An Improved Method for Mapping Tidal Flats Based on Remote Sensing Waterlines: A Case Study in the Bohai Rim, China

Abstract

Tidal flats form boundaries between marine and terrestrial environments, and reported rapid decreases in their extent require improved measures of their status. Current methods for delineating tidal flats have low accuracy because of the limited precision of available imagery, and they cannot identify zones within tidal flats. Three zones within tidal flats are ecologically distinct due to their varied inundation. These include the supratidal, intertidal, and subtidal zones. In this study, we present a more precise method for mapping tidal flats. We used Chinese HJ-1A/B satellite images with a semiautomatic extraction method to generate a sufficient number of waterline estimates taken at various tide heights to delineate the full extent of tidal flats. Then, the full set of waterline estimates is analyzed with the Digital Shoreline Analysis System (DSAS) using Jenks Natural Breaks to identify three zones within tidal flats. We demonstrated the utility of this approach by mapping tidal flats of the Bohai Rim, China. In 2014, the estimate of available tidal flats in the region was 3093 km2, among which the area of supratidal, intertidal, and subtidal zones accounted for 5.0%, 84.3%, and 10.7%, respectively. A confusion matrix for accuracy assessment revealed a 93.8% overall accuracy for the resulting tidal flat map. Our method enables tidal flats to be mapped and monitored precisely and comprehensively, providing baseline data on the extent of this disappearing ecosystem that can be used to inform coastal planning and sustainable development.