Our nation's coastal zones form a vital link between land and sea.
They provide transportation corridors for maritime commerce - generating 2.4 billion in annual benefits to the economy,
supply critical wildlife habitat, and are home to a third of the nation's population.
They are also highly dynamic areas regularly in flux:
shorelines ebb and flow with the tides, barrier islands morph from storms,
and local water levels rise or fall as land shifts and the climate changes.
To keep pace with these changing conditions and plan for future needs,
coastal communities rely on scientific data,
tools, and services provided by the National Oceanic and Atmospheric Administration or NOAA.
NOAA tracks the relationship
between land and sea by measuring water levels using a sophisticated network of tide gauges
to collect oceanographic data and generate related products like tidal datums.
Tidal datums are local reference planes determined by averaging a phase of the tide such as high or low tide.
Common tidal datums include Mean High Water, Mean Sea Level, and Mean Lower Low Water.
They are used by federal and state agencies as well as private companies for a range of purposes:
To protect life and property, engineers
set water level elevations above the Mean Higher High Water line to plan and construct levees.
NOAA forecasters predict storm surge relative to Mean Higher High Water
and rely on verified tidal datums to develop accurate storm models.
To ensure safe navigation, hydrographers reference Mean Lower Low Water when generating nautical charts.
Mariners use those charts to navigate safely through busy ports and shipping channels.
To protect natural resources, ecologists use Mean High Water
to determine inundation for marsh restoration projects, while aquatic biologists use
Mean Low Water to calculate dilution zones in marinas and determine safe habitat.
To plan development in the coastal zone and prepare for the future,
surveyors map coastlines at the Mean High Water mark to determine marine boundaries,
and federal and state policy makers reference tidal datums for resilience planning.
With so many industries relying on tidal datums to carry out their activities,
a consistent and comprehensive tidal datum network is critical for our coastal communities.
This is where the National Tidal Datum Epoch, or NTDE, comes into play.
The National Tidal Datum Epoch, NTDE,
is essentially a 19-year time period that spans the Earth's longest lunar cycle.
To collect water level data and track tidal conditions,
lunar cycles play a contributing role in the rise and fall of tides.
By using a full 19-year NTDE cycle, scientists can capture
all seasonal and environmental variations that occur at coastal locations over time.
This ensures that NOAA's resulting tidal datums can be meaningfully compared to one another.
Established in 1941, NTDE has historically been updated every 20 to 25 years
to address changes in sea levels and vertical land movement.
The process to update NTDE spans many years. After the data collection period closes,
NOAA field crews travel to the tide gauges around the country and verify data.
NOAA oceanographers then take the newly verified data to recompute each tidal datum for every location.
Beginning with NTDE 2002 to 2020, this process will be partially automated for the first time.
Automation will increase the speed and efficiency of future NTDE computations.
This process change was initiated to address long-term trends in sea level rise, which are accelerating
in many areas of the nation, and may indicate that a more frequent review of NTDE is needed
if tidal datums are to remain representative of actual conditions in coastal locations.
Visit our website at tidesandcurrents.noaa.gov for more information and station data.