.
This report develops maps that distinguish shores that are likely to be protected from the sea from those areas that are likely to be submerged, assuming current coastal policies, development trends, and shore protection practices. Our purpose is primarily to promote the dialogue by which society decides where people will yield the right of way to the inland migration of wetlands and beaches, and where we will hold back the sea. We obtained the land use and agriculture preservation data for the state's coastal zone and floodplain boundaries for New Castle and Kent counties, and consulted with planners representing the state's three counties on how to best interpret the data given existing statutes, regulations, and policies. The result is a statewide series of maps that uses existing data, filtered through the county governments who coordinate land use planning activities.
By "shore protection" we mean activities that prevent dry land from converting to either wetland or water. Activities that protect coastal wetlands from eroding or being submerged were outside the scope of this study. This study does not analyze the timing of possible shore protection; it simply examines whether land would be protected once it became threatened. Nor do we analyze whether shore protection is likely to be a transitional response or sustained indefinitely.
The sea level rise planning maps divide the dry land close to sea level into four categories of shore protection:
---Shore protection almost certain (brown);
---Shore protection likely (red);
---Shore protection unlikely (blue); and
---No shore protection; i.e., protection is prohibited by existing policies (light green).
For reasons related to data quality, our study area includes lands within 16 feet (5 meters) above the tides. (We did not project the fates of secured federal installations but depicted them in red so that they stand out.)
One can also view these maps as representing three shore protection scenarios. For example, in an "enhanced wetland migration" scenario, only the areas depicted in brown would be protected; but in an "enhanced shore protection" scenario, only the areas depicted in light green would be submerged. Thus the prospects for shore protection are best understood in the areas shown in brown and light green, while those shown in red and blue are most amenable to coastal planning.
The summary map shows our assessment of the likelihood of shore protection for the coastal zone of Delaware, and adjacent areas of Pennsylvania, New Jersey, and Maryland. Table 1 quantifies the area of land within approximately 3 feet (1 meter) above the tides for each of the shore protection categories by county. Table 2 quantifies the length of Delaware's shoreline along the Atlantic Ocean, Delaware Estuary, and back barrier bays by likelihood of shore protection.
The Delaware Estuary has a long history of shore protection. Partly because of the large tide range and partly because of Dutch tradition, dikes were constructed along the Delaware River in New Castle County, Delaware, during the 1670s. By 1885, land reclamation had converted 10,000 out of 15,000 acres of the marsh in New Castle County to agricultural lands, as well as 8,000 acres in the other two counties. Since the turn of the 20th century, however, these land reclamation efforts have been reversed. In many cases, the dikes were abandoned because of reduced prices for the crops that had been grown on the reclaimed land, so that only a few of the dikes remain. Numerous efforts are under way to restore the hydrology of these lands. (See Chapter 1, Overview.)
The momentum of these environmental restoration efforts has continued. Kent County does not permit subdivisions-and generally discourages most development in the 100-year coastal floodplain, as does New Castle County south of the Chesapeake and Delaware Canal. The State is purchasing agricultural preservation easements in the coastal zone, and more than one-third of the shore is in Prime Hook or Bombay Hook National Wildlife Refuge. More than 80 percent of the shore south of the canal is part of some form of preservation or conservation land.
Along the Atlantic Ocean, more than three-quarters of the barrier islands and spits are part of Delaware Seashore State Park, while the mainland coast is about evenly divided between Cape Henlopen State Park and intensively used resorts such as Rehoboth, Dewey Beach, and Bethany Beach. Because the major coastal highway from Ocean City (Maryland) to Rehoboth passes through the state park, shore protection may eventually be necessary; the highway is set back enough for shore protection to be unlikely along the ocean for the foreseeable future. Thus, shore protection is likely along less than half of the ocean coast. Most low-lying lands along the barrier bays are expected to be developed, however.
1. The 25-mile Atlantic Ocean shoreline is nearly evenly divided between lands that are likely or certain to be protected and those where shore protection is unlikely or precluded by environmental policies.
---Resort communities that are almost certain to be protected account for 10 miles of developed coastline. An additional 2 miles are likely to be protected, including the southern portion of Cape Henlopen state park..
---Twelve miles of the coast, including the portion of Delaware Seashore State Park beach seaward of Route 1, are unlikely to be protected.
---Shore protection is very unlikely (i.e., no shore protection) along an additional mile of shore within Cape Henlopen state park.
2. Along the inland bays of Sussex County, the prospects for shore protection appear to be largely established along 70 percent of the 53-mile shoreline.
---Nearly 51 percent (27 miles) of the shores of Rehoboth, Indian River, and Little Assawoman bays is already developed and therefore almost certain to be protected.
---State conservation policies within parks and preservation areas preclude shore protection along 19 percent (10 miles) of the inland bay shore.
3. South of Cape Henlopen, 64 percent (12 square miles) of the land within 3 feet of the tides is likely or certain to be protected.
---Eight square miles of land are developed and almost certain to be protected.
---With substantial development pressures along the coast, an additional 4.2 square miles are likely to be protected.
---Nevertheless, more than 6 square miles of dry land and nontidal wetlands within 3 feet of the tides are likely to be available for the landward migration of tidal wetlands.
4. The prospects for protection appear largely established for nearly 73 percent of the Delaware Bay shoreline; however, the prospects for landward migration of tidal wetlands are less clear.
---Although 65 percent of Sussex County's Delaware Bay shoreline is developed and almost certain to be protected, shore protection is less than certain for nearly 90 percent of the area. within 3 feet above the tides.
---Thirteen miles of the shore are conservation areas, including the Prime Hook and Bombay Hook National Wildlife Refuges. Although shore protection is very unlikely, managers of these lands may continue to use engineering approaches to manage the water levels and preserve particular vegetation and animal species.
---Our options still appear to be open in the portion of Kent County and New Castle County south of the Chesapeake and Delaware Canal. Within the 100-year floodplain, the area is unlikely to be developed or protected due to regulations that prevent new development. Agriculture preservation easements also further reduce the potential for development. The regulations and easements, however, do not preclude landowners from protecting the land.
---In Kent County and Sussex County, 13.9 square miles of land less than 3 feet above of the tides are unlikely to be protected or conservation lands. An additional 7.5 square miles of nontidal wetlands exist in the area. Thus, the area of land likely to be available for wetland migration is small compared to more than 90 square miles of current tidal wetlands.
5. The portion of New Castle County north of the Chesapeake and Delaware Canal is more heavily developed than the rest of the Delaware estuary, but also contains several state preservation areas.
---Along New Castle's 34-mile Delaware River shoreline, nearly 26 percent of the shore is either likely or almost certain to be protected.
---An additional 13 miles of shore are part of public parks and state preservation areas that preclude shore protection.
Governments Plan for Development of Most Land Vulnerable to Rising Sea (PDF, 7 pp., 1.3 MB) was originally published in Environmental Research Letters , Issue 3, Volume 4 (2009).