Water Quality Modeling in the Snake and Columbia River Basins
Assessment of dissolved gas abatement alternatives for dams on the Lower Columbia and Snake Rivers
The purpose of the Dissolved Gas Abatement Program (DGAS) is to study options to reduce total dissolved gas supersaturation (TDG) associated with spill at hydroelectric dams on the Lower Columbia and Snake rivers. In order to address the complex nature of TDG transport, fate, and exposure, PNNL developed numerical models of flow, water temperature, gas transport, gas mixing, and dynamic gas bubble trauma.
The models are general purpose one-dimensional (MASS1) and two-dimensional depth-averaged (MASS2) hydrodynamic and water quality codes that are written in FORTRAN90 and run on Windows and Linux systems. In addition, an individual fish model (FINS) was developed to assess the exposure of migrant fish to TDG. These models couple hydrodynamics, temperature, DGS production, DGS transport, and fish distribution information with a dynamic gas bubble trauma mortality model that will provide an estimation of cumulative mortality in fish populations passing Columbia and Snake River dams. As part of this program, an extensive database of existing Columbia and Snake River bathymetry was developed using ARC/Info GIS. To speed development of computational grids and cross-sections for 1D, 2D, and 3D models a set of tools to extract and format the required bathymetry from the ARC/Info GIS and to merge it with the output of grid generation tools such as GRIDGEN. Hydrology, meteorology, project operations data, and dissolved gas production were assembled into a database (PostGresSQL) system. Using this database system model input files for the various simulation scenarios could be generated with a minimal amount of “hand” preparation. A large number of scenarios had to be simulated and the results stored for additional analysis. PNNL implemented a distributed processing and data management scheme to meet this requirement. Model results were visualized using a variety of tools that included TECPLOT, GNUPlot, and ARC/Info. Frequency of exceeding water quality standards and impacted habitat area were used to compare the performance of the different abatement alternatives.
Results and Benefits
Total Dissolved Gas Plume downstream of Ice Harbor Dam on the Snake River. Click for a larger version.
Reductions in total dissolved gas supersaturation are needed to move toward meeting water quality criteria and might also reduce mortality in both anadromous and resident fish populations caused by gas bubble trauma (GBT). To achieve this goal, a number of design alternatives are being examined that include modifications to dam structures as well as dam operations. The PNNL developed models are being utilized to evaluate the effectiveness of the different gas abatement alternatives. In addition, the models have been applied to study other issues such as adult migrant fallback, predator fish habitat, and water temperature assessment.