VDatum for Eastern Louisiana and Mississippi Coastal Waters: Tidal Datums, Marine Grids, and Sea Surface Topography.

摘要

A vertical datum transformation software tool, VDatum, is developed for an area covering the coastal waters of Mississippi and the eastern half of Louisiana. The area encompasses major embayments (Atchafalya, Terrebonne, and Barataria Bays, LA), the Mississippi River Delta, and sounds (Breton, Chandeleur, and Mississippi Sounds, MS). VDatum provides spatially-varying conversions between tidal, orthometric, and ellipsoid-based 3D reference frames. The tidal datums fields were derived from tidal simulations using ADCIRC, a finite element hydrodynamic model which uses unstructured triangular grids. A grid consisting of 167,646 nodes and 306,749 cells was created for this project. The model was forced with nine tidal constituents (M2, S2, N2, K2, K1, P1, O1, Q1, and M4) and integrated for 65 days. Various tidal datum fields, including mean lower low water (MLLW), mean low water (MLW), mean high water (MHW), and mean higher high water (MHHW), were derived using the water level time series from the final 55 days of the simulation. Model results were validated by comparing with observations at 70 water level stations maintained by the NOAAs Center for Operational Oceanographic Products and Services (CO-OPS). Discrepancies between model results and observational datums were attributed to model errors and interpolated over the whole model domain using TCARI (Tidal constituent And Residual Interpolation), a spatial interpolation tool based on solution of Laplaces equation. The error fields were applied to the direct model results to derive corrected tidal datums on the model grid. These final tidal datum fields were interpolated onto a regularly structured marine grid to be used by the VDatum software. The Topography of Sea Surface (TSS), defined as the elevation of NAVD88 relative to local mean sea level (LMSL), was developed based on interpolation of bench mark data maintained by CO-OPS and the National Geodetic Survey (NGS). The NAVD88-to-LMSL values were derived either by fitting tidal model results to tidal bench marks leveled in NAVD88 or by calculating orthometric-to-tidal datum relationships at NOAA tidal gauges. Results by both methodologies were coupled to create the final TSS grids using spatial interpolation. Operationally, this particular VDatum grid will have to be updated at least every 5-years in order to account for rapid elevation changes in tidal datums and NAVD88 due to land subsidence in the region. CO-OPS has formal Modified 5-Year Tidal Epoch procedures for updating tidal datums and NGS is developing a Vertical Time Dependent NAVD system for Louisiana.