Impacts of Sea Level Change and Vertical Land Movement on Tidal Datums gill_MTS... ·...

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICESNOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES

Impacts of Sea Level Change and Vertical

Land Movement on Tidal Datums

Stephen Gill, Senior Scientist

NOAA Center for Operational Oceanographic Products and Services

TechSurge: Technical Support for Coastal Resiliency

June 4, 2014

Norfolk, VA


Outline

Tidal datums and their applications

101

National Tidal Datum Epoch (NTDE)

NTDE updates and sea level change

Effects of rapid rates of vertical land

motion


TIDAL DATUMS

In general, a datum is a base elevation used as a reference from which to reckon heights or depths.

A tidal datum is a standard elevation defined by a certain phase of the tide from observed data.

Tidal datums are used as reference elevations for local water levels and should not be extended into areas having differing tidal and oceanographic characteristics without substantiating measurements.

In order that they may be recovered when needed, such datums are referenced by leveling surveys from the sensors to fixed points known as bench marks.


Tidal Datum Application to Nautical Services

The vertical water level datum to which charted

soundings on a nautical chart are referred. This is

called Chart Datum.

For USA nautical Chart Datum is Mean Lower Low

Water (MLLW) for tidal areas or a Low Water Datum

(LWD) for non-tidal areas.

NOAA tide table and tide prediction products and

elevations are referenced to MLLW

Mean High Water (MHW) defines the official shoreline

for the USA.


Maritime Boundary Applications of Tidal Datums


• A specific 19 year period (currently 1983-2001) that includes the longest

significant periodic tidal variation caused by the astronomic tide-

producing forces.

• Averages out meteorological, hydrologic, and oceanographic fluctuations.

• Provides a nationally consistent tidal datum network by accounting for

seasonal variations and apparent trends in sea level that affect the accuracy

of tidal datums.

• The National Water Level Observation Network (NWLON) provides the

data required to maintain the NTDE and make primary and secondary

determinations of tidal datums.

NATIONAL TIDAL DATUM EPOCH (NTDE)A common time period to which tidal datums are referenced


The National Water Level Observation Network

210 NWLON Stations


TIDE STATION HEIRARCHY FOR

DATUM DETERMINATION

SECONDARY TIDE STATION:

• minimum of 1 yr but less than 19 yrs

PRIMARY TIDE STATION:

• minimum of 19 years

• allows statistical and datum control for short term stations

TERTIARY TIDE STATION:

• more than 30 days but less than a year


Errors in Tidal Datums and Duration of Observation


Sewells Point, Hampton Roads Tide Station


Tabulation of the Tide and Monthly Means


A First Reduction Determination of Tidal Datums – 19-year NTDE


Published Sheet for Tidal Bench Mark Elevations – Sewells Point, Hampton Roads


Why 19-years for a Tidal Datum Determination?

The 18.61 – year Lunar Nodal Cycle

http://tidesandcurrents.noaa.gov/publications/Understanding_Tides_by_Steacy_finalFINAL11_30.pdf

Axis of earth’s rotation is inclined 23.452 degrees to the plane of ecliptic;

Plane of the orbit of the moon is inclined 5.145 degrees to the plane of the ecliptic;

Maximum monthly declination of the Moon varies between 28.597 and 18.307 degrees every 18.61 - years


The 18.61-year Lunar Cycle – the Node Factor for Tidal Constituent Amplitudes used in Tidal Analysis and

Prediction


Long-term Variations in Monthly and Annual Mean Range of Tide


Effects of 18.61-year Lunar Nodal Cycle at Hampton Roads


NATIONAL TIDAL DATUM EPOCH UPDATES

1983-01 EPOCH

NTDE UPDATES ARE REQUIRED IN ORDER TO ACCOUNT FOR

RELATIVE SEA-LEVEL CHANGE OVER TIME


Relative Sea Level Trends at National Water Level

Observation Network (NWLON) Stations

http://tidesandcurrents.noaa.gov/sltrends/sltrends.html


The Relative Mean Sea level Trend at Sewells Point, Hampton Roads


Impacts of Sea Level Rise on Tidal Heights


History of Tidal Datum Updates at Sewells Point, Hampton Roads


NTDE Updates at Sewells Point, Hampton Roads


Effects of Vertical Land Motion and

Relative Sea Level Trends

http://tidesandcurrents.noaa.gov/sltrends/sltrends.html

* Tide stations measure sea level variation

relative to the local land.

* Sea level variations at any station can be due

to global sea level rise from climate change;

from shelf and estuarine changes in

hydrodynamics and circulation patterns due

to decadal climate oscillations; and changes

in freshwater flow into estuaries

* Long-term sea level variations can also be

due to local and regional vertical land

movement: either subsidence or uplift

• Tide gauge records do not distinguish as to

cause, but simply provide sea level variation

relative to the local land – a very important

piece of coastal intelligence for decision-making.


Vertical Land Subsidence in Lower

Chesapeake Bay Region

Source: USGS Circular 1392


Vertical land Motion – Subsidence in Louisiana

Source; NASA, JPL, LSU


Tidal Datum Updates at Grand Isle LA


Vertical Land Motion – Isostatic Rebound – Southeast Alaska


Tidal Datum Updates at Juneau AK


Tidal Datums and Vertical Land Motion – Recent Reports

http://tidesandcurrents.noaa.gov/publications/


Tidal Datums and Future Sea Level Projections

83-0102-20

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Impacts of Sea Level Change and Vertical Land Movement on Tidal Datums gill_MTS... ·...

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