Sediment transport trends and cross-sectional stability of a lagoonal tidal inlet on the Central Coast of Vietnam

Lagoonal tidal inlets are a typical morphology of the Central Coast of Vietnam. Recently, navigation channels in these inlets have become increasingly threatened by siltation. This study analyses the relations between sediment distribution and transport trends (using the technique of Sediment Trend Analysis-STA■) in the lagoonal system of the De Gi inlet and then proposes appropriate countermeasures against sand deposition in the navigation channel. The STA identified three types of transport trends in the De Gi inlet, namely dynamic equilibrium, net accretion, and net erosion. Processes associated with the tidal prism have resulted in trends of sediment transport and deposition across the flood and ebb tidal shoals, which maintain a present cross-sectional area of about 1000m^2. However, longshore sediment transport from north to south resulting from northeast waves cause additional sand deposition in the channel. In addition, the effects of refraction associated with a nearby headland and jetty also increase sedimentation. These processes provide the main reasons for sediment deposition in the De Gi inlet. Short term and regular dredging helps to maintain the navigation channel. A system comprised of three jetties (north, south, and weir) is necessary to ensure the longterm cross-sectional stability of the navigation channel.     

Formation of Abnormal Gas-Geochemical Fields of Methane,Helium, and Hydrogen in Northern Vietnam,Its Coastal and Adjacent Water Areas

Lithology and Mineral Resources - Gas-geochemical studies of the methane, helium, and hydrogen concentrations in the water of thermal and mineral springs, as well as the subsurface atmosphere, of...     

Tectonic implications of Mesozoic magmatism to initiation of Cenozoic basin development within the passive South China Sea margin

International Journal of Earth Sciences - The South China Sea (SCS) is one of the classical example of a non-volcanic passive margin situated within three tectonic plates of the Eurasian,...     

A coupled poroplastic damage model accounting for cracking effects on both hydraulic and mechanical properties of unsaturated media

Damage induced by microcracking affects not only the mechanical behaviour of geomaterials but also their hydraulic properties. Evaluating these impacts is important for many engineering applications, such as the safety assessment of radioactive waste disposal facilities. This paper presents a new constitutive model accounting simultaneously for the impact of damage on hydraulic and mechanical properties of unsaturated poroplastic geomaterials. The hydro‐mechanical coupling is formulated by means of the thermodynamic framework for partially saturated media, extended by taking into account isotropic damage and plasticity. State and complementary laws are governed by the so‐called plastic effective stress and equivalent pore pressure. Assuming a bimodal pore size distribution for cracked porous media, the hydraulic part (water retention curve and hydraulic conductivity) is modelled using phenomenological functions of damage variable. The participation of damage on both mechanical and hydraulic part enables this model to describe bilateral couplings between them. This coupled model is then validated against a number of experimental data obtained from Callovo‐Oxfordian argillite, which is the possible host rock for a radioactive waste disposal in France. Parametric studies are also carried out to check the consistency and to better demonstrate the bilateral couplings in the model. Copyright © 2015 John Wiley & Sons, Ltd.     

A simple approach to estimating freshwater discharge in branched estuarine systems

In tidal estuaries, quantifying freshwater discharge is still a difficult problem that has not yet been overcome due to the inherent difficulty in measuring and analysing the tidal discharge, especially during periods of low river flow. Because observations are often made in the stations further upstream, where the ratio of river to tidal discharge is large, it remains difficult to determine the discharge rate in the saline region. Freshwater discharge estimation is even more difficult in a branched estuary system having multiple diversion channels that connect with each other at a junction. To date, several methods have been developed for estimating freshwater discharge in estuaries. The most widely used are analytical and conceptual models that employ salinity as the principal trace and numerical simulations. However, these methods are very time consuming and costly as they require large sets of observations before the computations can take place. This paper presents a simple approach to investigating the discharge distribution over branched channels by considering the energy loss due to friction. We develop an analytical model that can obtain the discharge rate quantitatively at a junction where the main flow bifurcates into two branches. The model uses the bed roughness, tidal water level, and cross‐sectional profile under tidally averaged conditions as input data. Two selected estuarine systems in the Hiroshima delta in Japan and the Mekong delta in Vietnam have been investigated. Computations of the newly developed model show good agreement with earlier published results computed by sophisticated analytical and numerical models.     

Effects of Social Learning on Rural Farmers’ Adaptive Capacity: Empirical Insights from the Vietnamese Mekong Delta

Abstract

Environmental challenges in the Vietnamese Mekong Delta characterized by adverse impacts of climate change, upstream hydropower development and localized dyke expansion present imperatives for rural farmers to “learn to adapt.” However, little is known about how learning contributes to improving their capacity in adapting to these “wicked” problems. This study investigates potential effects of farmers’ learning on their adaptive capacity, utilizing nine focus group discussions, 33 interviews, and a structured survey of 300 farmers. The exploratory factor analysis produced two factors for social learning: (1) learning through social interactions and (2) self-reflection, and one factor for adaptive capacity. The regression results show that the social learning factors have significantly positive effects on adaptive capacity. Farmers with a higher level of social learning are likely to demonstrate higher adaptive capacity. The findings call for policy considerations to promote learning in a broader context of the delta to enhance local capacity.     

Use of stable isotopes to understand run‐off generation processes in the Red River Delta

This paper presents the use of stable isotopes of water for hydrological characterization and flow component partitioning in the Red River Delta (RRD), the downstream section of the Red River. Water samples were collected monthly during 2015 from the mainstream section of the river and its right bank tributaries flowing through the RRD. In general, δ¹⁸O and δ²H river signatures were depleted in summer–autumn (May–October) and elevated in winter–spring (November–April), displaying seasonal variation in response to regional monsoon air mass contest. The Pacific equatorial–maritime air mass dominates in summer and the northern Asia continental air mass controls in winter. Results show that water of the RRD tributaries stems solely from local sources and is completely separated from water arriving from upstream subbasins. This separation is due to the extensive management of the RRD (e.g., dykes and dams) for the purposes of irrigation and inundation prevention. Mainstream river section δ¹⁸O and δ²H compositions range from ?10.58 and ?73.74‰ to ?6.80 and ?43.40‰, respectively, and the corresponding ranges inside the RRD were from ?9.35 and ?64.27‰ to ?2.09 and ?15.80‰. A combination of data analysis and hydrological simulation confirms the role of upstream hydropower reservoirs in retaining and mixing upstream water. River water inside the RRD experienced strong evaporation characterized by depleted d‐excess values, becoming negative in summer. On the other hand, the main stream of the Red River has d‐excess values around 10‰, indicating moderate evaporation. Hydrograph separation shows that in upstream subbasins, the groundwater fraction dominates the river flow composition, especially during low flow regimes. Inside the RRD, the river receives groundwater during the dry season, whereas groundwater replenishment occurs in the rainy season. Annual evaporation obtained from this hydrograph separation computation was about 6.3% of catchment discharge, the same order as deduced from the difference between subbasin precipitation and discharge values. This study shows the necessity to re‐evaluate empirical approaches in large river hydrology assessment schemes, especially in the context of climate change.     

Early Mesozoic lamprophyres in Gorny Altai: petrology and age boundaries

The Mesozoic Chuya dike complex was recognized by R.V. Obolenskaya based on the similar mineral composition of dikes and their age characteristics. Lamprophyres occur along the large Terekta–Tolbonur and Kurai–Kobda shear zones. The Chuya complex was studied by the example of two areas, South Chuya and Yustyd, with different levels of erosional truncation. The dikes of the first area are localized in the South Chuya Ridge, where they cut Cambrian–Ordovician metamorphic rocks, and the dikes of the Yustyd area occur in the Devonian terrigenous blackshale deposits of the Yustyd trough. The dikes of these areas differ in structures, textures, the degree of carbonatization, and mineral composition. The performed studies of rocks and minerals confirmed that the dikes of both areas belong to the same complex. They helped to establish the regularities of the lithologic composition of the entire complex and its local areas and to substantiate the recognition of areas not only from their geologic position but also by the composition, structures, and textures of rocks and their mineral composition. Geochronological data show two stages of the complex formation: 236–234 and 250–242 Ma. The results of studies also demonstrate that the lamprophyres and coeval syenites of the Tarkhata massif are fractionates of the same parental melt and can be united into a hypabyssal-plutonic complex. Comparison with other Permo-Triassic lamprophyre complexes showed that the wide variations of the composition of the Chuya rocks and its trend as well as the geochemical anomalies are specific features of complexes of high-K lamprophyres.     

Ecological risk assessment of heavy metals sampled in sediments and water of the Houjing River,Taiwan

The Houjing River flows through Kaohsiung, the most industrialized city in southern Taiwan. In this study, heavy metal concentrations in water and sediments from samples along the river were investigated to illustrate metal contamination levels and call for the awareness of industrial pollution prevention. The heavy metal concentrations in the water samples were low and appear to pose little direct risk to aquatic life and irrigation, but heavy metal concentrations in the sediments are locally very high and present an environmental risk. Cadmium, Cu, and Zn were found in higher concentrations in the river sediments than those recommended in some sediment quality guidelines and findings of river sediments in similar studies worldwide. Hence, the ecological risk of heavy metal contamination in sediments was assessed using the pollution load index (PLI) and potential ecological risk index (RI). Three of the eleven sites sampled were found to have PLI values higher than 1 and 8 of them had ‘considerable’ to ‘very high’ RI values, suggesting a considerable ecological risk. These findings provide an insight into elemental metal contamination of the Houjing River and present a baseline data set, which will be critical for future development and environmental protection plans devised for the region.     

Adopting the downward approach in hydrological model development: the Bradford catchment case study

The paper presents the development of a lumped conceptual rainfall‐runoff model [Transformation of rainfall to runoff, Variability across timescales and Model parsimonization (TVM)] and a series of tests on various levels of model structure at different time resolutions. It is applied to the Bradford catchment in the United Kingdom. The TVM model is developed with a flexible structure through various relationships in each module that can be modified depending on the study catchments. Adopting the downward approach, parsimonious models are developed to examine at what level of complexity the model is able to capture runoff variability. The approach aims to compromise between parsimonious and complex alternatives in model development. This study shows that model structure requires data at different aggregation levels of timescales depending on its complexity. It reveals that the absence of the infiltration excess strongly affected all models. The analysis shows that the time resolution of hourly downwards must be used for the study catchment. The investigation of model complexity indicates that the combination of the most complicated model structure and timescale of quarter‐hourly is adequate to capture the catchment runoff characteristics. The downward approach in the TVM model helps to gain a deeper understanding of water balance and runoff process in the study catchment. The approach could be applicable to other catchments to obtain parsimonious models. Copyright © 2010 John Wiley & Sons, Ltd.