**4.1 Astronomical tide**

### **4.1.1 Tidal elevation**

Figures 7 and 8 are the comparison of simulated and observed tidal elevations at 5 stations (T2, T3, T4, T5 and T6) in spring tide and neap tide, respectively. The x-coordinate of these figures is in the unit of day, and, for example, the label '21.25 August 2005' indicates '6:00am of 21/08/2005'. Both the numerical simulation and field observation for spring and neap tides show that the tidal range increases significantly as it travels from the lower estuary (about 6.2 m in spring tide and 3.1 m in neap tide at T6) towards the middle estuary (about 8.1 m in spring tide and 3.7 m in neap tide at T4), mainly due to rapid narrowing of the estuary. The tidal range reaches the maximum at Ganpu station (T4) and decreases as it continues traveling towards the upper estuary (about 4.4 m in spring tide and 2.5 m in neap tide at T2). In general, very good agreement between the simulation and observation is obtained. There exists, however, a slight discrepancy between the computed and observed tidal elevations at T2 (Yanguan). The reason for this may be ascribed to that the numerical model does not consider the tidal bore, which may have significant effect on the tidal elevations at the upper reach. Such impact on tidal elevations, however, decreases and becomes negligible at the lower reach of the estuary.

Fig. 6. A sketch of triangular grid for modeling typhoon-induced storm tide

It is clearly seen from Figures 7 and 8 that the maximum tidal ranges occur at the Ganpu station (T4). Thus, it is expected that the maximum tidal current may occur near this region. The tidal currents were measured at four locations H1-H4 across the estuary near Ganpu. These measurements are used to verify the numerical model. Figures 9 and 10 are the comparison between simulated and measured depth-averaged velocity magnitude and direction for the spring and neap tidal currents, respectively. It is seen that the flood tidal velocity is clearly greater than the ebb flow velocity for both the spring and neap tides. The maximum flood velocity occurs at H2 with the value of about 3.8 m/s, while the maximum ebb flow velocity is about 3.1 m/s during the spring tide. During the neap tide, the maximum velocities of both the flood and ebb are much less than those in the spring tide with the value of 1.5 m/s for flood and 1.2 m/s for ebb observed at H2. The maximum relative error for the ebb flow is about 17%, occurring at H2 during the spring tide. For the flood flow the maximal relative error occurs at H3 and H4 for both the spring and neap tides with values being about 20%. In general, the depth-averaged simulated velocity magnitude and current direction agree well with the measurements, and the maximal error percentage in tidal current is similar as that encountered in modeling the Mahakam Estuary (Mandang & Yanagi, 2008).

Astronomical Tide and Typhoon-Induced Storm Surge in Hangzhou Bay, China 187

**4.1.2 Current velocity**

Fig. 5. A sketch of triangular grid (upper) and locally zoomed in mesh near Ganpu station (lower) for modeling astronomical tide

Fig. 6. A sketch of triangular grid for modeling typhoon-induced storm tide
