Modeling, mapping and consequences13 December 2013
Profiling dam safety work recently undertaken by AMEC for the US Army Corps of Engineers and Pacific Gas and Electric .
For more than 50 years, AMEC has engineered and built dams and levees for water supply, mining, recreation, hydropower, and flood control needs globally. It has worked with dams and retaining facilities ranging from small earthen dikes to award-winning, roller-compacted concrete dams hundreds of feet high.
AMEC offers a broad range of integrated services, from project planning and siting studies through full permitting, geologic and geotechnical investigations, hydraulic studies, design, construction management and testing, and instrumentation. Following construction, the team conducts dam safety evaluations, monitors instrumentation, and prepares emergency action plans.
Two of the company's recent projects for the US Army Corps of Engineers (USACE) and Pacific Gas and Electric (PG&E) demonstrate the breadth of its capabilities.
Modeling, Mapping and Consequences program
Since 2008 AMEC has assisted USACE with the national Modeling, Mapping and Consequences (MMC) program to evaluate the risks associated with dams it owns and maintains. The USACE Office of Homeland Security is tasked to develop dam break flood inundation mapping on approximately 600 USACE dams throughout the US. This project has been in support of the National Critical Infrastructure Protection and Resilience (CIPR) team led by the USACE Vicksburg District. The goals of this multi-year effort are to consistently measure the risk associated with the nation's dams and levees, as well as assist in mitigating this risk at the local level. The primary deliverables, which include consequence estimation information such as loss-of-life, economic impact, and population at risk, are rolled up into an updated digital emergency action plan (EAP).
AMEC assisted USACE in defining the baseline mapping, modeling and consequence determination procedures to evaluate failure and inundation risk. This task included the standardization of paper and digital maps, database format, inundation modeling procedures for probable maximum flood (PMF) and sunny-day conditions, Flood Impact Analysis (HEC-FIA), and included revisions to RD-13 for EAP development considering the modern day mapping formats.
The procedures developed in this initial phase of work was combined with existing USACE documentation to form the initial Standard Operating Procedures Document (SOP). The revised SOP outlines modeling, mapping, and consequence estimation procedures and also revises aspects of USACE's Emergency Action Plan (EAP) guidance, titled RD-13, to accommodate revised MMC mapping and figures. Changes not only refer to physical mapping components but also to the planning components tied to the map results.
USACE has provided exceptional leadership and value to the program by taking a holistic approach to estimating the risks associated with Corps owned dams and levees. For example, rather than modeling all low head navigation dams, USACE and AMEC developed a screening process which eliminates the need to develop hydraulic models for some low-head dams when specific criteria are met. Another valuable byproduct of the MMC program is a revised Probable Maximum Precipitation (PMP) tool. The new GIS-based tool still relies on the HMR methodology but drastically reduces the amount of effort to find the PMP by automating the process in a user interface.
AMEC's involvement has resulted in program activity and interaction with nearly all of the USACE districts and divisions. The products of these modeling and mapping efforts include data and documentation deliverables back to the USACE Office of Homeland Security as well as digital and paper map books that go to the dam operator for emergency planning purposes. The standardisation with which these dams are modeled account for most scenarios and allow the risk associated with dam failure to be measured consistently across the portfolio of assets.
Crane Valley Dam Seismic Retrofit for Pacific Gas and Electric Company
Crane Valley Dam is a 44m high earth and rockfill embankment built in the early 1900s. The dam and reservoir are located on the North Fork of Willow Creek in Madera County, California, in the Sierra Nevada foothills and are owned and operated by Pacific Gas and Electric Company (PG&E) of San Francisco, California.
In 2004, to accommodate changes in the state-of-practice in seismic stability analyses and an increase in the estimated local seismic hazard over the nearly 25-year interval since the last evaluation, PG&E initiated a re-evaluation of the seismic stability of the dam.
AMEC provided design and engineering analyses, embankment retrofit design and technical support during construction.
AMEC's design and engineering analyses of the dam included the development of design earthquake ground motions, implementation of field investigation and laboratory testing programs and selection of static and dynamic material properties for use in the stability analyses.
To evaluate potential liquefaction concerns, AMEC utilized modern, state-of-practice procedures and updated earthquake ground motions for the dam site seismic stability evaluation. Using dynamic finite-element methods to analyze representative cross sections of the dam, AMEC estimated earthquake-induced accelerations and stresses within the dam embankment. The cyclic resistance of the embankment material was estimated using standard penetration test results from recent and previous field investigations.
The results of seismic stability analyses performed in 2005 and 2006 showed that the dam's hydraulic fill embankments would liquefy and experience large deformations during and after the earthquake shaking postulated for the site. The magnitude of the deformations was found to be excessive and would likely cause an uncontrolled release of the reservoir water at the current normal maximum operating level. Accordingly, it was concluded that retrofit measures were necessary to improve the seismic stability and performance of the dam.
AMEC was the design engineer-of-record for the seismic retrofit of the dam embankment, which included new rockfill buttresses on the upstream and downstream slopes of the dam, internal drainage improvements, raising the dam crest by approximately 1.5m, and installing new seepage monitoring instrumentation.
AMEC's stability evaluation and retrofit designs were reviewed and accepted by the California Department of Water Resources, Division of Safety of Dams (DSOD), and the Federal Energy Regulatory Commission (FERC). Construction of the retrofit began in October 2010 and was completed in November 2012.
Crane Valley Dam is located in an environmentally sensitive area supporting fish and other aquatic and terrestrial species as well as wetland and riparian habitats. To reduce potential impacts to these habitats, retrofit measures and approaches were developed to limit the project footprint.
Due to the heavily forested, steep, rocky and remote project environment, flexible design approaches were necessary to accommodate the limited availability of subsurface information and the potential for discrepancies between design assumptions and the foundation conditions encountered during construction.
Bass Lake is a popular vacation destination, and the local economy is heavily dependent on use of the lake for recreational activities. The lake also serves as a municipal water supply for nearby communities. Reservoir operations are controlled by these factors, as well as power generation objectives, environmental requirements and agreements with downstream water users. Therefore, it was necessary to adhere to a challenging lake level schedule to ensure safe water levels for continued operations.
To limit interaction with local residents and recreational visitors as well as reduce transportation costs and environmental impacts, retrofit approaches were designed to utilise on-site resources and limit the amount of waste material requiring off-site disposal.
Due to concerns for public safety and the need to protect critical hydroelectric and flood management infrastructure from seismic hazards, PG&E established an aggressive year-round schedule for design and construction.
AMEC worked closely with DSOD, FERC, and PG&E to complete the project on time despite the unique physical and environmental challenges of the project.
Following the seismic retrofit project, Crane Valley Dam now meets current state and federal dam safety standards for seismic performance.
"AMEC's long-term relationship with the client, and its staff's history of successful interaction with the regulatory agencies, contributed to the timely completion of this seismic remediation project," said Faiz Makdisi, AMEC's project manager for the project.
"Dam Safety is maturing into a risk-based exercise where the focus is shifting from the engineering model to reducing the risk associated with water resource assets. This promises to be really exciting work that blends AMEC's Information Management and Water expertise to estimate, communicate and mitigate consequences below our nation's largest dams," remarked Jeff Albee, AMEC's project manager for the MMC project.
Future concerns - for which there appears to be some planning at the federal level - are related to the frequency and duration of storm events associated with climate change. Instead of looking at individual dams, owners and operators need to begin planning for extreme rain events at the watershed level and operating the dams as a system to mitigate the risk of flooding.
The authors would like to acknowledge the contributions of David Ritzman, Joseph Sun and Charles Ahlgren of Pacific Gas & Electric Company; and Joseph Windham and Ron Goldman with the Modeling, Mapping and Consequences Program from USACE's Vicksburg District for their permission to publish this article.