## 4 Discussion  
In this section, we further discuss the impacts of Typhoon Fongwong, including the tidal level, current velocity and salinity in the NB of the YRE through comparison with a scenario of normal wind force (5.8 m/s in 75\({}^{\circ}\) at 0:00 of 15 September 2014) compared to the wind force of Typhoon Fongwong after 0:00 of 15 September 2014. Figure 15 is the time history of the difference in tidal level between the two scenarios of Typhoon Fongwong and a normal wind force action at NB1 and NB2 in the NB. To clearly reflect the typhoon function impact on the estuary, Figure 16 shows the close-up view from 22 to 26 September (covering the whole stable stage and part of the falling stage) of the differences in tidal level, the current speed and surface salinity between the two scenarios of Typhoon Fongwong and the normal wind force action at NB1 and NB2 in the NB. The change processes in the difference of tidal levels at NB1 and NB2 have similar trends, while the change starting time at NB1 begins earlier than that at NB2, which corresponds to the tidal level change. The maximum tidal level difference at NB2 is 0.61 m at 9:00 on 23 September, which is higher than the maximum value of 0.46 m at NB1. The tidal level difference increases toward the upstream area in the NB corresponding to the tidal level range change (shown in Figures 9 and 11a), which is controlled by the funnel-shaped plane geometry of the NB and the typhoon action. For the current speed, the difference in the typhoon stable period is much larger than that in the typhoon rising and falling period. The maximum current velocity difference is 0.13 m/s during the flood tide, while it is 0.12 m/s during the ebb tide at NB1; it has a similar characteristic at NB2 with smaller change values. Table 3 shows that the typhoon  
Figure 16: Difference in tidal level (DTL), current speed (DCS) on the surface and surface salinity (DSS), from 22 to 26 September, between the two scenarios of Typhoon Fongwong and normal wind force action at NB1 and NB2 in the NB in 2014. Figure 15: Time history of difference in tidal level (DTL) between the two scenarios of Typhoon Fongwong and normal wind force action at NB1 and NB2 in the NB in 2014.4.  
enhances the flood current and weakens the ebb current, and this effect is obvious at the mouth of the NB. It implies that the typhoon has a stronger impact on the mouth than the upstream area of the river, due to both the typhoon becoming weaker after landing and an increasing bottom roughness toward the upstream area. [PERSON] et al. (2019), who studied the current induced by a hurricane in a semi-enclosed bay, found that after the storm's passage, the bay relaxed with a rapid movement of the entire water column in the opposite direction, which could explain the difference in current wind speed after 25 September in our study. We computed the flood and ebb discharge of the NB and the SB through two selected cross-sections in the NB and the SB at the separation point (positions shown in Figure 1b) under two scenarios of Typhoon Fongwong and a normal wind force; the results and the flow split ratio of the NB are also shown in Table 3. The typhoon slightly increases flood discharge and decreases ebb discharge in the SB, but a significant change happens in the NB, i.e., the flood discharge increases by 29.38% and ebb discharge decreases by 84.01%, respectively. This changes the flow split ratio of the NB, that is, the flood flow split ratio increases from 3.31% to 4.51% with an increase ratio of 26.61%, while the ebb flow split ratio decreases from 0.99% to 0.54% with a decrease ratio of 83.33%. This further supports that the typhoon enhances the flood current and weakens the ebb current in the NB of the YRE. For the surface salinity, this difference in the change process is similar to the change trend of salinity shown in Figure 8. The change range of the surface salinity difference at NB1 is much larger than that at NB2.