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New Shoreline Change Data Reveal Massachusetts is Eroding...continued

Figure 3, below, shows the status of shoreline change for Cape Cod, Nantucket and Martha's Vineyard communities. Note that eroding transects predominate in most communities. The highest rates of erosion and the longer expanses of eroding shoreline within a community are generally located along high-wave energy, open-ocean shores. For example, the Eastham shore exhibits the highest number of eroding transects at 98 percent (2 percent accreting), followed by Truro at 83 percent eroding, (16 percent accreting), and Wellfleet at 81 percent eroding, (18 percent accreting). These communities are exposed to both predominant wind and waves from the northeast, and prevailing winds and waves from the west. Other communities have less severe erosion problems, such as Falmouth at 67 percent eroding (29 percent accreting) and Mashpee at 69 percent eroding (30 percent accreting), due to the sheltering effects from ocean storm waves by the islands of Martha's Vineyard and Nantucket.

Click here for a larger version of this image.

Figure 3: Percent of linear length of shoreline eroding, accreting and stable,
by town, for Cape Cod, Nantucket, Martha's Vineyard, Massachusetts,
based on the long-term shoreline change data.
Credit: Jim O'Connell, WHOI Sea Grant/CCCE

Only three Cape Cod communities have a greater number of accreting transect locations than eroding transects, including Harwich at 63 percent accreting (36 percent eroding), which is protected from ocean storm waves by Monomoy Island. Also Provincetown at 62 percent accreting (37 percent eroding), which receives a large volume of sand from the eroding Cape Cod National Seashore bluffs.

A number of factors determine whether a community exhibits greater long-term erosion or accretion:

• exposure to high-energy storm waves,
• sediment size and composition of eroding coastal landforms feeding adjacent beaches,
• near-shore bathymetric variations which direct wave approach,
• alongshore variations in wave energy and sediment transport rates,
• relative sea level rise, and
• human interference with sediment supply (e.g. revetments, seawalls, jetties).

Interpreting Shoreline Change Data: Proceed with Caution!

A word of caution when reading long-term shoreline change rates: always analyze the short-term data that were used to calculate the long-term shoreline change rate. If short-term trend reversals in shoreline change have occurred (accretion to erosion or vice versa), it may be more appropriate to use the most recent short-term shoreline change rate than the long-term rate for siting a structure or for planning purposes.

For example, transects along the Codfish Park area of Nantucket's eastern shore show a long-term accretion rate of approximately +1.5 feet per year. However, the shoreline has been eroding since the 1950s, and erosion has accelerated since 1978 to 7-10 feet per year (Figure 1, above). The long period of accretion that took place from the mid-1800s to the 1950s biases the long-term rate, making the data suggest that the area is stable or accreting. The trend reversal and continuing erosion since the 1950s, however, illustrates the importance of analyzing short-term data and its potential utility in determining present-day construction setbacks and for planning purposes.

The widespread construction of coastal engineering structures, such as revetments, seawalls, jetties and groins -- particularly since the 1940s and 1950s -- has also affected shoreline change rates. In many areas, these coastal engineering structures have contributed to a trend reversal or accelerated downdrift erosion rates, and therefore their effects must be factored into analyzing long-term shoreline rates. The northern area of Humarock Beach in Scituate is a case in point, where erosion rates have accelerated in recent years due to both natural and human effects. The shoreline area east of Sandwich Harbor in Sandwich shows erosion has accelerated due, in part, to the effects of jetties.

Human activity, however, is not the sole reason for trend reversals and shoreline changes. In some areas, such as the southeastern shore of Nantucket, natural processes are responsible for large trend reversals (accretion to erosion back to accretion to erosion) over the 150-year study period. In this area, the data reveal that the shoreline has fluctuated between 50 to 100 feet of both erosion and accretion resulting in a long-term average suggesting stability. The shoreline is, however, exceptionally variable.

Ongoing Shoreline Change Analyses

WHOI Sea Grant and Cape Cod Cooperative Extension are conducting a detailed analysis of the recent shoreline change data to better understand why some areas are eroding and others accreting. They are also documenting areas where the use of short-term data may be more appropriate than long-term rates for planning and safe set-backs of buildings and other structures.

For more information on shoreline change, coastal processes, or erosion control alternatives contact WHOI Sea Grant or Barnstable County Cape Cod Cooperative Extension.

The Massachusetts shoreline change update project was completed by Dr. E. Robert Thieler, U.S. Geological Survey, Coastal and Marine Geology Program, Woods Hole, MA; Courtney Schupp, Virginia Institute of Marine Science, VA; and, Jim O'Connell, Woods Hole Oceanographic Institution Sea Grant Program and Cape Cod Cooperative Extension, Woods Hole, MA. The project produced shoreline change maps and an accompanying detailed technical report, "The Massachusetts Shoreline Change Project: 1800s to 1994: Technical Report," available as a USGS Administrative Report.

The shoreline change maps and data can be viewed on the Massachusetts CZM web site (http://www.state.ma.us/czm/czm.htm).

References

• Leatherman, S.P., 1993, Modes of Shoreline Behavior: Erosion Rate Analysis Using Geomorphic Principles, Proceedings of International Coastal Symposium, Hilton Head Island, S.C.
• O'Connell, J.F., Thieler, E.R., and Schupp, C., 2002, New Shoreline Change Data and Analysis for the Massachusetts Shore with Emphasis on Cape Cod and the Islands: Mid-1800s to 1994, Environment Cape Cod, Vol. 5, No.1.
• Pilkey, O.H. and Thieler, E.R., 1992, Erosion of the United States Shoreline, Quaternary Coasts of the United States: Marine and Lacustrine Systems, SEPM, Special Publication No. 48.
• Schupp, C., Thieler, E.R., and O'Connell, J.F., 2001, Mapping and Analyzing Historic Shoreline Changes in Massachusetts, Proceedings of CoastGIS '01: Managing the Interfaces, Halifax, Nova Scotia, June 18-20, 2001.
• Thieler, E.R., O'Connell, J.F., and Schupp, C., 2001, The Massachusetts Shoreline Change Project: 1800s - 1994 Technical Report, U.S.G.S. Administrative Report, Woods Hole, MA.

This bulletin should be referenced as follows: WHOI Sea Grant Program, 2003, Marine Extension Bulletin, "New Shoreline Change Data Reveal Massachusetts is Eroding," by Jim O'Connell, WHOI Sea Grant Program and Cape Cod Cooperative Extension.