## 2 Materials and Methods  
### Study Area and Data  
The geomorphic system from the lower reaches of the Yangtze River to the estuarine delta has undergone varying degrees of adaptive adjustment in recent years. The lower reaches of Datong (located in Chizhou City) are affected by the tide of the East China Sea, and they thus belong to the tidal reach area [26]. Numerous engineering construction projects have been implemented in the Yangtze River Estuary, and these have caused extension of the fluctuating section of the tidal zone [27]. The rise in relative sea level will ultimately lead to a rise in the tide level and changes in the dynamic conditions of the Yangtze River Estuary area. In addition, aggravation of river channel erosion in the mouth of the Yangtze River has seriously impacted the evolution of the river channel in the middle and lower reaches of the Yangtze River [28; 29]. Therefore, in the past decade, many underwater micro geomorphology surveys and researches using multi-beam sounding systems have been conducted in the tidal reach area [16; 17; 18; 30; 31]. With respect to the importance of studying this area, the tidal reach region is also the focus of this study (see Figure 1a).  
Figure 1: (**a**) Location of the two bathymetric survey areas in Yangtze River, (**b**) geomorphological environment surrounding Chizhou Reach, and (**c**) that of the Yangtze Estuary. (**d**) Sample data for Chizhou Reach and (**e**) Yangtze Estuary. Six cross-sections were set in the sample zones and used to compare the water depth profiles with the distribution of mapped geomorphic elements.  
Two bathymetric datasets were collected from the end and start of the tidal reach respectively, and were used as study cases (Figure 1a). The measurements of the first set were collected in Chizhou Reach within the Yangtze River (Figure 1b) where two flows converge in one direction, and the channel width ranges from 1 to 2 km (approximately), and those for the second set were collected in the North Port Channel of the Yangtze Estuary (Figure 1c) where the flow diverges in two directions and the channel width ranges from 4 to 10 km (approximately). Bathymetric point cloud data were acquired using a SeaBat 7125 multibeam system operating at a frequency of 400/200 kHz and DGPS (Differential Global Position System) -positioned. Data were input into PDS2000 software (Version 4.2.16, Teledyne RESON, Rotterdam, The Netherlands) to generate a grid model with a resolution of \(1\times 1\) m. Two sample zones (Figure 1d,e), both with an area of \(450\times 360\) m\({}^{2}\), were cut from the grid model and used to compare the feasibility of the three geometric element mapping methods. The two aforementioned zones were representative of dune and relatively flat areas. Interpretation by manual inspection separated dune areas and relatively flat areas, as shown in Figure 1d,e.  
The water depth for the sample data of Chizhou Reach ranges from 6.155 to 15.761 m (Figure 1d), and the depth of the Yangtze Estuary ranges from 8.607 to 19.007 m (Figure 1e). Both sample zones contained blank areas with NODATA value that is always represented by \(-\)9999 in files in ASCII format. The actual areas excluding NODATA value were 131,041 and 138,623 m\({}^{2}\), respectively.