Researchers at St. Anthony Falls have been interested in aeration of flow at hydraulic structures for decades. The basic process is simple: as water flows over a weir or spillway (or a rapids or waterfall), atmospheric gasses (mainly nitrogen and oxygen) can dissolve into the water. Likewise, dissolved gasses in the water can come out of solution at these structures, in effect “de-gassing” the water. The direction and rate of the gas transfer depends on the amount of gas already dissolved in the water, the limiting (or saturation) concentration, the surface area for gas exchange, and the turbulence of the system.
The process has environmental importance along all of the rivers of the world. For example, if there is too little dissolved oxygen in a river, then aquatic life such as fish can die, and the river may have odor problems. At the other end of the spectrum, too much dissolved nitrogen can also be harmful to organisms such as fish. Thus, there is a range of dissolved gas concentrations that define an acceptable level of water quality.
One recent project involved a combination of numerical and physical models to predict the concentration of dissolved gasses downstream of a spillway. Researchers at St. Anthony Falls developed a two-dimensional computer program to calculate the dissolved gas concentrations for different flow conditions. Input to the numerical code included the flow field information, which was measured on a physical model of the spillway and stilling basin.
Wanapum
Dam is located on the Columbia River in east-central Washington State,
with 12 spillway bays and ten hydroelectric generating units. The dam is
owned and operated by Public Utility District
No. 2 of Grant County, Washington. The dam owner has experienced elevated
gas levels downstream of the spillways. In an effort to lower dissolved
gas concentrations, the face of one spillway bay was modified to include
a “flow deflector” in 1996. The purpose of the deflector is to prevent
flow over the spillway from entering the deepest parts of the stilling
basin. This prototype deflector was field tested in the summer of 1996,
and was found to reduce the dissolved gas levels downstream of the spillway
when compared with the original spillway design. When the dam owner decided
to install a second prototype deflector for field testing in 1997, they
were interested in the possibility of installing one with a different design
that might provide better performance.
Unfortunately, a reduced-scale physical model can not simulate the entrainment of air into the stilling basin. While the mean flow fields can be well reproduced at a reduced scale, the distribution of air bubbles (including their size and quantity of air entrained) can not be accurately simulated. This implies that the air-water mass transfer (and thus concentration of dissolved gas downstream of the spillway) can not be determined directly from the physical model. However, information from the physical model can be used as input to a computational model which represents the turbulent mixing and air-water gas transfer, and can thus be used to predict dissolved gas concentrations for differing flow conditions.
The model was used to evaluate the performance of the existing spillway and three different spillway modifications for a range of operating conditions. The spillway modifications consist of deflectors of differing shapes, placed at a variety of elevations on the spillway face.
The computer model was used to calculate the dissolved gas concentrations downstream of the spillways for a variety of flows and water levels. A representative flow field (based on the physical model measurements) for one of the conditions evaluated is shown below.
The dissolved gas concentrations calculated as a function of distance downstream from the spillway and elevation. A representative concentration field is shown below.
Mannheim, C.O.M. [1997] "A Unique Approach of Modeling Gas Supersaturation Using a Physical Model," MS Thesis, University of Iowa, Iowa City.
Orlins, J.J. and J.S. Gulliver [1997] "Prediction of Dissolved Gas Concentrations Downstream of a Spillway," Proceedings, XXVII IAHR Congress, San Francisco, CA. Pub. by ASCE, New York. pp. 524-529.
Orlins, J.J., and J.S. Gulliver [1997] "Modeling Dissolved Gas Concentrations Below Wanapum Dam Spillways," Consultant's Report for R.A. Elder and Public Utility District No. 2 of Grant County, WA, Minneapolis, MN.
Weber, L.J. and C.O.M. Mannheim [1997] "A Unique Approach of Modeling Gas Supersaturation Using a Physical Model," Proceedings, XXVII IAHR Congress, San Francisco, CA. Pub. by ASCE, New York. pp. 518-523.