The Hydraulic Design of an Arced Labyrinth Weir at Isabella Dam

Abstract

The Sacramento District Corps of Engineers is designing modifications to the Isabella Dam located on the Kern River in the Tulare Lake Basin in the southern portion of the San Joaquin Basin, in Kern County, California. These modifications include raising the elevation of two high head dams (185 ft and 100 ft respectively) by 16 ft, modifying a service spillway to better suit the needs of flood capacity, and creating a new arced labyrinth weir emergency spillway in order to safely pass the probable maximum flood (PMF). The rating curve for the arced labyrinth weir emergency spillway was based on information published in a PhD dissertation (Crookston, 2010) and a Master’s Thesis (Christensen, 2012) from Utah State University. The proposed prototype weir geometry was a 10-cycle arced labyrinth with a 16o sidewall angle. As the published hydraulic performance data were limited to arced labyrinth weirs with 6o, 12o, and 20o sidewall angles, a spreadsheet program was developed to predict the rating curve for the prototype 16o weir using a double quadratic spline interpolation of the 12o and 20o discharge coefficient data. The spreadsheet was used to tailor the design of the arced labyrinth weir to the specific project needs. The spreadsheet was also used to evaluate the hydraulic and economic implications of over six-thousand different arced labyrinth weir/apron designs. The prototype design verification program used a composite spillways modelling approach, which included a 1:45 scale physical model study at the Utah Water Research Laboratory (Utah State University) and CFD modelling conducted by the USACE Portland District (Hydraulic Design Section). The goal of the composite modelling was to validate the theoretical rating curve for the arced labyrinth weir given a non-ideal approach condition with the potential for submergence effects decreasing the discharge over the weir. The selected arced labyrinth weir theoretical rating curve was compared to the physical model and the CFD model results of the labyrinth weir. The physical model and CFD model results confirmed that the rating curves developed using traditional methods (discharge coefficients, velocity head, etc.) are valid and that the designed configuration of the service spillway and labyrinth weir will safely pass the PMF within the target pool elevation.