Ming-Hsi Hsu

Modelling of hydrodynamics and cohesive sediment transport in Tanshui River estuarine system, Taiwan

A laterally averaged two-dimensional numerical model is used to simulate hydrodynamics and cohesive sediment transport in the Tanshui River estuarine system. The model handles tributaries as well as the main stem of the estuarine system. Observed time series of salinity data and tidally averaged salinity distributions have been compared with model results to calibrate the turbulent diffusion coefficients. The overall model verification is achieved with comparisons of residual currents and salinity distribution. The model reproduces the prototype water surface elevation, currents and salinity distributions. Comparisons of the suspended cohesive sediment concentrations calculated by the numerical model and the field data at various stations show good agreement. The validated model is applied to investigate the tidally averaged salinity distributions, residual circulation and suspended sediment concentration under low flow conditions in the Tanshui River estuarine system. The model results show that the limit of salt intrusion in the mainstem estuary is located at Hsin-Hai bridge in Tahan Stream, 26 km from the River mouth under Q75 flow. The null point is located at the head of salt intrusion, using 1 ppt isohaline as an indicator. The tidally averaged sediment concentration distribution exhibits a local maximum around the null point.

Simulation of water quality and plankton dynamics in the Danshuei River estuary, Taiwan

An ecosystem model was developed to simulate the water quality and plankton dynamics in the Danshuei River estuary, Taiwan. The model simulates the hydrodynamics with a laterally integrated 2-dimensional intratidal numerical model, which supplies the physical transport processes for simulation of water quality and plankton state variables. The application of the model to the Danshuei River estuary indicates that the point source loadings are mainly responsible for the degraded water quality and very high nutrient concentrations in the estuary. The impacts of wastewater discharges are tightly controlled by the transport processes. Frequent occurrence of high river flow and flood events rapidly cleanses the estuary by flushing out both pollutants and plankton populations. The plankton is allowed to grow to significant populations if low river flow lasts for a period much longer than the biological time scale.

Application of different turbulence closure model for stratified tidal flows and salinity in an estuarine system

Transient stratification in estuaries reflects competition between the stratifying influences of the vertical gravitational circulation and longitudinal density gradient by vertical shear, set against the mixing influence of, principally, tidally generated turbulence. A vertical (laterally averaged) two-dimensional model of an estuary, using seven different parameterizations of vertical mass and momentum mixing coefficients from the literature, is used to make general predictions about the nature of the time-dependent stratification, velocity field and salinity in an estuary. The downstream boundary, at the river mouth, is an M2 tide with amplitude being half of the mean tidal range to force the model runs for numerical experiments. The results show the Mellor and Yamada scheme products larger stratification, density gradients and have less vertical mixing. Model calibration and verification is performed to use parameterization of mixing coefficients against observational data of salinity. The root-mean-square (RMS) errors and mean absolute errors are used as qualitative and quantitative criteria. The results show Park and Kuo scheme performs best. Mellor and Yamada scheme over-predict the amplitude of the stratification signal. The other schemes, such as Thompson, Lehfeldt and Bloss, Pacanowski and Philander, and Munk and Anderson, over-predict the maximum salinity.

A MODELING STUDY OF WATER QUALITY IN MAIN CHANNEL AND ESTUARINE WETLAND

A simple computation framework is applied to include estuarine wetland and their interaction with main channels in estuarine modeling. The concept and the model implementation of the scheme are explained using a vertical two-dimensional model of estuarine hydrodynamics and water quality. The model was applied to the Tanshui River estuary and Kuan-Du wetland. The model is calibrated and verified by the available measured data. Simulations are also conducted for various upstream freshwater discharges to predict water quality in the main channel and estuarine wetland. The results show that the inclusion of estuarine wetland in a water-quality model not only provides a framework for computing water-quality conditions but also accounts for the interaction between wetland and main channel. The model provides a useful tool for environmental planning, protection and proposed wetland restoration works.