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Coastal Marine Geology - Frequently Asked Questions
Sea level, tides, and datums
Currents, waves, and storms
Beaches, Dunes, and Morphology
Bluffs and Landslides
Sea level, tides, and datums
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).
Historic sea level changes can be determined from the geologic record using radiocarbon dating of fossil shells and wood from borings conducted in marshes and in the nearshore ocean. About 13,000 years ago, relative sea level was about 70 m (230 ft.) above present. As glacial ice retreated inland, the land rebounded, but the sea was able to invade the shoreline creating a vast inland sea, the DeGeer Sea (named for a famous Swedish geologist). Ultimately, the release of the weight of the ice allowed rebound of the land (isostatic rebound) to outpace the rate of global sea-level rise. Thus, local relative sea level fell, at rates up to 43 mm/yr (1.7 in/yr), until it reached a local lowstand 55 m (180 ft.) below present sea level at about 11,000 years ago. This caused the coastline to move well offshore of the present coast. After this lowstand, global sea-level rise rates again became dominant as the rate of isostatic rebound waned. Local relative sea-level rose at rates up to 22 mm/yr (0.9 in/yr), until the rate of rise slowed again, dramatically, about 9000 BP. This probably related to large-scale warping of the crust at a distance from the ice sheet. Finally, sea-level rise resumed, and approached its present level over the past 5000 years. The record of sea-level changes in Maine over late Holocene (the past 5000 years) is much more detailed than the record discussed above. Indeed, Maine has one of the best-studied Holocene sea-level curves in the world. We use salt marsh peats, which grow almost precisely up to mean high tide level, as markers of sea level. Salt marsh peats are extremely well preserved in many marshes in Maine, sometimes to thicknesses as great as 5 m (16 ft.) and, even if not connected with a modern marsh, fragments of older marshes can be found in cores offshore. Radiocarbon dating supplies the time information to construct a sea-level curve from these preserved peats.
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.
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.
A tide chart will provide a tidal elevation and time (e.g., high tide of +8.7 ft at 10:35 a.m.) for a known tidal station, such as at the Portland tide gauge. The chart will also provide tidal corrections for different locations along the coastline (e.g., Presumpscot River Bridge, high water height *1.01 and time +0:01). This information tells you that to calculate the time and elevation for high tide at the Presumpscot River, you must multiply the provided high tide for Portland by the correction, and add time to the given tide, since a plus sign is given. Thus, the high tide at the Presumpscot River Bridge would be +8.79 ft (+8.7 ft * 1.01) at 10:36 a.m. (10:35 + 0:01).
The National Oceanic and Atmospheric Administration (NOAA) National Geodetic Survey (NGS) provides a simple online conversion site called VERTCON. Simply input the latitude, longitude, and elevation (in meters) in one datum to get the elevation in the other. U.S. Army Corps of Engineers software CORPSCON can be downloaded and run on a personal computer and also make the conversions.
Maine's coastline varies dramatically in terms of tidal fluctuations. Tides in Maine are semi-diurnal, meaning there are 2 highs and 2 lows in a 24-hour period. Tides range from 5.6 meters (18.4 feet) in Eastport to 2.6 meters (8.7 feet) in Kittery. The tidal range increases with the new and full moon phases to create spring tides that are 10 to 15% larger than the mean range. Online tidal forecasts and historical records for Maine are available from the National Ocean Service.
The Bay experiences a natural tidal funneling effect (and sloshing effect, known as a seiche) which causes tidal ranges of over 10-15 meters. The seiche has a period of 13 hours (780 minutes), which corresponds almost exactly with the lunar tidal cycle of the Atlantic (745 minutes), creating resonance conditions that the tidal flow amplifies the seiche, causing the highest tides in the world. Visit the Parks Canada website for more information.
Currents, waves, and storms
The Gulf of Maine has several different real-time monitoring programs that utilize buoys to provide information on wave height, period, wind speed, wind direction, barometric pressure, and other hydrodynamic and meteorological conditions that are updated every hour. Several of these buoys are located close to shore (within 12 miles), while others are located farther offshore. See the National Data Buoy Center and the Northeastern Regional Association of Coastal and Ocean Observing Systems (NERACOOS) websites. Wave models are also used to predict conditions in the North Atlantic by the Navy's Fleet Numerical Meteorology and Oceanography Center, and marine forecasts for coastal and offshore waters are available through NOAA's National Weather Service.
Usually, hurricanes are either relatively weak (Category 1), or of tropical storm strength when they threaten the Maine coastline. However, the chance of Maine being hit by a hurricane does exist. The United States Landfalling Hurricane Probability Project is part of the Tropical Meteorology Research Project at Colorado State University and the GeoGraphics Laboratory at Bridgewater State College. It provides information on probabilities of landfall, including past landfalling hurricanes and tropical storms.
The cold waters of the Labrador Current flow southeastward from the Arctic, along the eastern coast of Canada and into the Gulf of Maine, bringing with it arctic waters year-round. Visit the Northeastern Regional Association of Coastal and Ocean Observing Systems (NERACOOS) website for more information.
In addition, complex bathymetry in the Gulf of Maine, including the presence of channels, basins, and banks, influences subtidal circulation. Subtidal circulation in the Gulf includes a nearshore surface component ("buoyant" water with freshwater input, at depths above -75 m) and deep component (more dense, "salty" water, at depths below -150 m). During summer, a nearshore component brings colder water of the Labrador current and wraps it around Nova Scotia into the Gulf. Additionally, a more dense and cold deep component enters the Gulf through the Northeast Channel into the deeper basins (e.g., Jordan Basin), where it is mixed through the water column by upwelling, overturning, and boundary mixing. Wind-driven upwelling (due to surface winds blowing offshore) also occurs in summer along the Maine coastline, pushing warmer surface waters offshore and bringing colder, deeper water closer to shore (Gulf of Maine Scientific Workshop, 1991).
A rip current is a strong current that flows seaward from the shore. They typically form in response to patterns of breaking waves and sand bars in the surf zone. If caught in a rip current, never try to swim against it; instead, swim parallel to the beach. More information on rip currents is provided by NOAA.
A 100-year storm is an event of the magnitude that has a 1 percent chance of occurrence in any given year, which means it has a 63% chance of occurring in 100 years. One hundred year storms have certain flood/runup elevations (called Base Flood Elevations) that are expected with this type of an event. Depending on the location on the coast, flood elevations vary from place to place in Maine. These BFEs are provided in Federal Emergency Management Agency Flood Insurance Rate Maps (FEMA-FIRMs). These maps, which show mapped flood zones, are available in hard copy from FEMA, and can be viewed at your local town office. If you have access to a computer, maps are available at the Maine State Planning Office's Floodplain Management Program, and also in digital format from the Maine Office of Geographic Information Systems.
Beaches, Dunes, and Morphology
Only about 2% (70 miles) of Maine's coast is considered beach (sand and cobble), with about 35 miles of sandy beach concentrated in the southern portion of the state, from Portland south to Kittery. The majority of Maine's beaches are bound by rocky headlands, making them called pocket beaches or closed littoral cells.
Rivers provide sediment to some beaches in Maine, but there are only a few large rivers (i.e., the Saco or the Kennebec), and must of the sediment that used to naturally flow down the rivers into the ocean has been trapped by numerous dams upstream. Most beaches are enclosed littoral cells and have a limited sediment supply available (reworked offshore material or material eroded from cliffs and bluffs).
Characteristics of a typical beach system illustrates the dominant features of the typical beach system, from the breaker zone landward to the marsh.
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.
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.
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).
Saco Bay has about 7 miles of contiguous sandy beach. It is bound by Fletcher Neck and Biddeford Pool in the south, and Prouts Neck to the north. It has 2 main rivers - the Saco River in the south and Scarborough River at its north end. The dominant direction of sand transport in the bay is from south to north. 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.
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.
The State of Maine Beach Profiling Project is a cooperative program run by the Maine Sea Grant/University of Maine Cooperative Extension, Maine Coastal Program, and the Maine Geological Survey. This program utilizes volunteers who monitor sandy beaches on a monthly basis. The Project employs simple yet relatively accurate methods to collect beach profiles along Maine's sandy beaches.
The Maine Geological Survey has been monitoring specific areas, such as Camp Ellis Beach, using its Nearshore Survey System (NSS). The NSS employs a personal watercraft outfitted with state-of-the-art positioning and depth sounding technology to survey bathymetry and currents in the nearshore and surf zone.
Q25. How do I know where the dunes are in Maine, or if my house is located in mapped dune areas? Back
Annotated aerial photos showing Beach and Dune Geology are available for the majority of the sandy and cobble beaches within the State. These maps identify areas of beach, frontal dune and back dune, and other geologic environments. Visit our publications page to purchase copies of these photos or view them online.
Activities within the coastal sand dune system are regulated by the Department of Environmental Protection. You may need a permit under the Natural Resources Protection Act. Please contact the Department for more information (also, see the answer to the next question: "How are Maine's beaches regulated?").
Activities on Maine beaches are regulated by the Department of Environmental Protection under the Natural Resources Protection Act (NRPA), which has several chapters dealing with beach-related activities, including Chapter 355 - Coastal Sand Dune Rules, Chapter 305 - Permit-by-Rule, and Chapter 310 - Wetlands Protection.
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.
Volunteer to become part of the State of Maine Beach Profiling Project, which is a cooperative program run by the Maine Sea Grant/University of Maine Cooperative Extension, Maine Coastal Program, and the Maine Geological Survey. This program utilizes volunteers who monitor sandy beaches on a monthly basis. The Project employs simple yet relatively accurate methods to collect beach profiles along Maine's sandy beaches.
You will probably need a Permit-by-Rule from the Department of Environmental Protection to create a dune. You should contact a qualified coastal geologist or engineer for information on how to design and build a dune. Also, native vegetation (usually American beach grass) should be used to help stabilize the dune, along with strategically placed sand fencing. MGS hopes to release information in the near future that would help provide guidelines on "how to create a sand dune."
Q31. I'm rebuilding my house, which is in the dune system. Why should I put my house up on posts? Back
MGS highly recommends that anyone rebuilding their home within the coastal sand dune system, in both frontal and back dune areas, place their home on posts. By placing your house on posts, you are moving the majority of the structure out of any floodplains or erosion hazard areas. If waves were to overtop a seawall or sand dune and encroach on your lot, the minimized surface area of posts will allow for the natural transport of sand underneath your house, while at the same time protecting your house by moving the home up and away from the energy associated with waves. The amount of energy hitting a solid foundation is much higher than the amount of energy hitting several 1-foot diameter posts. We highly recommend that you follow the FEMA Coastal Construction Manual for any home rebuilding within the coastal floodplain or the coastal sand dune system.
Bluffs and Landslides
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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