Implementation and Development of Three-dimensional Hydrodynamic and Suspended Sediment Transport Model for Estuarine System and Its Adjacent Coastal Sea
Date Issued
2012
Date
2012
Author(s)
Chen, Wei-Bo
Abstract
Estuaries are coastal waters where the mouth of the river meets the ocean and where the freshwater of the river mixes with the saline water from the ocean, and represent the complex nonlinear interaction of tides, current, salt, water and suspended sediment. The primary factors controlling transport processes in estuaries are tides and freshwater from upstream. Hydrodynamic processes in estuaries are generally three-dimensional; hence, implementation of the three-dimensional model for estuarine system is imperative.
In the present study, a three-dimensional hydrodynamic model incorporating suspended sediment module was developed and applied to the Danshuei River estuarine system and its adjacent coastal sea. The model was calibrated with measured water surface elevation, tidal current, salinity and suspended sediment concentration (SSC). The overall model simulation results are in qualitative accordance with the available field data. The validated model was then adopted to investigate the physical properties of the Danshuei River estuary.
The vertical profiles of salinity, velocity, and eddy diffusivity show a marked asymmetry between the flood and ebb tides; moreover, the tidal currents, turbulent mixing and vertical stratification show large fortnightly fluctuations over the spring-neap cycle. The circulation strengths were stronger during the neap tides than during the spring tides. The Eulerian transport is always seaward, whereas the Stokes transport is the main mechanism driving the up-estuary. According to the numerical experiments, the residence time is less than three days under mean flow conditions and the density-induced circulation play an accelerating role for mass transport.
The particle-tracking model indicates that the water ages were approximately 320, 100 and 485 hours at the mouths of the Danshuei River-Tahan Stream, the Hsintien Stream and the Keelung River under the Q60 flow condition. Particles released experiments show that compare to the tidal current, wind influences were insignificant in the river channel. However, particle distribution was affected by density-induced circulation. The trajectories of oil spill were sensitivity to the leaking timing and wind direction, no matter oil spill even occurred in the river channel or harbor, because the oil particles always drift on the water surface.
The estuarine turbidity maximum appears around Kuan-Du Bridge, it may be due to the high bottom stress and deeper topography there. The SSC has slight raise with increasing turbulent eddy diffusivities but significant increase with high freshwater discharge. The SSC is over 1.5 times higher at spring tide than that at neap tide. The estuarine turbidity maximum extends its range from 7.0~12.0km to 7.0~16.0km (distance from the Danshuei River mouth) under the condition of bathymetric change.
Subjects
Estuaries
Hydrodynamic model
suspended sediment module
Particle-tracking model
the Danshuei River system
estuarine turbidity maximum
Type
thesis
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