Flow Rates Through Joints and Cracks in Spillway Chutes

ABSTRACT ONLY - Flow entering joints and cracks in the concrete spillway chute at Oroville Dam was identified as a contributing factor to the 2017 hydraulic jacking failure. Post-event forensic estimates of flow rates through the joints suggest the drainage system below the slab was overwhelmed by the flow. Testing performed in the Bureau of Reclamation hydraulics laboratory is now providing experimental measurements of discharge through joints at which there is an open gap and an offset into the flow.
The pressure head driving flow into an offset joint is generated by the stagnation of the main flow against the face of the offset, which propagates through the joint to generate uplift below the slab. Pressures under the slab are maximized when there is no subsurface drainage. This uplift pressure is being measured in the laboratory under sealed conditions and related to chute flow parameters and the geometry of the offset joint. Following the sealed-joint measurements, valves below the joint are opened to allow partial drainage of the foundation. The reduced uplift pressure and flow rate through the joint in the partially vented condition are measured and used to compute a discharge coefficient for the joint. This discharge coefficient varies consistently for very wide and very narrow joints as a function of the flow conditions in the chute. The coefficient exhibits more variability for joints with a moderate gap width to offset height ratio.