A team from Tottori University led by Dr. Hiroshi Yajima has been using the Nortek Vector in a study aimed at improving the operational efficiency and water quality of Japan’s freshwater reservoirs. This velocimeter collects high-resolution water velocity and pressure data in rapidly changing environments.
Finding the right depth from which to draw water
Many of the world’s man-made reservoirs present stratified conditions, where water at depth may have a significantly different temperature and oxygen content than water closer to the surface. This condition varies throughout the year and between different basins. Reservoir operators therefore drain water from different depths depending on these conditions. However, because of the temporal and spatial variability in these conditions, knowing the right depth from which to drain water is an imprecise endeavor: too much cold (or warm) water can negatively affect the fauna and flora, and the quality of the water that is distributed to the population.
Water intake structures at reservoirs are often built with gates or openings at different levels to deal with this variability in water conditions. But knowing just the temperature or oxygen content is not enough. Operators must also take into consideration the approach velocity of the water, as it has a significant impact on how much of each water mass gets taken, as well as how well these masses mix.
Obtaining precise measurements of 3D velocity
Water velocity inside a reservoir is often very slow – less than 1 cm s-1 on average. But in the immediate vicinity of an intake opening, this normally gentle flow gets rapidly accelerated over a short distance (typically 20–60 cm), not unlike water going over a waterfall. Therefore, precise measurement of 3D velocity must be obtained within a small sampling volume – just the task for the Nortek Vector.
The team led by Dr. Yajima designed a system to deploy the Nortek Vector at different depths over the face of the intake tower at the Tono Dam reservoir, operated by the Japanese Ministry of Land, Infrastructure, Transport and Tourism. This allowed Dr. Yajima to generate a profile of 3D approach velocity around the intake tower, especially at the opening of each intake siphon. Although Doppler profiling systems can be used to give 3D velocities over multiple depth layers, the divergent geometry of their beams makes Doppler profilers impractical for this application, but perfect for the Vector’s small sampling volume. Additionally, the Vector’s optional Inertial Motion Unit (IMU) sensor ensured that the instrument’s own motion during the deployment did not affect the quality of the very low velocities at the sampling points farthest away from each intake opening.
A tool for supplying high-quality water
The data from the Nortek Vector, together with temperature and other parameters, is being used by Tottori University researchers to develop a 3D reservoir water quality prediction model. The goal of the model is to understand the processes involved in improving water quality at Japan’s reservoirs, and to give reservoir operators the tools they need to predict future conditions and to be able to adjust their operational parameters to guarantee a steady supply of high-quality water to the population.