EVALUATION OF SHORELINES AND LEGAL BOUNDARIES CONTROLLED BY WATER LEVELS ON SANDY BEACHES

Integration of beach profiles and water-level measurements at three si tes on a microtidal, wave-dominated coast reveals that tide-gauge reco rds systematically underestimate the actual elevations and horizontal positions that water reaches on the beach as a result of wave runup. O n low-gradient sandy beaches, natural morphological beach features, su ch as the erosional scarp and vegetation line accurately reflect the p ositions of frequent maximum high water levels and the berm crest refl ects the position of more frequent ordinary high water levels, whereas tide-gauge records consistently predict lower maximum and average lev els of beach flooding. The discrepancies between predicted and actual water positions on the beach have important scientific and legal impli cations. The scientific implications involve the need to map shoreline features that closely track the long-term trends in beach movement, b ut are insensitive to short-term fluctuations in water level. Neither the instantaneous high water line (wet beach-dry beach boundary) or th e berm crest satisfy this requirement, and therefore, they are not rec ommended for monitoring shoreline position either in the field or inte rpreted from aerial photographs unless there is no reliable alternativ e. The legal implications pertain to land ownership and property bound aries in the United States that currently are surveyed from tide-gauge records but were originally defined by common law on the basis of hig h water levels that leave physical marks on the upland property. Becau se water levels are actually higher on the beach than predicted by tid e gauges, land surveys based on a tidal datum allocate more littoral p roperty to the upland owner than is justified by the physical facts or was intended by law. Consequently, the publicly-owned state submerged lands encompass less of the beach than that area which is regularly f looded by marine water.