LONG-TERM EQUILIBRIUM TIDES

Extreme equilibrium tides, or ''hypertides,'' are computed in a new eq uilibrium tidal model combining algorithms of a version of the Chapron t ELF-2000/82 Lunar Theory with the BER78 Milankovitch astronomical ex pansions. For the recent past, a high correspondence exists between co mputed semidiurnal tide levels and a record of coastal flooding demons trating that astronomical alignment is a potential influence on such f looding. For the Holocene and near future, maximum tides demonstrate e yelid variations with peaks at near 5000 B.P, and 4000 A.P. On the lat e Quaternary timescale, variations in maximum equilibrium tide level d isplay oscillations with periods of approximately 10,000, 100,000 and 400,000 years, because of precessional shifts in tidal maxima between vernal and autumnal equinoxes. While flooding occurs under the combine d effects of tides and storms via ''storm surges,'' the most extensive flooding,will occur with the coincidence of storms and the rarer hype rtides and is thus primarily influenced by hypertides. Therefore we su ggest that astronomical alignment's relationship to coastal flooding i s probabilistic rather than deterministic. Data derived from this mode l are applicable to (1) archaeological and paleoclimatic coastal recon structions, (2) longterm planning, for example, radioactive waste site selection, (3) sealevel change and paleoestuarine studies or (4) ocea n-meteorological interactions.