Rock Scour Assessment for Dams, Spillways and other Water Conveyance Structures
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Erodibility of dam foundations, spillways and other water conveyance structures is a critical issue for the safe operation of dams world-wide. The February 2017 events at the Oroville spillways that resulted in the evacuation of nearly 200,000 downstream residents were a recent reminder of the need for meaningful quantification of scour. The course will offer examples of rock scour events impacting the safety of dams as well as provide an overview of scour mechanisms including discussion of key geologic controls and hydraulic drivers of the scouring process. Methods that can be reliably used to predict and assess such scour will be demonstrated, which include the Erodibility Index Method, a semi-empirical approach, and a more physics-based approach utilizing Block Theory. Both methods analyze the engineering properties of the rock mass and quantify the erosive capacity of flowing water based on principles of hydraulic engineering. These methods will be explained and their use illustrated by example. Finally, incorporation of high-resolution remote sensing tools in scour applications will be explored.
Five takeaways/learning objectives attendees will receive from this course:
- Scour of rock is a dam safety concern that requires meaningful quantification
- It is possible to predict the scour potential of rock using data normally available for dam design
- High-resolution remote sensing and monitoring can facilitate improved scour assessment
- Course attendees should develop an understanding of processes and key drivers/controls for scour of rocks
- Course attendees should be able to execute rock scour analysis following examples presented
George William Annandale
President, George William Annandale, Inc.
Dr. Annandale has more than 40 years of experience specializing in water resources engineering. He is known for the development of the Erodibility Index Method that has globally been accepted by the engineering profession for design and safety assessment of water resource infrastructure, particularly dams. The method is contained in numerous dam safety guidelines, including those of FERC, USACE, USBR, Australian Dam Safety Guidelines, French Dam Safety Guidelines, and others. The book "Scour Technology" he authored, published by McGraw-Hill in 2006, has become a standard reference in this field of specialization. Dr. Annandale consults internationally, has worked on projects in more than 25 countries and was named by International Water Power and Dam Construction as one of 20 engineers who globally made a significant contribution to dam engineering.
Michael F. George
Senior Geological Engineer/BGC Engineering, Inc.
Dr. George is a geological engineer specializing in water resources and dam engineering with focus on foundation / spillway erodibility, scour remediation, rock mechanics, hydraulics, high-resolution remote sensing monitoring, and reliability methods. He has worked as a consulting engineer and researcher in the United States and abroad and has developed methodologies for evaluation of rock mass erodibility, delivered training to FERC/USACE on scour, and authored over 25 papers on the subject including sections of the recently updated FERC Engineering Guidelines for Arch Dams (2018). Dr. George is also co-chair to the newly formed International Working Group on Overflowing & Overtopping Erosion (IWGOOE) hosted through ICOLD.
- Examples of projects where scour of rock occurred
- Kariba, Boondooma, Paradise, Bartlett, …
- Historic – empirical equations like Mason, etc.
- Single equations / developed in laboratory / not cause and effect / regression equations
- Modern approach
- Cause and effect / What is the resistance of rock to scour? / What is the erosive capacity of flowing water?
- Rock scour mechanisms
- If erosive capacity of flowing water > resistance offered by rock >>> scour
- Three methods available / Erodibility Index Method (EIM) / Block Theory / Comprehensive Scour Model / we are only presenting EIM and Block Theory
- Scour Prediction Concept
- Threshold condition
- Conceptual application of threshold conditions to assess scour potential / applied and threshold stream power / What is stream power / why use stream power
- Quantification of scour resistance by rock
- Erodibility Index
- Conversion to threshold stream power
- Spatial distribution of scour threshold under surface
- Quantification of erosive capacity of flowing water
- Plunging jets / jet breakup / energy loss / concepts
- Dissipation in plunge pool / concepts
- Video of Ricobayo physical model study / benefit of jet breakup /
- Quantification of plunging jet stream power and dissipation in plunge pool
- Jet breakup / nappe jets / flip bucket jets / Castillo and its interpretation / stream power at surface of plunge pool / change of stream power below surface / dynamic and fluctuating pressures
- Other flow conditions / circular jets / hydraulic jump / knickpoints / headcuts / no detail / concepts and equations
- Calculating stream power along boundaries / using shear stress / using flow velocity
- Use of CFD Modeling
- Assessment of scour potential using threshold conditions
- Compare applied and threshold stream power
- Maximum scour depth
Block Theory
- Geologic controls on rock erodibility
- Block theory basics (removability, kinematics, stability)
- Hydraulic loading for block stability
- Block erodibility threshold
- Directional resistance, block removal mechanics
- Laboratory tests, field blocks
- Assessment of scour potential using Block Theory method
- Rock characterization
- LiDAR/Photogrammetric methods
- Discontinuity orientations, roughness, block detection, geometry
- Probability distributions
- Flow characterization
- Use of video/LSPIV for remote measurement of flow velocity/turbulence
- Change detection – repeat monitoring to benchmark performance, develop erosion rates in rock
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