Jihn-Sung Lai

Finite-volume component-wise TVD schemes for 2D shallow water equations

Abstract Four finite-volume component-wise total variation diminishing (TVD) schemes are proposed for solving the two-dimensional shallow water equations. In the framework of the finite volume method, a proposed algorithm using the flux-splitting technique is established by modifying the MacCormack scheme to preserve second-order accuracy in both space and time. Based on this algorithm, four component-wise TVD schemes, including the Liou–Steffen splitting (LSS), van Leer splitting, Steger–Warming splitting and local Lax–Friedrichs splitting schemes, are developed. These schemes are verified through the simulations of the 1D dam-break, the oblique hydraulic jump, the partial dam-break and circular dam-break problems. It is demonstrated that the proposed schemes are accurate, efficient and robust to capture the discontinuous shock waves without any spurious oscillations in the complex flow domains with dry-bed situation, bottom slope or friction. The simulated results also show that the LSS scheme has the best numerical accuracy among the schemes tested.

Real-time monitoring of local scour by using fiber Bragg grating sensors

Local scour is one of the major factors for bridge failure. Scour failures tend to occur suddenly and without prior warning or sign of distress to the structure. Two types of real-time monitoring systems for bridge scour, using fiber Bragg grating (FBG) sensors, have been developed and tested in the laboratory. These FBG scour-monitoring systems can measure both the processes of scouring/deposition and the variations of water level. Several experimental runs have been conducted in the flume to demonstrate the applicability of the FBG systems. The experimental results indicate that the real-time monitoring system has the potential for further applications in the field.

Using mems sensors in the bridge scour monitoring system

Abstract Scour failures tend to occur suddenly and without warning. Hydraulic scouring is one of the major factors in bridge failure. Bridges that are subject to periodic flooding should be monitored during high‐flow seasons for the safety of the public. Wireless sensor networks have been applied widely, and in many fields. In this study, micro‐electro‐mechanical system (MEMS) pressure sensors are integrated with the wireless Zigbee network on a sensor board for real‐time bridge scour monitoring. A wireless MEMS scour monitoring system has been developed and tested in the laboratory. This system can measure the scouring/deposition process and the variations of water levels at a bridge pier. A scour model is employed to calculate the scour‐depth evolution and validated with real‐time measured data. The results indicate that the proposed system has potential applicability to monitor scour evolution in the field.

Maximum Local Scour Depth at Bridge Piers under Unsteady Flow

The temporal effect of hydrograph on local scour depth is investigated under clear-water scour condition. By analyzing the characteristics of scour-depth evolutions at bridge piers for different rising hydrographs, a relation for estimating the maximum scour depth in uniform sediment is proposed. In the relation, the flow unsteadiness effect is taken into account by an unsteady flow parameter combining the peak-flow intensity and time-to-peak factors. For nonuniform sediment with d84 employed as the effective sediment size, this relation can yield reasonably good results of the maximum scour depth under rising hydrograph.

Houfeng Bridge Failure in Taiwan

Failures of bridges that span a waterway often result from scour and channel instability near the bridge foundations. The Houfeng Bridge, which crosses the Da-Chia River in central Taiwan, collapsed in the 2008 typhoon flood event. On the basis of the historical records and survey of related data just after the collapse of the bridge, a methodology for the assessment of scour depth, including long-term general scour caused by earthquake and impinging jet scour generated by a concrete encased pipeline, is illustrated in the present study. The proposed method provides reasonable estimates for various scour components, which implies that before constructing a new or rebuilding an old bridge, one should use proper methodology and formulas to evaluate the scour potential and improve the bridge design, especially for bridges that are founded around the Houfeng Bridge. This case study also highlights the important effect of long-term general scour on bridge stability. In addition, a lesson is learned from this case study regarding the importance of bridge operation.

Development of a 3D virtual environment for improving public participation: Case study - The Yuansantze Flood Diversion Works Project

The hydraulic design of the Yuansantze Flood Diversion Works (YFDW) project was considerably important for flood mitigation in Taiwan, and positive outcomes of the public hearings played a major role in the success of the project. In the beginning of the project, communication between decision makers, engineers and the public was tenuous due to excess engineering details and a lack of integrated presentations during the public hearings. An ideal solution was then proposed and developed whereby a 3D virtual environment was used to present. In this paper, experimental data obtained from the physical model of the YFDW under design flood conditions was used to validate a 3D flow numerical model. The verified 3D flow numerical model was applied to simulate water surface variation and flow velocity field. The simulated outcomes were used as input data to build an interactive 3D virtual environment, which required the integration of 3D modelling, virtual reality, and human computer interaction techniques. The 3D virtual environment possessed engineering accuracy and overcame challenges in flood flow visualization. An implementation in some public hearings resulted in increased interactivity between stakeholders and improved communication efficiency from public participation. Finally, the YFDW project was concluded smoothly, largely thanks to strong public support.

Coupling typhoon rainfall forecasting with overland-flow modeling for early warning of inundation

Taiwan suffers from an average of three or four typhoons annually, and the inundation caused by the heavy precipitation that is associated with typhoons frequently occurs in lowlands and floodplains. Potential inundation maps have been widely used as references to set up non-structural strategies for mitigating flood hazards. However, spatiotemporal rainfall distributions must be addressed to improve the accuracy of inundation forecasting for emergency response operations. This study presents a system for 24-h-ahead early warning of inundation, by coupling the forecasting of typhoon rainfall with the modeling of overland flow. A typhoon rainfall climatology model (TRCM) is introduced to forecast dynamically the spatiotemporal rainfall distribution based on typhoon tracks. The systematic scheme for early warning of inundation based on the spatiotemporal downscaling of rainfall and 2D overland-flow modeling yields not only the extent of inundation, but also the time to maximum inundation depth. The scheme is superior to traditional early warning method referring to the maximum extent and depth of inundation determined from conditional uniform rainfall. Analytical results show that coupling TRCM with an overland-flow model yields satisfactory inundation hydrographs for warning of the extent and peak time of inundation. This study also shows that the accuracy of forecasting spatiotemporal rainfall patterns determines the performance of inundation forecasting, which is critical to emergency response operations.

Flow Structure around Bridge Piers of Varying Geometrical Complexity

Piers with back-to-back stems or columns and piers for which part of the foundation becomes exposed as a result of the development of scour over large periods of time or because of severe flood events are fairly common at bridge waterways. The present paper uses eddy-resolving numerical simulations to study flow and turbulence structure at piers of complex shape and/or with multiple components. In particular, the study considers cases with one and two back-to-back pier columns for which the section of the main column is neither circular nor rectangular. In addition to a design case for which the foundation of each pier column is submerged, the study analyzes a case when scour exposes part of the foundation of the main column. The results show that the shape and size of the pier column have a significant effect on the spatial and temporal distributions of the bed friction velocity induced by the horseshoe vortex system. The large-scale shedding behind the main column greatly influences flow structure and increases bed friction velocity around the downstream column for piers with two back-to-back columns that are aligned with the incoming flow direction. The present study shows that the presence of large-scale unsteady coherent structures in the vicinity of the bed around piers of complex shapes results in very complex distributions of the bed friction velocity and in large-scale temporal oscillations of the bed friction velocity. The results of eddy-resolving simulations strongly suggest the need to account for the effect of these large-scale oscillations around the mean value when bed friction velocity distributions are used to estimate the flux of entrained sediment in movable bed simulations that do not resolve the large-scale turbulent flow structures.

High-resolution TVD schemes in finite volume method for hydraulic shock wave modeling

High-resolution total variation diminishing (TVD) schemes in the framework of the finite volume method are presented and evaluated for hydraulic shock wave modeling. Three approximate Riemann solvers, namely the FVS, Roe and Osher schemes, are extended to high-resolution TVD schemes based on the direct MUSCL-Hancock (DMH) slope limiter approach. The TVD schemes are then used to develop numerical models to compute water depth and velocity. The numerical models developed are then verified through simulations of the dam-break flows, the oblique hydraulic jump, and the shock-on-shock interaction. The numerical models with TVD schemes are capable of capturing discontinuous shock waves without any spurious oscillation. A comparison of numerical efficiency shows that the Osher-DMH scheme coupled with van Leer limiter performs the best among the proposed TVD schemes. Applications of the Osher-DMH scheme to flows of partial dam-break experiments have shown that the simulated water depths agree well with the measured data.

Improvement of a drainage system for flood management with assessment of the potential effects of climate change

Abstract Runoff discharge in the Tuku lowlands, Taiwan, has increased with land development. Frequent floods caused by extreme weather conditions have resulted in considerable economic and social losses in recent years. Currently, numerous infrastructures have been built in the lowland areas that are prone to inundation; the measures and solutions for flood mitigation focus mainly on engineering aspects. Public participation in the development of principles for future flood management has helped both stakeholders and engineers. An integrated drainage–inundation model, combining a drainage flow model with a two-dimensional overland-flow inundation model is used to evaluate the flood management approaches with damage loss estimation. The proposed approaches include increasing drainage capacity, using fishponds as retention ponds, constructing pumping stations, and building flood diversion culverts. To assess the effects on the drainage system of projected increase of rainfall due to climate change, for each approach simulations were performed to obtain potential inundation extent and depth in terms of damage losses. The results demonstrate the importance of assessing the impacts of climate change for implementing appropriate flood management approaches. Editor Z.W. Kundzewicz Citation Chang, H.-K., Tan, Y.-C., Lai, J.-S., Pan, T.-Y., Liu, T.-M., and Tung, C.-P., 2013. Improvement of a drainage system for flood management with assessment of the potential effects of climate change. Hydrological Sciences Journal, 58 (8), 1581–1597.