## 3 Results
### Underwater Delta Erosion and Deposition Evolution
From 1989 to 2012, the overall waterway outside Shantou Port was in a state of slight erosion, with a net erosion volume of 0.40 \(\times\) 107 m\({}^{3}\), and the deposition area and erosion area were 5.83 \(\times\) 10\({}^{7}\) m\({}^{2}\) and 5.38 \(\times\) 10\({}^{7}\) m\({}^{2}\), respectively. The deposition area and erosion area were roughly equal. During this period, the average erosion thickness was 0.04 m, and the average deposition rate was \(-\)0.15 cm/a. The erosion and deposition situation is shown in Figure 2b. Ninety-four percent of this area had an erosion and deposition range within two meters, and there was no obvious deposition and erosion center. At 116\({}^{\circ}\)48\({}^{\prime}\)31\({}^{\prime\prime}\) E and 23\({}^{\circ}\)14\({}^{\prime}\)24\({}^{\prime\prime}\) N, the deposition amplitude was the largest, the deposition thickness at the center was 9.1 m, and the average deposition rate was 39.7 cm/a. At 116\({}^{\circ}\)48\({}^{\prime}\)14\({}^{\prime\prime}\) E and 23\({}^{\circ}\)20\({}^{\prime}\)11\({}^{\prime\prime}\) N, the erosion amplitude was the largest, the erosion thickness at the center was 7.9 m, and the average deposition rate was \(-\)34.2 cm/a. From 1989 to 2012, the average annual sediment flux into the sea from the Hanjiang River was 3.844 million t, a 54.4% decrease compared with that in 1980-1989. The average annual sediment flux into the sea from the Rongjiang River was 363,000 t, a 29.5% decrease compared with that in 1980-1989 (Table 3). During this period, the significant reduction in the sediment fluxes from the Hanjiang and Rongjiang Rivers into the sea was the main reason for the transition of the outer channel of Shantou Port from deposition (1980-1989) to a balance between erosion and deposition (1989-2012). From 2012 to 2016, the Shantou Port Outer Channel was in the erosion period, with a net erosion volume of 4.14 \(\times\) 10\({}^{7}\) m\({}^{3}\) and deposition and erosion areas of 2.04 \(\times\) 10\({}^{7}\) m\({}^{2}\) and 9.15 \(\times\) 10\({}^{7}\) m\({}^{2}\), respectively. The erosion area accounted for 82% of the total area. During this period, the average erosion in this area was 0.37 m, and the average deposition rate was \(-\)9.24 cm/a. The erosion and deposition situation in this area is shown in Figure 2c. The deposition amplitude was the largest at 116\({}^{\circ}\)48\({}^{\prime}\)31\({}^{\prime\prime}\) E and 23\({}^{\circ}\)14\({}^{\prime}\)24\({}^{\prime\prime}\) N; the deposition thickness at the center was 6.3 m; and the average deposition rate was 156.9 cm/a. The erosion area was mainly distributed on the north side of the study area. The erosion amplitude was the largest at 116\({}^{\circ}\)45\({}^{\prime}\)37\({}^{\prime\prime}\) E and 23\({}^{\circ}\)19\({}^{\prime}\)15\({}^{\prime\prime}\) N; the erosion thickness at the center was 8.0 m; and the average deposition rate was \(-\)201.0 cm/a. From 2012 to 2016, the average sediment flux into the sea from the Hanjiang River was 2.257 million t, a 41.3% decrease compared with that from 1989 to 2012. The average annual sediment flux into the sea from the Rongjiang River was 269,000 t, a 25.9% decrease compared with that from  
Figure 3: Variation in sediment fluxes into the sea from the Hanjiang and Rongjiang Rivers and the average deposition rate of the channel outside Shantou Port. 1989-2012 (Table 3). During this period, the continuous reduction in the sediment fluxes from the Hanjiang and Rongjiang Rivers into the sea was the main reason for the change in the external waterway of Shantou Port from erosion and deposition balance (1989-2012) to erosion (2012-2016). The waterway area outside Shantou Port in the Hanjiang underwater delta was in a slight erosional state from 1980 to 2016 (Figure 2d, Table 2), with an annual average deposition rate of \(-\)0.71 cm/a.