Hydraulic Structures – Learning from Recent (Partial) Failures and the Opportunities They Present

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. The review, specific for the IFT, determined that some USACE concrete chute spillways have the potential vulnerabilities (i.e. erodible foundations, no water stops between slabs, inadequate reinforcement in slabs, unpinned/unkeyed slab joints, unanchored slabs, issues with foundation drainage design details, no cutoffs at slab joints, etc.). This information combined with the damage noted to the Guajataca Dam spillway as a result of Hurricane Maria as well as the request made by the United States Congress, led to the determination that a more in-depth assessment should be conducted of the existing concrete chute spillways in the USACE portfolio. A team of six engineers, including the author, with relevant engineering experience applicable to the review of existing concrete chute type spillways in the USACE portfolio was assembled from the USACE to perform the review. The screening process was to identify all of the concrete chute spillways in the USACE portfolio:

 714 dams in USACE portfolio  508 high hazard dams in USACE portfolio1  253 high hazard dams in USACE portfolio with concrete spillways1  115 (118 spillways) high hazard dams in USACE portfolio with concrete chute type spillways

After the screening process, the 118 concrete chute spillways were screened further to identify those spillways that are most likely to be susceptible to damage/failure during operation. The review team took into account the vulnerabilities that contributed to the extensive spillway damage at Oroville Dam and Guajataca Dam (refer to second case study presented below) and concluded that the spillways founded on erodible foundations with thin chute slabs (chute slab thickness less than 600mm) were likely the most vulnerable to damage/failure during operation. It was recognized that spillways with slabs thicker than two feet, or other design details could result in a high susceptibility to failure, however in order to meet the aggressive schedule and budget constraints, the erodible foundation and slab thickness were utilized as the initial screening criteria. Spillways that did not meet these criteria were kept in the database for future assessment. Structures that remained after this initial screening were advanced to more detailed assessment. The initial screening results may be summarized as:  118 concrete chute spillways in the USACE portfolio at high hazard dams  81 concrete chute spillways in the USACE portfolio at high hazard dams with erodible foundations (earth, earth and rock, and/or shale)  29 concrete chute spillways in the USACE portfolio at high hazard dams with erodible foundations (earth, earth and rock, and/or shale) and thin chute slabs (chute slab thickness less than two feet) Subsequently, a more detailed assessment of the 29 spillways deemed to be the most vulnerable to damage/failure during operation was conducted. To accomplish this task, the review team carefully examined the design, construction, and history of each spillway, and then assigned each spillway a susceptibility ranking for vulnerability to damage/failure during operation. The key design details and visual observations from previous inspections that the assessment team considered included:  Potential for erosion of the foundation – foundation type and potential for erosion  Concrete slab: slab dimensions, reinforcement (single or double mats, size of reinforcement), observations of cracking or movement  Slab joint details: presence of waterstops, continuous reinforcement or dowels between slabs 6 joints, type of joint (expansion, contraction, construction, control), keyed joints, cutoff walls  Anchors into foundation: number of anchors, size of anchor bars, corrosion protection  Drainage under slabs: type of drains, size of drains, ease of inspection and cleanout and if inspection and maintenance had been performed, embedment into concrete slabs, historic issues with drains such as broken drains or observations of material migration  Spillway chute walls details: wall type, drainage from behind the walls, connection to spillway chute, observations of movement of walls  Energy dissipation structure details: headcut erosion cutoff walls, slab and wall details, anchors, drainage details  Performance of the spillway if had operated historically and any post-inspection observations  Historic repairs to the spillway chute

The key findings from the assessment showed that many of the 118 concrete chute spillways in the USACE portfolio at high hazard dams have characteristics similar to those that contributed to the extensive spillway damage at Oroville Dam and Guajataca Dam:  Approximately 84% do not have water stops between all chute slab joints  Approximately 69% founded on erodible foundations (earth, earth and rock, and/or shale)  Approximately 64% have only one or no mat of reinforcement in chute slabs  Approximately 42% have thin chute slabs of less than two feet in thickness  Approximately 41% have no dowels, keys, or continuous reinforcement between slabs to prevent differential movement  Approximately 34% are not anchored to foundation  Approximately 7% do not have a drainage system under or through the chute slabs There were several design details that were common amongst many of the 29 spillway chutes assessed that led to potentially higher susceptibilities of failure.

 Spillway chute only had one line of defense, such as only anchors, or only a drainage system, or relying on the dead weight of the concrete slabs against uplift, as shown in Figure 3

Figure 3. Example of spillway chute with single line of defense (drainage system under chute slabs)  Spillway chute slabs with thickness of 0.45 m or less with a single mat of reinforcement typically showed extensive surface cracking (example shown in Figure 4)

Figure 4. Example of extensive surface cracking of chute slabs

 Offset slab joints into flow without waterstops at the lateral joints, as shown in Figure 5

Figure 5. Example of joint offsets without waterstops at slab joints  Drains protruding in the stilling basin, such as upstream of baffle blocks, where high pressures could be injected into the drainage system, thereby increasing the uplift on the slabs, as shown in Figure 6

Figure 6. Example of drainage system with openings upstream of baffle blocks Many of the concrete chute spillways in the USACE portfolio have characteristics similar to those that contributed to the extensive spillway damage at Oroville Dam but most have never operated. The Oroville Dam concrete chute spillway is of the service type and has seen regular operation over its lifespan. The IFT report indicates that the Oroville service spillway has operated over 25 times over its lifespan with maximum discharges up to approximately 3,800 cubic meters per second. This is in contrast to the majority of the concrete chute type spillways in the USACE portfolio, which are of the auxiliary or emergency type and rarely, if ever, see flow, which can make it difficult to assess how the spillway would perform in future operations and relies on observations and assessments to determine potential susceptibilities to failure. Of the 118 concrete chute type spillways in the USACE portfolio at high hazard dams, 89% have only seen intermittent operation or never operated:  Regular Use (operated more than 10 times over lifespan) = 11%  Intermittent Use (operated 1-10 times over lifespan) = 30%  No Use (never operated) = 58%  Missing (unknown operational history) = 1%

Lessons learned during the USACE concrete chute spillway review are outlined below:  Many USACE concrete chute spillways have one or more design vulnerabilities relative to current best design practices, however, most have never operated, which raises the importance of these type of assessments to determine potential susceptibilities to failure. When the Oroville Emergency Spillway operated for the first time, it rapidly head-cut scoured several hundred feet to within a short distance of the emergency spillway crest triggering evacuation of a large downstream population.  Spillway vulnerability does not necessarily reflect the age of construction. Some spillway chutes from the 1940s were designed with details similar to modern best-practice, while some spillways designed in the 1970s and 1980s have single lines of defense and are missing multiple modern best-practice details.  Thorough spillway inspection practices and documentation are not uniform across USACE, and modernization and standardization of inspection protocols is needed to improve this practice  Attention needs to be paid during inspections to review of design/as-built data, chute slab cracking, joint condition, joint offsets into flow, and condition of foundation drainage systems  Consider incorporating spillway operational risk into current best practices for risk analysis and assess risk for economic damages as well as life safety for non-breach scenarios  A multidisciplinary team of experienced spillways experts, surveillance inspectors and spillway/project operators should be engaged for spillway inspections, design reviews, risk analyses, and construction oversight  Many of the spillway design documents and drawings do not reflect as-built conditions  Some of the spillways designed by individual districts have similar designs and thus share similar susceptibilities

What are the opportunities in Hydraulic Structures as a result of Oroville Dam Spillway Failure?

 We as the hydraulic structures community of practice should be familiar with the Oroville Dam spillway IFT report and the identified likely causes of the partial failure. This should apply to all hydraulic structures practitioners, whether researcher, designer (hydraulic, structural), modeler (physical, computational fluid dynamics, structural), construction engineer, or project manager, as the end product of most if not all hydraulic structure projects is to construct or upgrade the structure.  Are you learning from (partial) failures of hydraulic structures? Are you learning from your failures (are you teaching/mentoring others about your failures)? Are you learning from other’s failures? As professionals, we are not out to assign blame or judge, but to learn and become better engineers and scientists.  Are there (partial, near) failures within your hydraulic structures community that can be shared with the greater community? The author presented some of USACE’s interesting findings about some of our spillways in our inventory, many of which have features that could lead to potential issues during operation and encourages others to follow suit. You can find additional details regarding USACE’s spillway review in the proceedings of the 2019 NZSOLD/ANCOLD conference in Auckland, New Zealand.  For those working on spillways for dams and high head structures, there are careers to be made in upgrading spillways from the lessons learned from the Oroville partial failure. New spillway details and RCC mixtures, etc. that are now considered state of the practice for design and construction.  As our infrastructure ages, and those that designed them are retiring or passing away, how do we understand why they were designed and how they were constructed. Why are spillway chutes from different eras designed and constructed in certain manners? One of the team members has 45+ year experience, and he was able to offer his insight into historic design and construction practice. o Why are concrete chute slabs typically designed and constructed at 10m x 10m? o Are there case studies that have never been published that would provide insight identifying potential failure in the spillway review? o Have anchors on spillway chute slabs always been a standard? o Some construction materials, while robust at the time, have deteriorated, failed, or fallen out of favor for design and construction  Are you developing your skill set to be a Hydraulic Structure engineer/scientist, not just a hydraulic engineer, or structural engineer, modeler, etc.? Understanding the bigger picture is key to reducing the potential for failure.  I have traveled the world for many projects and, in all instances, there has been a knowledge gap of hydraulic structure design from those in their 60s to 90s to those in their 20s to 30s, with few in between

to bridge that gap. I have learned from my mentors (who are generally in their 70s, 80s and a few in their 90s) throughout my career and continue to learn from them.

1.2. Are You Mentoring or Being Mentored in Hydraulic Structures? Case Study Guajataca Dam Spillway Repair

The case study of Guajataca Dam was not as widely publicized as the partial failure as Oroville Dam, however the lessons to be learned are no less important for the hydraulic structure community of practice. There were no fatalities as a result of the service spillway chute failure or outlet works blowback. Similar to Oroville Dam, we as the hydraulic structures community of practice are fortunate that the dam owner has allowed the USACE to publish the history of the dam and emergency spillway and outlet repairs for the hydraulic structures community of practice to read and learn from the contents. The case study below presents a brief synopsis of the Guajataca Dam spillway partial failure, and the mentoring of hydraulic structures engineers of the design of the subsequent emergency repairs to the spillway and outlet works. The author encourages the reader to consider whether you are truly being mentored by highly experienced design engineers and/or are you in a position to be mentoring future generations.

Guajataca Dam is an embankment dam located on the Rio Guajataca in northwest Puerto Rico. The main embankment is 37 meters high with a maximum reservoir capacity of 48 million cubic meters of water. The dam was designed and constructed in the 1920’s and is owned and operated by the Puerto Rico Electric Power Authority (PREPA). The main purpose of Guajataca Lake is to provide water supply for over 250,000 people in northwestern Puerto Rico. The project includes an embankment dam across the Rio Guajataca, a service spillway on the left abutment of the dam, and an outlet works conduit that extends through the right abutment/embankment interface where it splits into two separate conduits that discharge into an outlet structure and a water supply canal, as shown in Figure 7.

Figure 7. Aerial View of the Post-Hurricane Maria Site Conditions

The service spillway is an uncontrolled concrete chute with a length of 214 meters, a width of 31 meters, and a discharge capacity of 1,200 cubic meters per second at a reservoir elevation at top of dam crest. The service spillway is used to pass flows in excess of the outlet works capacity. Prior to Hurricane Maria, it is unknown how many times the auxiliary spillway has operated over its lifespan but records from 1999-2017 indicate the spillway operated 7 times with maximum discharges up to approximately 45 cubic meters per second. The center of circulation for Hurricane Maria crossed onto the southeast corner of the island on the morning of September 20, 2017 and exited later that afternoon on the northwest corner of the island. The storm caused catastrophic damage as it crossed the island, from both wind and flooding. Rainfall totals on the island varied widely from about 25 to more than 127 cm (10 inches to more than 50 inches) with recorded wind gusts of 130

to 150 miles per hour. Hurricane Maria generated approximately 63 cm (25 inches) of rain in the three days that the storm and outer bands impacted the Guajataca basin, with an additional 20 cm (8 inches) of local rainfall in the 21 days following the hurricane. Rainfall from this storm and subsequent wet season precipitation resulted in the spillway operating for approximately 23 consecutive days. The Guajataca Dam auxiliary concrete spillway chute was extensively damaged in September 2017 while passing excess flows resulting from the intense precipitation produced by Hurricane Maria, as shown in Figure 3. The peak reservoir level during Hurricane Maria is unknown due to the reservoir gauge malfunctioning during the event; however, based on debris on the embankment, it was estimated that the peak reservoir head over the spillway crest was about 2m with an estimated maximum discharge of 227 cubic meters per second (8020 cubic feet per second, which is about 43 percent of the spillway capacity). Although there is some uncertainty in exact rainfall totals, Hurricane Maria was estimated to be equivalent to approximately a 1,000 year 72 hour rainfall event based on NOAA Atlas 14. Damage likely initiated when flow within the spillway caused failure of the downstream end of the concrete spillway chute and stilling basin due to undermining from erosion. As the spillway continued to flow, progressive head cutting and undermining of the foundation at the end of the spillway chute caused sliding of the upstream chute slabs along the foundation and then overturning failure, which progressed in a similar manner over half the chute length, as shown in Figure 8. The spillway had previously experienced erosion at the downstream end of the chute in past flow events and the spillway was in poor condition with extensive cracking of the concrete chute. No known forensic assessments have been completed for the Guajataca Dam partial spillway failure, however, based on a USACE review of the available information (design drawings, design and construction memorandums, inspection reports, photographs), the likely design, construction, and operational issues that led to the damage were:  The entire spillway, including the stilling basin were founded on an active landslide, comprised of highly erodible material.  A relatively thin concrete chute slab (0.229 m thick) with a single layer of reinforcement, as shown in Figure 9.  A “stilling basin” with 2 m deep cutoff wall at the downstream end was provided. The stilling basin was more of a runout apron type and would likely operate without major erosion for only relatively low flows over the spillway.  Although concrete transverse cutoff walls and a drainage system under the chute slabs are shown in the drawings, the existence of these features could not be verified.  No joint details of the chute slabs have been located to date.  No anchors of the chute slabs were shown in the drawings.  Known voids located below the concrete chute slabs as noted in previous inspection reports.  Open joints between slabs  Erosion of the fill placed in the stilling basin from previous events  Frequent operation of the spillway in the past 5 to 7 years

Figure 8. Guajataca Dam Service Spillway Damage

Maximum spillway discharge around the time of failure was estimated to be approximately 128 cubic meters per second, compared to its intended capacity of 1,200 cubic meters per second. Repairs to the Guajataca Dam concrete chute auxiliary spillway are completed. Total costs to repair the spillway are estimated at approximately $50 million US dollars.

Figure 9. Guajataca Dam Service Spillway cross-section (1926) – note 0.229 m thick slab with single mate of reinforcement

Starting on October 9, concrete “Jersey” barriers and large sand bags were air dropped from helicopters to arrest the headcutting and to quickly construct the end sill for a temporary stilling basin. These bags are heavy duty nylon sacks that contain about 0.7 cubic meters of a sand-gravel mixture. Figure 10 is a photograph taken on October 19 of the progress of Jersey barrier and large sand bag placement. Placement of these materials was completed on October 21 which included 502 Jersey barriers and 1,338 large sand bags.

Figure 10. Photograph taken on October 19 of Jersey barriers and large sand bags.

The USACE assembled a team to prepare designs for the repair of the auxiliary spillway and reduce the potential for blow back at the outlet tower.

The key challenges to the design of the spillway repair were:

 The existing spillway is located on a known active landslide and the erosion at the toe removed portions of the buttress keeping it relatively stable. Re-establishing a buttress of the landslide was critical to the long-term success of the spillway structure and protecting the many residential structures located further up the landslide.  The construction timeline for the spillway was in flux, meaning that the although the goal was to complete the spillway repair before the next hurricane season, there were no guarantees this could be accomplished, as such the spillway could operate again with another large rainfall event, further degrading the spillway. Therefore, the selected design should allow for spillway flow during construction  Re-establishing the water supply conduit, located under the spillway, which was severed during the flood event was paramount.

For Guajataca Dam spillway, the spillway chute was to be replaced in-kind and repaired with a similar geometry, however the energy dissipation structure would prove the biggest challenge. The following alternatives were considered for the dissipation structure with discussions on the benefits and risks associated with each:

 Conventional hydraulic jump basin – the basin would perform for the full range events up to the dam crest, however it would require excavation deeper into the toe of the landslide to achieve required tailwater conditions and would not achieve the re-establishment of the buttress to the landslide with potentially further movement of the landslide.  Concrete buttress with flip bucket – a concrete buttress would be constructed approximately 7 m above the eroded surface to re-establish the landslide buttress and the flip bucket would direct the flood flows away from the repaired spillway structure. However the energy from the flood waters, for all events, would be dissipated by either forming a pre-formed plunge pool or naturally eroding a plunge pool. The plunge pool would be located deeper into the toe of the landslide to achieve required tailwater conditions and could activate the landslide in a different area.  Concrete buttress with hydraulic jump basin - a concrete buttress would be constructed approximately 7 m above the eroded surface to re-establish the landslide buttress and a hydraulic jump basin with high end sill would dissipate the energy for the floods slightly greater than those from Hurricane Maria, and would act like a flip bucket for extreme events and direct the flood flows away from the repaired spillway structure. This spillway configuration would be unconventional. Additionally, the energy from the extreme event flood waters would be dissipated by eroding a plunge pool. The plunge pool would be

located deeper into the toe of the landslide to achieve required tailwater conditions and could active the landslide in a different area.  Constructing a new spillway and abandoning the existing spillway – this was considered however the scope of works from FEMA was to replace in-kind.

Spreadsheet hydraulic calculations were performed to estimate the key hydraulic parameters for sizing the structure as well as using engineering judgment for the expected performance of the structure. Sketches were developed to demonstrate the extent of the structure.

The concrete buttress with hydraulic jump basin was ultimately selected based on risk informed decision making (i.e. it presented the least risk to the project of the alternatives proposed) and comparison of the expected hydraulic performance. Another key reason for selecting the mass concrete buttress was the scour from the flood event demonstrated that the most hydraulically efficient path to pass flood waters was to follow the channel created during the flood, resulting in minimal excavation, thus simplifying the design and construction.

A mass concrete buttress would constrain both time and budget. However, rock could be sourced from a quarry approximately 10 km away. The combination of large rock and cementicious grout would was suggested based on the successful operation of the grouted rock that was placed just prior to operation of the Oroville Dam emergency spillway a few months earlier. At the peak of the event, the hydraulic head from the Oroville Dam reservoir to the toe of the emergency spillway chute was approximately 16 m, resulting in velocities of around 15 m/s. The emergency spillway operated for approximately 36 hours and the grouted rock performed well. At Guajataca Dam, the hydraulic head from the reservoir, at approximately 2 m of head, to the energy dissipation basin would be approximately 25 m, similar to that at the Oroville Dam emergency spillway. As such, the mass of grouted rock (or cyclopean concrete as commonly referred), was utilized for the construction of the repaired spillway chute and energy dissipation basin and would be expected to perform well under similar hydraulic conditions as the Oroville Dam emergency spillway.

Time constraints on the emergency repairs to the Guajataca Dam spillway precluded the construction of a physical hydraulic model, nor was one required. A computational fluid dynamics (CFD) model was developed to confirm the performance of the selected design and make adjustments to alignments and wall heights based on the model results. The CFD model was first validated to the available data from the flood a month earlier, including debris lines at two locations (indicating approximately 2 m of head over the spillway crest), and photos of the spillway operating, with results presented in Figure 11. The CFD model was then modified for select reservoir heads (2 m, 4 m, 6 m) to simulate the hydraulics of the flow in the proposed dissipation basin. The results showed that the flood water would serpentine down the spillway chute due to the angle of approach to the spillway before entering the energy dissipation basin. The hydraulic jump basin would perform as expected from the hand calculations up to a reservoir head of approximately 3 m, or 2 times (~520 m3/s) that of the peak head during Hurricane Maria, as shown in Figure 12. Also as expected, the end sill would act as a flip bucket for the extreme events of 4 m of head and higher, also shown in Figure 12. A semi-quantitative risk assessment of the repaired project showed a reduction in the risk to the project as related to the spillway since the repaired spillway should safely pass flows the more frequent flood events.

Figure 11. CFD Model Validation Results to Flood Discharge from Hurricane Maria

Figure 12. CFD Model Results of Repaired Spillway Dissipation Basin for 250 m3/s and 1,100 m3/s

Opportunities in Hydraulic Structures to Learn from the Guajataca Spillway (partial) Failure

 Lessons learned from the Oroville Dam Emergency Spillway, pre-operation construction demonstrated that grouted boulders can withstand high velocities and shear stresses. By applying this knowledge to Guajataca, the team knew from this recent experience that with similar hydraulic head and flow rates, the Guajataca Dam spillway repair could utilize the grouted rock (cyclopean concrete) for the spillway chute and energy dissipation basin repair design and it would perform in a similar manner as the Oroville spillway did  Mentoring younger engineers in design of hydraulic structures (for spillways): It was demonstrated that spreadsheet hydraulic calculations were likely sufficient to design the repaired spillway, but that the CFD modelling could be utilized as a design tool (not the only method of design) to confirm the spreadsheet calculations and hydraulic performance. Too often, inexperienced engineers want to jump straight to CFD or physical hydraulic models without understanding the likely outcome, instead these models should be used as tools for design and confirmation of the design.

 Are you mentoring others? Everyone can mentor, we all have specialties/expertise in a subject. That is why you are all here at this conference because you are mentors of others (i.e. presenting) or you are looking to be mentored (attending the presentations, asking questions, networking)

1.3. Who Are the Hydraulic Structure Experts or Peers You Can Call at a Moment’s Notice? Case Study: Mosul Dam Bottom Outlet Dentate Commissioning and the Hydraulic “Issue” that Wasn’t

The case study of the successful design and commissioning of the Mosul Dam bottom outlet dentates has not been previously published, however the lessons from the author’s own “near” failure during the commissioning process are shared to demonstrate that we all need peers and hydraulic structure experts in our careers that can be called upon at a moment’s notice without fear of judgement or career impact. The hydraulic structures community of practice are fortunate that the USACE has allowed the author to publish the history of the dam and upgrade to the bottom outlet for the hydraulic structures community of practice to read and learn from the contents. The case study below presents a brief synopsis of the Mosul Dam bottom outlet erosion issue and the hydraulic “issues” related to the commissioning of the new dentated flip bucket in an active construction site and the lessons learned from obtaining prototype measurements. The author encourages the reader to consider which experts (national or international) or peers you can call upon anytime to resolve an issue without worry of judgement or career impacts.

Mosul Dam is located in Nineva, Iraq on the Tigris River, and is owned by the Government of Iraq and operated and maintained by the Ministry of Water Resources (MoWR). The dam is a 110 m high earth and rockfill embankment with primary purposes of water supply, hydropower, and flood control. The Mosul Dam Bottom Outlets were designed to divert the river during construction, to manage the reservoir when the lake level is sufficiently low that hydropower cannot be generated, or to lower the reservoir during emergencies. The bottom outlet, as shown in Figure 13, consists of a submerged intake tower which bifurcates the flow into two identical conduits, each with 12 m diameter reinforced concrete culvert sections, an access and emergency regulation tower, followed by 10 m diameter steel lined tunnel sections, and regulated by 7 m high by 5 m wide top seal tainter gates. Each bottom outlet has a maximum discharge capacity of 1,225 m3/s with 55 m of reservoir head. The energy dissipation structure for the bottom outlets consists of a flip bucket located downstream of the tainter gates and a partially concrete lined plunge pool.

Figure 13. Profile of Mosul Dam Bottom Outlets

The bottom outlet plunge pool has generally performed as designed (i.e. dissipating the energy from the bottom outlets), however, erosion of the plunge pool bottom and sides has created additional maintenance issues since initial use. The erosion of the plunge pool is a result of modifications to the design of the plunge pool invert during original construction in order to meet the aggressive schedule of impounding the reservoir. The primary

modification was to raise the proposed invert of the plunge pool by 15 m, as shown in Figure 14. The originally proposed invert was determined by physical hydraulic modelling many years prior. The MoWR engineering staff at Mosul Dam appear to have adequately managed the erosion of the East/left side of the plunge pool by repairs as required, and managing the erosion on the plunge pool floor by regulating the outflows from the bottom outlet, however erosion on the bottom and sides has continued as shown by bathymetric surveys over the past 40 years.

Figure 14. Modifications to Bottom Outlet Plunge Pool during Original Construction

The US Army Corps of Engineers (USACE) was selected to support and work in conjunction with the Ministry of Water Resources (MoWR) Engineers to assess the erosion of the Mosul Dam Bottom Outlet Plunge Pool, conduct an alternatives analysis, and develop conceptual designs that would reduce the extent of erosion on the East/left side of the bottom outlet plunge pool and the associated maintenance. Each of the alternatives were developed to a conceptual level of design using conventional hydraulic calculations and a couple of the designs were subsequently modeled using a Computational Fluid Dynamics (CFD) model to estimate the potential reduction in scour/erosion potential of the plunge pool and compared with the results from the physical model study results (mobile bed model) from the 1970s and 1980s. The author discussed with MoWR engineers the various alternatives and the selected alternative was the construction of two dentates on each of the bottom outlet flip bucket structures, as shown in Figure 15. The purpose of the dentated flip bucket is to modify the concentrated jet trajectory from each conduit such that the surface area of the plunging jet increases, thereby decreasing the erosive stream power of the jet at the plunge pool surface and decreasing (but not eliminating) the potential for erosion at the bottom surface.

Figure 15. Bottom Outlet Dentates Profile and Section View In order to validate the CFD model and reduce the hydraulic performance uncertainties in detailed design, in May 2017, prototype measurements were taken by the author of the jet of water from the top of the bottom outlet structure down to the top of the jet of water. The primary purpose was to ensure that the modified jet trajectory would not impact the existing tainter gate trunnions and concrete trunnion block. Prototype measurements were taken at the West (right) Bottom Outlet at four different gate openings, 25%, 50%, 75%, and 100% open, with the reservoir at EL 321.21 m, or approximately 50 m of head to the center of the tainter gate opening. Measurements were obtained using a 100 m flexible measuring tape that was tied to a 0.75 m long heavy duty linked chain, as shown in Figure 16. The chain and measuring tape were lowered down until the bottom chain link visually appeared to intersect the top of the aerated portion of the jet, and then further lowered until the chain reached the top of the solid core of the jet, which was obvious when the chain and measuring tape rapidly increased velocity to that of the jet of water, at approximately 30 m/s.

Figure 16. Measuring Tape and Heavy Duty Link Chain and Prototype Measuring of West (right) Bottom Outlet Jet from top of Bottom Outlet Structure

The CFD model was then simulated with the existing flip bucket and the model results were compared to the prototype measurements at the downstream side of the West Bottom Outlet in May 2017. The purpose of this simulation was to verify that the CFD model was simulating the jet from the bottom outlet. The results, as presented in Table 1 and in Figure 17, show that:

 The CFD model resulted in similar water surface elevations of the jet core (i.e. solid water) as the prototype measurements at the downstream side of the bottom outlet structure. The top of the solid jet core follows the standard trajectory equation from an orifice (red line in Figure 17).  The solid core of the jet would not impact the trunnions or trunnion block  The CFD model did not accurately model the highly aerated section at the top of the jet profile, as shown in Figure 18. However, a modified trajectory calculation was utilized to estimate the highly aerated area at the top (blue line in Figure 17).

Based on a comparison of the results of the CFD model to the prototype measurements, the model was considered verified and could be used for further refinement of the dentate design and estimating changes in erosion potential in the plunge pool.

Table 1: CFD Model Comparison to Prototype – West Bottom Outlet – No Dentates – Reservoir at EL 321.21 masl Gate Opening EL of Jet Solid EL of Jet Solid Difference in Estimated EL to Core at D/S of Core at D/S of Elevation, m Aerated Area at Bottom Outlet Bottom Outlet D/S of Bottom (Prototype), masl (Model), masl Outlet, masl

100% 273.4 273.4 0.0 275.2

Figure 17. CFD Model Results and Prototype Measurements of Existing Flip Bucket (Res EL 321.1 masl; 100% Gate Open), solid jet core trajectory profile (red line) and highly aerated trajectory profile (blue line)

Figure 18: West Bottom Outlet (Res EL 321.1 masl; 100% Gate Open) Discharge Approximately 1,100 m3/s on 15 May 2017 with Existing Flip Bucket (Note solid jet core and highly aerated trajectory profile at the top) The CFD hydraulic model was then modified to include the proposed dentate configuration, as shown in Figure 15:

 Two dentates that extend 6 m downstream at a 20 degree angle, or 2.2 m higher than the existing flip bucket lip.  Each dentate is 1 m wide, with 1 m spacing either side  Side walls that extend 7.2 m downstream

The CFD model was simulated for the maximum design reservoir level of EL 330 masl, both gates open in a balanced discharge, and for a range of discharges: 2,435 m3/s (both gates open 100%), 825 m3/s (both gates open 43%), and 250 m3/s (both gates open 14%). The CFD model results, as shown in Figure 19, indicate that:

 The CFD model appeared to better simulate the highly aerated area at the top of the jet trajectory (blue line in Figure 19) based on the modified trajectory equation from the verification modelling described in the previous section.  The jet trajectory should not impact the trunnions and trunnion block for the full range of gate openings for reservoir levels up to EL 330 masl.  The flow leaving the bottom outlet is much more aerated and has a longer trajectory, as expected.  The left training wall was raised from its initial level to its final configuration to contain flows that could land directly on the left plunge pool concrete wall and gravel covered platform.

Figure 19: East Bottom Outlet (Res EL 330 mas; 100% Gate Open) Discharge 1,217 m3/s (Note solid jet core – red line and highly aerated trajectory profile at the top – blue line) The construction of the East Bottom Outlet dentates, as shown in Figure 20 and Figure 21, were completed in January 2019. It was recommended to MoWR that the dentates be commissioned (i.e. open the Bottom Outlet gate from 0% to 100% open) in order to:

 Confirm that the dentates will operate as intended, both at the Bottom Outlet structure and in the plunge pool.  Measure the jet trajectory at the downstream end of the bottom outlet structure and confirm the verification of the jet trajectory calculations  Obtain photographs and videos of the flow patterns in the plunge pool for comparison to the flow patterns from the West Bottom Outlet without the dentates, and  Perform pre- and post- operation inspections to observe any structural changes, if any, from operation of the dentates.

Figure 20: Completed East Bottom Outlet Dentates; looking upstream

Figure 21: Completed East Bottom Outlet Dentates; looking downstream

Prior to commissioning the dentates on the East Bottom Outlet, prototype measurements were first obtained from the West Bottom outlet again so that there were measurements of with and without dentates at the same reservoir level. The author again obtained the same 100 m long flexible measuring tape from nearly two year earlier, however the heavy duty chain could not be located, so a heavy solid weight (a 0.2 m diameter steel ball with hook) was utilized.

The weight and measuring tape were lowered in a similar manner as nearly two year earlier and with a nearly identical reservoir level, similar measurements were expected. Except this round of measurements resulted in a higher elevation for the top of the jet core, nearly 2 m higher. The author asked many questions, including, were the gate openings truly as indicated, was the gate at 100% open or was it closer to 95% open, etc. Needless to say, the author was quite confused regarding the new measurements, particularly with the commissioning of the East side dentates less than 12 hrs away.

Figure 22: Hydraulic Calculation Results and Prototype Measurements of Dentated Flip Bucket (Res EL 319.2 masl; 100% Gate Open), solid jet core trajectory profile (red line) and highly aerated trajectory profile (blue line) The author performed many jet trajectory calculations based on the new measurements and it showed that it would impact the trunnions and concrete trunnion block! At this point, the author called the expert hydraulic structure engineers and mentors in the U.S (very early morning in the US and late at night in Iraq) that worked with him on the dentates and design to work through the problem and determine if there is a real issue. After a long night of discussions and calculations, the answer lied within the surface area of the two measuring devices. The chain link traversed through the aerated portion of the jet, then when sufficient links were within the jet core, the device would quickly accelerate and follow the jet core. For the solid steel ball, it had sufficient surface area that the aerated section would cause it to accelerate and follow the aerated portion of the jet and not reach the jet’s solid water core. We all agreed that the commissioning could go on as scheduled the next day and the hydraulics would be as expected. The commissioning of the East dentates went well with hydraulics matching the CFD model. A comparison of the with and without dentates cases are presented in Figure 23, which shows a wider and more aerated jet.

The author’s intent with sharing this case study is to demonstrate that “issues” (real or not) sometimes require you to have experts/peers that you can call at a moment’s notice (even the middle of the night), without judgment or fear of career impact. My family of expert hydraulic structure engineers and peers have been there for me my entire career and even in the middle of the night or early morning, as shown by this “issue” that wasn’t. They have also been there for me in real project emergencies. Who are your experts/peers you can call?

Figure 23: Discharge jet profile (looking West); West Bottom Outlet without dentates (top photo) and East Bottom Outlet with dentates (bottom photo); 24 January 2019; 100% gate open (1,100 m3/s) Opportunities in Hydraulic Structures to Learn from the “Issue” that Wasn’t

 Who can you call at a moment’s notice when you have a hydraulic structures issue or want to ask a question that might be awkward to ask of others? These experts/peers/mentors could be located in your organization, outside your organization, nationally, or internationally. Most international experts are natural teachers and enjoy working with younger colleagues, sharing their experiences, participating in solving interesting problems, and working through the night when required.  Things don’t always go as planned: You have performed calculations, double checked them with models and reviewed it. However, sometimes the conditions you encounter are not expected.  Don’t be afraid to share your “horror” stories, because others might learn from them (or in my case, know that even “experts” can get it wrong sometimes)

1.4. Conclusions

There have been several hydraulic structure (partial) failures over the past several years that have been identified, investigated and documented for the wider hydraulic structures community to learn from and take note of the causes of the failure. However, there are likely many more that go un-reported, that could benefit the hydraulic structures community. These un-reported failures could range from prototype failures during operation, model failures, or construction failures. The author would encourage those working on these types of structures to document and present them to wider hydraulic structures community as a mentoring opportunity and a lesson learned opportunity. We should always consider mentorship throughout (and after) our careers to grow current and future generations of hydraulic structure practitioners

Table 2. Opportunities for Learning and Mentoring out of Hydraulic Structure Failures

Issue Opportunity

Failure or partial failure of Learn the cause of the failure, apply it to your practice. For example: The hydraulic structure partial failure of the Glen Canyon spillway tunnel was an opportunity to improve design of hydraulics structures for dams. This partial failure improved the understanding of cavitation causes and practical, elegant methods to reduce and eliminate cavitation. For example, spillway aeration ramps and air slots. Be knowledgeable on the significant failures or near failures that influenced our designs today. Look at similar projects you have worked on or know about, do they have similar features that could lead to a failure?

Failure was a result of a Seek advice from other multi-disciplinary experts about what led to the different discipline failure (geology, geotechnical, structural, hydraulic). Failures and repairs require multi-disciplinary teams and approaches. Understand the big picture of the problem at hand

I have questions about… Seek out a mentor (or mentors) both within your discipline and other disciplines – I have Dr. Falvey, Mr. Todaro, Dr. Crookston, Dr. Tullis, Dr. Annandale, and others

I need to assemble a team for Build your skill set to be both Hydraulic Structure Engineers or Scientists a hydraulic structures and Dam Safety Engineers - take the time to learn from each other. Who problem were/are the significant hydraulic structure and dam designers at your organization, in your community of practice? Be able to rattle off their names. Are they still around? Bring them in to do brown bags, to talk through your work. Re-establish the connections between generations of hydraulic engineers and dam safety engineers.

Difficult to understand New technology making it easier (use of drones to take measurements or problem in real time photographs)

2. ACKNOWLEDGMENTS

The author would like to knowledge several individuals and groups:

 The ISHS 2020 committee members for recommending and allowing me to publish this keynote lecture to the broader international hydraulic structure community.  Puerto Rico Electric Power Authority (PREPA), and Jose Bermudez, the Interim Chief, Division of Irrigation, Dams and Reservoirs, San Juan, Puerto Rico for allowing the author to present the lessons learned from the events at Guajataca Dam during and post-Hurricane Maria.

3. REFERENCES

France, John W., et al. (2018). Independent Forensic Team Report Oroville Dam Spillway Incident.

National Weather Service, Hurricane Irma. 2017. September 5-7, 2017, San Juan, Puerto Rico Weather Forecast Office, https://www.weather.gov/sju/irma.

Storm News. 2017. Lake Guajataca Dam Failure Aerial View Footage. YouTube video, 1:09, September 24, 2017, https://www.youtube.com/channel/UCxIgEf-tm9j-2fu-_ixHEUg.

U.S. Army Corps of Engineers (USACE). 2019. East Bottom Outlets Dentates Commissioning, RMC-TR-2019.

U.S. Army Corps of Engineers (USACE). 2018. Guajataca Dam Semi-Quantitative Risk Assessment, Jacksonville District.