Garrison Dam is located on the Missouri River as the river flows through central North Dakota. The Colorado State University (CSU) Hydraulics Lab has conducted this study of Garrison Dam to investigate the conditions impacting the stilling basin of the prototype. The objective of this report was to design a 3D computational fluid dynamics (CFD) spillway model in FLOW-3D to accurately match to USACE water surface elevation data approaching the stilling basin to aid the design of the physical model being built in the CSU Hydraulics Lab. In the original design, the concrete slabs in the stilling basin failed when they experienced uplift forces due to seepage flow under the slabs. In the new design, concrete overlays have been added to the stilling basin to prevent this issue. Using numerical modeling, part of the study involved testing between different slopes in the entrance slope section to see if this made a difference on hydraulic flow depths, velocity vector magnitudes and velocity vector angles at different points along the model. It was important to check to see if there were large differences which could cause further erosional problems in the stilling basin. This numerical model was created at 1:24 length scale of the prototype and is 1:1 to the physical model created in the Engineering Research Center (ERC).  

Many simulations were created and run for different design flow rates, having baffles or excluding them, changing the approach slope, and also changing the geometry of the inflection point between the main chute and the approach slope. Each run took approximately 12 hours to complete and a total of 16 simulations were made. Data from the FLOW-3D was compared to the 2D model created by USACE for model validation. After the completion of the physical model in the lab, the collected data will be compared to the numerical model results for further validation.