Tamsui river is the largest river of north Taiwan. Spatio-temporal variation of morphology in the Tamsui River estuary and its adjacent coasts is highly dynamic and therefore complex. Primary reason of the variations is the complicity of the hydrodynamic and morphodynamic processes in the area. Quick morphological changes often occur in a tropical storm or typhoon. Sediment releases from reservoirs at upstream also contribute to the morphological changes in the estuary.
To better understand hydrodynamic and morphodynamic processes in the estuary driven by river flood flows, waves, tides, and winds, an integrated coastal process model, CCHE2D-Coast, is used to simulate flows and morphological changes due to multiple typhoons, i.e., Fong Wong (2008), Sinlako (2008), Jangmi (2008), and Morakot (2009). The computational grid was generated by using the bed elevations and the Lidar data measured before the typhoon season. The computed hydrodynamic variables (e.g., tidal levels, flow velocities, and waves) at several gage stations are carefully compared with the observation data. The simulated morphological changes in the estuary including its adjacent coasts and tidal river reaches are also compared with the survey data. Preliminary simulation results indicate that this integrated coastal process model is able to reproduce hydrodynamic and morphodynamic processes such as tidal flows, waves, river flood flows, and sediment transport in the estuary and coasts in a typhoon season.
To assess the impact of multiple typhoons (Aere (2004), Fong Wong (2008), Sinlako (2008), Jangmi (2008), Morakot (2009) and Saola (2012) ) on the estuary, the computational conditions were assumed to include six historical typhoons which made landfalls at the east coasts of Taiwan. The sediment flux at the Kuando bridge is computed by a 1-D model (CCHE1D).
Two computational grids were generated by using the bathymetric data observed in 2013: one without the bridge piers of the planned Dan Jiang Bridge; another with the two bridge piers installed. In the Aere typhoon without Dan Jiang Bridge the significant morphological changes occurred at the river mouth, and the range of the changes at the middle of the river is from -0.1m to +0.1m. The maximum bed changes in the river mouth (not along with the longitudinal transect in the middle) were found to be within a range from -0.50 m to +0.50 m. However, the difference of the bed changes in the other cases (sediment released form Shihmen Reservoir Project C, and Project D) is quite small.
The case 3 (with Dan Jiang Bridge) is to compute the morphological changes under the given conditions of the selected storms (without the additional sediment release from the Shihmen Dam). Computed flows and waves in the coasts and the offshore are quite similar to the above-mentioned cases. The comparisons of the wave heights show that after the bridge piers installed, the maximum wave heights may slightly increase up to approximately 0.1 m. It implies that after the bridge piers are constructed, the morphology may vary, the deposition at the river mouth may increase up to 0.3m, and the erosion may grow quickly. The comparison indicates that in Case 4C (sediment released form Shihmen Reservoir Project D), more erosion at the river mouth may occur. But the difference in bed changes is not significant.
The results of multiple typhoons for all cases are also obtained. It can be seen that the bar-like morphology in the river mouth has a trend to move upstream, due to the installation of the bridge and the sediment release from the reservoir.