Stephen M. Dickson
Maine Geological Survey
Natural processes cause coastal hazards when shoreline recession threatens coastal residents or properties. Change of the coastline itself is not a hazard until something of value is threatened. The major causes of coastal hazards are storm surge, sea-level rise, erosion, and inlet migration.
Storm Surge. Storm surge is the temporary elevation (or depression) of the sea surface caused by changing atmospheric pressure and strong winds. Coastal Maine sea levels can become elevated from 1 to 3 feet under a storm's center and may persist for the duration of the storm. This elevation is superimposed on and is independent of the tides. The threat of coastal flooding and erosion is greatest when a storm surge is superimposed on spring or perigean high tides. Coastal flooding of 1.5 meters (5 ft) above mean high water (MHW) should be expected in southern Maine once a century and 1.4 m (4.5 ft) above MHW twice a century. Based on past storms, millions of dollars of coastal property damage result from 1.5 m (5 ft) of coastal flooding.
Sea-Level Rise. Historic sea-level rise along the Maine coast has been measured by tide gauge stations. Sea level in Maine has risen erratically over the last several decades. Since 1912, when the Portland tide station was established, sea level has risen at an average rate of 9 inches per century (2.2 mm/yr; Lyles et al., 1988). Over the last millennia sea level (as measured in the geologic record of coastal Maine) has risen at a rate of about 2 inches per century (0.5 mm/yr, see Maine's History of Sea-Level Changes).
Future sea-level rise is expected to accelerate as a result of global warming. The combined effects of coastal sinking and global warming may cause sea level to rise 0.5 to 1.0 m (20 to 40 in) along the Maine coast by the year 2090. These estimates will be refined further by scientific studies.
Erosion. Erosion of beaches and dunes is driven by both storm action and long-term sea-level rise. As ocean levels rise, coastal storm flooding is able to reach farther inland and overtop low-lying dunes more frequently. Even without global warming and accelerated sea-level rise, more frequent flooding of dunes is expected based on the historic record. Erosion and accretion of beaches tend to be variable due to short-term changes in sea level and regional storm histories.
With continuing long-term sea-level rise, erosion should dominate over accretion in most beach locations. Net loss of sand to the seafloor in offshore areas may result from coastal erosion. However, some beach and dune sand may be preserved in the sand dune system if storm waves overwash the dunes and carry sand in a landward direction. In time, the erosion and landward deposition of sand may shift the frontal dune landward, over the back dune environment. In some locations back dunes may form over adjacent salt or freshwater marshes. Consequently, it is important to allow sand-laden flood waters to pass from the beach into and through the dune system. By keeping dune areas open to the flow of flood waters, the dunes can naturally migrate and build upward as sea level rises.
Inlet Migration. Inlets where streams and rivers pass through the dunes and enter the sea are perhaps the most dynamic portion of the sand dune system. Dunes and low energy beaches along the margins of tidal inlets are affected by changes in channel position. The complex tidal mixing of freshwater with saltwater in estuarine channels and the geometry of constricted inlets between dunes result in a complex series of sand bars adjacent to the dunes. These sand bodies form a dynamic network of channels and shoals. As sand bars and channels shift position, erosion and accretion of adjacent dunes and beaches may take place rapidly. Over time there is exchange of sand between the dunes, beach and channel bars.
The dynamic nature of inlet margins makes projection of future shoreline positions difficult. Positions of past shoreline provide some guidance of the variability and rates of shoreline migration along inlet margins. Hazards from channel meandering and erosion exist adjacent to tidal inlets.
In summary, the changes forced upon the beach and dunes by storms, sea-level rise, erosion and inlet migration are natural and necessary for the future continuation of these environments. The dynamic nature of the sand dune system strongly contrasts with the fixed nature of human development on dunes and can lead to the creation of coastal hazards.
Anticipatory Planning for Sea-Level Rise Along the Coast of Maine, 1994: Maine State Planning Office (38 State House Station, Augusta, Maine, 04333).
Belknap, D.F., Andersen, B.G., and 12 others, 1987, Late Quaternary Sea-Level Changes in Maine, in Nummedal, D., Pilkey, O.H., and Howard, J.D. (eds.), Sea-Level Fluctuation and Coastal Evolution, Society of Economic Paleontologists and Mineralogists, Special Publication No. 41, p. 71-85.
Lyles, S.D., Hickman, L.E., Jr., Debaugh, H.A., Jr., 1988, Sea Level Variations for the United States, 1855-1986, U.S. Department of Commerce, Rockville, Maryland, 182 p.
Last updated on April 23, 2012