[Federal Register Volume 78, Number 207 (Friday, October 25, 2013)]
[Notices]
[Pages 63966-63967]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2013-25139]


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DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration


Notice of Change to the Nation's Tidal Datums With the Adoption 
of a Modified Procedure for Computation of Tidal Datums in Area of 
Anomalous Sea-Level Change

AGENCY: National Ocean Service (NOS), National Oceanic and Atmospheric 
Administration (NOAA), Department of Commerce.

ACTION: Notice to advise the public of periodic updates to tidal datums 
due to the adoption of modified procedures for computation of accepted 
tidal datums in areas of anomalous relative sea-level trends using a 5 
year time period for determination of tide level datums.

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SUMMARY: NOAA has typically updated tidal datum elevations for the 
nation to new National Tidal Datum Epoch (NTDE) time periods every 20-
25 years. Updates are necessary due to long-term sea level change. In 
1998, NOS recognized the need for a modified procedure for 
determination of tidal datums for regions with anomalously high rates 
of relative sea level change. This modified procedure is necessary at 
selected stations to ensure that the tidal datums accurately represent 
the existing stand of sea level.
    The procedure is limited only to those stations in areas with high 
rates of vertical land motion that have documented anomalous relative 
sea level trends exceeding 9.0 millimeters per year. Sea level analyses 
in these anomalous regions are conducted approximately every 5 years to 
determine if the mean sea level difference exceeds the established 
threshold tolerances in order to qualify for a special update. 
Anomalous relative sea level trends are seen along the western Gulf 
Coast, southeast Alaska, and southern Cook Inlet, AK. For example, the 
magnitude of the sea level trends in these areas is +9.24 millimeters 
per year in Grand Isle, LA; -12.92 millimeters per year in Juneau, AK; 
and -9.45 millimeters per year in Seldovia, AK.
    This procedure is necessary to provide the most accurate 
information available for applications that are essential to supporting 
Federal, State and private sector coastal zone activities, including 
hydrographic surveys and coastal mapping, navigational safety, wetland 
restoration, marine boundary determinations, coastal engineering, storm 
warnings and hazard mitigation, emergency management, and hydrodynamic 
modeling.
    While maintaining the 19 year NTDE computational period for tidal 
mean range and diurnal range, a shorter more recent 5 year 
computational period is used to compute the mean tide level datums to 
better reflect the current elevation of mean sea level relative to the 
land. Consequently, tidal datums at stations exhibiting anomalous 
trends are computed from mean sea level, diurnal tide level and mean 
tide level values for the most recent 5 year time period, and tidal 
ranges (GT and MN) based on the most recent full 19 year NTDE at 
stations.
    The average absolute difference between 19 year NTDE time periods 
across the nation of 0.03 meters (0.10

[[Page 63967]]

foot) is generally used as the threshold difference to warrant 
consideration of a 19 year NTDE update, and a 20-25 year review cycle 
has been adequate to capture the changes of 0.03-0.04 meters for most 
locations. To meet this target at locations with anomalous rates of sea 
level change, tidal datum elevation updates must occur more frequently. 
In general, the vertical changes in datum elevations which result from 
these more frequent special tidal datum updates every 5 years are kept 
as close to the 0.03 meters (0.10 foot) to 0.05 meter target as 
possible. In comparison to the overall accuracy of hydrographic-
cartographic processes and scale and resolution and accuracy of 
soundings on the NOAA nautical charts, these elevations changes will 
not necessitate a correction or update to the charts every time a datum 
update is issued. However, in regions that have experienced rapid land 
movement, the changes to actual soundings and shoreline depiction may 
need to be updated on the next regularly scheduled chart edition. 
Although depictions of the datum changes will not be evident on the 
largest scale NOAA nautical charts, the datum changes will be 
noticeable when establishing or re-occupying tide stations using 
accepted surveying techniques and updating elevations on tidal bench 
marks provided by NOAA. Appropriate outreach will be conducted per 
office guidelines prior to performing each update.

FOR FURTHER INFORMATION CONTACT: Visit the Web site (http://www.tidesandcurrents.noaa.gov) or contact the Center for Operational 
Oceanographic Products and Services (CO-OPS) at the following address: 
NOAA, National Ocean Service, CO-OPS, Oceanographic Division, 1305 
East-West Highway, Silver Spring, MD 20910-32821, U.S.A., Telephone: 
301-713-2890 x149, Fax: 301-713-4437, Email: [email protected].

    Dated: October 18, 2013.
Holly A. Bamford,
Assistant Administrator for Ocean Services and Coastal Zone Management.
[FR Doc. 2013-25139 Filed 10-24-13; 8:45 am]
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