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Lowest Astronomical Tide in the North Sea derived from a vertically referenced shallow water model, and an assessment of its suggested sense of safety.

D.C. Slobbe
Water level reduction with GNSS in bathymetric surveying requires
knowledge of the ellipsoidal heights of Lowest Astronomical Tide
(LAT). The traditional approach uses tidal water levels of a
hydrodynamic model, which are subtracted from Mean Sea Level (MSL).
This approach has two major drawbacks: the modeled water levels refer
to an equipotential surface, which differs from MSL, and MSL may not
be known close to the coast. Here, we propose to model LAT directly
relative to an equipotential surface (geoid). This is conceptually
consistent with the flow equations and allows the inclusion of
temporal MSL variations into the LAT definition. Numerical experiments
for the North Sea show that significant differences between the
traditional and the pursued approach exist if monthly variations in
MSL are included. A validation of the modeled LAT using LAT derived
from tide gauge records reveals systematic errors, which we attribute
to both the model and the tidal analysis procedure. We also show that
the probability that water levels drop below LAT is high, with maximum
frequency of once per week in the eastern North Sea. Therefore, we
propose to reconsider the deterministic concept of LAT by a
probabilistic Chart Datum concept, and we quantified the differences
between them.