Large-scale PIV surface flow measurements in shallow basins with different geometries

Shallow depth flow fields and low velocity magnitudes are often challenges for traditional velocity measuring instruments. As such, new techniques have been developed that provide more reliable velocity measurements under these circumstances. In the present study, the two-dimensional (2D) surface velocity field of shallow basins is assessed by means of Large-Scale Particle Image Velocimetry (LSPIV). The measurements are carried out at the water surface, which means that a laser light sheet is not needed. Depending on the time scales of the flow and the camera characteristics, it is even possible to work with a constant light source. An experimental application of this method is presented to analyze the effects of shallow basin geometry on flow characteristics in reservoirs where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics. The flow and boundary conditions that give rise to asymmetric flow are presented. Asymmetric flow structures were observed starting from basin shape ratios that are less than or equal to 0.96. By decreasing the basin length and increasing the shape ratio to greater than 0.96, the flow structure generally tends towards a symmetric patter

Reservoir Sediment Management Using Replenishment : A Numerical Study Of Nunome Dam

Improving the integration of dams in the natural environment and recovering their storage capacity lost to sedimentation are two topics of growing concern. To address these two issues, the accumulated sediments are being relocated down into the dam's tail water. This process is called sediment replenishment (SR), and has been tested in Japan and other parts of the world. More study is required to enhance its effectiveness, and the present research thus offers a practical method to assess positive outcomes of SR on the downstream ecosystems. It is shown through a 2-D numerical model of Nunome River that SR positively influences the river's morphology by generating riffle-pool structures and sand bars. These patterns increase the channel's global heterogeneity, and create hydraulically favorable habitats for fish and spawning. The habitat quality was quantified by applying suitability indexes to the computed hydraulic variables, and the SR-induced geomorphologies were designed according to field observations of past SR tests and validated by the 2-D mode

Tidal amplification and salt intrusion in the Mekong Delta driven by anthrogenic sediment starvation

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

Channel formation during flushing of large shallow reservoirs with different geometries

Depositional and flow patterns are first described to gain an understanding of the erosion patterns during hydraulic flushing in a reservoir. Considering the importance of this issue, two modes of flushing operation for control of sedimentation were performed in several experiments with different reservoir geometries. In order to investigate the effect of flushing and the effectiveness during free and drawdown flows, ten experiments have been conducted. The final bed morphology formed previously was used as the initial bed topography for the two modes of flushing. The entire experiments lasted for two days. Investigations of the flow pattern and the associated bed topography for free flow with normal water depth and drawdown flushing in various shallow reservoir geometries are presented. To effectively apply the flushing processes for the removal of sediment deposits, the location, depth and width of the flushing channel can be changed by modifications to the reservoir geometry. The channel formed during flushing attracts the jet and stabilizes the flow structures over the entire surface. Empirical formulae to describe the relationship between the reservoir geometry and flushing efficiency for the two modes of flushing were developed. Flushing at normal water level allows only a relatively small part of the deposited sediment to be evacuated. As deposits could be flushed out of the basin

Large-scale PIV surface flow measurements in shallow basins with different geometries

Shallow depth flow fields and low velocity magnitudes are often challenges for traditional velocity measuring instruments. As such, new techniques have been developed that provide more reliable velocity measurements under these circumstances. In the present study, the two-dimensional (2D) surface velocity field of shallow basins is assessed by means of Large-Scale Particle Image Velocimetry (LSPIV). The measurements are carried out at the water surface, which means that a laser light sheet is not needed. Depending on the time scales of the flow and the camera characteristics, it is even possible to work with a constant light source. An experimental application of this method is presented to analyze the effects of shallow basin geometry on flow characteristics in reservoirs where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics. The flow and boundary conditions that give rise to asymmetric flow are presented. Asymmetric flow structures were observed starting from basin shape ratios that are less than or equal to 0.96. By decreasing the basin length and increasing the shape ratio to greater than 0.96, the flow structure generally tends towards a symmetric pattern