Evaluation of Shorelines and Legal Boundaries Controlled by Water Levels on Sandy Beaches

Abstract

Integration of beach profiles and water-level measurements at three sites on a microtidal, wave-dominated coast reveals that tide-gauge records systematically underestimate the actual elevations and horizontal positions that water reaches on the beach as a result of wave runup. On low-gradient sandy beaches, natural morphological beach features, such as the erosional scarp and vegetation line accurately reflect the positions of frequent maximum high water levels and the berm crest reflects the position of more frequent ordinary high water levels, whereas tide-gauge records consistently predict lower maximum and average levels of beach flooding.

The discrepancies between predicted and actual water positions on the beach have important scientific and legal implications. The scientific implications involve the need to map shoreline features that closely track the long-term trends in beach movement, but are insensitive to short-term fluctuations in water level. Neither the instantaneous high water line (wet beach-dry beach boundary) or the berm crest satisfy this requirement, and therefore, they are not recommended for monitoring shoreline position either in the field or interpreted from aerial photographs unless there is no reliable alternative. The legal implications pertain to land ownership and property boundaries in the United States that currently are surveyed from tide-gauge records but were originally defined by common law on the basis of high water levels that leave physical marks on the upland property. Because water levels are actually higher on the beach than predicted by tide gauges, land surveys based on a tidal datum allocate more littoral property 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 flooded by marine water.