Predicted Extreme High Tides for Mixed-Tide Regimes

Abstract

Abstract There are a number of published studies of the astronomical conditions prevailing when extreme high tides are predicted, but usually these are directed toward tidal regimes that are dominantly semidiurnal. The published criteria are found to be inadequate for mixed regimes (diurnal tides roughly the same order of magnitude as the semidiurnal tides in the area). For the mixed tides on the California coast, added consideration must be given to tropic tides (diurnal tides larger than average when the moon is near maximum declination) and to the 18.61-year period of the lunar-node cycle. Furthermore, extreme high diurnal tides tend to occur when the sun is near maximum declination (summer and winter) whereas comparable semidiurnal tides ordinarily occur near the equinoxes (spring and fall). Because of these added complications, harmonic tide predictions were prepared for four California ports up to the year 2000 so that information on extreme high tides could be tabulated. These data help to alleviate public fears that tidal contributions to the disastrous flooding in California in 1982?83 would be exceeded in the following decade.