Physical Model Study of the Intake Structure of the Prairie Du Sac Hydroelectric Project

A physical model study of the intake structure of the Prairie du Sac Hydroelectric Project was conducted to assess the distribution of velocities across the intake as an aid in identifying possible modifications to meet resource agency fish protection criteria. The model was built at a scale of 1 to 14. In the model study, approach velocity characteristics were evaluated for a new trash rack placed upstream of the intake structure replacing the existing trash rack inside the structure. Two modification scenarios were studied: a trash rack with 1.75-inch openings (68% opening ratio), and a trash rack with 1.0-inch openings (53% opening ratio). Flow velocities were measured at 40 locations upstream of the model trash rack, corresponding to points located approximately one foot upstream of the trash rack in the full-scale (prototype) structure. Flow velocity measurements were conducted under three flow conditions: 1300 cfs (representing typical flow through the project’s 3 larger generating units) 1000 cfs (representing typical flow through the project’s 5 smaller generating units), and 2170 cfs (representing maximum flow possible through the project’s largest generating unit). Under the 1300 cfs flow condition, the approach flow velocities did not exceed 1.1 and 1.2 fps with the 1.75- and 1.0-inch opening trash racks, respectively. The maximum through flow velocities for the 1300 cfs flow condition were estimated to be 1.9 and 2.4 fps, respectively.