8th IAHR ISHS 2020 Santiago, Chile, May 12th to 15th 2020 DOI: 10.14264/uql.2020.515 Keynote Lecture: Hydraulic Structures – Learning from Recent (Partial) Failures and the Opportunities They Present Mike Phillips1 1U.S. Army Corps of Engineers Risk Management Center Lakewood, Colorado USA E-mail:[email protected] ABSTRACT There have been numerous partial, near, or complete failures of hydraulic structures on dams around the globe over the past 10 years. I have been fortunate in my consulting and US Army Corps of Engineers (USACE) career to have contributed to the post-failure analysis and design of the repair for many of these dams and structures throughout the world. Out of these (and other) hydraulic structure failures, come great opportunities to learn, teach, mentor, and apply lessons learned to future projects. The purpose of this paper is to highlight three case studies of partial or near complete failure of hydraulic structures that I have been involved in as part of USACE including: 1) the USACE Review of Spillways – post Oroville Dam; 2) the Guajataca Dam Spillway Repair; and 3), the Mosul Dam Bottom Outlet Flip Bucket Dentates commissioning In this paper, I will discuss why sharing these case studies is one way we maintain safe and operational structures for society and industry, and is an important component to allowing the dam safety community to learn from hydraulic structure failures, and why mentoring is critical to the future of hydraulic structure design and operation. Keywords: Failure, hydraulic structures, case studies, mentoring 1. INTRODUCTION Over the past 10 years, there have been multiple hydraulic structure failures for dams in many locations around the world, as listed below. Many of these have been reported in national and international news and media outlets, however there are likely many others that go unreported. Although they resulted in major repairs and expense, no fatalities were recorded for each of these partial failures. Fort Peck Spillway – US (2011) Paradise Dam Spillway – Australia (2013) Oroville Dam Spillway – US (2017) Guajataca Dam Spillway – Puerto Rico (2017) Ituango Hydropower Dam Diversion Tunnels and Powerhouse – Colombia (2018) Lake Dunlap Spillway Gate – US (2019) Dicle Dam Spillway Gate – Turkey (2017) Whaley Bridge Dam Spillway – UK (2019) As hydraulic structure engineers and scientists, whether our careers are centered on research, design, construction, operation, or modelling, we should each be endeavoring to learn and establish: How hydraulic structures can fail How they have failed (i.e. case studies and forensic studies) How existing hydraulic structures were designed, constructed, and operated, many of which are nearly 100 years old How to ensure we are mentoring and being mentored in hydraulic structures Who is in your family of experts, mentors, or peers that you can call at a moment’s notice without fear of judgment or career impact. I have been fortunate in my career to have been mentored and guided by some of the world’s experts in hydraulic structures for dams. Similarly, I have travelled internationally and worked on many large dams projects, and have generally found common themes within the hydraulic structure industry: In the countries where I have worked on projects, there is a generational gap in knowledge of hydraulic structure engineering, typically with engineers in their 60s and older and junior engineers in their 20s and 30s, with few in-between. As our more experienced engineers retire or pass on, the knowledge of past failures and the reasons for changes in the state of the practice gets muddled or lost. There is a general reluctance to share lessons learned from failures or partial failures so that the greater hydraulic structures industry can benefit from the insight gained by the team. As an industry we like to say that we are mentoring or being mentored by those closest to us in our career, but often we need to take a step back and self-reflect. Mentoring should be about sharing our knowledge freely and honestly, including our own failures and lessons learned. 1.1. Are You Learning From and Sharing (Partial) Failures of Hydraulic Structures? Case Study: USACE Spillway Review as a Result of Oroville Dam Spillway Failure The case study of Oroville Dam is one of the largest, widely publicized, partial failures in recent history. There were no fatalities as a result of the service spillway chute failure and the operation of the emergency spillway. We as the dam and hydraulic structures community of practice are fortunate that the dam owner, dam regulator, and forensic team published their findings for the world to read and learn from the contents. The case study below presents a brief synopsis of the Oroville Dam spillway partial failure and the findings that USACE found within its own spillway portfolio. The author encourages others to follow suit and share/publish (partial) failures that have occurred in projects for the broader community to learn from the findings. Oroville Dam is an embankment dam located on the Feather River in northern California. The main embankment is 235 meters high and the reservoir behind it has a maximum capacity of 4.3 billion cubic meters of water storage. The dam was designed and constructed in the 1960’s and is owned and operated by the California Department of Water Resources (DWR) with state dam safety oversight provided by DWR – Division of Safety of Dams (DSOD) and federal dam safety oversight provided by FERC. The project includes an embankment dam across the Feather River, a service spillway and an auxiliary spillway on the right abutment/reservoir rim, and a powerhouse in the left abutment adjacent to the embankment. The service spillway is a gated concrete chute with a length of 915 meters, a width of 55 meters, and a discharge capacity of 8,400 cubic meters per second at a maximum reservoir elevation of 280 meters. The service spillway is used to pass flows in excess of the powerhouse capacity. These excess flows generally occur during the winter/spring months and are dependent on the snowpack in the Sierra Nevada Mountains as well as winter/spring precipitation. The service spillway has operated over 25 times over its lifespan with maximum discharges up to approximately 3,800 cubic meters per second The Oroville Dam concrete chute service spillway was extensively damaged in February 2017 while passing excess winter/spring flood flows, as shown in Figure 1. Damage initiated when one or part of one of the chute slabs failed causing the erodible foundation to be exposed to flow. As the spillway continued to flow, progressive head cutting erosion of the chute caused toppling failure of additional chute slabs. According to the Independent Forensic Team (IFT) report (2018) on the Oroville Dam spillway incident, the design and construction issues that likely led to the spillway damage were: “The foundation drainage system lacks the redundancy of intermediate longitudinal collector drains. It relies on all flows being collected on one side of the spillway chute or the other, where surface runoff is collected. Any plugging of either collector drains or the individual herringbone drains could cause backups in the drainage system.” “The design of the herringbone drains to be placed within the chute slab thickness resulted in a reduce[d] concrete section over the drains where cracking could occur. Since the drains were only designed to collect groundwater seepage, flow into cracks in the concrete chute slab could potentially exceed the drain capacity and pressurize the drainage system” (resulting in uplift pressures on the spillway slab) (refer to Figure 2). “Concrete cutoffs beneath the chute slab, had they been installed, could have helped minimize slab movement in areas of weaker foundation material.” “The chute slab thickness of 15 inches seems to be thin for a spillway chute on one of the tallest dams in the United States. However, the single layer of light reinforcement and the reduced thickness at drains are of greater concern” (and would likely not provide adequate embedment length to mobilize the fully structural capacity of the hooked foundation anchors). “The keys at slab joints, particularly the transverse joints could have been more robust if they were coupled with foundation cutoffs. However, the biggest problem at the slab joints is the lack of waterstops, which were generally not added to chute slab designs until Oroville Dam spillway was constructed.” “The foundation anchor design strength was not well documented in the bid specifications. Embedment lengths into the foundation were to be tested in the field. At the time Oroville Dam was designed, chute anchors bars were typically only being designed for minor uplift pressures.” Figure 1. Oroville Dam Service Spillway Damage Maximum spillway discharge around the time of failure was estimated to be approximately 1,400 cubic meters per second, compared to its intended capacity of 8,400 cubic meters per second. Repairs to the Oroville Dam concrete chute service spillway are complete with total costs to repair the spillway in excess of $1 billion US dollars. Figure 2. Oroville Dam Drain Detail – note herringbone drain protrudes into the concrete slab (from Figure E-3 of the Oroville Dam IFT report, 2018) In the months following the damage to the Oroville service spillway in February 2017, the USACE completed a cursory review of USACE concrete chute spillways designed and constructed around the same time as Oroville Dam (1960’s) as part of a request from the Oroville Dam IFT. The review consisted of collecting data on approximately 20 USACE concrete chute spillways that were designed and constructed around the same time as Oroville Dam and screening those spillways using an internal panel of experts. The objective was to determine if any of the concrete chute spillways had design and construction flaws similar to Oroville Dam that would make them susceptible to similar damage or failure during operation.