Coastal reclamation shaped narrower and steeper tidal flats in Fujian, China: Evidence from time-series satellite data

Abstract

As one of the most ecologically and economically important ecosystems, tidal flats are under severe threat from extensive coastal reclamation. The lack of time-series topographic data limits the understanding of the evolution of tidal flats under stress. Existing approaches to tidal flat topographic mapping are complex and weak in their representation of micro-geomorphic features. In this study, we propose a pixel-by-pixel inundation-based topographic mapping algorithm (PITMA) to accurately reconstruct tidal flat topography from full-time-series satellite imagery and to assess multi-decadal changes in the tidal flats of Fujian Province, which has undergone intensive coastal reclamation. The results show that the proposed algorithm can generate accurate tidal flat topography with an RMSE of 0.30 m in Fujian province. The total areas of tidal flats were 1311.27 km2, 1136.33 km2, and 1048.18 km2 in 2000, 2010, and 2020, respectively. Coastal reclamation in Fujian Province has resulted in the loss of more than 20% of the tidal flats, but the average elevation of the tidal flats has increased by 40% with mean elevations of −0.68m, −0.82m, and −0.41m in 2000, 2010, and 2020, respectively. Meanwhile, the slope of tidal flats also increased in most tidal flats in bay areas. Topographic profiles in 2000, 2010, and 2020 showed that tidal flats in bay areas gained deposition at higher-elevation areas but suffered erosion at lower-elevation areas, due to insufficient sediment availability and extensive land reclamation, then the tidal flats have become narrower and steeper. In contrast, estuarine tidal flats exhibited seaward expansion with stable slopes because of available sediment provision from the upstream river. Such tidal flat morphology would be at increased risk of erosion as sea level rises and sediment supply decreases. This quantitative assessment of the impact of coastal reclamation on tidal flat evolution provides scientific guidance for sustainable coastal management.