## 3 Results  
### Underwater Delta Erosion and Deposition Evolution  
By digitizing and comparing the four seaway charts outside Shantou Port, the overall erosion and deposition results (Table 2) and the spatial distribution maps of erosion and deposition in the region in different periods (Figure 2) were obtained. From 1980 to 1989, the channel outside Shantou Port was in a state of accumulation, with a net deposition volume of 1.24 \(\times\) 10\({}^{7}\) m\({}^{3}\) and a deposition area and an erosion area of 7.40 \(\times\) 10\({}^{7}\) m\({}^{2}\) and 3.81 \(\times\) 10\({}^{7}\) m\({}^{2}\), respectively; thus, the deposition area accounted for 66% of the total area. The average deposition thickness during this period was 0.11 m, and the average deposition rate was 1.23 cm/a. The erosion and deposition distribution is shown in Figure 2a. Ninety-nine percent of the study area had an erosion and deposition range within two meters, and there was no obvious deposition and erosion center. At 116\({}^{\circ}\)50\({}^{\prime}\)4\({}^{\prime\prime}\) E and 23\({}^{\circ}\)18\({}^{\prime}\)29\({}^{\prime\prime}\) N, the deposition amplitude was the largest, the deposition thickness at the center was 5.5 m, and the deposition rate was 61.4 cm/a. At 116\({}^{\circ}\)48\({}^{\prime}\)30\({}^{\prime\prime}\) E and 23\({}^{\circ}\)14\({}^{\prime}\)24\({}^{\prime\prime}\) N, the erosion amplitude was the largest, the erosion thickness at the center reached 6.3 m, and the deposition rate was \(-\)70.0 cm/a. From 1980 to 1989, the average annual sediment flux of the Hanjiang River into the sea was 8.431 million t, and the average annual sediment flux of the Rongjiang River into the sea was 515,000 t (Table 3, Figure 3). \begin{table}
\begin{tabular}{c c c c c} \hline  & **1980–2016** & **1980–1989** & **1989–2012** & **2012–2016** \\ \hline Period length (a) & 36 & 9 & 23 & 4 \\ Total area (10\({}^{7}\) m\({}^{2}\)) & 11.14 & 11.22 & 11.22 & 11.19 \\ Sedimentation area (10\({}^{7}\) m\({}^{2}\)) & 5.10 & 7.40 & 5.83 & 2.04 \\ Sedimentation volume (10\({}^{7}\) m\({}^{3}\)) & 3.44 & 2.18 & 3.24 & 0.79 \\ Sedimentation thickness (m) & 0.68 & 0.29 & 0.55 & 0.39 \\ Erosion area (10\({}^{7}\) m\({}^{2}\)) & 6.04 & 3.81 & 5.38 & 9.15 \\ Erosion volume (10\({}^{7}\) m\({}^{3}\)) & 6.29 & 0.94 & 3.63 & 4.93 \\ Erosion thickness (m) & 1.04 & 0.25 & 0.67 & 0.54 \\ Net erosion volume (10\({}^{7}\) m\({}^{3}\)) & \(-\)2.84 & 1.24 & \(-\)0.40 & \(-\)4.14 \\ Net deposition thickness (m) & \(-\)0.26 & 0.11 & \(-\)0.04 & \(-\)0.37 \\ Average net erosion and deposition volume & \(-\)0.08 & 0.14 & \(-\)0.02 & \(-\)1.03 \\ of the period (10\({}^{7}\) m\({}^{3}\)/a) & \(-\)0.71 & 1.23 & \(-\)0.15 & \(-\)9.24 \\ Average deposition rate over time (cm/a) & \(-\)0.71 & 1.23 & \(-\)0.15 & \(-\)9.24 \\ \hline \end{tabular}
\end{table} 
Table 2: Calculated results of erosion and deposition in the Shantou Port Outer Channel in different periods. Figure 2: Erosion and deposition situations of the Shantou port outer waterway in different periods. (Thickness of erosion and siltation (m)). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Period** & **1980–1989** & **1989–2012** & **2012–2016** \\ \hline Average annual sediment flux into the sea (10,000 t) & Han Jiang & 843.1 & 384.4 & 225.7 \\ \hline \multirow{3}{*}{Shatou Port Outer Channel} & Silt thickness (m) & 0.11 & \(-\)0.04 & \(-\)0.37 \\  & Average deposition rate & 1.23 & \(-\)0.15 & \(-\)9.24 \\ \hline \multirow{3}{*}{ZZY07} & Silt thickness (m) & 0.38 & 0.31 & \(-\)0.11 \\  & Average deposition rate & 4.22 & 1.36 & \(-\)2.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Sediment fluxes from Hanjiang and Rongjiang into the sea and sedimentation rate in the channel outside Shantou Port during the same period.