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Typhoon is the most frequent natural disaster that causes widespread damage during summer and autumn in Taiwan. On average, each year the island suffers four typhoons, which result in disastrous flash floods and losses in a short time because of steep terrains and intense rainfall. The Tamsui River Basin is located in northern Taiwan about 2,726 square kilometers and inhabited by eight million people. During flooding events, the emergency managers rely on accurate flood forecasting to take proper actions for damage reductions. The flood forecasting and warning system based on hydraulic models play an important role in flood risk management. This study first establishes river stage routing model based on dynamic wave theory. Then, both the real-time observed river stages and the least squares method are used to adjust the model currently flow conditions as the data assimilation. Finally, The Ensemble Kalman Filter method carries out the data correction with the computation of minimum error-covariance between the model prediction and the observation. The simulation results found the root-mean-square error of forecasted river stage using the data assimilation at the gauged stations of Taipei Bridge and Tudi-Gong-Bi for 1-3 hours lead time is 0.862m, 0.892m, 0.903m, and 0.281m, 0.326m, 0.345m, respectively. When the Ensemble Kalmen Filter is added in the model, the root-mean-square error reduces to 0.191m, 0.375m, 0.612m, and 0.062m, 0.090m, 0.145m at described gauged stations. It is found that the data assimilation and the Ensemble Kalmen Filter give reliable forecast water stages with a small root-mean-square error which successfully corrects the forecasted river stage at each time step of the flood routing process. The results reveal that the integrated model gains a better accuracy of the water-stage profiles with probabilistic uncertainties. The model provides reliable forecasts of the water-stage profiles for 1–3 hours lead time along the Tamsui River for specific locations in emergency response for flood risk management.

This study presents an incorporation and application of a three-dimensional, unstructured-grid hydrodynamic model with a suspended sediment transport module in the Danshui-River estuarine system, Taiwan, and its adjacent coastal sea. The model is verified with data observed in 2010: water surface elevations, longitudinal velocities, salinities and suspended sediment concentration (SSC). The simulations agreed quantitatively with the observations. The validated model was applied to numerical experiments to investigate the formation of the estuarine turbidity maximum (ETM) and the influence of vertical mixing, different freshwater-discharge conditions and bathymetric change on suspended sediment dynamics in the main Danshui-River estuary. The model results reveal that the suspended sediment may be trapped in the deeper section of the river. Higher vertical diffusivity slightly increased SSC because of greater bottom stress during the ebb tide. Under extreme freshwater-discharge conditions, SSC increased considerably at both the surface and bottom layers of the water column, and ETM shifted its location upstream. When we change the bathymetry near the Guandu Bridge in our model, ETM extends its range a further 5.0 km from the river estuary because of higher velocity.

A flash flood forecasting model including a state-of-the-art data assimilation method was developed to provide a precise water stage forecast for flood emergency response. The model integrates a flash flood routing model (FFRM) coupled with an ensemble Kalman filter (EnKF) and an artificial neural network (ANN) submodel. In the model, the ANN forecasts river water stages at gauge stations first. Then, these are used as the initial and boundary conditions of the FFRM. The water stages, simulated from the FFRM, are then corrected by the EnKF for lead time. The model was applied to the Tanshui River watershed in northern Taiwan during past typhoons. The model forecasts almost covered the data observed during a typhoon period to within 95% confidence intervals. Compared with the use of FFRM without EnKF, the forecast water stages from the EnKF improved the accuracy at the conjunctions between upstream and downstream channels and the steep slope location.

Due to the special hydrographic and physiographic conditions in Taiwan, flooding is likely to occur in the middle and lower reaches of a plain whenever serious rainstorm events occurred. Note worthily, the loss of lives and property caused by flooding are always most considerable in a metropolitan area, and the densely distributed buildings would, not only increase the impervious area, but also decrease the water storage area. Furthermore, a large number of intensive buildings have changed the original land flow conditions, resulting in a beam shrinking flow and the additional form drag phenomenon, which makes the flooding phenomenon more serious. The main purpose of this research is to find the correlation between building coverage and the Manning’s coefficient n through a water flume model experiment. To probe into this issue, the Manning’s roughness adjustment is further divided into a part caused by the surface impedance and a part caused by the building impedance. Thus, building coverage can be added to the general computing grid to reflect the flooding situation with buildings. The two-dimensional inundation model, based on this research, was applied to Taichung City for an actual case simulation. The simulation result of Typhoon Kalmaegi showed that the presented model can obtain a more accurate flooding situation in urban area by considering the blockage effects of buildings and adjusting the surface roughness.

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.

After the catastrophic disaster brought by Typhoon Morakot in 2009, the enhancement of flood warning technology cannot wait in Taiwan. In recent years, ensemble flood warning has exhibited advantages in extending lead time, quantifying uncertainty and raising confidence in issuing warnings. Unlike most ensembles aimed at integrating meteorological variations, this study generates the ensemble through the combination of multiple conceptually different hydrological models in order to avoid possible bias by applying a single model for a flood forecast. Taking Typhoon Morakot as the study case, the townships in Chiayi City/County are selected as the study areas to compare the performance of ensemble warning with that given by individual models. The results indicate that the ensemble warning shows better accuracy than individual models by giving higher overall correctness, revealing the fact that hydrological ensemble is no less important than meteorological ensemble in acquiring better flood warning performance.

Recent developments in turbine technology allow the extraction of kinetic energy from tidal flows. This method of renewable energy generation is gaining popularity because of the high predictability of tides, the low environmental impact with no land occupation, and the lower investment needed for tidal turbines. In the present study, an existing three-dimensional finite element numerical model was refined to include an algorithm for computing the power density and mean power density around Kinmen Island of Taiwan. The refined model was validated against measured water levels and tidal currents at different gauging stations. The model results are in reasonable agreement with the measured data. The validated model was then used to assess the potential tidal current energy resources, including the distributions of power density at the mid-flood and mid-ebb of a mean spring tide and the mean power density. Three potential points at the east and west coasts of Kinmen Island were evaluated. The tidal current and corresponding power density at these points were computed with the model for a 15-day period to cover the spring-neap tide cycle. The annual energy output was obtained based on the power density curve to be numerically integrated. In light of the topographic considerations, the integration of the power supply, and the existing harbor/port facilities, the location near Suetau would be an appropriate site for establishing a tidal current power plant.

Tidal stream speeds in straits are accelerated because of geographic and bathymetric features. For instance, narrow channels and shallows can cause high tidal stream energy. In this study, water level and tidal current were simulated using a three-dimensional semi-implicit Eulerian-Lagrangian finite-element model to investigate the complex tidal characteristics in the Taiwan Strait and to determine potential locations for harnessing tidal stream energy. The model was driven by nine tidal components (M 2 , S 2 , N 2 , K 2 , K 1 , O 1 , P 1 , Q 1 , and M 4 ) at open boundaries. The modeling results were validated with the measured data, including water level and tidal current. Through the model simulations, we found that the highest tidal currents occurred at the Penghu Channel in the Taiwan Strait. The Penghu Channel is an appropriate location for the deployment of a tidal turbine array because of its deep and flat bathymetry. The impacts of energy extraction on hydrodynamics were assessed by considering the momentum sink approach. The simulated results indicate that only minimal impacts would occur on water level and tidal current in the Taiwan Strait if a turbine array (55 turbines) was installed in the Penghu Channel.

Precise predictions of storm surges during typhoon events have the necessity for disaster prevention in coastal seas. This paper explores an artificial neural network (ANN) model, including the back propagation neural network (BPNN) and adaptive neuro-fuzzy inference system (ANFIS) algorithms used to correct poor calculations with a two-dimensional hydrodynamic model in predicting storm surge height during typhoon events. The two-dimensional model has a fine horizontal resolution and considers the interaction between storm surges and astronomical tides, which can be applied for describing the complicated physical properties of storm surges along the east 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 simulated results of the hydrodynamic model indicate that this model fails to predict storm surge height during the model calibration and verification phases as typhoons approached the east coast of Taiwan. The BPNN model can reproduce the astronomical tide level but fails to modify the prediction of the storm surge tide level. The ANFIS model satisfactorily predicts both the astronomical tide level and the storm surge height during the training and verification phases and exhibits the lowest values of mean absolute error and root-mean-square error compared to the simulated results at the different stations using the hydrodynamic model and the BPNN model. Comparison results showed that the ANFIS techniques could be successfully applied in predicting water levels along the east coastal of Taiwan during typhoon events.

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.

Accurately predicting tidal levels, including tidal and freshwater discharge effects, is important for human activities in estuaries. The traditional harmonic analysis method and numerical modeling are usually adopted to simulate and predict estuary water stages. This study applied artificial neural networks (ANNs) as an alternative modeling approach to simulate the water stage time-series of the Danshui River estuary in northern Taiwan. We compared this approach with vertical (laterally averaged) 2D and 3D hydrodynamic models. Five ANN models were constructed to simulate the water stage time-series at the Shizi Tou, Taipei Bridge, Rukuoyan, Xinhai Bridge, and Zhongzheng Bridge locations along the Danshui River estuary. ANN models can preserve nonlinear characteristics between input and output variables and are superior to physical-based hydrodynamic models during the training phase. The simulated results reveal that the vertical 2D and 3D hydrodynamic models could not capture the observed water stages during an input of high freshwater discharge from upstream boundaries, while the ANN could match the observed water stage. However, during the testing phase, the ANN approach was slightly inferior to the 2D and 3D models at the Xinhai Bridge, Zhongzheng Bridge, and Rukouyan locations. Our results show that the ANN was able to predict the water stage time-series with reasonable accuracy, suggesting that ANNs can be a valuable tool for estuarine management.

This paper presents the development of a dynamic optical tracking system based on a PTZ camera platform to stably track any selected object and keep the targeted object image in the center of video images. The system is based on the Compressive Tracking algorithm which provides dimensionality reduction to achieve real-time tracking. To enhance reliability and avoid failure due to morphological variation, disappearing and blocking of the targeted objects on image frames, the developed tracking algorithm is enhanced with an Adaptive Boosting (Ada-Boost) evaluation strategy to improve the reliability of the weak Naive Bayes classifier. Besides, a heuristic threshold is applied to select and update object templates to avoid drifting of the templates. Furthermore, potential locations of target patches are searched evenly for based on the Halton sequence. Experiments on practical scenarios demonstrate the robustness and effectiveness of the proposed scheme.

A three-dimensional hydrodynamic model that includes a Lagrangian particle-tracking simulation was applied to the Danshuei River estuarine system in northern Taiwan. The model's 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.

A three-dimensional water quality model has been developed and applied to the Danshuei River estuarine system and adjacent coastal sea. The model considered various species of nitrogen, phosphorus, and silicon, organic carbon, phytoplankton, and zooplankton as well as dissolved oxygen and was driven by a three-dimensional hydrodynamic model. The hydrodynamic and water quality model was validated with observational tidal range, water surface elevation, velocity, salinity, and water quality state variables. According to the analyses of statistical error, predictions of hydrodynamic, salinity, dissolved oxygen, and nutrients from the model simulation quantitatively agreed with the observed data. Based on the validated model, model sensitivity analyses were conducted with sediment oxygen demand (SOD), benthic flux of ammonium nitrogen, and freshwater discharge at upstream boundaries. If SOD value increased 50%, the maximum rates for decreasing dissolved oxygen were 47.9% and 94.2% at the surface and ...

The effective response actions can significantly reduce the damage caused by disasters, but the emergency managers require plenty of information before setting up appropriate strategies. The data gathering and analyzing processes are complex and the time is constrained during emergency. A GIS-based decision support system was developed to enhance the emergency operations during typhoon attacks in Taiwan. The system integrates the real time rainfall monitor and forecast information, the hazard potential, and the basic spatial database to help the emergency managers making decisions efficiently.

A flash flood routing model with artificial neural networks predictions was developed for stage profiles forecasting. The artificial neural network models were used to predict the 1–3 h lead time river stages at gauge stations along a river. The predictions were taken as interior boundaries in the flash flood routing model for the forecast of longitudinal stage profiles, including the un-gauged sites of a whole river. The flash flood routing model was based on the dynamic wave equations with discretization processes of the four-point finite difference method. Five typhoon events were applied to calibrate the rainfall-stage model and the other three events were simulated to verify the model’s capability. The results revealed that the flash flood river routing model conjunction with artificial neural networks can provide accurate river stages for flood forecasting.

Typhoon Aere swept over Taiwan with heavy rain, which induced huge discharge in the Danshuei River in August 2004. The flood in the Danshuei River intruded Sanchung through a culvert that was under construction. The deluge inundated thousands of premises and resulted in severe damage. This study reconstructs the event scenario using hydrologic and hydraulic methods to analyse the causes of the disaster. We integrated the radar rainfall estimations and rain gauge observations to recreate the temporal and spatial varied precipitation inputs; estimated the influent volume from the culvert using hydrologic equations; and simulated the flood dynamic within the study area during the event with a coupled overland and sewer flow model. The evidence showed that both the rainfall and the culvert flow contributed similar flood volume to the study area, but culvert discharge concentrated at single location within short time period such that the local drainage system could not cope with and notable damage was incurred.

A three-dimensional hydrodynamic model was created to study the Danshuei River estuarine system and adjacent coastal ocean in Taiwan. The model was verified using measurements of the time-series water surface elevation, tidal current, and salinity from 1999. We conclude that our model is consistent with these observations. Our particle-tracking model was also used to explore the transport of particles released from the Hsin-Hai Bridge, an area that is heavily polluted. The results suggest that it takes a much longer time for the estuary to be flushed out under low freshwater discharge conditions than with high freshwater discharge. We conclude that the northeast and southwest winds minimally impact particle dispersion in the estuary. The particles fail to settle to the bottom in the absence of density-induced circulation. Our model was also used to simulate the ocean outfall at the Bali. Our experimental results suggest that the tidal current dominates the particle trajectories and influences the transport properties in the absence of a wind stress condition. The particles tend to move northeast or southwest along the coast when northeast or southwest winds prevail. Our data suggest that wind-driven currents and tidal currents play important roles in water movement as linked with ocean outfall in the context of the Danshuei River.

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.

A three-dimensional, time-dependent, baroclinic, hydrodynamic and salinity model, UnTRIM, was performed and applied to the Danshuei River estuarine system and adjacent coastal sea in northern Taiwan. The model forcing functions consist of tidal elevations along the open boundaries and freshwater inflows from the main stream and major tributaries in the Danshuei River estuarine system. The bottom friction coefficient was adjusted to achieve model calibration and verification in model simulations of barotropic and baroclinic flows. The turbulent diffusivities were ascertained through comparison of simulated salinity time series with observations. The model simulation results are in qualitative agreement with the available field data. The validated model was then used to investigate the influence of wind stress and freshwater discharge on Dasnhuei River plume. As the absence of wind stress, the anticyclonic circulation is prevailed along the north to west coast. The model results reveal when winds are downwelling-favorable, the surface low-salinity waters are flushed out and move to southwest coast. Conversely, large amounts of low-salinity water flushed out the Danshuei River mouth during upwelling-favorable winds, as the buoyancy-driven circulation is reversed. Wind stress and freshwater discharge are shown to control the plume structure.

Numerical models are often used to evaluate the potential impact of human alteration of natural water bodies and to help the design the alternation to mitigate its impacts. A vertical (laterally integrated) two-dimensional hydrodynamic model was expanded to include the capability of simulating river loops as well as tributaries. The model was performed and applied to the Danshuei River estuarine system in northern Taiwan which consists of three major tributaries: the Tahan Stream, Hsintien Stream, Keelung River, and one river loop under the Chung-Hsin Bridge. The expanded model was reverified with observational field data of 2000. The verified model was then used to hindcast the river hydrodynamic conditions with a loop connection between the Danshuei River and Keelung River, which existed prior to 1965. It was found that the configuration of river loop connection has significant impacts on the residual transport along the connecting channel and the salinities in the connected river branches. The results show that the model may provide an ideal tool for management decision.

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.

A 3-D, time-dependent, baroclinic, hydrodynamic and salinity model was implemented and applied to the Danshuei River estuarine system and the adjacent coastal sea in Taiwan. The model forcing functions consist of tidal elevations along the open boundaries and freshwater inflows from the main stream and major tributaries in the Danshuei River estuarine system. The bottom friction coefficient was adjusted to achieve model calibration and verification in model simulations of barotropic and baroclinic flows. The turbulent diffusivities were ascertained through comparison of simulated salinity time series with observations. The model simulation results are in qualitative agreement with the available field data. The validated model was then used to investigate the influence of freshwater discharge on residual current and salinity intrusion under different freshwater inflow condition in the Danshuei River estuarine system. The model results reveal that the characteristic two-layered estuarine circulation prevails most of the time at Kuan-Du station near the river mouth. Comparing the estuarine circulation under low- and mean flow conditions, the circulation strengthens during low-flow period and its strength decreases at moderate river discharge. The river discharge is a dominating factor affecting the salinity intrusion in the estuarine system. A correlation between the distance of salt intrusion and freshwater discharge has been established allowing prediction of salt intrusion for different inflow conditions.

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.

The urban areas have been rapidly developed in Taiwan in recent years. The expanding urban areas and the increasing population, especially in the metropolitan Taipei city, result in surface runoff discharge during typhoons or rainstorms. When the surface runoff exceeds the capacity of drainage systems of the city, the urban inundation and property losses occur. The urban flooding risk assessment is a useful tool for the decision-making in flood damage mitigation. In general, hazard and vulnerability are two main factors for the risk assessment. In the present study, the Center Taipei City (CTC) is chosen as the study area. The A1B scenario simulations proposed by IPCC are used to compare the flood risks between the present situation and future condition which is under climate change scenario. The ArcGIS is used to yield the flood potential information and the flood risk for the return period of 10, 25, 100 and 200-year flood. The simulated results revealed that the increasing water storage to meet the regulation of the Taipei City government would effectively reduce the effects of climate change on the decreasing inundation areas. The adaptation strategies will also reduce the high-risk areas in the CTC according to risk assessment. The adaptation strategies composed of increasing the water storage in urban areas and reducing the social vulnerability of flooding area are the effective way for the flood risk reduction in urban area.

Flows in the lakes are driven in most cases by shear stresses resulted from wind and freshwater inflow. The effect of wind-induced current has been studied through field observations, laboratory small-scale physical models, and numerical models to evaluate the impacts on hydraulic structures, mixing, and stratification. Typhoon-induced inflow, especially in Taiwan, is an important factor to affect the mixing in the water column. In the present study, a three-dimensional, time-dependent hydrodynamic model was performed and applied to the alpine Yuan-Yang Lake in northeastern region of Taiwan. The model was driven with discharge inflow, heat, and wind stress to simulate the hydrodynamics of lake. The model was validated with measured water surface elevation, current, and temperature in 2008. The overall model simulation results are in quantitative agreement with the observed data. The validated model was then used to investigate mean circulation and residence time in the YYL. The simulated mean current reveals that the surface currents flow toward the southwest direction and form a clockwise rotation. The calculated residence time is strongly dependent on the inflows and wind effects. The calculated residence time is approximately 2~2.5 days under low inflow with wind effect.

The river basins in Taiwan are small with steep bed slopes, which result in short time of concentration and high flood discharges during the typhoon or heavy rainfall. Consequently, the inundation in local area happens quickly and causes the very short time for taking emergency responses. Numerical models are widely adopted for predicting the flood risks. The study integrates the HEC-HMS hydrological model, the 2D inundation model, and the real time rainfall monitor information from the Central Weather Bureau for building a real time distributed inundation computation system. The system is applied to whole Taiwan which includes 54 simulation areas and the results can be finished between 3 hours lead time before inundation occurs. The results help the emergency decision agencies forecasting the inundation during typhoon, warning the residents, and taking proper actions to mitigate hazard damages.

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.

The structural measure was the major solution for flood defense in Taiwan. However, the measure is always limited to the design standard and cannot prevent the damages when floods exceed certain scale. Therefore, non-structural measures for flood mitigation are the indispensable complements to structural solutions. The study introduces the establishment of inundation potential database that provides required information for the non-structural measures in Taiwan. The database was built by numerical simulations, based on different rainfall scenarios, and has been applied by the local governments of Taiwan for land use managements, flood warning systems, emergency responses, and flood insurance programs to reduce the flood damages and impacts.

The increasing natural disasters, especially floods during the last quarter century, are raising the economic losses in Taiwan. The most severe hazard in Taiwan is flooding induced by typhoons and storms in summer and autumn. By comparing the rivers around the world, the ones in Taiwan have the steepest slopes, the largest discharge per unit drainage area, and the shortest time of concentrations. Rapid urbanization without proper land uses managements usually worsen the flood problems. Consequently, flood hazards mitigation has become the most essential task for Taiwan to deal with. Although the government keeps improving flood defense structures, the flood damage grows continuously. In this article, possible flood mitigation strategies are identified for coping with complex environmental and social decisions with flood risk involved.

Midstream of the Keelung River Basin in Northern Taiwan has become highly urbanized and densely populated area. Flood inundation along riversides frequently occurred during typhoons or rainstorms. Three protection measures, including constructions of high-level protection levees, a diversion channel, and a detention reservoir, were proposed for flood mitigation. The main purpose of this study is to evaluate the flood mitigation performance of the three proposed structural measures by using combined hydrologic analyses and hydraulic routings. A semi-distributed parallel-type linear reservoirs rainfall-runoff model was used for estimating the surface runoff. Furthermore, a 1-D dynamic channel routing model was coupled with a two-dimensional inundation model to simulate the hydraulic characteristics of river flooding and overland flow. Simulation results of flood stages, runoff peak discharges, and inundation extent under design rainfall scenarios were chosen as the criteria for evaluation. The results showed a diversion channel is superior to the other two measures for flood mitigation of the study area. After the process of environmental impact assessment, a revised diversion channel approach has been approved for construction as the major structural measure.

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 numerical model is developed in this study with various components for simulating the complex flow phenomena in urban drainage basins. The model integrates the HEC-1 model, a 1-D dynamic channel-flow model, a 2-D non-inertia overland-flow model and the SWMM model to reflect the hydraulic processes in areas with different characteristics. The inundation of underground infrastructure during flood is also considered in the model. The typhoon Nari event in 2001, which resulted in severe flood in downtown Taipei, is simulated by the model. The result is compared with the survey records of flooded areas, which reveals the storage effect of underground infrastrucures is significant to the simulation results of highly developed urban areas.

The optimal transition for channel contraction with supercritical Hows in a horizontal straight-wall channel is investigated. A new optimal contraction diagram is developed based on the fundamental relationship for oblique shock waves. The diagram may replace the commonly used trial-and-error procedures for the determination of Ihe optimal contraction. Two equations are also proposed to approximate the functional relationship in the optimal contraction diagram. Two application tests are presented to demonstrate that the optimal contraction determined from the newly developed diagram agrees with those calculated by the trial-and-error method. Numerical simulations are also used to examine the validity of the new optimal contraction diagram.

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.

The residence time of an estuary is defined in this study as the average time the initially existing water parcels reside in the system before they are flushed out. The residence time of the Danshuei River is calculated through a series of numerical experiments using a laterally integrated two-dimensional hydrodynamic eutrophication model (HEM-2D). The results show that the residence time is on the order of 1e2 days under the mean river flow to zero river flow condition, which is very short compared with most time scales of biogeochemical processes. A procedure is developed to quantify individual contribution to flushing by each of the three major physical transport mechanisms: tide, river discharge, and the density induced circulation. The results indicated that, in general conditions, tidal flushing exerts the greatest influence to the flushing of the Danshuei River estuary, while the density induced circulation hardly contributes any. Tidal transport contributes more than 50% of the flushing when river discharge is below its longterm mean. The suitability of applying the tidal prism method, the modified tidal prism methods, and the fraction of freshwater method in this estuary is also investigated. The relatively short residence time is likely to be one of the limiting factors that result in low phytoplankton biomass in spite of extremely high nutrient concentrations, and causes a significant fraction of pollutants to exert their effects in the coastal waters outside the estuary. 2004 Elsevier Ltd. All rights reserved.

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.

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.

An improved urban inundation model, coupling a 2D non‐inertia overland flow model with a storm water management model, is adopted to simulate inundation in urban areas. The model computes, not only the overland runoff and the water overflow through manholes where surface runoff exceeds the capacity of storm sewers, but also the bidirectional flow interactions between sewers and overland runoff. The model was verified by a typhoon event in Nov. 2000, which resulted in serious inundation in the Mucha area of Taipei City. The result shows that the present model indeed improves simulation accuracy over the earlier model, and can be used to provide a more reliable flood mitigation design.

Without any omen, massive landslides induced by the Chi-Chi earthquake blocked up gorges of Ching-Shui creek, and produced a new landslide lake. Although emergency spillways have been constructed to prevent dam failures, overtopping and possible breaching may still occur due to excessive inflows in raining seasons. As a result, the downstream valleys will have serious inundation and the safety of people and properties will be in immediate danger. The purpose of this work is to simulate and to analyze the inundation potentials downstream of Tsao-Ling landslide lake using a hydrologic/hydraulic approach and GIS (Geographic Information System) technology. Hydrologic analysis is employed to describe regional rainfall-runoff characteristics and to design rainfall/runoff scenarios. One-dimensional dam break flood routings are performed with different return periods of rainfall events and dam failure durations for downstream creeks. The depletion hydrographs of dam break routings are applied into two-dimensional overland flow simulations for downstream lowlands. The results of hydraulic computations are evaluated with GIS maps for inundation potentials analysis, which can be used to assist the planning of emergency response measures.