Implementation of Procedures for Computation of Tidal Datums in Areas with Anomalous Trends in Relative Mean Sea Level.

Abstract

NOAA has typically updated tidal datum elevations for the nation to new National Tidal Datum Epoch (NTDE) time periods every 20-25 years. Updates at this frequency are necessary due to long-term global sea level change. In 1998, NOS recognized the need for a modified procedure that utilized more frequent time period updates, for determination of tidal datums for regions with anomalously high rates of local relative sea level change. These localized effects in relative sea level trends are typically due to different forces other than those responsible for global trends which can vary significantly from global trends in both time scales and magnitude. This modified procedure is necessary at selected stations to ensure that the tidal datums accurately represent the existing stand of sea level relative to land on which these datums are held fixed. Bench mark monuments are typically used as reference points for numerous applications requiring tidal datum references. The modified procedure is limited only to those stations with documented anomalous relative sea level trends due to high rates of vertical land motion. Anomalous relative sea level trends are seen along the central Louisiana, the southern Cook Inlet, and the southeastern Alaska coasts. For example, the magnitude of the sea level trends in these areas are +9.24 mm/yr at Grand Isle, LA; - 9.45 mm/yr at Seldovia, AK; and -12.92 mm/yr at Juneau, AK. Following the first implementation of the modified procedure in 1998, using the time series for tidal datum computation of 1990-1994, s ea level analyses in these anomalous regions are now conducted approximately every five (5) years to identify stations that require datum updates using the modified procedure. NOAA’s mission is to provide the latest up-to-date tidal datum information available for applications that are essential to supporting Federal, State and private sector coastal zone activities. These include activities such as hydrographic surveying, coastal mapping, and the resulting nautical charts, general navigational safety, wetland restoration, marine boundary determinations, coastal engineering, storm warnings and hazard mitigation, emergency management, and multi-use hydrodynamic modeling.