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Coastal Erosion - Frequently Asked Questions
A tide gauge in Portland, ME has been monitoring tidal fluctuations and sea level since 1912. Based on this data, it appears that sea level has been rising at Portland at a rate of about 0.61 feet/century (1.87 mm/yr). Additional sea level records for other parts of Maine are available from the National Oceanic and Atmospheric Administration (NOAA) National Ocean Service (NOS).
Shoreline erosion is driven in part by the elevation of the high tides. As sea level rises, the height of the high tide rises and the height of the coastal flood plain rises. A higher floodplain will alter the frequency and inland extent of property damage from floods. Waves and currents can erode soil, bluffs, and beaches when they wash ashore at higher and higher levels. Salt water will reach farther inland and damage roots of trees, shrubs, and grasses. Underground salt water will flow farther inland and "intrude" on freshwater aquifers, perhaps turning some coastal wells salty. As the ocean rises, all coastal environments - salt marshes, mud flats, exposed ledge, and beaches - will attempt to migrate inland. If the transgression of marine environments over terrestrial ones is prevented, then some loss of coastal wetlands can be expected. Over decades, coastal infrastructure - docks, pipelines, roads, utilities, among others - will need to be rebuilt at higher levels or farther inland to provide an equal amount of protection or service.
The Intergovernmental Panel on Climate Change (IPCC) predicts that global sea level will rise at an accelerated rate in the next century. The IPCC Third Assessment projects that global sea level will rise an average of 9-88 cm (4-35 inches) with a range between 20-70 cm (8-28 inches) more likely by 2100. Using IPCC and US EPA information, the Maine coast has a 50% chance of experiencing a 55 cm (22 inch) sea-level rise by 2100 due to climate change. The Maine coast is likely to experience an additional sea-level rise of 36 cm (14 inches) over and above the current trend by 2100 due to climate change.
Similar to the winter-summer shape variation in beach profiles, beaches undergo changes during storms and in response to sea-level rise. During storms, waves attack the berm and dunes, causing overtopping of the dunes and overwash. At the same time, the berm and dunes are eroded and sediment is transported offshore and deposited in sandbars. This causes waves to break farther offshore, decreasing the wave energy that reaches the beach.
As sea level rises, the same process occurs: waves can attack the upper part of the beach profile, pushing sand over the dune in a process called overwash. At the same time, sand is pulled offshore. The barrier beach migrates landward, rolling landward over itself. The initial beach migrates landward over its own marsh into its second position. This is why you can find peat deposits, tree stumps, and oyster shells in the surf zone. Think of the beach as a tread on a tank rolling over itself in a landward direction.
Most beaches undergo seasonal variations from a lean, sediment-starved "winter" profile and a sediment-rich "summer" profile. The wave climate during the winter is much more energetic. Large, short-period waves associated with storms tend to erode the beach and berm and move sediment seaward into an offshore bar. The beach profile is typically steeper with a much smaller or non-existent berm. Smaller, longer-period waves during the summer tend to move sediment back onto the upper portion of the beach, rebuilding the berm and creating a less steep beach profile.
Major factors on beach morphology in Maine include sea-level rise, waves, currents, tides, and winds, underlying geology, sediment supply, shoreline stabilization, development pressure, and recreational usage. Sea level at Portland, Maine has risen about 0.6 ft in the last century. Global sea level is forecast to rise about 2 feet in the next 100 years. Beaches transgress, or move landward, in response to sea-level rise. If sea level rises faster than sediment supply can keep up, beaches can disappear. Waves, currents, tides and winds are daily forces that influence the shape of our shorelines on time-scales that humans can plainly see. Underlying geology can influence sediment supply, the pathways along which sediment may move, and where accretion or erosion takes place. Sediment supply is extremely important in determining whether a beach will erode or accrete. Beaches along the Maine coast are supplied with sediment from rivers and the erosion of existing materials in the nearshore and bluffs alongshore. Shoreline stabilization influences how a beach responds to hydrodynamic forces. About 50% of Maine's sandy beaches are stabilized with seawalls. Development pressure along sandy beaches in Maine is very high because it is a prime location to live. Finally, recreational usage is an important factor. One of every 2 Mainers live near the coast, and over 6 million people visit Maine's coastline each year, the majority of these tourists visit sandy beaches (MCP, 2002).
Some of the worst erosion, on the order of 2-3 feet/year, is occurring at Camp Ellis Beach in Saco. Over 30 homes have been lost to the sea since 1908. The erosion in the area is caused by a lack of natural sediment to adjacent beaches due to the presence of the northern jetty of the Saco River (placed in 1869), wave focusing on Camp Ellis Beach due to offshore bathymetry, and reflected wave energy that directs wave energy from the jetty onto the Beach. See our slide show on Coastal Processes and Beach Erosion (pdf format - 2.5 Mb) for more information on Saco Bay and Camp Ellis Beach.
Seawalls are shore-parallel coastal engineering structures that are meant to protect properties behind them, not the beach or sand dune system. They have been determined to harm the beach and sand dune system per the Coastal Sand Dune Rules (Chapter 355). About 50% of Maine's sandy beaches are lined with seawalls. Seawalls inhibit the natural migration of the beach and dune system by providing a solid barrier which deters the movement of sand from the dune and beach. When a wave strikes a seawall, the energy of the wave is dissipated on the wall and reflected both up, and down, into the beach. This leads to the movement of beach sand offshore, into quieter water conditions. Wave action on seawalls can lead to a gradual lowering of the beach and loss of dry beach. Along many seawalled beaches, there is no dry beach at high tide. New seawalls and the extension of existing seawalls on beaches are illegal in Maine.
Groins are shore-perpendicular coastal engineering structures. Groins are usually placed in groin "fields" (multiple groins) and are meant to inhibit the alongshore movement of sand and "catch" sand. This usually results in a wider, accretive beach on the updrift side of the groin, but a receding, erosive beach on the down-drift side of the groin. This is due to the interruption of the alongshore transport of sand by the groin. Groins are illegal structures in Maine.
Jetties are similar structures to groins but located on either sides of an inlet. They are usually longer than groins and are meant to stabilize the channel of the inlet for safe navigation, and deter the natural flow of sediment into the inlet. Large jetties along Maine's sandy coast are located at some of the larger tidal rivers, including the Scarborough River, Saco River, and Webhannet River. Some of the worst erosion problems in Maine are located next to a jetty (Camp Ellis Beach in Saco). See our slide show on Coastal Processes and Beach Erosion (pdf format - 2.5 Mb) for more information on Saco Bay and Camp Ellis Beach.
MGS is working to develop maps of historic shoreline changes along Maine beaches, including maps of Erosion Hazard Areas. When these are finished, they will be available on our website, in hard copy, and as data layers for Geographic Information Systems.
The Maine Geological Survey has been mapping the spatial extent of bluffs for the majority of the Maine coastline. A bluff is defined as a steep shoreline slope formed in sediment (loose material such as clay, sand, and gravel) that has three feet or more of vertical elevation just above the high tide line. Cliffs or slopes in bedrock (ledge) surfaces are not bluffs and are not subject to significant erosion in a century or more. Beaches and dunes do not form bluffs, except along the seaward dune edge as a result of erosion. Coastal Bluffs Maps show the shoreline type and relative stability of bluffs along the Maine coast. The slope, shape, and amount of vegetation covering a coastal bluff and the adjacent shoreline are directly related to the susceptibility of the bluff face to ongoing erosion. Bluffs are classified as stable, unstable, and highly unstable.
Unstable coastal bluffs can result in landslides. MGS has produced a series of Coastal Landslide Hazard Maps which show locations of known landslides and areas of potential landslide hazard on bluffs along the Maine coast.
Last updated on March 29, 2013
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