Numerical Simulation of Tidal Datum Fields for the Long Island Sound, New York Bight, and Narragansett Bay Area

In support of a vertical datum transformation tool called VDatum, a tide modeling application was developed for the Long Island Sound, New York Bight, and Narragansett Bay area to compute spatially-varying tidal datum fields. The tidal datum fields, when combined with orthometric and ellipsoidal models, are input to VDatum, a software tool developed by the National Oceanic and Atmospheric Administration's (NOAA) National Ocean Service (NOS) to transform bathymetry-topography data among approximately 30 vertical datums. Tidal datum fields were derived from numerically simulated time histories of tidal water levels using the two-dimensional, depth-integrated version of the ADCIRC model. The model was applied to simulate barotropic tides and depth-integrated tidal currents. A high-resolution grid with 332,524 triangular elements and 180,042 nodes was created to represent the water area up to the Mean High Water (MHW) shoreline. The spatial resolution of the elements, ranging approximately from 25 m to 5.5 km, resolves fine coastline features and preserves local bathymetric variability. The grid bathymetry was derived from NOAA/NOS sounding data and was adjusted from Mean Lower Low Water (MLLW) to Mean Sea Level (MSL) iteratively using the results of a series of tidal simulations. The model was forced with harmonic constants for seven tidal constituents (K₁, O₁, Q₁, M₂, S₂, N₂, and K₂) extracted from a tidal database of harmonic constants from a regional tide model of the Western North Atlantic. A quadratic formulation was used to calculate the bottom friction. To optimize model and observation agreement, the open boundary conditions were adjusted by increasing the M₂ tidal amplitudes along the northeast portion of the boundary. Spatially-varying bottom friction coefficients were also used with values increasing westward through Long Island Sound in an effort to mitigate spatially variable overpredictions of water levels. Tidal datum fields of Mean Higher High Water (MHHW), MHW, Mean Low Water (MLW), and MLLW were calculated from the model results based on a 30-day period of water levels. They were compared to observations at 123 NOAA/CO-OPS water level stations across the study area. Good agreement was achieved, with the average discrepancy of the above four tidal datums less than 4 cm.