## 1 Introduction  
Tidal flats are intertidal, soft sediment habitats, found between mean higher high water (MHHW) and mean lower low water (MLLW) lines [1, 2] and are present at various locations around the world. Tidal flats are an important part of the coastal zone and play an important role in improving coastal protection, increasing potential land resources, and conserving biodiversity [3, 4]. The monitoring of tidal flats has great importance for morphological evolution, sediment transformation, and hydrodynamic research [5]. Therefore, in order to understand the dynamic changes of intertidal areas, topographic monitoring provides the most objective and foundational bases. However, this topographic information remains limited in most coastal areas [6]. A tidal flat is a \"difficult area\" to recognize by traditional measurements, such as ground or shipboard surveys, that have high precision (centimeter level) [7]. Additionally, complete surveys and regular maintenance of updated digital elevation models (DEM) are costly and time-consuming. Data gaps are in the intertidal zone according to most global DEMs products due to the water-impermeable nature of existing remote sensing approaches (e.g., radar used for WorldDEMTM and Shuttle Radar Topography Mission DEM, and optical stereo-pairs used for ASTER Global Digital Elevation Map Version 2) [8]. Airborne survey (e.g., light detection and ranging (LIDAR) and radar interferometry (InSAR)) used in land surveying usually have relatively high resolution (0.5-1 m) [9]. Due to the high-cost to cover a large area and the favourable tidal conditions when flying overhead, Airborne survey methods are also not effective ways to obtain topographic data for tidal flats [10; 11]. From multiple images obtained over a range of tidal conditions, a set of heightened waterlines can be assembled in the intertidal zone, and from this a gridded DEM can be interpolated [12; 13]. According to the previous studies, DEMs of intertidal areas had been constructed around the world using waterline method. A DEM having a spatial resolution of 50 m and height accuracy of about 40 cm was formed using the waterline technique applied to satellite Synthetic Aperture Radar (SAR) images from 1991-1994 in Morecambe Bay, U.K [12]. [PERSON] et al. (2008) succeeded in generating DEMs with an accuracy of 10.9 cm for the years 1991 and 2000 in the tidal flats of Gomso Bay, Korea [10]. [PERSON] et al. (2020) applied the waterline method in the Gulf of Tonkin, Vietnam for multi-temporal satellite images to build DEMs with the vertical accuracy of 0.144 m \(\pm\) 0.05 m in 1989, 2000, and 2014 [6]. The Yangtze River mouth was also selected as the case example to test the suitability of the approach, and the maximum error for each time slice occurred under lower tide condition (elevation \(<\) 100 cm, i.e., 56 cm in 1987, 25 cm in 1993, and 76 cm in 2004) [14]. In Jiangsu coast, [PERSON] et al. (2012) generated a DEM which contained the error within 47 cm over a large offshore sandbank Dongsha integrating the conventional waterline method with a hydraulic model and multitemporal satellite images [15]. [PERSON] et al. (2017) explored an iterative waterline method to construct a DEM covering an area of approximately 1900 km\({}^{2}\) of tidal flats in the Radial Sand Ridges (RSRs) and the height accuracy (root mean square error) was 0.182 m [13]. To construct a DEM, the waterline method usually requires specific conditions. The most important of all is sufficient and high-quality remote sensing images at various tidal levels. In the process of DEM construction, more multi-images should be collected in a shorter time (e.g., a single month or season) so that the change of topography can be neglected. However, limited by weather or image quality, this collection is difficult to achieve in practice. In previous studies, the time span of images was half a year, at least, extending up to 3-4 years [16].