Estimating muddy intertidal flat slopes under varied coastal morphology using sequential satellite data and spatial analysis

Muddy intertidal flats are important resources for coastal development activities, such as coastal shoal reclamation and mudflat cultivation. Understanding changes in intertidal flat topography is essential for intertidal zone development, management, and protection. As an essential topographic factor, the slope of an intertidal flat can effectively express profile morphology in the cross-shore direction, reflect topographic undulation in the long-shore direction, and be indicative of intertidal flat erosion and deposition. Previously, intertidal flat slopes have been estimated by the two-temporal average gradient (TTAG) method using two spatially separated waterlines derived from satellite remote sensing data and their relevant water elevations. However, this method does not adequately reflect the profile morphology of different coastline types, especially sinuous coastlines, and is highly sensitive to the selected waterlines. This study proposes an effective strategy for estimating slope considering not only the characteristics of the vertical terrain undulation but also the horizontal plane-form shape of the coastline. Using waterlines extracted from sequential satellite images and their corresponding tidal height information, the slopes were estimated utilizing a profile morphology discriminant inferential (PMDI) method on straight coasts and a digital elevation model (DEM) method on sinuous coasts. We obtained the following results: (a) for straight coasts, by judging the various shapes of the coastal profiles and curve-fitting separately, the PMDI method achieved significant improvements in accuracy and robustness of slope estimates compared to results obtained using the TTAG method; and (b) for sinuous coasts, the DEM-based method performed better for addressing the intersecting waterline issue and accurately retrieved the slope, although this accuracy is strongly dependent on the areal coverage of the DEM and the precision of the terrain inversion. Using this new paired methodology, we estimated that the average slope of the intertidal flats in the Mid-Jiangsu Province between the Sheyang Estuary, the Liangduo Estuary, and the Lianxing Port was 0.96‰. In general, the slopes from north to south in this coastal area exhibited a steep-gentle-steep pattern, which was consistent with in situ observed data. We conclude that stratifying the coastlines according to their plane-form shapes and applying different methods to straight and sinuous coastlines can result in a more robust estimation of coastal slope for muddy intertidal flats than previous TTAG methods. This work demonstrates the utility of satellite-based remote sensing for retrieving critical coastal geomorphological information in dynamic terrains where in situ data are difficult to obtain.