Buzzards Bay NEP Study of Salt Marsh Expansion with Sea Level Rise

Last update: September 20, 2013.

Introduction

The Buzzards Bay National Estuary Program (BBNEP) is evaluating the potential expansion and migration of existing salt marshes, particularly those that are in tidally restricted areas, within the Buzzards Bay watershed. Similar to our study of the likely expansion of the one-percent ("100-year") floodplain, we are evaluating salt marsh expansion and migration with 1-foot, 2-foot, and 4-foot increases in sea level. Because tidal regimes differ slightly around Buzzards Bay, the real world elevation of the high marsh boundary (in the NAVD88 datum), also varies around the bay slightly. Consequently, in this study, the 1-foot, 2-foot, and 4-foot increases in sea level are added to the elevation of the specific upper boundary of the marsh in that part of Buzzards Bay.

The Buzzards Bay National Estuary Program will utilize LiDAR data released in 2009 and 2012 to evaluate high priority sites in the Atlas of Tidally Restricted Salt Marshes in the Buzzards Bay Watershed and most of the rest of the coast, including several large unaltered salt marshes. To define the existing salt marsh boundary, we will use the regulatory definition of salt marshes under state regulations, namely the "high tide line," which is defined by the highest predicted tide of the year. This elevation is also referred to as the "annual high tide" and "King Tide" on our site. The Buzzards Bay NEP will use NOAA tidal models including a program called VDatum to calculate the precise elevation of the high tide line in each part of Buzzards Bay. This elevation will be applied to LiDAR digital elevation models, to delineate the existing upper boundary of the salt marsh. To this base conditions elevation we will add 1-, 2-, and 4-feet increases in sea level. This data can be used by towns, state, and federal agencies to identify and prioritize restoration sites that will meet the challenges of climate adaptation. Towns can also use this information to target land acquisitions, or the removal of wetlands fill and anthropogenic obstructions that will impede inland migration of salt marshes. Maps will be prepared, and salt marsh area expansion will be calculated. Below is an example of what the marsh expansion maps will look like.

example salt marsh expansion along Rt. 6 in Wareham, MA.

Salt marsh expansion potential in an area of Wareham (along Rt. 6 near the Agawam River). The green shaded area shows elevations below the high tide line. Red = +1 foot sea level rise, yellow is +2 feet, and purple is +4 feet.


Principal staff involved in these tasks are the Executive Director, Wetland Specialist, and Administrative Assistant. The BBNEP will coordinate with MCZM in its broader efforts under StormSmart Coasts to avoid duplication of effort, and the CZM South Coastal Regional Coordinator will participate in the data analysis. This effort will lead to improved public and governmental understanding of the vulnerabilities of Buzzards Bay salt marshes to sea level rise and their ability to migrate. This information will be communicated through maps and GIS data that will also identify inundation zones. These maps and data sets can be used for planning and climate adaptation efforts, and to set priorities for habitat restoration and land acquisition projects. This work also meets the long-term goals contained in the Shifting Shoreline Action Plan in the 1992 and 2012 draft Buzzards Bay Comprehensive Conservation and Management Plan.

Importance of Salt Marshes

Coastal salt marshes are an important habitat and nursery for many marine species of plants, aquatic and terrestrial vertebrates, and invertebrates. Other functions provided by salt marsh ecosystems include storm damage prevention, prevention of pollution, protection of marine fisheries and wildlife habitat, and a source of primary production carbon that is the basis of coastal food webs. Salt marshes also contribute to aesthetic values of the coast. Historically, many salt marshes in Buzzards Bay have been filled or otherwise adversely affected by human activities.

Since the end of the last ice age, sea levels have risen hundreds of feet, and salt marshes have migrated inland in concert with the coastline. In recent centuries, it is believed that most salt marshes in protected areas have been able to accrete vertically to keep pace with rising sea level that stands at roughly 10 inches per century relative to the local rate. With global warming, the rate of sea level rise may increase. If vertical accretion within salt marshes cannot keep pace with rising sea levels, and if they cannot migrate inland, they will be lost.

Besides the potential threat of increased rates of sea level rise, two impairments are common in surviving salt marshes. First, many salt marshes have had tidal exchange with the ocean restricted by road construction and undersized culverts. Second, road construction and other fill have created hard boundaries to the inland migration of salt marshes caused by sea level rise.