Wen-Cheng Liu

Eutrophication modelling of reservoirs in Taiwan

Two reservoirs in Taiwan were modeled to simulate the hydrodynamics and water quality in the water column. The modelling effort was supported with data collected in the field for a 2-year period for both reservoirs. Spatial and temporal distributions of temperature in the water column of the two reservoirs were well reproduced by the hydrodynamic model. Model calculated concentrations of key water quality constituents such as nutrients, dissolved oxygen, and algal biomass matched the measured values closely in both reservoirs. Most importantly, vertical stratification of temperature and dissolved oxygen in the Tseng-Wen Reservoir was mimicked by the model throughout this 2-year period. The calibrated model was then used to simulate water quality response to various nutrient reduction scenarios. Model results reveal that a 30-55% reduction of the phosphorus loads will upgrade the existing eutrophic/mesotrophic to oligotrophic conditions in the Te-Chi Reservoir.

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.

Using a three-dimensional particle-tracking model to estimate the residence time and age of water in a tidal estuary

A three-dimensional hydrodynamic model that includes a Lagrangian particle-tracking simulation was applied to the Danshuei River estuarine system in northern Taiwan. The models accuracy was validated with data from 1999; the results from the model agreed well with empirical observations of water surface elevation, tidal currents, and salinity. The validated model was then used to investigate the residence time and water age in response to different levels of freshwater discharge. A regression analysis of the model results revealed that an exponential equation best explained the correlation between residence time and freshwater input. We found that the residence times during the low and high freshwater discharge episodes were 4.4 and 2.5 days, respectively. The water age during the low-flow periods was greater than that during the high-flow periods. The modelled residence time and water age values without density-induced circulation were higher than those with density-induced circulation, which indicates that density-induced estuarine circulation may play a significant role in the estuary.

Impact of phosphorus load reduction on water quality in a stratified reservoir-eutrophication modeling study.

Monitoring data collected from the Mingder Reservoir in Taiwan indicate that the water quality is between mesotrophic and eutrophic. Chlorophyll a concentration is higher in the summer and anoxic conditions occur in the bottom. The data also reveal that a pronounced vertical thermal gradient in summer and vertical mixing the end of fall. A vertical two-dimensional, laterally averaged hydrodynamic and water quality model (CE-QUAL-W2) was adopted to simulate the water surface elevation, water temperature, and water quality conditions in the water column. The modeling effort was supported with monitoring data collected in the field for a 2-year period in the reservoir. The hydrodynamic model reproduced the time series water surface elevation. Spatial and temporal distributions of temperature in the water column of the reservoir were also well reproduced by the hydrodynamic model. Model-calculated concentrations of key water quality constituents such as nutrients, dissolved oxygen, and algal biomass matched the measured values closely in the reservoir. The calibrated model was then applied to simulate water quality response to various nutrient reduction scenarios. Results of the model scenario runs reveal that a 20% and 80% reduction of the phosphorus loads will improve the water quality from eutrophic to mesotrophic and oligotrophic conditions, respectively. The modeling effort has yielded valuable information that can be used by decision makers for the evaluation of different management strategies of reducing watershed nutrient loads.

Development and Application of an Automated River-Estuary Discharge Imaging System

AbstractAn automated river-estuary discharge imaging system (AREDIS) is developed to measure the velocity of a wide tidal estuary. The system contains near- and far-field cameras that capture the entire 370-m-wide channel from an oblique angle with high image resolution. A new rotational scheme is developed to calibrate the cameras. Wakes generated by flow past bridge piers are found to be natural tracers for large-scale particle image velocimetry (LSPIV) to reliably and accurately measure the surface velocity, confirmed by buoy tracking velocimetry (BTV). The success of AREDIS is demonstrated in three field measurements on the Danshui River, the largest estuary in Taiwan. First, AREDIS is used to measure discharge over the entire tidal cycle under normal flow, and a 10% difference is found between discharges measured with AREDIS and a boat-mounted acoustic Doppler profiler. Second, AREDIS is employed to measure discharge under a typhoon event, resulting in discharges 45% greater than those at normal flow...

Artificial neural network modeling of dissolved oxygen in reservoir

The water quality of reservoirs is one of the key factors in the operation and water quality management of reservoirs. Dissolved oxygen (DO) in water column is essential for microorganisms and a significant indicator of the state of aquatic ecosystems. In this study, two artificial neural network (ANN) models including back propagation neural network (BPNN) and adaptive neural-based fuzzy inference system (ANFIS) approaches and multilinear regression (MLR) model were developed to estimate the DO concentration in the Feitsui Reservoir of northern Taiwan. The input variables of the neural network are determined as water temperature, pH, conductivity, turbidity, suspended solids, total hardness, total alkalinity, and ammonium nitrogen. The performance of the ANN models and MLR model was assessed through the mean absolute error, root mean square error, and correlation coefficient computed from the measured and model-simulated DO values. The results reveal that ANN estimation performances were superior to those of MLR. Comparing to the BPNN and ANFIS models through the performance criteria, the ANFIS model is better than the BPNN model for predicting the DO values. Study results show that the neural network particularly using ANFIS model is able to predict the DO concentrations with reasonable accuracy, suggesting that the neural network is a valuable tool for reservoir management in Taiwan.

Modeling fecal coliform contamination in a tidal Danshuei River estuarine system

A three-dimensional fecal coliform transport model was developed and incorporated into a hydrodynamic model to obtain a better understanding of local microbiological water quality in the tidal Danshuei River estuarine system of northern Taiwan. The model was firstly validated with the salinity and fecal coliform data measured in 2010. The concentration comparison showed quantitatively good agreement between the simulation and measurement results. Further, the model was applied to investigate the effects of upstream freshwater discharge variation and fecal coliform loading reduction on the contamination distributions in the tidal estuarine system. The qualitative and quantitative analyses clearly revealed that low freshwater discharge resulted in higher fecal coliform concentration. The fecal coliform loading reduction considerably decreased the contamination along the Danshuei River-Tahan Stream, the Hsintien Stream, and the Keelung River.

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.

Modelling hydrodynamics and water quality in the separation waterway of the Yulin offshore industrial park, Taiwan

Abstract Numerical models are often used to evaluate the potential impact of human alternation of natural water bodies and to help the design of the alternation to mitigate its impacts. This paper describes a case study in which an estuarine hydrodynamic and water quality model was used to help the design of the artificial waterway for an offshore industrial park. Yulin offshore industrial park is located at the central western coast of Taiwan. To mitigate its impacts on the nearby coastal area, the industrial park is buffered by a waterway separating it from the main island. The original design of the separation waterway has a width of 500 m. However, this width has been a controversial issue for the past 10 years. Since the separation waterway receives discharges from two streams in the main island, the Shu-Chu-Liao Stream and Hsin-Hu-Wei Stream, the potential inundation during flood events and water quality conditions during low flow periods are two of the major considerations for the width of the waterway. A vertical two-dimensional, real-time model of estuarine hydrodynamics and water quality was modified and used to determine the rational width of the separation waterway. The model modification is necessary to account for the multiple outlets into the sea when the whole system of the separation waterways is completed. Model calibration and verification were conducted with current, residual velocity, salinity, and water quality variables measured in the completed portion of the waterway. The overall performance of the model was in qualitative agreement with the available field data. The model was then used to evaluate several scenarios of different widths for the yet-to-be completed system of separation waterways. Based on the model simulation results, a 200 m wide waterway, bordered by 300 m wide flood plain, is recommended for adoption. The model simulation indicated that coastal water quality standard may be attained and inundation by the 50-year flood may be avoided by such a separation waterway.

Prediction of water temperature in a subtropical subalpine lake using an artificial neural network and three-dimensional circulation models

Water temperature is considered as a dominant factor in controlling the stratification, water quality, and ecological environment of lakes because many biological processes are temperature-dependent. Therefore accurate prediction of water temperature is crucially important for lake management. Traditional three-dimensional circulation models have been widely adopted to predict lake water temperatures spatially and temporally. An artificial neural network (ANN) technique is used as an alternative in water temperature simulation studies. The present study compares the performance of the ANN technique with a physically based three-dimensional circulation model. Four ANN models were established to simulate the time-series water temperature at a buoy station of the Yuan-Yang Lake (YYL) in north-central Taiwan at various measured depths. To evaluate the performance of the ANN and the three-dimensional circulation model, three different statistical indicators were used, including the root mean square error, the mean absolute error, and the coefficient of correlation. The simulated results reveal that the three-dimensional circulation model provides a better prediction of water temperature at different layers, except at the 3m below water surface, during the calibration phase. For the validation phase, the three-dimensional circulation model can predict water temperature satisfactorily at different layers than ANN model. Overall, the performance of water temperature prediction with three-dimensional circulation model is better than that with the ANN model. However ANN is a black box model and fails to simulate the internal physical processes in the lake, while three-dimensional circulation model is physical model which can be used to predict water temperature in spatial and temporal variations simultaneously.