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Weather and Tides

Tidal Datums and Benchmarks for Buzzards Bay

Overview

Whether you are interested in historical storm surges, flood plain mapping, relative sea level rise, or are an engineer designing structures along shore, it is important that you have a good understanding of tidal datums, geoids datums, local benchmarks, and how they relate. Over the years we have come to realize that many scientists and engineers that work on the coast do not have a full understanding of these elevations, their relationships, or how definitions have changed over time. Although differences in the various datums are less than a foot or two, such discrepancies in designing structures like culverts under roads can have profound implications on tidal flushing and flooding characteristics if these structures are not installed at their appropriate elevations.

We have set up this page to help planners, town engineers, government officials, and scientists quickly find the data and information they need to relate tidal elevations, local benchmarks, and vertical elevation datums for sites they are interested in. We have also have included sections below that explain the definition of coastlines and maps, and summarize historical storm surges recorded in Buzzards Bay

Relationship Between Tidal Datums, Vertical Datums, and Local Benchmarks

The relationship between tidal datums (average tide heights), vertical datums (land elevation), and local benchmarks (brass elevation markers found on bridges, roads, and occasionally large rocks) varies up and down the coast of the US and elsewhere.

Below is a generalized diagram of this relationship for our area. Refer to this diagram as you read the sections below it.

Tidal Datums

In our area we have diurnal tides. These are slightly asymmetrical tides that occur twice daily. In a 24 hour period, we have two high tides and two low tides. One of these low tides is lower than the other, and one of the high tides is higher than the other. These two tides are described as the "lower low" tide and the "higher high" tide respectively. The heights of these diurnal tides change over the course of the month because of the relative alignment of the moon and sun which both exert gravity on the oceans to create the tides. When the moon and sun are aligned (new moons and full moons), the higher highs are higher and the lower lows are lower, and these are referred to as "spring tides." New moon spring tides are more extreme than full moon spring tides. These occur every two weeks). The mid point between spring tides (also every two weeks), are called "neap tides", which are the periods of the smallest time changes.

Tidal datums are 19-year averages of all tides in an area. Mid tide level is the average tide elevation over the period. The MLW, MHW, and MLLW and MHHW is based on the average of each of these terms for the 19 year period. Every 25 years or so, the federal government revises these tidal datums by examining new "epochs" of data to account for relative sea level rise.

Mean Higher High Water (MHHW) tidal datum: The average of the higher high water height of each tidal day observed over the National Tidal Datum Epoch.
Tidal elevations recorded at the Woods Hole tide station, including highest values recorded can be found at: http://140.90.121.76/cgi-bin/station_info.cgi?stn=8447930+WOODS+HOLE,+BUZZARDS+BAY+,+MA It is not the same as "mean sea level" which is actually a term tied to the NGVD29 tidal datum, which was really the average sea level at 26 stations in North America. Some time in the 1960s it was realized that mean sea level was not really the mean sea level at all, and they changed the term to " ". If you see a engineering plan that labels "mean sea level", recognize that this is not the same as mid tide level for the current tidal epoch.

What is the Coastline on Maps?

On both USGS topographical "Quad" maps and NOAA nautical charts (as well as GIS data files derived from these), the coastline is generally the Mean High Water (MHW) mark. The coastline is not the 0 ft land elevation contour. The land elevation varies around the coast. In Buzzards Bay, MHW is roughly at the 2 ft land elevation (NAVD88 datum).

We say the coastline is "generally" the MHW mark because there are important exceptions. One of these exceptions is where salt marshes occur. Where there is a salt marsh, the "apparent coastline" is shown, not the MHW mark. The apparent coastline is the extent of the salt marsh vegetation. Because salt marsh can grow down to just below the mid-tide level, the coastline on maps is closer to the 0 foot land elevation in Buzzards Bay. The apparent and MHW coastline is illustrated in the portion of a nautical chart shown below.

Land elevations versus bathymetric depth

Land elevations on federal government maps produced after 1990 are generally given as feet above NAVD88 (North America Vertical Datum -1988), which is the most recently accepted zero elevation for North America. Earlier maps use NGVD29 (National Geodetic Vertical Datum-1929). In our area there is about a 10-inch difference between the two elevations.

Bathymetric depths for nautical charts and topographic quads in our area are generally given as feet below mean low water (MLW) on maps prior to 1990, and depth below mean lower low water (MLLW) after 1990, however, always check the tidal datum definition for the chart because it varies. In Buzzards Bay, the difference between the MLW mark and MLLW mark is generally around 4 inches in Buzzards Bay, but varies around the bay.

As might be apparent from the above definitions, there are areas in the intertidal which have negative land elevations and negative bathymetric elevations.

Past Hurricanes Tidal Surges in Buzzards Bay

NOAA Summaries for the four worst storms affecting southern New England are in the links below.

The 1938 Hurricane

Hurricane Carol, 1954

Hurricane Edna, 1954

Hurricane Bob, 1991

Environmental Management Issues Sea Level Rise

Sea level has repeatedly risen and fallen over geologic time with the formation of large glaciars during ice ages, and the melting of this glaciers and polar ice during intervening warm periods. Since the last ice age 10,000 years ago, sea level has continued to rise, with the rate of relative sea level rise in recent centuries of roughly 10 inches per century. This rate of sea level rise is expected to increase with global warming caused by green house gas emissions, with rates projecting to possibly increase to 20 inches or more per century.

Storms and high seas erode the coast and flood low-lying areas. For environmental managers and regulators today, construction and development in storm flood zones, and areas of high erosion is an important concern, especially in terms of threats to human safety. Also important are the high public financial costs and economic impacts associated of replacing roads, loss of personal property, and public financing of flood insurance programs and emergency response, particularly in areas repeated damaged by storms. The links to the articles below discuss some of these public policy issues.

Massachusetts Coastal Zone Management Office Shoreline Change Project.

What would be the impacts of a Hurricane like Isabel on Buzzards Bay? We need only consider the impacts of Hurricane Bob in 1991. Read this Cape Cod Times special Cape Cod Times Special Report: Hurricane Bob Ten Years Later.

Massachusetts Coastal Zone Management Office Coastal Hazards Assessment (pdf file).

National Assessment of Coastal Vulnerability to Sea-Level Rise (pdf file).

National Academy of Sciences: Sea Level Rise and Coastal Disasters (pdf file).

WHOI case study on beach erosion..

URI fact sheet on coastal Erosion.

Controversial Beach-nourishment project in Virginia in the news.

Buzzards Bay National Estuary Program