A generalized methodology, relevant for a wide variety of shoreline change analyses, is developed to estimate the horizontal offset between proxy-based high water line (HWL) type shorelines and datum-based mean high water (MHW) type shorelines. The ability to compute this term is critical for change analyses that incorporate variously defined and derived shoreline estimates because this horizontal offset nearly always acts in one direction; HWL shorelines are landward of MHW shorelines. Not accounting for this offset will cause shoreline change rates to be biased toward slower shoreline retreat, progradation rather than retreat, or faster progradation than in reality (for the typical case where datum-based shorelines are collected after proxy-based shorelines), depending on actual changes at a given site. It is also demonstrated that by computing the uncertainty associated with this proxy datum shoreline bias, we are quantifying, for the first time, the uncertainty of HWL shorelines due to water level fluctuations. Complete accounting of the uncertainty of shoreline position estimates is necessary for determining the statistical significance of shoreline change rate computations. The proxy-datum bias and the bias uncertainty are estimated to be approximately 18 and 9 m, respectively, on average for the sandy beaches of the California coast (and significantly larger on the milder sloping beaches of the U.S. Pacific Northwest). The importance of accounting for the bias in calculating shoreline change rates is confirmed as its inclusion along the California coast changes the coastwide decadal-scale (1970s to present) shoreline change rate from net progradation to net shoreline retreat.
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