Over the years, we have received requests from Conservation Commissions and planners to review the accuracy of land and tidal elevations specified on engineering plans submitted with local permit applications. While land surveyors have long-established practices for tying land elevations on a property to local benchmarks, it turns out that tying “tidal datums” like the “high tide line” and “mean high water” (two very different elevations by the way) to land elevations is complicated and nuanced. This is because the real world elevation of tidal datums differs all around Massachusetts, and can even differ appreciably between the upper and lower reaches of a large tidal estuary. Field estimates vary with the lunar and solar cycles, and weather conditions. Exactly where a tidal datum lies on a property can affect the cost of a project, or even whether an activity is permittable under state or local regulations. Disputes over the location of these tidal datums have been the subject of lawsuits about property ownership and regulatory jurisdiction.

Because tying tidal datums to land elevations is complicated, many municipal boards and their staff often accept at face value tidal datums specified on site plans. However, these lines may be wrong, or the elevation definition applied by the surveyor may not match the definition used in an applicable regulation. In Connecticut, the state agencies were so frustrated by inconsistent high tide line elevations reported by surveyors and engineers, the state legislature passed a law defining its own coastal jurisdiction high tide line boundary for municipalities and water bodies in the state (see the CT Coastal Jurisdiction Line Fact Sheet page).

This past summer, CZM’s North Shore regional coordinator, Kathryn Glenn, came to us with a problem. She reported that the chair of a conservation commission complained to her that over the years the commission received plans for coastal projects citing a wide range of elevations for mean high water and the high tide line. The commission was frustrated because they did not understand why the elevations on the plans were different, or how they could objectively evaluate them.

Because we had tackled similar issues in the Buzzards Bay watershed, we decided to do all the calculations necessary to create an interactive tidal datum viewer map of the entire Massachusetts coast to enable anyone to lookup the precise land elevation of any tidal datum in their area. We also added a tidal datum information page to help both specialists and the public understand why these elevations are relevant, and which are used to draw the coastline on maps. To create the viewer, we used tidal data and a tidal datum model from NOAA (VDatum).

While much of the information on these pages will be of interest only to engineers and regulators, one product of this effort that will be of interest to a broader audience, are the statewide tidal data maps at the bottom of our tidal datum information page, like the one below. Massachusetts, or more specifically, Cape Cod, is at the boundary between the Gulf of Maine tidal regime and an upper mid-Atlantic tidal regime. Because these tidal regimes are offset by about 3 1/2 hours, where the two regimes meet (roughly a line east of Woods Hole through Martha’s Vineyard, to Nantucket), tides are suppressed. This is why, along the Shining Sea Bikeway in Falmouth, the mean tidal range is only 1 foot. In contrast, on the Cape Cod Bay side of Wellfleet, Brewster, and Orleans, the mean tidal range is 10 feet.

Mean tidal range (MN = MHW -MLW) in feet around Massachusetts.

These differences in tidal ranges also have profound effects on the kinds and extent of tidal habitats found along the Massachusetts coast, like salt marshes. Generally, salt marshes grow between the high tide line and local mean sea level. To aid scientists studying salt marsh migration using LiDAR data and field measurements, we created a Salt Marsh Lower Boundary page to explain how our tidal datum calculations can be used in studying salt marsh loss and migration in the face of rising sea levels from climate change.

On September 6, 2017, the Buzzards Bay National Estuary Program announced the award of $135,000 in federal grant money for land protection projects and water quality monitoring in Buzzards Bay. The funding includes three grants to help towns in the Buzzards Bay watershed protect important habitat, including an Atlantic white cedar swamp, and funding to support long-term monitoring efforts in Buzzards Bay. All three land-protection projects are in the Mattapoisett River Valley watershed, which provides drinking water to four municipalities. The grants are being matched by $65,700 in municipal and private contributions that will support the water quality monitoring effort, help permanently protect important habitat and promote passive recreation for the enjoyment of natural resources. The following grants were awarded:

The Town of Acushnet will receive $35,000 to work with its partner, the Buzzards Bay Coalition, to protect 38-acres in the Towns of Acushnet and Fairhaven. The project will protect land that benefits water resources associated with Tripps Mill Brook and the Mattapoisett River. It will also protect wildlife habitat, expand trail-based recreational opportunities and protect a large contiguous undeveloped block of forest.

The Town of Mattapoisett will receive $35,000 to work with its partners, Mattapoisett Land Trust and Buzzards Bay Coalition, to protect a 53-acre property, also in the Mattapoisett River Valley, designated as habitat for rare species. The property also contains an old historic quarry with deep ties to New England history and culture. The Town and its partners intend to use the property for environmental and historical educational purposes.

The Town of Rochester will receive $35,000 to work with its partners, Rochester Land Trust and Buzzards Bay Coalition, to permanently protect a 78.6-acre property, which contains extensive wetlands, including an Atlantic white cedar swamp. The property, located in a large undeveloped forest area near the dividing line of the Mattapoisett and Sippican River watersheds, will provide trail-based recreational opportunities for the public.

The Buzzards Bay Coalition will receive $35,000 to support an additional season of monitoring of the Baywatchers program. Baywatchers is a comprehensive volunteer-based water quality monitoring program, which has been in existence for 25 years. With the help of trained volunteers, basic water quality measurements of dissolved oxygen, temperature, salinity and water clarity will be made at approximately 150 stations in and around Buzzards Bay. The water quality data collected will be used to track nutrient pollution effects and will be made available to federal, state and local decision makers.

Additional details about the award are found on this EEA press release.

Last year we received a call from a Bourne resident concerned about a die-off of vegetation in the Patuisset salt marsh in Bourne. A review of aerial images shows a continuous loss of the marsh over many decades, but the rate of loss appeared more pronounced during the past decade.  The March 1995 and October 2016 aerial photographs of the Patuisset marsh in Fig. 1 show a stark difference in both channel widths, and the amount of vegetation near the marsh channel banks.

Fig. 1. Comparison of the extent of Patuisset salt marsh in Bourne between March 1995 (top), and October 2016 (bottom). Note both the widening of channels, and the dramatic loss of vegetation away from the channels.

Recently CZM’s South Coast Regional Coordinator, Dave Janik, provided these photographs of an apparent die back of vegetation away from the creek banks of a salt marsh in Mattapoisett (Fig. 2).

Fig. 2. Loss of vegetation along a salt marsh bank in Mattapoisett, observed June 21, 2017. This pattern of loss is consistent with damage caused by the purple marsh crab. Click to enlarge. Photo credit: Dave Janik.

As noted in the Buzzards Bay Coalition’s article Study shows Westport Rivers losing salt marshes at an accelerating rate, and our own posting, the loss of salt marshes can occur for many reasons.  Since the last ice age, sea level rise has caused the continued loss or migration of salt marshes inland, but at many sites, new losses exceed that expected from sea level rise alone.  Studies elsewhere in the region of rapid marsh loss and die-off have pointed to causes that include population explosions of the purple marsh crab (Sesarma reticulatum) and nitrogen pollution. The articles below have information about the subject for a general audience. If you know a site in Buzzards Bay with salt marsh die-off, please email photographs or information to tracy.warncke@state.ma.us.

Additional Reading

Salt Marsh Dieback and the Purple Marsh Crab on Nantucket

The Not-So-Mysterious Loss of Salt Marshes and Ecosystem Services

Wikipedia: Salt marsh die-off