DEEP: Coastal Hazards Glossary

Coastal Hazards Glossary

The following is a glossary of selected terms pertinent to coastal hazards in Connecticut:

Astronomical High Tide: The local elevation of the sea is constantly changing. In Connecticut, two high tides and two low tides occur every 24 hours and 50 minutes. This semi-diurnal cycle of high and low tides is driven by astronomical forces: the moon and the sun exerting force on the earth. The absolute levels of these tides can be significantly altered as a result of meteorological forces like wind and barometric pressure. Astronomical high tide is thus a more specific term that refers to a high tide level that can be expected according to astronomical forces alone.

Bulkhead: Wall or other structure, often of wood, steel, stone, or concrete, designed to retain or prevent sliding or erosion of the land.

Coastal Flooding: A temporary rise in sea level due to wind, low barometric pressure, astronomical high tides, or waves. 

Coastal Hazards: Coastal hazards include both natural and man-made events (chronic and episodic) that threaten the health of coastal ecosystems and communities. This definition includes, but is not limited to, hurricanes, tsunamis, erosion, oil spills, harmful algal blooms, and pollution.  (NOAA Coastal Services Center)

Erosion: Wearing away of land by natural forces.

Flood: An overflowing of water onto normally dry land.

Floodplain: Any land area susceptible to being inundated by floodwaters from any source.

Freeboard: A factor of safety usually expressed in feet above a flood level for purposes of floodplain management. "Freeboard" tends to compensate for the many unknown factors that could contribute to flood heights greater than the height calculated for a selected size flood and floodway conditions, such as wave action, bridge openings, and the hydrological effect of urbanization of the watershed. Freeboard is not required by NFIP standards, but communities are encouraged to adopt at least a one-foot freeboard to account for the one-foot rise built into the concept of designating a floodway and the encroachment requirements where floodways have not been designated. Freeboard results in significantly lower flood insurance rates due to lower flood risk. (

Groin: A short, shore-perpendicular structure designed to trap littoral sediments.

Jetty: Wall built out into the water to restrain currents or protect a structure. (FEMA)

Littoral Transport: The movement of sediment (mud, sand, silt, gravel) waves and currents. Includes movement parallel (longshore transport) and perpendicular (onshore-offshore transport) to the shore.

Mean High Water (MHW): The average high tide level at a particular location, usually determined from hourly height observations at a permanent tide gauge over the National Tidal Datum Epoch.

Mean Sea Level (MSL): Average height of the sea for all stages of the tide at a particular location.   MSL is usually determined from hourly height observations at a permanent tide gauge over the National Tidal Datum Epoch.

National Tidal Datum Epoch (NTDE): The specific 19-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values (e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of periodic and apparent secular trends in sea level. The present NTDE is 1983 through 2001 and is actively considered for revision every 20-25 years. Tidal datums in certain regions with anomolous sea level changes (Alaska, Gulf of Mexico) are calculated on a Modified 5-Year Epoch. (NOAA: Tidal Datums)

NAVD88: North American Vertical Datum of 1988.  A vertical control datum designed to replace NGVD29.

NGVD29: National Geodetic Vertical Datum of 1929.  (Known as the Sea Level Datum prior to 1976).

Revetment: Facing of stone, cement, sandbags, or other materials placed on an earthen wall or embankment to protect it from erosion or scour caused by flood waters or wave action.

Saffir-Simpson Hurricane Intensity Scale: A system, developed by the National Weather Service, of classifying hurricanes according to intensity:

Seawall : Solid, usually vertical barricade designed to prevent erosion and protect property from waves and flooding. (FEMA)

Special Flood Hazard Area (SFHA):  A FEMA-identified high-risk flood area where flood insurance is mandatory for properties. An area having special flood, mudflow, or flood-related erosion hazards, and shown on a Flood Hazard Boundary Map or a Flood Insurance Rate Map as Zone A, AO, A1-A30, AE, A99, AH, AR, AR/A, AR/AE, AR/AH, AR/AO, AR/A1-A30, V1-V30, VE, or V.  (

Spring Tide: A tide roughly coincident with full or new moon. During the full and new moon phases of the moon, the sun, moon and earth are roughly in line with each other. Tides occurring when the moon is new or full are called spring tides. They are generally characterized by high tides that are higher than normal and low tides that are lower than normal.

Storm Surge: A temporary increase in the height of the local sea level, formally defined as the difference between the observed water level and expected water level in the absence of the storm, according to the normal astronomical tide. Storm surge is caused by low atmospheric pressure and onshore winds pushing water towards shore.

Tidal Current: A horizontal movement of water caused by the rising and falling of the tide.

Tidal Range: The difference in height between high and low tide.

Tides: Very long-period waves that move through the oceans in response to the forces exerted by the moon and sun. Tides originate in the oceans and progress toward the coastlines where they appear as the regular rise and fall of the sea surface. When the highest part, or crest of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough. (

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Content Last Updated January 3, 2012