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One of the central problems in creating integrated bathymetric/topographic elevation models is how to generate the spatially-varying tidal datum fields. For example, NOS charted depths are relative to the Mean Lower Low Water (MLLW), so they must be adjusted to Local Mean Sea Level (LMSL). At present, tidal datums, which are elevations referenced to a mark on the tide staff, are known only for specific locations (i.e., NOS' water level gauges) along the coast. But tidal datums are actually two-dimensional surfaces that vary in elevation with latitude and longitude over horizontal space. To solve this problem in Tampa Bay, datums were generated by two methods: a numerical hydrodynamic circulation model of the region, and spatial interpolation using values at the water level stations. 
Figure 1. The model-generated Mean Lower Low Water field, relative to the model's Mean Sea Level. The Hydrodynamic Circulation Model The numerical circulation model is a version of the Princeton Ocean Model (POM) that was previously applied to Tampa Bay by NOS (Hess, 1993). The POM has been under development and refinement for more than twenty years and has been widely applied to many estuarine and coastal regions; it has become a standard tool in the oceanographic research community. The model uses a terrain-following vertical coordinate and orthogonal curvilinear coordinates in the horizontal to simulate currents, salinities, and temperatures over depth and at numerous locations throughout the Bay. The grid spacing varies from 100 to 1000 m (Figure 2). For this project, the model was run with wind, shelf tides, and river flows to simulate a 365-day period for which extensive data are available (June 1990 to June 1991). Water levels from the model have been validated to be accurate to within 4 cm root mean square (RMS) error. Four steps were involved in the hydrodynamic modeling process: (1) making the hydrodynamic model runs, (2) completing the tidal analysis of the model-generated water levels, (3) generation of the uniform regional grid and datum fields, and (4) generation of datums for the offshore area outside the model grid. 
Figure 2. Grid used by the Tampa Bay numerical circulation model. 
Figure 3.
A portion of a modeled water level time series showing the selected higher high
waters (triangle with "+"), Extraction of Tidal Extremes A computer program was developed in NOS to select the high and low waters from a time series of water level values at 6-minute intervals from the hydrodynamic model. The program picks out the highs and lows, and then the higher highs and lower lows. The program computes the precise time and elevation by cubic interpolation. Mean Sea Level (MSL) is computed by saving and averaging all values. Then the tidal datums (MHHW, MHW, DTL, MTL, MLW, and MLLW) are found by averaging the appropriate elevations at high and low waters. The last step in the extraction process is to adjust the datum values from the modeled period (in 1980 and 1981) to the Tidal Epoch of 1960-1978. Interpolation to a Uniform Spatial Grid Once the model datum fields were obtained, these were transferred to a rectangular grid with uniform spacing of 1 arc minute in latitude and longitude. In addition, since bathymetric data for locations outside of Tampa Bay had to be corrected, this area of the West Florida Shelf outside of Tampa Bay and northward toward Clearwater needed to be assigned tidal datum values. It is known that tidal datums based on observed time series at various locations along the coast north of the Bay's entrance show a general increase with latitude. Therefore, values in the coastal area outside of Tampa Bay and northward toward Clearwater were filled as follows. For any specific latitude, the value was computed by linearly interpolating between the appropriate pair of datum values selected from the northmost model grid cell, the value at Indian Rocks Beach, and the value at Clearwater Harbor. All grid cells at that latitude, whatever the longitude, were given the same value. This process is described in more detail in Hess (2001). 
Figure 4. The field of MHHW relative to MLLW (m). Publications
Hess, K. W., 2001: Generation of Tidal Datum Fields for Tampa Bay and the New York Bight.
Proceedings, International Conference on HydroScience and Engineering, Washington, DC, June 8-11, 1993, pp. 1499 - 1506. For further information on tidal datum modeling for Tampa Bay, contact Dr. Kurt Hess |
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Revised Tuesday December 20 2005