Search

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.

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.

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.

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.

A numerical model based on a finite-difference scheme is presented for simulation of two-dimensional transient flow in open channels. The Arbitrary Lagrangian-Eulerian (ALE) kinematic description, and the techniques of moving grids and boundary-fitted coordinate system are adopted for the computation of free surface flows. An iterative flow field solver using an algorithm of velocity divergence with an adjusted pressure is employed to ensure mass conservation. The proposed model is demonstrated on three well-known problems, which include wave propagation caused by dam-break, viscous damping of a solitary wave, and reflection of a wave running against a vertical wall. The simulated results show good agreement with available experimental data or theoretical solutions.

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

On September 21, 1999, the Chi‐Chi (Taiwan) Earthquake with a local magnitude of 7.3, struck west‐central Taiwan and resulted in tremendous disasters with a death toll over 2,000. Hydraulic facilities such as river banks, levees, dams, weirs, channels, and irrigation systems were seriously damaged in the counties of Miaoli, Taichung, Changhua, Yunlin, and Nantou. In addition, some waterways were blocked by landslide materials, and thus, new lakes were formed. The damage to hydraulic facilities and the causes are thoroughly investigated. The performance of dams under this earthquake is discussed in view of earthquake resistant design. The inundation maps for the downstream lowlands of the blocked creeks are analyzed by using a dam‐break flow numerical model. As a result, the damaged facilities are identified and suggestions for restoration are proposed. The results can be used as a reference in proposing strategies for hazard mitigation and recovery plans.

Large temporal and spatial variations in chlorophyll concentration found in estuarine and coastal waters have been attributed to a diverse set of physical, chemical and biological factors. The Keelung River has the largest tidal excursion and the most serious pollution problem among the three tributaries of the Tanshui River system in northern Taiwan. It receives extremely high anthropogenic nutrient loadings from municipal and industrial discharges with little or no treatment. The observed data show very high nutrient concentrations in the river, with total nitrogen of order of several milligrams per liter and total phosphorus of 1 mg/1. However, the summer chlorophyll ‘a’ concentrations are generally less than 10 μg/1. A time‐dependent, laterally averaged, two‐dimensional hydrodynamic and water quality model (HEM‐2DT) was applied to the tidal reach of the Tanshui River estuarine system to simulate the phytoplankton dynamics, nutrient cycling and dissolved oxygen distribution. The numerical mode...

A vertical two‐dimensional numerical model was developed to simulate water quality processes in branched estuarine systems. The model consists of hydrodynamic and water quality submodels. The model was applied to the Tanshui estuarine system of northern Taiwan, and calibrated and verified using field data collected from 1994 to 1996. Favorable model calibration and verification results were obtained. After calibration and verification, the model was used to predict future water quality conditions under various hydrological conditions as the result of the Tanshui River estuarine system clean‐up project which is being implemented. Due to the clean‐up project by the interception of a large amount of wastewater to the ocean outfall system, the freshwater flow in the estuarine system will decrease. On the basis of the model prediction the daily minimum dissolved oxygen (DO) concentration in the Tanshui River‐Tahan Stream and Keelung River will not meet the water quality standard under low freshwater f...

A vertical two‐dimensional estuarine hydrodynamic and salinity model is developed to simulate the hydrodynamic characteristics and salinity distribution of the tide‐influenced wetland. The Kuan‐Du plain, an alluvial fan with vast wetlands and transition of fresh‐salt water, situated at the confluence of the main Tanshui River and Keelung River in northern Taiwan is selected as the study case. For the purpose of land development, 85% of the wetlands in the Kuan‐Du plain was carved out by a tidal dike constructed in 1968. The municipal government of Taipei recently decided to downgrade the plain of land development and to restore a portion of the carved‐out wetland. Thus, the aim of the study is to provide information to help planning wetland restoration, and to evaluate hydraulic impact of wetland restoration with removal of the tidal dike. The model is first calibrated and verified by the existing reliable measurement data. Simulations are conducted for various combinations of tidal conditions an...