Study of a semi‐active stiffness damper under various earthquake inputs

Semi‐active stiffness damper (SASD) is one of many semi‐active control systems with the capability to mitigate the dynamic response using only a small amount of external power. The system consists of a hydraulic damper connected to the bracing frame in a selected story unit. In this paper, study of a SASD in two building models of five‐stories under four benchmark earthquake records is reported. The purpose of this study is to evaluate the effectiveness of the control system against structure type and varying earthquake inputs. Various control laws are chosen to work with SASD, such as: resetting control, switching control, linear quadratic regulator (LQR) and modified LQR, and the results are compared with no control and passive control cases. Numerical results show that the use of a SASD is effective in reducing seismic responses. Control effectiveness is dependent on the type of structure and earthquake excitation. Passive control is less effective than other control cases as expected. Resetting control, switching control and LQR generally perform similarly in response reduction. While modified LQR is more efficient and robust compared with other control algorithms. Copyright © 2002 John Wiley & Sons, Ltd.     

Simulation of acid water movement in canals

An attempt to tackle the problem of the propagation of acid water in canals is described, and a mathematical model to simulate the acid water movement is developed, in which the jurbanite equilibrium is found to prevail. The processes of settling owing to sedimentation, precipitation and redissolution have been considered in the modelling. Data available from Tan Thanh, in the Plain of Reeds of the Mekong Delta in Viet Nam, are used as a case study.     

Effects of riverbed incision on the hydrology of the Vietnamese Mekong Delta

The hydrogeomorphology of the Vietnamese Mekong Delta (VMD) has been significantly altered by natural and anthropogenic drivers. In this study, the spatiotemporal changes of the flow regime were examined by analysing the long-term daily, monthly, annual and extreme discharges and water levels from 1980 to 2018, supported by further investigation of the long-term annual sediment load (from the 1960s to 2015), river bathymetric data (in 1998, 2014 and 2017) and daily salinity concentration (from the 1990s to 2015) using various statistical methods and a coupled numerical model. Then, the effects of riverbed incision on the hydrology were investigated. The results show that the dry season discharge (i.e., in March–June) of the Tien River increased by up to 23% from the predam period (1980–1992) to the postdam period (1993–2018) but that the dry season water level at My Thuan decreased by up to −46%. The annual mean and monthly water levels in June at Tan Chau and in January and June–October at My Thuan in the Tien River decreased statistically, even though the respective discharges increased significantly. These decreased water levels instead of the increased discharges were attributed to the accelerated riverbed incision upstream from My Thuan, which increased by more than three times, from a mean rate of −0.16 m/year (−16.7 Mm³/year) in 1998–2014 to −0.5 m/year (−52.5 Mm³/year) in 2014–2017. This accelerated riverbed incision was likely caused by the reduction in the sediment load of the VMD (from 166.7 Mt/year in the predam period to 57.6 Mt/year in the postdam period) and increase in sand mining (from 3.9 Mm³ in 2012 to 13.43 Mm³ in 2018). Collectively, the decreased dry season water level in the Tien River is likely one of the main causes of the enhanced salinity intrusion.     

A new portable in situ flume for measuring critical shear stress on river beds

A new portable in situ flume(PISF)for measuring critical bed shear stress(CBSS)was developed in this study.The PISF consists of an open bottom sediment erosion chamber and an electrically-driven pump.Unlike most existing in situ flumes with similar designs,the new PISF does not rely on monitoring the flow conditions or particle density in the sediment erosion chamber;instead,it is a pre-calibrated flume.The calibration was performed by first determining CBSS of various selected sediment samples of known particle size and density(using the law of the wall),based on flow velocity-depth profiles measured in a 6 m straight open-channel flume using a Particle Image Velocimetry(PIV)system.These same particles of known CBSS were then used in the new in-situ flume under controlled lab conditions to obtain a series of calibration curves of CBSS vs.pump electrical power.A wide variety of particle types and sizes(simulated sediments)were used in this two-step calibration procedure to widen CBSS measurement range and simulate cohesive force effects.The size of the PISF is much smaller and more practical than other similar devices as lamellar flow conditions are not required and it can be applied to a wide range of sediment types including cohesive sediments.     

Enhancement of Seismic Data Processing and Interpretation of Fracture Zones on the Upper Part of Granitic Basement in Cuu Long Basin,Vietnam

The fractured granite basement is the primary oil and gas reservoir in the Cuu Long Basin, Vietnam. Due to the complexity of this nonlayered unconventional target, combined with complicated fault and fracture systems, the seismic data quality near and within the basement section is very low. For this reason, it is important to apply improved seismic data processing workflows, filtering and migration techniques, as wells as attribute processing methods to enhance the imaging quality.Our studies show that applying different types of filters, including the f-k, Radon transform and Tau-P, improves signal to noise ratio, removing multiples, revealing basement’s top and its related fractured and fault zones. In addition, the application of multi-arrival-solution migration algorithms, such as Kirchhoff Migration and Controlled Beam Migration, provides improved imaging for identifying basement top and faults and fractures within the basement. Furthermore, the application of seismic attributes such as curvature, apparent dip, or energy gradient, is important in locating faults and fractures, whereas mapping of intensity and orientation of such structures assists the delineation of “sweet spots” and assists the planning of exploration.     

Natural Radioactivity at the Sin Quyen Iron-Oxide-Copper-Gold Deposit in North Vietnam

The field radiometric and laboratory measurements were performed at the Sin Quyen copper deposit in North Vietnam. The field gamma-ray spectrometry indicated the concentration of uranium ranging from 5.5 to 87 ppm, thorium from 5.6 to 33.2 ppm, and potassium from 0.3 to 4.7%. The measured dose rates ranged from 115 to 582 nGy/h, the highest doses being at the copper ore. Concentrations in the solid samples were in the range of 20–1700 Bq/kg for uranium, 20–92.7 Bq/kg for thorium, and 7–1345 Bq/kg for potassium. The calculated doses were from 22 to 896 nGy/h; both measured and calculated dose rates are mostly related to uranium. Concentrations of radium in water samples were below 0.17 Bq/L. Uranium in water samples was significantly higher than the hydrogeological background; the maximum of 13 Bq/L was at the waste zone pool, but neither radium nor uranium were present in tap water. Radon concentration in the dwelling air was from 42 to 278 Bq/m³ for ²²²Rn and from 8 to 193 Bq/m³ for ²²⁰Rn. The estimated committed dose rates were principally related to ²²²Rn concentration and ranged from 1.1 to 8.1 mSv/y.     

Geophysical techniques to aquifer locating and monitoring for industrial zones in North Hanoi, Vietnam

Geophysical methods were applied for hydrogeological targets in many countries including Vietnam. This paper presents results of using complex geophysical techniques as well as 2D electrical resistivity imaging (ERI), vertical electrical sounding (VES), very low frequency (VLF), and seismic refraction for geological structure investigation for locating the aquifers and assessing the hydrogeological conditions for groundwater potential in industrial zones of North Hanoi, Vietnam. The locations of two aquifers are determined by their depth and thickness on the basis of resistivity and seismic velocity values which were proved by stratifications of three boreholes to 40–60 m of depth on the study area. There are connections from surface water to shallow aquifer by hydraulic windows, as follows from VLF data. The deeper aquifer can be considered as a potential groundwater supply, but the water level is descending in time, as shown by hydrological monitoring. However, with careful use and by reducing sources of pollution, groundwater can continue to be an important natural resource for future.     

Floodplain hydrology of the Mekong Delta,Vietnam

The Mekong Delta is one of the largest and most intensively used estuaries in the world. Each year it witnesses widespread flooding which is both the basis of the livelihood for more than 17 million people but also the major hazard. Therefore, a thorough understanding of the hydrologic and hydraulic features is urgently required for various planning purposes. While the general causes and characteristics of the annual floods are understood, the inundation dynamics in the floodplains in Vietnam which are highly controlled by dikes and other control structures have not been investigated in depth. Especially, quantitative analyses are lacking, mainly due to scarce data about the inundation processes in the floodplains. Therefore, a comprehensive monitoring scheme for channel and floodplain inundation was established in a study area in the Plain of Reeds in the northeastern part of the Vietnamese Delta. This in situ data collection was complemented by a series of high‐resolution inundation maps derived from the TerraSAR‐X satellite for the flood seasons 2008 and 2009. Hence, the inundation dynamics in the channels and floodplains, and the interaction between channels and floodplains, could be quantified for the first time. The study identifies the strong human interference which is governed by flood protection levels, cropping patterns and communal water management. In addition, we examine the tidal influence on the inundation in various parts of the Delta, since it is expected that climate change‐induced sea level rise will increase the tidal contribution to floodplain inundation. Copyright © 2011 John Wiley & Sons, Ltd.