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SHORELINE CHANGE IS A NATURAL, NECESSARY PROCESS.
photo by Rebecca Haney
RAPIDLY ERODING COASTLINE IN NANTUCKET.
(In PDF Format, 361K)
MASSACHUSETTS SHIFTING SHORELINES: NEW DATA ON SHORELINE CHANGE
Coastal shorelines change constantly in response to wind, waves, tides, sea level fluctuation, seasonal and climatic variation, human alteration, and other factors that influence the movement of sand and material within a shoreline system. Major storms, such as hurricanes, can have dramatic and immediate impacts. The loss (erosion) and gain (accretion) of coastal land is a visible result of the way shorelines are reshaped in the face of these dynamic conditions.
To help make informed and responsible decisions,
coastal managers, shore front landowners, and
potential property buyers need information on both
current and historical shoreline trends, including
reliable measurements of erosion and accretion rates
in non-stable areas. The goal of the Massachusetts
Office of Coastal Zone Management (CZM)
Shoreline Change Project is to develop and distribute
scientific data that will help inform local land
A word of caution to shorefront landowners, potential buyers, and others interested in this information as it relates to a particular property—the Shoreline Change Project presents both long—and short-term shoreline change rates at 40-meter intervals along the entire Massachusetts coast. In a broad sense, this information may provide useful insight into the erosional forces at work along the Massachusetts coast. But care must be used when applying this information to specific property or local sections of coastline. Due to the multitude of natural and human-induced factors that can influence shoreline position over time, correct interpretation of the data requires an in-depth knowledge of coastal processes. Those without extensive backgrounds in coastal geology, while encouraged to explore the historical migration of Massachusetts shorelines, are strongly advised to consult with a professional when attempting to use the Shoreline Change Project data for planning purposes. In no case should the long-term shoreline change rate be used exclusively before the short-term rates and contributing factors are understood and assessed.
HOW AND WHY SHORELINES CHANGE
Erosion, transport, and the accretion that results are continuous and interrelated processes. Every day, wind, waves, and currents move sand, pebbles, and other small materials along the shore or out to sea. Shorelines also change seasonally, tending to accrete slowly during the summer months when sediments are deposited by relatively low energy waves and erode dramatically during the winter when sediments are moved offshore by high energy waves and storm surge. Hurricanes, Northeasters, and other major storms, of course, can cause severe erosion whenever they strike.
SHORELINE CHANGE AND COASTAL PROPERTY
While erosion and flooding are necessary and natural, they do have the potential to damage coastal property and related infrastructure, particularly when development is sited in unstable or low-lying areas. These dynamic and powerful processes can expose septic systems and sewer pipes, contaminating shellfish beds and other resources; release oil, gasoline, and other toxins to the marine environment; and sweep construction materials and other debris out to sea. Public safety is also jeopardized when buildings collapse or water supplies are contaminated.
Shoreline change can result in significant economic and emotional loss in a system of fixed property lines and ownership. Attempting to halt the natural process of erosion with seawalls and other hard structures, however, simply shifts the problem, subjecting downdrift property owners to similar losses. Also, without the sediment transport associated with erosion, some of the Commonwealth's greatest assets and attractionsbeaches, dunes, barrier beaches, salt marshes, and estuariesare threatened and will slowly disappear as the sand sources that feed and sustain them are eliminated.
The challenge, therefore, is to site coastal development in a manner that allows natural physical coastal processes, such as erosion, to continue. To meet this challenge, coastal managers, property owners, and developers must work with erosionnot against itby understanding the magnitude and causes of erosion, and applying appropriate management techniques that will allow its beneficial functions to continue.
THE CZM SHORELINE CHANGE PROJECT
It is very important to note that due to necessary adjustments in the baseline for this project, the location of current transect numbers are not consistent with those reported on the 1997 shoreline maps or data tables. Therefore, shoreline rates of change noted at the end of numbered transects on these shoreline change maps and data tables should not be compared directly with previous numbered transects.
HOW TO CORRECTLY INTERPRET THE DATA
For example, a transect along Springhill Beach in Sandwich that is downdrift from a jetty shows a long-term annual erosion rate of -2.82 feet per year. From 1860-1952, the average rate of erosion at this transect was -3.74 feet per year. From 1952-1994, however, the annual erosion rate was only -0.20 feet per year. These rates show that, following construction of the jetty in 1914, there was an accelerated short-term rate of erosion. Once the shoreline adjusted to the presence of the jetty, however, the erosion rate decreased and leveled off. If the jetty was properly engineered and is properly maintained, the short-term erosion rate of -0.20 feet per year is more likely to be representative of how this shoreline is functioning currently and should be used for planning purposes, rather than the long-term rate, which dampens the effect of the jetty installation.
In contrast, an area along the southeastern shore of Nantucket exhibits a long-term annual shoreline change rate of +0.07 feet per year with a net movement of the beach of only 34.6 feet from 1846-1994. This beach is far from stable, however, as illustrated by an analysis of the short-term rates of change. Between 1846 and 1887 the beach accreted 215 feet; from 1887-1955 it eroded 12 feet; from 1955-1978 it eroded 113 feet; and from 1978-1994 this same beach eroded 56 feet. Despite the apparent long-term statistical stability of the beach, any buildings constructed on the accreting beach would have been threatened when the erosional trend returned, a situation that is presently occurring at Low Beach on Nantucket.
In addition, in many cases human attempts to stop erosion result in a change to the natural equilibrium of the shoreline. Where segments of the shoreline have been armored with sea walls and other structures to stop erosion, the shoreline change data must be looked at very closely to determine what affect these structures are having on short- and long-term erosion rates. For example, much of the sand sources for Humarock Beach in Scituate have been eliminated due to seawall and revetment construction in the 1940s and 1950s. Consequently, the recent trend of erosion that began in the 1950s is not only continuingit is accelerating.
This review will give you a good sense of how a particular shoreline has behaved over time, and may provide an indication of future shoreline behavior. However, professional expertise is necessary when attempting to use these maps and data for planning purposes.
FOR MORE INFORMATION . . .