Prediction of dissolved gas concentration downstream of a spillway

The increase in dissolved gas concentration downstream of hydraulic structures such as dam spillways can be harmful to juvenile salmonids. Such increases have been noted at Wanapum Dam on the Columbia River in Washington State. Modifications to the spillways at this dam will be installed to help lower the concentration of total dissolved gas downstream of the dam. These modifications were designed and optimized using a combination of physical and numerical models. The physical model provided information about the hydraulics associated with different spillway modifications. The numerical model calculated the concentration of total dissolved gas based upon hydrodynamic data from the physical model and mass transport relations for air-water flows. This article describes the numerical model development and application. A companion article describes the physical modeling efforts and field measurements made to evaluate the performance of the modifications installed at the dam.