SPECIAL PUBLICATION NO. 43
OPEN-OCEAN WATER LEVEL DATUM PLANES:
USE AND MISUSE IN COASTAL APPLICATIONS
by
James H. Balsillie, P. G. No. 167
ABSTRACT
Swanson (1974) notes that tidal datum planes "... are planes of reference derived from the rise
and fall of the oceanic tide". There are numerous tidal datum planes. Commonly used datums in the United States include the planes of mean higher high water (MHHW), mean high water (MHW), mm didelevfd (MTL), nei a- Aevaf (MSL), nwa Jew water (MLW), and m~ean Jlower low water (MLLW). Each datum is defined for a specific purpose or to help describe some tidal phenomenon. For instance, MHW high water datums have been specified by cartographers in some states (e.g., Florida) as a boundary of property ownership. Low water datum planes have been used as a chart datum because it is a conservative measure of water depth and, hence, provides a factor of safety in navigation. High water tidal stages have historically been of importance because they identified when sailors should report for duty when 'flood tide"
conditions were favorable for ocean-going craft to leave port, safely navigate treacherous ebb tidal shoals, and put to sea. Not only do tidal datum specifications vary geographically based on local to regional conditions for purposes of boundary delineation, cartographic planes, design of coastal structures, and land use designations, etc., but they have changed historically as well. Moreover, given ongoing technological advancements (e.g., computer-related capabilities including the advent of the personal computer), how we approach these data numerically is
highly important from a data management viewpoint.
IN7RODUC77ON Florida are problematic because there are a limited number of gauging stations to Tide gauges are usually located in water represent astronomical tidal phenomena. bodies connected to the oceans, such as While it has been standard practice to estuaries and rivers, and may even be used to linearly interpolate open ocean tidal datums record seiches such as those occurring in the between gauges, such an approach is not Great Lakes. Here, however, the concern is recommended should the gauges be spaced with open ocean es. Open ocean tide further apart than about 6.2 miles (Balsillie gauges are defined "... as those gauges sited and others, 1987a). Of the 33 currently directly upon the open ocean nearshore available open ocean gauges in Florida (Table waters and subject to the influence of ocean 1), only three pairs of stations meet this processes, excluding those under the constraint. In fact, the average distance influence of inlet hydrodynamics..." (Balsillie between Florida open ocean tide gauges is and others, 1987a, 1987b, 1987c). The 27.4 miles. Ostensibly, the 6.2-mile latter constraint in the definition is included constraint is recommended since even though it is difficult to determine the concurrently similar tidal stage datum extent of influence from inlet to inlet, elevations can vary significantly over segments of the coastline when this distance Open ocean tidal datum applications in
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