Discharge through Open Offset Joints and Cracks in Spillway Chutes

Abstract

Spillway chutes containing offsets into the flow at joints or cracks are susceptible to hydraulic jacking from injected high pressures and erosion of foundation materials caused by flow through the openings. Consequently, drainage systems are often provided below the chute floor, but there is a lack of reliable experimental data on expected flow rates. To address this need, uplift pressure and joint discharge tests were performed in a supercritical flume equipped with a model joint whose gap width and offset height into the flow were adjusted over a 725∶1 range of aspect ratios. A previous article gives relations for computing the uplift pressure generated in undrained foundation conditions. This article provides relations for computing discharge through a joint or crack acting as an orifice when the foundation is partially or fully drained. When drainage is unable to fully vent the foundation, these relations can also be used to estimate the residual pressure obtained from partial drainage. Application to the 2017 spillway failure at Oroville Dam shows that open joints can admit much larger flows than previously estimated. This emphasizes the importance of preventive measures such as joint details that prevent the development of offsets into the flow, maintenance to remediate existing offsets, and waterstops to prevent water intrusion. Aging spillway chutes often exhibit cracks or displaced joints that create offsets into the flow. Water entering these areas can cause erosion of the foundation leading to failures such as the one experienced at Guajataca Dam in Puerto Rico in 2017. Drainage systems beneath a slab are one method for addressing such problems. Testing performed in a high-velocity laboratory flume was used to develop equations for calculating the flow rate into open offsets. The equations can also be used to estimate the residual pressure remaining in the foundation as a result of partial drainage. An example application to the 2017 spillway failure at Oroville Dam shows that open joints can admit much more flow than previously estimated. The most effective methods for preventing such failures in the future are the design of joints with details that prevent the development of offsets into the flow, grinding down displaced joints or cracks in existing spillways to eliminate offsets into the flow, and the use of waterstops in original construction to prevent water intrusion.