The Hydraulic Design of an Arced Labyrinth Weir at Isabella Dam

The Sacramento District Corps of Engineers is designing modifications to the Isabella Dam located in Kern County, California. These modifications include 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 was based upon information published in a PhD dissertation (Crookston, 2010), and a Master’s Thesis (Christensen, 2012) from Utah State University and a subsequent journal article (Crookston and Tullis 2012). The proposed prototype weir geometry was a 12-cycle arced labyrinth weir 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 using a cubic spline interpolation was developed to predict the rating curve for the prototype 16º weir. The spreadsheet was also used to evaluate the hydraulic and economic implications of over six-thousand different arced labyrinth weir/apron variations. The design process included composite modelling with use of a 1:45 scale physical model study at the Utah Water Research Laboratory (Utah State University) and CFD modelling with Star-CCM+ conducted by the Corps of Engineers Portland District. The goal of the composite modelling was to validate the design 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 physical model and CFD modelling confirmed that the rating curves developed using traditional design methods is valid and the design will safely pass the PMF