Jau-Yau Lu

Temporal Variation of Scour Depth at Nonuniform Cylindrical Piers

The paper proposes a semiempirical model to estimate the temporal development of scour depth at cylindrical piers with unexposed foundations. A cylindrical pier with a foundation is considered as nonuniform pier. The concept of primary vortex and the principle of volumetric rate of sediment transport are used to develop a methodology to characterize the rate of evolution of the scour hole at nonuniform cylindrical piers. The model also simulates the entire scouring process at nonuniform cylindrical piers having the discontinuous surface located below the initial bed level. The scouring process includes three zones; viz Zone 1 having the scouring phenomenon similar to that of a uniform pier, Zone 2 in which the scour depth remains unchanged with its value equal to the depth of the top level of foundation below the initial bed level while the dimensions of the scour hole increase, and in Zone 3 the geometry pier foundation influences the scouring process. A concept of superposition using an effective pier diameter is proposed to simulate the scouring process in Zone 3. In addition, the laboratory experiments were conducted to utilize the laboratory results for the validation of the model. The simulated results obtained from the proposed model are in good agreement with the present experimental results and also other experimental data. Also, the effect of unsteadiness of flow is incorporated in the model and the results of the model are compared with the experimental data. The model agrees satisfactorily with the experimental data.

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.

Measurement and simulation of turbulent flow in a steep open‐channel with smooth boundary

Abstract Turbulence measurements were performed in a steep open‐channel with a smooth boundary using a fiber‐optic laser Doppler velocimeter (FLDV). A sophisticated Reynolds‐Stress‐Model (RSM) was chosen to simulate the fully developed turbulent open‐channel flow. The computed mean velocities and turbulence statistical quantities were compared with the high‐accuracy turbulence measurements to explore the capabilities and limitations of the existing turbulence model. The measured mean velocity profiles exhibit a clear retardation effect near the free surface, a feature absent from the numerical simulations. In general, the simulated longitudinal turbulence intensities yielded by the RSM were slightly lower than the experimental results, and the simulated vertical turbulence intensities were fairly consistent with the measured data except for the regions near the side walls. Increase of the turbulence intensities for both the longitudinal and the vertical components near the free surface were found for flows with steep slope gradient and small width‐to‐depth ratio due to the increase of the intensity of secondary current. The relative magnitude of the longitudinal, vertical and transverse turbulence intensities simulated from RSM was consistent with that for hot film measurements under mild slope conditions.