Sequoyah Computer and GIS Lab

Campus/Tribal Projects: Passmaquoddy Bay

Tide and sea level combine

On coastlines with large tides, the daily change in tide can easily dwarf long-term predicted increases in sea level rise. One such coastline is the shore of the Passmaquoddy Reservation on Passmaquoddy Bay, between Maine and New Brunswick. In order to understand coastal inundation, it is necessary to combine sea level rise with tides.

Passmaquoddy Bay

Passmaquoddy Bay

Sea level rise, measured by change in mean sea level, does not explain the impact on coastal communities. Sea level rise plus high tide and storm surges determines which land is lost to use. Sea level rise at low tide determines which tidal flats are lost.

To measure inundation of land and loss of tidal flats, we require a dataset that combines elevation and bathymetry data. The State of Maine provides USGS digital elevation model data on 10m x 10m cells with values in integer meters and bathymetry data on 30m x 30m cells with values to the tenth of a meter. These data were combined into a single raster dataset with a common vertical datum at mean sea level. Data on tidal ranges at the Passamaquoddy Reservation were obtained from NOAA.

From these two data products, we produced a set of raster files indicating sea level at a range of tide and sea level rise conditions. These rasters are then compared to maps of exposed land at high and low tide to identify areas where dry land will be inundated at high tide and areas where tide flats will be lost at low tide.

Passmaquoddy Bay

Sea level rise of as little as a half meter results in immediate loss of tidal flats, changes to sedimentation of tide flats and disruption of commercially valuable clam, mussel, seaweed, and bloodworm populations. An increase of four meters results in complete loss of the existing tidal flats.
At high tide, an increase of sea level of two meters breaches the causeway between the Reservation and the Town of Eastport. An increase of three meters separates the Reservation from the mainland.

Studies of sea level rise alone can document areas that will be lost to human use as a result of increases in mean sea level. Combining sea level rise with tidal data reveals additional areas that would be lost to human use or in which plant and animal communities would be disturbed. These results therefore support studies of sea level rise by suggesting that their conclusions concerning impact on human activities – developed using mean sea level -- are conservative.

This model can be refined further by adding processes other than tides. We hope to extend our modeling to add the action of storm surges, freshwater runoff, and sedimentation. Additional geographic research will include consideration of impacts of tidal changes on cultural resources, including those of Native American reservations, and fieldwork to collect data on both natural and cultural resources in the study area.

For more Information on the Possmaquaoddy Bay Project, contact Dave McDermont at