High Water Mark Determination Based on the Principle of Spatial Continuity of the Swash Probability

This study presents a model that determines the position of the high water mark (HWM) based on the spatial continuity of inundation probability due to swash for a range of HWM indicators. These indicators include mean high water (MHW), high water line (HWL), and, a number of shoreline features, such as the vegetation line. HWM identifies the landward extent of the ocean and is required for cadastral boundary definition, land-use and infrastructure development along the foreshore,and for planning associated with climate change adaptation. In this paper, shoreline indicators are extracted using an object-oriented image analysis (OOIA) approach. Ten-year hourly swash heights (shoreline excursion length) are fitted into a cumulative distribution function. The probability that swash will reach the various HWM indicators over a 10 y period is then estimated. The spatial continuity distances of the swash probability of HWM indicators are calculated using semivariogram models that measure similarity of swash probability. The spatial continuity distance is defined as the distance between the lower bound of sampling position (the most seaward HWM indicator) and the position where autocorrelation, or the similarity of swash probability of the various MAIM indictors, approaches zero. The latter is considered as the HWM position in this study. This HWM determination method is evaluated at two study sites at different latitudes and with distinct coastal features.