Modeling Morphological Changes in the Danshui River Estuary during Typhoon Season in 2008

Spatiotemporal variation of morphology in the Danshui River estuary in Taiwan and its adjacent coasts is highly dynamic during typhoon seasons and therefore complex. Primary reason of the morphological variations is due to interactions of multiscale hydrodynamic and morphodynamic processes under extreme hydrological conditions driven by tides, waves, and river floods. Sediment releases from reservoirs at upstream also contribute to the morphological changes in the estuary. To better understand sedimentation processes in the estuary driven by the multiscale hydrodynamic forcing, an integrated coastal process model is used to simulate flows and seasonal morphological changes due to multiple typhoons in 2008. The high-resolution computational grid was generated by using bed elevations and Lidar data measured before the typhoon season. For the study, three typhoons (i.e. Fong Wong, Sinlako, and Jangmi), which made landfalls close to the study site, were simulated. The hydrodynamic model was validated by comparing the computed hydrodynamic variables (e.g. tidal levels, flow velocities, and waves) at gage stations with observation data. The simulated morphological changes in the estuary including its adjacent coasts and tidal river reaches were also compared with survey data. The computed morphodynamic processes reveal complex bed change patterns in the estuary induced by riverine and coastal processes and influenced by harbors, fishing ports, and sea walls. The simulated results and the computational model is able to facilitate systematic planning and design of the estuary and coasts by assessing impacts of both hydrological forcing (waves, tides, and river floods) and sediment transport in rivers and coasts in a typhoon/hurricane season.