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The effects of environmental factors on suspended sediment plumes in the continental shelf out of the Danshuei River estuary were numerically investigated using an unstructured-grid three-dimensional hydrodynamic model (SCHISM) together with a suspended sediment (SS) module. The coupled model (SCHISM-SS) was calibrated and validated against the in situ measurement data in 2016. Consistent with the observation results, the model simulations satisfactorily reproduced the water levels, velocities, salinities, and suspended sediment concentrations. The model was then applied to explore the role of various environmental factors in the dynamics of suspended sediment plumes from the estuary to the adjacent coastal seas. These factors include tidal forcing, salinity, river discharge, and wind stress. Analysis and comparisons of different scenario results indicated that the suspended sediment plume was greatly affected by tides, e.g., a longer plume distance resulted from a larger flux under tidal motions. A higher sediment concentration in the plume in the offshore area was also found during the neap tide, relative to that observed during the spring tide. In addition, salinity affects the movement of density currents and the spread of the sediment plume, i.e., the plume distance is longer due to the residual circulation when a salinity difference is present. Further, an extreme river flow could occur during typhoon periods and would discharge a greater water volume into the coastal region, causing the suspended sediment plume to expand from the near shore. Finally, the directions of prevailing winds can slightly influence the sediment plumes.

This study mainly aims to hindcast historical Typhoon Nepartak (2016) and to analyze tide–surge and wave interactions. To achieve this goal, a coupled tide–surge–wave model was built around the coast of Taiwan, and it was applied to simulate storm surges and waves during historical typhoon event. To demonstrate its capability, the coupled model was validated with observed water levels, wind speeds, atmospheric pressures, wave heights, and wave periods at different stations. Quantitatively, the model results reasonably reproduced the observed data. Furthermore, the model was utilized to explore the tide, surge, and wave interaction using historical Typhoon Nepartak (2016). The model results revealed that onshore wind enhanced the surge height, while offshore wind decreased the surge height. The atmospheric pressure consistently displayed a positive contribution to surge height. The wind-induced surge height contributing to the total surge height was 136.83% at Taichung Harbor, because the offshore wind mainly dominated the surge height compared to the effect of atmospheric pressure. The atmospheric pressure-induced surge heights contributing to the total surge height were 100.59%, 96.26%, and 99.74% at Kaohsiung Harbor, Hualien Harbor, and Taitung Fugang, respectively, as a result of the atmospheric pressure dominating the surge height. However, the contribution of the wave setup to the total surge height was minor. The path of typhoons at different time frames significantly affected the spatial distributions of surge height and wave height around the coast of Taiwan. The contributions of water level and current to wave height reached a maximum value of approximately 0.3 m at Suao, while the contributions at Penghu, Qigu, and Eluanbi were all less than 0.2 m. This signified that the influence of the water level and current on wave height was not negligible.

Tidal estuaries provide crucial pathways for contaminant transport. The salinity levels in estuaries and coasts are conserved substances that function as natural tracers to easily understand the offshore transport of substances that are subject to environmental factors. A three-dimensional (3D) circulation and mass transport model were utilized to delineate the salinity plume in a tidal estuary and continental shelf. The numerical modeling results were compared with the tidal amplitudes and phases, velocities, and salinities at different gauging stations in 2017. Quantitatively, the simulation and measurement results are in reasonably good agreement. Furthermore, the validated model was adopted to estimate the recovery times in tidal estuaries that are subjected to extreme freshwater discharges that come from the upstream reaches during typhoon events and to analyze the influences of freshwater discharge and wind stress on the river plume around the continental shelf. The simulated results revealed that the salinity recovery time at the river mouth due to Typhoon Saola in 2012 was less than 8 days. Increased inputs from freshwater discharge resulted in changes in the distances and areas of the river plumes. Linear regression relationships between the plume distance/plume area and the total freshwater discharge inputs were established. Neap and high slack tides were associated with the maximum plume distances and areas. Excluding tidal forcing resulted in larger plume distances and areas compared to the case in which tidal forcing was considered. The southward-favorable and northward-favorable plumes were controlled by northeasterly winds and southwesterly winds, respectively. The relative importance of freshwater discharges and wind forcing was explored. The results indicate that freshwater discharges frequently dominated the river plume, except when strong southwesterly or northeasterly winds prevailed.

Large-scale particle image velocimetry (LSPIV) provides a cost-effective, rapid, and secure monitoring tool for streamflow measurements. However, surveys of ground control points (GCPs) might affect the camera parameters through the solution of collinearity equations and then impose uncertainty on the measurement results. In this paper, we explore and present an uncertainty analysis for image-based streamflow measurements with the main focus on the ground control points. The study area was Yufeng Creek, which is upstream of the Shimen Reservoir in Northern Taiwan. A monitoring system with dual cameras was set up on the platform of a gauge station to measure the surface velocity. To evaluate the feasibility and accuracy of image-based LSPIV, a comparison with the conventional measurement using a flow meter was conducted. Furthermore, the degree of uncertainty in LSPIV streamflow measurements influenced by the ground control points was quantified using Monte Carlo simulation (MCS). Different operations (with survey times from one to nine) and standard errors (30 mm, 10 mm, and 3 mm) during GCP measurements were considered. Overall, the impacts in the case of single GCP measurement are apparent, i.e., a shifted and wider confidence interval. This uncertainty can be alleviated if the coordinates of the control points are measured and averaged with three repetitions. In terms of the standard errors, the degrees of uncertainty (i.e., normalized confidence intervals) in the streamflow measurement were 20.7%, 12.8%, and 10.7%. Given a smaller SE in GCPs, less uncertain estimations of the river surface velocity and streamflow from LSPIV could be obtained.

A three-dimensional hydrodynamic, suspended sediment, and fecal coliform transport model was developed and applied to characterize the fecal coliform plume in the Danshuei River estuarine system and its adjacent coastal sea. The numerical model was calibrated and validated with time-series water levels and spatial distributions of suspended sediment and fecal coliform concentrations in 2012 and 2013, respectively. Three indices (i.e., mean absolute error, root-mean-square error, and skill score) were used to compare the predicted results and measured data for assessing the performance of the model. Quantitatively, there was good agreement between the simulation and the measurement results. The validated model was later applied to explore the influence of river discharge and wind stress on the fecal coliform plume in the Danshuei River coastal area. In the absence of wind stress, no cyclonic circulation occurs as a result of river discharge under flood conditions. However, anticyclone circulation occurs on the east coast of the Danshuei River mouth at low river discharge. The plume distances of fecal coliform from the Danshuei River mouth reach 17 km and 5 km under flood flow and Q50 flow conditions, respectively. In the absence of river discharge, the plume distance of fecal coliform is confined to the Danshuei River mouth. Consequently, the wind directions control the upwelling and downwelling plumes. If the river discharge and wind stress are included in the model simulations, the surface fecal coliform concentrations are flushed out of the Danshuei River mouth and move to the southwestern coast as a result of the wind’s favoring downwelling. Conversely, the fecal coliform concentrations are flushed out of the Danshuei River mouth and flow to the northeastern coast, with the wind favoring upwelling. Comparing simulated results with and without wind stresses, the plume distance/area of fecal coliform decreases with the inclusion of wind stress.

Storm surge induced by severe typhoons has caused many catastrophic tragedies to coastal communities over past decades. Accurate and efficient prediction/assessment of storm surge is still an important task in order to achieve coastal disaster mitigation especially under the influence of climate change. This study revisits storm surge predictions using artificial neural networks (ANN) and effective typhoon parameters. Recent progress of storm surge modeling and some remaining unresolved issues are reviewed. In this paper, we chose the northeastern region of Taiwan as the study area, where the largest storm surge record (over 1.8 m) has been observed. To develop the ANN-based storm surge model for various lead-times (from 1 to 12 h), typhoon parameters are carefully examined and selected by analogy with the physical modeling approach. A knowledge extraction method (KEM) with backward tracking and forward exploration procedures is also proposed to analyze the roles of hidden neurons and typhoon parameters in storm surge prediction, as well as to reveal the abundant, useful information covered in the fully-trained artificial brain. Finally, the capability of ANN model for long-lead-time predictions and influences in controlling parameters are investigated. Overall, excellent agreement with observations (i.e., the coefficient of efficiency CE &gt, 0.95 for training and CE &gt, 0.90 for validation) is achieved in one-hour-ahead prediction. When the typhoon affects coastal waters, contributions of wind speed, central pressure deficit, and relative angle are clarified via influential hidden neurons. A general pattern of maximum storm surge under various scenarios is also obtained. Moreover, satisfactory accuracy is successfully extended to a much longer lead time (i.e., CE &gt, 0.85 for training and CE &gt, 0.75 for validation in 12-h-ahead prediction). Possible reasons for further accuracy improvement compared to earlier works are addressed.

Anthropogenic changes in tidal estuaries have significantly altered bathymetry and topography over the past half century. The geomorphic-driven changes in estuarine hydrodynamics and salt transport remain unclear. To explore this issue, a SELFE (Semi-implicit Eulaerian-Lagrangian Finite Element) -based model was developed and utilized in a case study in the Danshui River, Taiwan. The model was calibrated and validated using observed water level, current, and salinity data from 2015, 2016, and 2017. The performance of the SELFE model corresponded well to the measured data. Furthermore, the validated model was utilized to analyze the hydrodynamics, residual current, limit of salt intrusion, and residence time under the predevelopment (1981) and present (2015) conditions. The predicted results revealed that the time lag of water surface elevation at both high tide and low tide under the present condition was approximately 0.5&ndash, 2 h shorter under the predevelopment condition. The residual circulation under the predevelopment condition was stronger than under the present condition for low flow, causing the limit of salt intrusion to extend further upstream under the predevelopment condition compared to the limit of salt intrusion under the present condition. The calculated residence time under the predevelopment condition was longer than the residence time under the present condition. The freshwater discharge input is a dominating factor affecting the salt intrusion and residence time in a tidal estuary. A regression correlation between the maximum distance of salt intrusion and freshwater discharge and a correlation between residence time and freshwater discharge were established to predict the limit of salt intrusion and residence time under the predevelopment and present conditions with different scenarios of freshwater discharge input.

Thermal stratification is a critical physical process controlling carbon (C) flux from lakes into the atmosphere. In general, vertical water temperature profiles in shallow subtropical lakes can vary significantly because typhoons frequently induce vertical mixing across the entire lake due to strong winds and rapid flushing from river inputs. Since C fluxes are driven by dissolved inorganic C (DIC), it is necessary to understand stratification's influence on DIC dynamics in shallow subtropical lakes. Therefore, we aimed to clarify the impact of stratification on DIC in Yuan-Yang Lake, a typical shallow subtropical mountain lake, by developing a net ecosystem production (NEP) model. We measured the vertical profile of water temperature and DIC once or twice a month from July 2004 to December 2017. We applied a three-dimensional hydrological model to estimate residence time and NEP, which revealed that large amounts of DIC are stored in the lower layer from spring to summer due to the suppression of vertical mixing by stratification. In autumn and winter, the lake was well mixed, and DIC was evenly distributed in the water column due to vertical mixing. This was confirmed by vertical DIC profiles.

An integrated storm surge and flood inundation modeling system was developed and applied to simulate storm surges and inundation in the Gaoping River basin and its adjacent coastal area in southern Taiwan. An unstructured grid, three-dimensional hydrodynamic model was driven by the tidal elevation at the open boundaries and freshwater discharge at the upriver boundary. The model was calibrated and validated against the observed water levels for five typhoons. The results indicate a reasonable agreement between the model simulation and the observational data. A frequency analysis was used to fit the maximum surge height of historical typhoons induced by storm surges and yield the storm surge height for different return periods. The validated model was then applied to assess the potential hazard maps for different return periods as well as the influence of the sea level rise on the potential hazard maps. The typhoon-induced storm surge under the 200-year return period matched the 200-year return period peak flow, resulting in the maximum extent and depth of inundation in the low-lying area. Furthermore, the simulated results reveal that the sea level rise would enhance the flooding areas. The inundation hazard maps developed in this study could be a useful tool for planning the evacuation of residents living in low-lying areas and the decision-making of flood risk management on the coasts and floodplains.

We investigated how the lack of usual winter and spring monsoons, effectively representing consecutive drought events, affected the dynamics of ecosystem metabolism in a shallow mesotrophic seepage lake in northeastern Taiwan. An instrumented buoy provided high-frequency free-water dissolved oxygen measurements, water temperature profiles, and meteorological data, which we used to estimate daily values of gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP). Results revealed that the disappearance of monsoons decreased lake level and volume, concentrated dissolved nutrients, stimulated the development of algal biomass, promoted stratification, and resulted in a major shift in lake metabolism. Offshore GPP and R were both initially stimulated but then decreased due to shallower mixing depths in the water column. The lake rapidly shifted from a heterotrophic state to a highly autotrophic status when the water level dropped to the lowest level. A return to autotrophy was caused by a greater decline in R than an increase in GPP. This study demonstrates the dramatic effect that drought events can have on lake ecosystem function and suggests that nutrient control may be important in mitigating the effects of a predicted warmer and drier climate and increased water withdrawal in this region.

An unstructured grid, two-dimensional hydrodynamic model was established and applied to the coast of Taiwan to investigate the tide-surge interaction. Tidal elevations at the open boundaries coupled with a global ocean tidal model and the meteorological conditions using a cyclone model are used to drive the model. The model was calibrated and verified with the observed tidal levels at six tidal stations for seven typhoon events to ascertain the capability and feasibility of the model. The results show reasonable agreement between the simulated and observed tidal levels. The validated model was then applied to probe the influence of tide-surge interaction on phase, water levels, and storm surge height. We found that the tide-surge interaction influenced both the magnitude and timing of the surge, which depended on the typhoon path. The storm surge heights at different tidal stations were significantly influenced by wind stresses and directions. The water level rise due to the storm surge during high tide was greater at neap tide than at spring tide. Changing tidal ranges altered the prediction of the surge enough to induce the changes in peak water levels.

The accurate prediction of hourly runoff discharge in a watershed during heavy rainfall events is of critical importance for flood control and management. This study predicts n-h-ahead runoff discharge in the Sandimen basin in southern Taiwan using a novel hybrid approach which combines a physically-based model (HEC-HMS) with an artificial neural network (ANN) model. Hourly runoff discharge data (1200 datasets) from seven heavy rainfall events were collected for the model calibration (training) and validation. Six statistical indicators (i.e. mean absolute error, root mean square error, coefficient of correlation, error of time to peak discharge, error of peak discharge and coefficient of efficiency) were employed to evaluate the performance. In comparison with the HEC-HMS model, the single ANN model, and the time series forecasting (ARMAX) model, the developed hybrid HEC-HMS–ANN model demonstrates improved accuracy in recursive n-h-ahead runoff discharge prediction, especially for peak flow disch...

An automated discharge imaging system (ADIS), a non-intrusive and safe approach, was developed for measuring river flows during flash flood events. ADIS consists of dual cameras to capture complete surface images in the near and far fields. Surface velocities are accurately measured using the Large Scale Particle Image Velocimetry (LSPIV) technique. The stream discharges are then obtained from the depth-averaged velocity (based upon an empirical velocity-index relationship) and cross-section area. The ADIS was deployed at the Yu-Feng gauging station in Shimen Reservoir upper catchment, northern Taiwan. For a rigorous validation, surface velocity measurements were conducted using ADIS/LSPIV and other instruments. In terms of the averaged surface velocity, all measured results were in good agreement with small differences, i.e., 0.004 to 0.39 m/s and 0.023 to 0.345 m/s when compared to those from acoustic Doppler current profiler (ADCP) and surface velocity radar (SVR), respectively. The ADIS/LSPIV was further applied to measure surface velocities and discharges during typhoon events (i.e., Chan-Hom, Soudelor, Goni, and Dujuan) in 2015. The measured water level and surface velocity both showed rapid increases due to flash floods. The estimated discharges from ADIS/LSPIV and ADCP were compared, presenting good consistency with correlation coefficient R = 0.996 and normalized root mean square error NRMSE = 7.96%. The results of sensitivity analysis indicate that components till (τ) and roll (θ) of the camera are most sensitive parameter to affect the surface velocity using ADIS/LSPIV. Overall, the ADIS based upon LSPIV technique effectively measures surface velocities for reliable estimations of river discharges during typhoon events.

A three-dimensional hydrodynamic model was applied to the Danshuei River estuarine system in northern Taiwan to investigate the influence of flood-ebb, spring-neap tidal cycles, and salinity distribution on tidal mixing, residual circulation, stratification, and tidal asymmetry. The model was validated using observational data collected in 2008. The results from the model agreed well with observations of water surface elevation, tidal currents, and salinity. It was found that the depth-averaged tidal current during flood tide is weaker with a shorter duration than that during ebb tide in the estuary, which was attributed to tidal asymmetry. Vertical profiles of salinity, flow, eddy diffusivity, and Richardson number also showed a marked asymmetry between flood and ebb tides. Bottom boundary stresses were higher during flood tides than during ebb tides, resulting in more mixing occurrence and consequently decreasing the Richardson numbers. The tidally averaged salinity was more stratified during neap tides than during spring tides because the presence of the stronger vertical diffusivity and turbulent kinetic energy during spring tides. The modeling results also confirmed that the residual circulation was stronger during neap tides than during spring tides.

This paper documents a modeling investigation to comprehend the effect of future sea-level rise (SLR) on estuarine salinity and transport time scales, including the residence time and the water age of dissolved substances in a partially mixed estuary. A three-dimensional semi-implicit Eulerian–Lagrangian finite-element model was established and applied to the Tamsui River estuarine system and the adjacent coastal sea in northern Taiwan. The modeling results indicated reasonable agreement with the observed water levels, tidal currents, and salinity. The model was then applied to calculate the salt intrusion, residence time, and water age between the baseline (without SLR) and different scenarios, including SLRs of 0.34, 1.05, and 1.40 m for the year 2100. The numerical model results reveal that the average salt content and salt intrusion length will increase as the sea level rises. The 1 psu isohaline moves toward upstream reaches with an increase in SLR. The results reveal that the maximum increment of tidal-averaged and depth-averaged salinity would be 1.1, 2.4, and 3.0 psu, respectively, for the SLRs of 0.34, 1.05 and 1.40 m at the middle estuary under mean flow conditions. The regression between salt intrusion length and freshwater discharge are established corresponding to different SLR scenarios. The residence time of the entire Tamsui River system would increase from 6.3 to 23 % compared to the baseline under low flow conditions. The concentration of dissolved substances would have a longer transport time from upstream to downstream because water volume increases with SLR. This indicates that the water age will increase in the main Tamsui River estuary as the sea level rises.

Accurate simulations of river stages during typhoon events are critically important for flood control and are necessary for disaster prevention and water resources management in Taiwan. This study applies two artificial neural network (ANN) models, including the back propagation neural network (BPNN) and genetic algorithm neural network (GANN) techniques, to improve predictions from a one-dimensional flood routing hydrodynamic model regarding the water stages during typhoon events in the Danshuei River system in northern Taiwan. The hydrodynamic model is driven by freshwater discharges at the upstream boundary conditions and by the water levels at the downstream boundary condition. The model provides a sound physical basis for simulating water stages along the river. The simulated results of the hydrodynamic model show that the model cannot reproduce the water stages at different stations during typhoon events for the model calibration and verification phases. The BPNN and GANN models can improve the simulated water stages compared with the performance of the hydrodynamic model. The GANN model satisfactorily predicts water stages during the training and verification phases and exhibits the lowest values of mean absolute error, root-mean-square error and peak error compared with the simulated results at different stations using the hydrodynamic model and the BPNN model. Comparison of the simulated results shows that the GANN model can be successfully applied to predict the water stages of the Danshuei River system during typhoon events.

Global climate change has resulted in a gradual sea level rise. Sea level rise can cause saline water to migrate upstream in estuaries and rivers, thereby threatening freshwater habitat and drinking-water supplies. In the present study, a three-dimensional hydrodynamic model was established to simulate salinity distributions and transport time scales in the Wu River estuary of central Taiwan. The model was calibrated and verified using tidal amplitudes and phases, time-series water surface elevation and salinity distributions in 2011. The results show that the model simulation and measured data are in good agreement. The validated model was then applied to calculate the salinity distribution, flushing time and residence time in response to a sea level rise of 38.27 cm. We found that the flushing time for high flow under the present condition was lower compared to the sea level rise scenario and that the flushing time for low flow under the present condition was higher compared to the sea level rise scenario. The residence time for the present condition and the sea level rise scenario was between 10.51 and 34.23 h and between 17.11 and 38.92 h, respectively. The simulated results reveal that the residence time of the Wu River estuary will increase when the sea level rises. The distance of salinity intrusion in the Wu River estuary will increase and move further upstream when the sea level rises, resulting in the limited availability of water of suitable quality for municipal and industrial uses.

Studying mixing processes in a stratified lake is important for understanding the biological, chemical and physical processes occurring there. Statistical analyses were performed of data from a small, shallow, stratified lake in a subtropical alpine region (Yuan-Yang Lake in Taiwan) to determine the predominant physical factors in heavy-rainfall-induced mixing. This study focused on both vertical mixing in the entire water column and surface-layer mixing extending to the upper thermocline. The effects of meteorological driving forces, such as wind, heating/cooling and inflow on vertical mixing and surface layer mixing, were evaluated using the relationships between each driving force and the change in thermal stability between the pre-mixing and mixing periods. For surface layer mixing, a comparison between penetrative convection related to heating/cooling and wind-related friction velocity was conducted for each heavy rainfall event. A heat content parameter measuring thermal potential energy was introduced to further investigate inflow effects (e.g. effects of changes in discharge volume and temperature) on vertical mixing during heavy rainfall events. Results show that wind input affected vertical mixing more significantly than did other meteorological forcing factors during storm-dominant events. Indeed, wind energy input in the surface layer was more pronounced than was energy of heating/cooling for surface layer mixing. Furthermore, inflow effect was shown to be crucial during large scale and extreme weather events (i.e. lower air pressure events) in the vertical mixing process. Forcing by heating/cooling likely contributes less to mixing because it is likely less dynamic than the wind and inflow inputs with respect to internal response of the lake. In addition, a principal component analysis (PCA) modified by partial correlation was performed to verify the results quantitatively. The first and second components, which accounted for more than 90% of the total variance in the PCA, showed that the intensity of vertical mixing was affected primarily by wind-induced turbulence and inflow intrusion and was only weakly associated with the effect of net heat balance. Considering the interactions between chemical and physical processes, inflow intrusion may have an effect on dissolved oxygen concentration in the lake.

Climate change is one of the key factors affecting the future quality and quantity of water in rivers and tidal estuaries. A coupled three-dimensional hydrodynamic and water quality model has been developed and applied to the Danshuei River estuarine system in northern Taiwan to predict the influences of climate change on water quality. The water quality model considers state variables including nitrogen, phosphorus, organic carbon, and phytoplankton as well as dissolved oxygen, and is driven by a three-dimensional hydrodynamic model. The hydrodynamic water quality model was validated with observational salinity distribution and water quality state variables. According to the analyses of statistical error, predictions of salinity, dissolved oxygen, and nutrients from the model simulation quantitatively agreed with the observed data. The validated model was then applied to predict water quality conditions as a result of projected climate change effects. The simulated results indicated that the dissolved oxygen concentration was projected to significantly decrease whereas nutrients will increase because of climate change. Moreover, the dissolved oxygen concentration was lower than 2 mg/L in the main stream of the Danshuei River estuary and failed to meet the water quality standard. An appropriate strategy for effective water quality management for tidal estuaries is needed given the projected persistent climate trends.

Suspended sediment in the lakes can impact the aquatic ecosystem and water resource management. A three-dimensional hydrodynamic and suspended sediment transport model was performed to simulate temporal and spatial variations of suspended sediment and applied to the subtropical subalpine Yuan-Yang lake of Taiwan. The model was validated with measured water level and suspended sediment concentration in 2009 and 2010. The overall model simulation results are in quantitative agreement with the available field data. The validated model was then used to find out the important parameter which affected the suspended sediment concentration and to investigate the effect of wind stress on mean current and suspended sediment distribution in the lake. Modeling results of sensitivity analysis indicate that the settling velocity plays a crucial parameter in the suspended sediment transport. The simulated results also show that the bottom currents are in opposite direction as surface currents due to return flows. Remarkable lake circulation was found and affected by the wind speed and direction. Mean suspended sediment concentration at the bottom layer is less than that at the top layer. Strong wind would result in higher mean current and mean suspended sediment distribution at the top layer. The wind stress plays a significant influence on mean circulation and suspended sediment transport in a shallow lake.

Vertical mixing affects the vertical transport of biochemical materials and modifies the characteristics of basin-scale internal waves in subtropical, subalpine Yang Yuan Lake (YYL), in the north central region of Taiwan. Vertical mixing in YYL is generally caused by typhoons (strong winds and heavy rainfalls) during the spring, summer and early fall, or by cooling of the water column during late fall and winter. Vertical mixing caused by typhoons significantly affects internal thermal dynamics and biochemical processes, with basin-scale internal waves enhanced before complete mixing occurs, and temporal variations of water temperature dramatically changing after typhoon events. This study quantitatively determines when the complete mixing (defined as homogenous temperature in a water column) occurs on the basis of the balance between kinetic energy and potential energy and also shows what meteorological conditions contribute to the vertical mixing. Data on water column thermal profiles and meteorological variables were collected by a wireless, instrumented buoy in the deepest location of YYL and from a nearby meteorological station, from spring 2004 to summer 2006. This study also investigated how a physical process (internal waves) is affected by the mixing associated with typhoons. Signal processing and a two-layer model of the water column are used to understand the characteristics of basin-scale internal waves.

In this study, a numerical model of tide surge was developed and applied to the coast of Taiwan and the adjacent sea area. This two-dimensional model has a fine horizontal resolution and takes into account the interaction between storm surges and astronomical tides, which is suitable for describing the complicated physical properties of storm surges along the coast of Taiwan. The model is driven by the tidal elevation at the open boundaries using a global ocean tidal model and is forced by the meteorological conditions using a cyclone model. The numerical model was calibrated and verified against the observed tide and surge tide for five typhoon events. The results indicate reasonable agreement between the model simulations and the observed data. The model was then used to study the effects of both different typhoon paths and the intensity of the wind stress and pressure at the typhoon center on the surge height. The regression equations for the surge height and pressure difference were also established for predicting the surge height along the coast of Taiwan.

An 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...

A vertical, two-dimensional heavy metal (lead) transport model incorporated into the hydrodynamic, salt, and sediment transport modules was developed to simulate the lead concentration in the tidal Keelung River estuary of northern Taiwan. We validated the developed model with measured data, including longitudinal velocity, salinity, suspended sediment, and heavy metal (lead) concentration, obtained in 1998. An exponential relationship relating the salinity and suspended-sediment concentrations was established to calculate the partition coefficient of lead in the estuary. The simulated results of dissolved, particulate, and total lead concentrations agreed well with the measured data. A model sensitivity analysis indicated that the partition coefficient plays an important role in the distribution of dissolved and particulate lead concentrations along the tidal Keelung River estuary.

A nowcasting system integrating observations and a numerical model was developed to better understand the spatial distributions and temporal variations in the dynamics of a small, subtropical lake occasionally influenced by severe rainstorms. The nowcasting system was used to monitor real-time meteorological conditions and thermal structures, to provide spatial information on the thermal and flow dynamics from model predictions, and to compare the predictions with the observations. The system was also able to determine whether the instruments collecting field data were performing correctly using telemetry operations. The nowcasting system was initially operated in the spring of 2009 in Yuan-Yang Lake, Taiwan. Rainstorm-induced mixing occurred due to Typhoon Morakot during August 7–9, 2009. The mixing was observed by the instruments, and the spatial distributions and temporal variations during the mixing were successfully predicted by the three-dimensional hydrodynamic lake model. A quantitative comparison of the energy balances among the heat, wind, and water inflow inputs to the lake implied that the typhoon-induced mixing was primarily caused by strong winds. The model predictions showed that the lake experienced mixing and flooding (large amounts of inflow/outflow discharges), resulting in homogenous temperatures and flows that moved to the outlet of the lake.

Flow measurements in a tidal-affected river provide important information of cyclic flow pattern characteristics in temporal and spatial variations. Reliability and accuracy of measured flow data can provide for numerical model calibration and verification. In the present study, we measured tidally affected river flow in the Danshui River of northern Taiwan during the 2008 summer and fall. The measured flow data, produced by both state of the art and conventional instruments including particle tracking velocimetry (PTV), acoustic Doppler profiler (ADP), horizontal acoustic Doppler current profiler (H-ADCP) and Price-type current metre (PCM) were compared and correlated during several tidal cycles. The mean flows by ADP were similar to that by PCM and highly correlated with those by PTV and H-ADCP. Index velocity rating between ADP and H-ADCP separately computed during the flood and ebb tides shows good correlation for the ebb tide and poor correlation for the flood tide due to subsurface maximum velocity in the middle- to near-bottom layers.

There has been a yearly increase in precipitation in Taiwan, consistent with trends seen across the world. In the summer and fall, typhoons or tropical cyclones with torrential rainfall frequently occur as a result of Taiwan’s subtropical climate. Flash floods may cause a levee-break and/or the overtopping of banks at narrow neck locations in a river system, which may in turn produce inundation in urban areas. Therefore, a model that predicts flash floods is of vital importance for river management. The present study is based on a flash flood routing model, which incorporates levee-break and overbank functions to calculate the discharge hydrographs in the complicated Danshuei River system of northern Taiwan. The numerical model was calibrated and verified against observed water stages using three typhoon events. The results indicate reasonable agreement between the model simulations and the observed data. The model was then used to calculate the levee-break and overbank flow hydrographs due to Typhoon Talim (2005) and Typhoon Nari (2001), respectively. The simulated results indicate that several parameters significantly affect the flow hydrograph during a levee-break and should be carefully monitored when levee-break events occur in the river system. The simulated water stages at several stations are consistent with observed data from Typhoon Nari. The simulated overbank flow results quantitatively agree with reported information. The data also confirm that most of the overbank events occurred at the upper reaches of the Keelung River, consistent with the low levee height protection.

The amount of photosynthetically active radiation (PAR) in the water column is of fundamental importance in determining the growth of aquatic plant and aquatic primary production. Light attenuation in aquatic ecosystems has important ecological implication and water quality applications. In the present study, the light attenuation through the water column in the Shihmen Reservoir, Taiwan was measured. A light attenuation coefficient (Ke) can be derived from the PAR measurements at each stations. The linear regression analysis reveals that Secchi disk depth is a sample alternative measure of light transmitability and provides a reasonable estimate of the light attenuation coefficient in the Shihmen Reservoir. We conducted multiple-regression analysis for the Ke, chlorophyll a, and total suspended solids (TSS). Because the concentration of chlorophyll a is roughly

The Keelung River is one of the major branches of the Danshuei River estuarine system, which runs the metropolitan capital city of Taipei, Taiwan, and receives a large of amount of wastewater. The dissolved oxygen (DO) concentration is generally low in the tidal portion of the Keelung River. Hypoxia/anoxia occurs often, particularly during a low-flow period. The sediment oxygen demand (SOD) amounts to a significant value, hence increasing the total oxygen demand load of the river. The present work reports on laboratory SOD made on grab sediment core samples in situ that are undisturbed. The results reveal that SOD values fluctuate with variations in seasons and are higher in summer due to a high-temperature effect. It was found in the laboratory tests that the average SOD (at 20°C) value is 0.76g/m 2 /day and the maximum SOD (at 20°C) value reaches 1.58 g/m 2 /day. The mean values of measured SOD at each station were adopted in the vertical two-dimensional water quality model to simulate the DO distribution along the tidal Keelung River. The simulated results are in reasonable agreement with the measured DO distribution in the river. Model sensitivity analyses were also conducted with increasing and decreasing SOD. It reveals that SOD is an important parameter that affects the DO distribution in the tidal estuary.

A real-time flood-forecasting method coupled with the one-dimensional unsteady flow model was developed for the Danshuei River system in northern Taiwan. Based on the flow at current time, the flow at new time is calculated to provide the water stage forecasting during typhoons. Data, from two typhoons in 2000: Bilis and Nari, were used to validate and evaluate the model capability. First, the developed model was applied to validate and evaluate with and without discharge corrections at the Hsin-Hai Bridge in Tahan Stream, Chung-Cheng Bridge in Hsintien Stream, and Sir-Ho Bridge in the Keelung River. The results indicate that the calculated water stage profiles approach the observed data. Moreover, the water stage forecasting hydrograph with discharge correction is close to the observed water stage hydrograph and yields a better prediction than that without discharge correction. The model was then used to quantify the difference in prediction between different methods of real-time water stage correction. The model results reveal that water stages using the 1–6 h forecast with real-time stage correction exhibits the best lead times. The accuracy for 1–3 h lead time is higher than that for 4–6 h lead time, suggesting that the flash flood forecast in the river system is reasonably accurate for 1–3 h lead time only. The method developed is effective for flash flood forecasting and can be adopted for flood forecasting in complicated river systems.

The main parameters that affect the flow conditions and intrusion of salt water in an estuary system are tides and the seasonal variation of water discharge. A laterally averaged two-dimensional numerical model called MIKE 11 XZ is used to simulate the hydrodynamics and salinity intrusion of Danshuei River estuarine system. This model can simulate hydrodynamics and water quality in estuaries, reservoirs and lakes. MIKE 11 XZ solves the Reynolds-averaged Navier–Stokes equations by using Abbott–Ionescu finite difference scheme in a non-dimensional vertical σ-coordinate. Vertical eddy diffusivity in the model can be determined by a constant value, a mixing length theory and a k or k−ɛ turbulence closure scheme with Richardson number correction. A series of comprehensive field data obtained from Danshuei estuarine system is used for evaluation, calibration and verification of the model. The friction coefficient was calibrated and verified using water surface elevation and velocity measurements, respectively. Then the vertical eddy diffusivity was calibrated and verified through comparison of salinity measurements in different layers of several stations. Reasonable agreement was obtained between the model results and the observed data using k−ɛ turbulence closure scheme. The model application was investigated with different discharges and the effect of discharge variation on salinity intrusion was determined. The results showed that the fresh water discharge is the main parameter that affects the salinity intrusion in this system. Finally, simple power equations are suggested to predict the salinity intrusion due to the fresh water discharge in different tributaries of the system.

A three-dimensional, time-dependent hydrodynamic and salinity model was applied to the Danshuei River estuarine system and adjacent coastal sea in Taiwan. The model forcing functions consist of tidal elevations along the open boundary and freshwater flows from the main stem and tributaries in the Danshuei River system. The bottom roughness height was calibrated and verified with model simulation of barotropic flow, and the turbulent diffusivities were calibrated through comparison of time-series of salinity distributions. The overall model verification was achieved with comparisons of residual current and salinity distribution. The model simulation results are in qualitative agreement with the available field data. The model was then used to investigate the tidal current, residual current, and salinity patterns under the low freshwater flow condition in the modelling domain. The results reveal that the extensive intrusion of saline water imposes a significant baroclinic forcing and induces a strong residual circulation in the estuary. The downriver net velocity in the upper layer increases seaward despite the enlargement of the river cross-section in that direction. Strong residual circulation can be found near the Kuan-Du station. This may be the result of the deep bathymetric features there. Copyright © 2007 John Wiley & Sons, Ltd.

Taiwan usually suffers severe inundation disasters during typhoons and strong rainstorms, and therefore flood mitigation is considered an important issue. To assess the effect of the Yuansantze flood diversion tunnel (YFDT) on flood mitigation at the upstream reaches of the Keelung River, a three-dimensional, unstructured grid, Finite-Volume, primitive equation Community Ocean Model (FVCOM) was used. The model was validated with observed data for water levels and inundation extent during different typhoon events. The simulated results show a good agreement with field measurements of water level with three historical typhoon events but underestimated the measured inundation extent with Typhoon Nari. The validated model was then applied to assess the flood mitigation and economic loss with the YFDT. The results demonstrated that the river level decreases approximately 3 m with the YFDT and that the inundation extent decreases by more than 50% in the Ruifang District with YFDT. The YDFT aims to not only mitigate hazards but also reduce economic losses. The average annual expected benefit after construction of the YFDT is approximately 184 million NTD in the Ruifang District.

The Shihmen Reservoir, located in northern Taiwan, features the second storage volume of impoundments on the island. The watershed of the Shihmen Reservoir has been subjected to serious pollutants due to anthropogenic interference. This study applies a watershed model, BASINS, to simulate the flow and nutrients loads from the watershed. BASINS then drives the CE-QUAL-W2 model for water quality predictions in the reservoir. The watershed modelling results are compared with field data. They reveal that significant nutrient loads were generated from the watershed during storms. The model calibration and verification were achieved with water surface elevation, temperature, and water quality constituents including nutrients, dissolved oxygen, and chlorophyll a in the reservoir using the eutrophication model.

To comprehend the distributions of salinity, temperature, and suspended sediment in the Danshuei River estuary in Taiwan, monthly field surveys were conducted in 2003. These included several high and low slackwater surveys and intensive surveys. The results show that the Danshuei River estuary is predominately a partially mixed estuary. The highest concentration of suspended sediment is typically observed at the Chung-Hsin Bridge, the most upstream sampling station. The suspended sediment concentration exhibits a general decreasing trend in the downstream direction. It may be concluded that the sediments mostly come from the upstream reach. A locally high concentration of suspended sediment is found at the Kuan-Du station because of the local deep channel bathymetry and two-layered estuarine circulation. A vertical two-dimensional hydrodynamic and sediment transport model is applied to investigate the tidally averaged salinity distribution, residual circulation, and suspended sediment concentration. The modeling results reveal that, under the Q75 flow condition (i.e., low flow), a turbidity maximum occurs at the Kuan-Du station due to the strong estuarine circulation. The model simulation with a much higher river flow condition results in a weaker residual circulation and weaker turbidity maximum.

A flood routing model with time-varying roughness updating was developed to simulate flows through natural channels based on the dynamic wave theory. Taking observed stages as the targets, a roughness updating technique was developed using the Gauss-Newton method to update the Manning n in each time step of the routing processes. The technique provides a reasonable roughness coefficient estimate and reliable initialization of stage profile for the forecast. The examinations including the initialization of stage profile, conservation of mass, iteration convergence, effectiveness evaluation, and convergence with different initial values were conducted to verify the predictive capability of the roughness updating technique. The forecasting results show that the stage recalculated by updating the Manning n in current time has a good agreement with the observed stage. The presented model can improve the forecast for a lead time up to 6 h in the Tanshui River of northern Taiwan.

Eutrophication has been one of the most serious water quality problems for Taiwan's reservoir in recent years. As the water quality of a reservoir is closely related to its watershed development, the fate and transport of pollutants in a watershed is a key consideration to water quality studies in reservoirs. The Tseng‐Wen Reservoir, located in southern Taiwan, features the largest storage volume of impoundments on the island. The watershed of the Tseng‐Wen Reservoir has significant agricultural activities, such as cultivation of tea, mustard, and betel palm. Fertilizer use for these crops in the watershed is very popular, thereby discharging significant nutrient loads into the reservoir. This study uses a watershed model, BASINS, to simulate the flows and nutrient loads from the watershed; BASINS then drives the CE‐QUAL‐W2 model for water quality predictions in the reservoir. The watershed model results are compared with data measured at the Dapu Check Dam, in the main stem of the Tseng‐Wen Cree...

The penetration of sunlight into the water column plays a critical role in the aquatic ecosystem. The irradiance available for primary production in a water body depends on the incident light at the water surface, light extinction in the water column, and depth. In this study, the light attenuation through the water column of the Danshuei River–Keelung River estuary was estimated. The measurement of photosynthetically active radiation (PAR) indicates that the conventional exponential attenuation of light with depth is a very good model. A light attenuation coefficient may be derived from the PAR measurements at each location. The regression with salinity yields a good correlation, indicating that the fraction of seawater should be a good parameter for estimating the water column light attenuation coefficient (K d ). A laterally averaged two-dimensional finite difference model for hydrodynamic and water quality model was performed and applied to simulate the phytoplankton population at the lower reach of the Danshuei River estuary. In the process of phytoplankton population simulation, the regression model of K d and salinity was incorporated in the water quality model. The simulated results show that the modeled concentration of chlorophyll a matched the measured values at the lower reach of the Danshuei River estuary.

The penetration of sunlight into the water column plays a critical role in the aquatic ecosystem. This study investi- gates light attenuation through the water column of the Danshuei River estuary and explores the models for quantifying it. The mea- surement of photosynthetically active radiation (PAR) indicates that the conventional exponential attenuation of light with depth is a very good model. A light attenuation coefficient may be derived from the PAR measurements at each location. The simpler mea- surements of Secchi depth (SD) provide an approximate estimate of the attenuation coefficients through an inverse linear relationship. The results also reveal that the amount and kinds of materials, which are either dissolved or suspended in the water, control light attenuation through the water column. However, the linear regres- sion with such a model results in very poor correlation, and a rela- tively large constant term for the Danshuei River estuary. The regression with salinity yields a good correlation, indicating that the fraction of sea water might be a good parameter for estimating light attenuation coefficient for practical application in the Dan- shuei River estuary. (KEY TERMS: light attenuation; Danshuei River estuary; Secchi depth; salinity; suspended sediment.)

A vertical (laterally integrated) two-dimensional numerical model was applied to study the hydrodynamic characteristics, salt-water intrusion and residual circulation in the Danshuei River estuarine system. The cross-sectional profiles measured in 2001 and 1990 respectively represent the conditions after and before channel regulation in the Keelung River. The model was re-verified with the available hydrological data measured in 2001. Detailed model re-verification has been conducted with water surface elevations, tidal current, and salinity distributions measured. The overall performance of the model is in qualitative agreement with the available field data. The model was then used to investigate the change in tidal ranges, salt-water intrusion, and residual circulation as a result of channel regulation in the Keelung River. The model simulations indicate that more tidal energy propagates into the estuarine system before channel regulation because of the substantial increase in river cross-sections. The residual circulations before channel regulation are greater than those after channel regulation and result in the limits of the salt intrusion before channel regulation being extended farther inland than those after channel regulation. This may show that channel regulation for flood control in the Keelung River did not contribute to the expansion of the mangrove areas and the disappearance of freshwater marshes at the Kuan-Du wetlands. Copyright © 2005 John Wiley & Sons, Ltd.

Settling velocities of suspended cohesive sediment in estuaries vary over a range of several orders in magnitude. Variations in the suspended sediment concentration are often considered as the principal cause. Turbulence and the suspended sediment concentration, as well as other factors such as salinity, dissolved organic substances, flocculation ability, and the rate of floc growth affect setting velocities. A laterally−averaged finite difference model for hydrodynamics and cohesive sediment transport is developed and applied in the Tanshui River estuary, Taiwan. The model has been calibrated and verified with water surface elevation, longitudinal velocity, salinity, and cohesive sediment measured. The overall performance of the model is in qualitative agreement with the available data. The model is used to investigate the influence of settling velocity on cohesive sediment transport dynamics. The simulation indicates that the turbidity maximum zone is near Kuan−Du. When settling velocities increase the surface cohesive sediment concentration at Kuan−Du station trends to decrease and bottom cohesive sediment concentration increases. Both surface and bottom cohesive sediment concentrations decrease at Taipei Bridge and Pa−Ling Bridge. This implies that suspended sediment advected seaward and deposited. There is consequently a net seaward flux of suspended sediment near surface, and a net landward flux near the bed.

A vertical (laterally integrated) two-dimensional numerical model was applied to study the salt water intrusion in the Tanshui River estuarine system, Taiwan. The river system has experienced dramatic changes in the past half century because of human intervention. The construction of two reservoirs and water diversion in the upper reaches of the river system significantly reduces the freshwater inflow. The land subsidence within the Taipei basin and the enlargement of the river constriction at Kuan-Du have lowered the river bed. Both changes have contributed farther to the intrusion of tidal flow and salt water in the upstream direction. The model was reverified with the earliest available hydrographic data measured in 1977. The overall performance of the model is in reasonable agreement with the field data. The model was then used to investigate the change in salt water intrusion as a result of reservoir construction and bathymetric changes in the river system. The model simulation study reveals that significant salinity increases have resulted from the combined changes. It has been speculated by ecological researchers that the long-term increase in salinity might be the driving force altering the aquatic ecosystem structure in the lower reach of the estuary and the Kuan-Du mangrove swamp, particularly the enlargement of the mangrove area and the disappearance of freshwater marshes. However, concrete proof has not been available since no prototype salinity data were available prior to the reservoir construction. This case study offers the first quantitative estimate of the salinity changes due to human interference in this natural system.

Based on the dynamic wave theory of unsteady flow in open channels, a four-point implicit finite-difference method is employed to develop a flash flood routing model for the Tanshui River in Taiwan. The real-time observed river stages are specified as the internal boundary conditions for the routing model in order to adjust the computed real-time flow. The number of discritized equations for the conservations of mass and momentum, plus the internal boundary conditions, is more than that of flow variables to be solved for. The least-squares method is used to solve the over-determined problem. The model is calibrated and verified with the field records of several typhoon flood events. There is a reasonable good agreement between measured and computed river stages. Four typhoon flood events are simulated to confirm the accuracy of the forecasting method. The results reveal that the present model can provide accurate river stage for flood forecasting in the Tanshui River.

Field data collected from the Feitsui Reservoir in Tai- wan indicate that the water is at a stage between mesotrophic and eutrophic. Recent measurements such as total phosphorus, chloro- phyll a and Secchi depth levels suggest that the water quality has been declining. A two-dimensional, laterally averaged, finite differ- ence hydrodynamic and water quality model was used to simulate the water quality in the Feitsui Reservoir in an effort to determine sound water quality management strategies. The model was cali- brated and verified using data collected in 1996 and 1997. The cali- brated model was used to simulate algal biomass (in terms of chlorophyll a) levels under various wasteload reduction scenarios. Model results show that 50 percent reduction of the total phospho- rus load will improve the existing water quality, shifting the trophic status from eutrophic/mesotrophic to oligotrophic. 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. KEY TERMS: water quality modeling; hydrodynamics; CE-QUAL- W2 model; phosphorus; chlorophyll a; Feitsui Reservoir.)

The effects of mangrove trees on the flow field in a tidal estuary during high freshwater discharge events are investigated using a depth-averaged two-dimensional hydrodynamic model. The model is refined to include the effects of mangrove trees on flow resistance. The resistance is described by a set of two empirical equations depending on water depth and vegetative parameters. The vegetative parameters are investigated and tested using on-site samples of Kandelia plants. The model is calibrated and verified with experimental data measured in a physical model. The agreement between the measured and computed water surface elevation is good. The model is then applied to the entire tidal Tanshui River system, which includes mangrove swamps near the mouth of the Keelung River. The flow patterns and resistance distributions are investigated for several scenarios of high flow events. The results show that the refined model provides an ideal management tool for the mangrove swamps.

A large amount of accurate river cross-section data is indispensable for predicting river stages. However, the measured river cross-section data are usually sparse in the transverse direction at each cross-section as well as in the longitudinal direction along the river channel. This study presents three algorithms to resample the river cross-section data points in both the transverse and longitudinal directions from the original data. A two-dimensional (2D) high-resolution unstructured-grid hydrodynamic model was used to assess the performance of the original and resampled cross-section data on a simulated river stage under low flow and high flow conditions. The simulated river stages are significantly improved using the resampled cross-section data based on the linear interpolation in the tidal river and non-tidal river segments. The resampled cross-section data based on the linear interpolation satisfactorily maintains the topographic and morphological features of the river channel, especially in the meandering river segment. Furthermore, the performance of the 2D and three-dimensional (3D) models on the simulated river stage was also evaluated using the resampled cross-section data. The results indicate that the 2D and 3D models reproduce similar river stages in both tidal and non-tidal river segments under the low flow condition. However, the 2D model overestimates the river stages in both the tidal and non-tidal river segments compared to the 3D model under the high flow condition. The model sensitivity was implemented to investigate the influence of bottom drag coefficient and vertical eddy viscosity on river stage using 2D and 3D models based on the linear interpolation method to resample river bed cross-section. The results reveal that the change of bottom drag coefficient has a minor impact on river stage, but the change of vertical eddy viscosity is insensitive to river stage.

A vertical (laterally integrated) two-dimensional numerical model has been applied to study the hydrodynamic characteristics and salt water intrusion in the Tanshui River estuarine system. The cross-sectional profiles measured in 1978 and 1994 are schematized for model simulations. Detailed model calibration and verification have been conducted with water surface elevations, tidal current, salinity distributions, and residual velocities measured. The overall performance of the model is in qualitative agreement with the available field data. The model was then used to study how hydrodynamics and salt water intrusion change in response to changes in bathymetry. The model simulations indicate that more tidal energy propagates into the estuarine system in 1994 because of the substantial increase in river cross-sections. The limits of salt intrusion in 1994 extended farther inland than those in 1978. On the other hand, the extent of mangrove wetland in the lower estuary has increased over the past 20 years and is likely a result of the increased salinity in the estuary.

The net, long-term transport of materials in estuaries is governed primarily by the residual (or tidally averaged) circulation in the system. Knowledge of the residual circulation is essential for predicting the transport of algal patches, debris, and pollutants of concern in an estuary. A real-time, two-dimensional, laterally averaged hydrodynamic model is used to analyze residual current and salinity distributions in the Tanshui River estuary. The model results were filtered to arrive at the subtidal, or residual quantities. The characteristic two- layered estuarine circulation prevails for most of the time at Kuan-Du station near the river mouth. Though the circulation strengthens with increasing river discharge at low flow, its strength decreases at moderate river discharge and the two-layered circulation ceases at moderately high flow. This implies a weak trapping capacity in the estuary. At the upriver stations where salinity is lower, strength of circulation generally decreases with increasing river discharge. The response of residual velocity to the spring-neap cycle indicates stronger residual circulation during neap tide than spring tide.

In most hydrodynamic models, friction and turbulent diffusion/dispersion coefficients are the important calibration parameters affecting the calculation of surface elevation, velocity, and salinity distribution. This paper presents a rational approach to calibrate and verify a hydrodynamic model of partially stratified estuaries. The calibration procedures and verification requirements are demonstrated with the application of a vertical (laterally averaged) two-dimensional model to a branched estuarine river system. The friction coefficient is calibrated and verified with model simulation of barotropic flow, and the turbulent diffusion and dispersion coefficients are calibrated through comparison of salinity distributions. The overall model verification is suggested to be achieved with comparisons of Eulerian residual circulation and salinity distribution. In the case of the Tanshui River system in Taiwan, the available prototype current records are too short to calculate slowly varying residual currents. The snapshots of the model-computed and field-measured residual currents are provided to qualitatively agree with theoretical analysis. The overall performance of the model is verified with an additional set of salinity data.

A three-dimensional, time-dependent hydrodynamic and suspended sediment transport model was performed and applied to the Danshuei River estuarine system and adjacent coastal sea in northern Taiwan. The model was validated with observed time-series salinity in 2001, and with salinity and suspended sediment distributions in 2002. The predicted results quantitatively agreed with the measured data. A local turbidity maximum was found in the bottom water of the Kuan-Du station. The validated model then was conducted with no salinity gradient, no sediment supply from the sediment bed, wind stress, and different freshwater discharges from upstream boundaries to comprehend the influences on suspended sediment dynamics in the Danshuei River estuarine system. The results reveal that concentrations of the turbidity maximum simulated without salinity gradient are higher than those of the turbidity maximum simulated with salinity gradient at the Kuan-Du station. Without bottom resuspension process, the estuarine turbidity maximum zone at the Kuan-Du station vanishes. This suggests that bottom sediment resuspension is a very important sediment source to the formation of estuarine turbidity maximum. The wind stress with northeast and southwest directions may contribute to decrease the suspended sediment concentration. When the freshwater discharges increase at the upstream boundaries, the limits of salt intrusion pushes downriver toward river mouth. Suspended sediment concentrations increase at the upriver reaches in the Danshuei River to Tahan Stream, while decrease at Kuan-Du station.