Optimizing Spillway Rehabilitation: A Multi-Tool Hydraulic Approach at McCloud Dam

ABSTRACT ONLY - Spillways are complex hydraulic structures with materials and geometries that are often tailored to unique site conditions. This is certainly the case at Pacific Gas and Electric Company’s (PG&E) McCloud Dam, a 255- foot-high embankment that was initially completed in 1965. The existing concrete-lined chute spillway is located in a 300-foot-tall rock cut at the right abutment of the dam embankment and consists of an ogee-shaped gated control section, a 605-foot-long chute, and flip bucket. The chute converges from 91-feet-wide to 40-feet-wide and experiences 185 feet of elevation drop. A recent assessment of the spillway identified the need for structural remediation as well as a significant increase in discharge capacity, leading PG&E to pursue rehabilitation design for the structure. Given the unique site constraints, as well as constructability and operational considerations, spillway alternatives included several non-typical and hydraulically complex configurations (including various combinations of gated, non-linear, and side channel spillways). The alternative selection process consisted of three multi-day workshops to narrow 26 initial alternatives to the five most viable alternatives, and finally the selection of the preferred alternative: a concrete chute spillway with a hybrid control section (i.e., gated ogee weir combined with a passive labyrinth weir). This alternatives analysis and subsequent design required numerous hydraulic analyses using the full range of available tools including various empirical methods, one-and two-dimensional USACE HEC-RAS modeling, USBR SpillwayPro analyses, computational fluid dynamics (CFD) modeling using Flow-3D, and ultimately the construction and testing of a 1:35 scale physical model at Utah State University. For each of these analyses, methods were selected to be commensurate with the complexity of the structure and the intended use of the model results. This presentation will discuss the full range of hydraulic design methods including their limitations, strengths, and applicability in the context of the McCloud Dam Spillway Project. Design engineers and regulators will learn how to evaluate the adequacy of hydraulic design methods for common flow conditions. Insights will be shared that can be applied to any spillway, regardless of complexity or size.