Natural resources of the Mekong River are essential to livelihood of tens of millions of people. Previous studies highlighted that upstream hydro-infrastructure developments impact flow regime, sediment and nutrient transport, bed and bank stability, fish productivity, biodiversity and biology of the basin. Here, we show that tidal amplification and saline water intrusion in the Mekong Delta develop with alarming paces. While offshore M2 tidal amplitude increases by 1.2–2 mm yr−1 due to sea level rise, tidal amplitude within the delta is increasing by 2 cm yr−1 and salinity in the channels is increasing by 0.2–0.5 PSU yr−1. We relate these changes to 2–3 m bed level incisions in response to sediment starvation, caused by reduced upstream sediment supply and downstream sand mining, which seems to be four times more than previous estimates. The observed trends cannot be explained by deeper channels due to relative sea level rise; while climate change poses grave natural hazards in the coming decades, anthropogenic forces drive short-term trends that already outstrip climate change effects. Considering the detrimental trends identified, it is imperative that the Mekong basin governments converge to effective transboundary management of the natural resources, before irreversible damage is made to the Mekong and its population.

Free surface flows in several shallow rectangular basins have been analyzed experimentally, numerically and theoretically. Different geometries, characterized by different widths and lengths, are considered as well as different hydraulic conditions. First, the results of a series of experimental tests are briefly depicted. They reveal that, under clearly identified hydraulic and geometrical conditions, the flow pattern is found to become nonsymmetric, in spite of the symmetrical inflow conditions, outflow conditions and geometry of the basin. This non-symmetric motion results from the growth of small disturbances actually present in the experimental initial and boundary conditions. Second, numerical simulations are conducted based on a depth-averaged approach and a finite volume scheme. The simulation results reproduce the global pattern of the flow observed experimentally and succeed in predicting the stability or instability of a symmetric flow pattern for all tested configurations. Finally, an analytical study provides mathematical insights into the conditions under which the symmetric flow pattern becomes unstable and clarifies the governing physical processes.

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Large-Scale Particle Image Velocimetry (LSPIV) is an extension of a quantitative imaging technique to measure water surface velocities using simple and inexpensive equipment. This paper describes the implementation of imaged-based LSPIV in eight different environmental flow and hydraulic engineering applications for the investigation of complex configurations with and without sediment transport (bed and suspended loads). These applications include the investigation of sedimentation in shallow reservoirs, run-of-river hydropower plants, side weirs used to control bank overflow, flow fields in different spillway configurations with and without Piano Key Weir (PKW), oil spills with flexible and rigid barriers, groin fields, river confluence, and sediment flushing in reservoirs. The paper summarises some special problems encountered in such study cases. The selection and adjustments of the parameters to solve them properly were examined. The potential of LSPIV to measure surface flow velocities in the context of river and dam engineering projects is shown. Despite significant variations of natural and artificial illuminations and seeding tracers in the laboratory, field, wind, and water surface elevation, LSPIV was applied successfully to obtain velocity measurements. LSPIV has proven to be a reliable, flexible, and inexpensive flow diagnostic tool that can be employed successfully in many engineering applications. Ó

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Abstract:The catchment of the Dashidaira reservoir located on the Kurobe River has high sediment yield. Because of the sufficient available amount of water in the catchment during flood events, the free-flow sediment flushing operation with full water-level drawdown is employed every year to preserve the effective storage capacity of the Dashidaira reservoir. This paper focuses first on the numerical simulation of a previously conducted free-flow flushing operation in the Dashidaira reservoir using the available in situ obtained data. Afterwards, to improve the flushing efficiency, the effects of water and discharge manipulation and the construction of an auxiliary channel on the total volume of the flushed sediment were studied. A fully 3D numerical model using the finite volume approach in combination with a wetting/drying algorithm was utilized to reproduce the flow velocity field and simulate the movable bed variations. The outcomes revealed that increasing the average free-flow discharge during the free-flow stage by approximately 56%, in the form of multiple discharge pulses, can enhance the flushing efficiency by up to 13%, and the construction of an auxiliary channel in the wide midstream of the reservoir can locally increase the sediment erosion from this area.

Between 2015 and 2016, Vietnamese Mekong Delta (VMD) has undergone the most severe drought event over the last 90 years, causing damages to agriculture, aquaculture, and fresh water suply. Moreover, upstream Mekong River development by constructing hydropower dams will magnify the severity to the region. This research therefore aims at summarizing some damaged information caused by drought event 2015-2016 and analyzing the impacts of eleven proposed mainstream dams in Thailand, Lao PDR, and Cambodia on hydrology of Vietnamese Mekong Delta under the effect of sea level rise. Results show that the flow discharge is reduced by maximum 14.9% whereas the maximum increase in water level exceeds 220%. This leads to more intrusion of saltwater into the delta and reduction of fine sediment and natural nutrients settling in floodplains.