Dams and Extreme Events Reducing Risk of Aging Infrastructure under Extreme Loading Conditions

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1 Dams and Extreme Events Reducing Risk of Aging Infrastructure under Extreme Loading Conditions 34th Annual USSD Conference San Francisco, California, April 7-11, 2014 Hosted by San Francisco Public Utilities Commission

2 On the Cover Aerial view of the Calaveras Dam Replacement Project taken on January 27, The San Francisco Public Utilities Commission is building a new earth and rock fill dam immediately downstream of the existing dam. The replacement Calaveras Dam will have a structural height of 220 feet. Upon completion, the Calaveras Reservoir will be restored to its historical storage capacity of 96,850 acre-feet or 31 billion gallons of water. The project is the largest project of the Water System Improvement Program to repair, replace and seismically upgrade key components of the Hetch Hetchy Regional Water System, providing water to 2.6 million customers. Vision U.S. Society on Dams To be the nation's leading organization of professionals dedicated to advancing the role of dams for the benefit of society. Mission USSD is dedicated to: Advancing the knowledge of dam engineering, construction, planning, operation, performance, rehabilitation, decommissioning, maintenance, security and safety; Fostering dam technology for socially, environmentally and financially sustainable water resources systems; Providing public awareness of the role of dams in the management of the nation's water resources; Enhancing practices to meet current and future challenges on dams; and Representing the United States as an active member of the International Commission on Large Dams (ICOLD). The information contained in this publication regarding commercial projects or firms may not be used for advertising or promotional purposes and may not be construed as an endorsement of any product or from by the United States Society on Dams. USSD accepts no responsibility for the statements made or the opinions expressed in this publication. Copyright 2014 U.S. Society on Dams Printed in the United States of America Library of Congress Control Number: ISBN U.S. Society on Dams 1616 Seventeenth Street, #483 Denver, CO Telephone: Fax: stephens@ussdams.org Internet:

3 ANDERSON DAM SEISMIC RETROFIT PROJECT: UTILIZING PROJECT RISK ANALYSES TO SUPPORT MANAGEMENT DECISIONS Emmanuel Ayree, P.E 1 Brian Hubel, P.E., G.E. 2 Chris Mueller, P.E., Ph.D. 3 ABSTRACT Anderson Dam and Reservoir is a major water supply facility located about 18 miles southeast of San Jose, California. It is owned and operated by the Santa Clara Valley Water District (District). The dam was built in 1950, as a zoned rockfill embankment with a maximum height of approximately 240 feet at its maximum section. It is regulated by the California Division of Safety of Dams (DOSD) and the Federal Energy Regulatory Commission (FERC). Several dam safety deficiencies associated with seismic liquefaction, fault traces, PMF capacity, and emergency drawdown capabilities have been identified. As a result of these deficiencies, the District initiated the Anderson Dam Seismic Retrofit Project (ADRSP) to fix the dam, which is comprised of planning, design and construction phases. Components of the dam involved in the deficiencies include the embankment (upstream and downstream), outlet works, spillway and dam crest. During the planning phase of the ADSRP, remedial measures representing conceptual level repairs and fixes for each component were identified and developed for analysis and evaluation. These were combined in various permutations to for the project alternatives which were analyzed and evaluated to select the recommended alternative to repair the deficiencies within an aggressive schedule required by regulators. This paper highlights the importance risk modeling played in differentiating between alternatives that had similar component designs, similar costs and similar schedules. Deaggregation of the total project risk, a project risk register, and periodic risk elicitation workshops are tools management utilized to better understand the most important individual risks, which facilitated better scoping and scheduling of planning and design work to manage overall project risk. BACKGROUND Anderson Dam is located in Santa Clara County near the communities of Morgan Hill and San Jose. The dam was constructed in It has a clayey core and rockfill shells on both the upstream and downstream sides of the dam and is approximately 240 feet tall, has side slopes of approximately 2.5:1 (H:V) on both the upstream and downstream sides of the dam, and a crest width of about 40 feet. 1 Engineering Unit Manager, Santa Clara Valley Water District, 5750 Almaden Expressway, San Jose, CA, 95118, (408) , Eayree@valleywater.org 2 Senior Engineer, Black and Veatch, 353 Sacramento St, San Francisco, CA 94111, , hubelb@bv.com 3 Associate Vice President, Black and Veatch, 353 Sacramento St, San Francisco, CA 94111, , Muellercg@bv.com Anderson Dam Seismic Retrofit Project 1267

4 The dam has an ungated ogee spillway on the right abutment that currently is designed to pass 62,000 cfs of flow. The outlet works for the dam were constructed in a cut and cover trench in the same approximate alignment as the previous creek bed, and consists of a 49-inch diameter reinforced concrete pipe. The dam was modified in to raise the crest of the dam and modify the spillway to pass the PMF (estimated at about 62,000 cfs) as determined using HMR 36 standards. A small electric generation facility was added about ¼ downstream of the dam in The intake structure was also modified in which includes intake ports at elevations 488 feet, 528 feet, and 563 feet. Figure 1 illustrates the main features of the existing Anderson Dam. The reservoir impounded by the dam has approximately 91, 000 ac-ft of storage and is the largest water supply reservoir in the Santa Clara Valley Water District (SCVWD) inventory. In addition to water supply the reservoir also provides significant recreation benefit to the local community. Figure 1. Anderson Dam Major Features Anderson Dam is owned and operated by the SCVWD and falls under the regulatory jurisdiction of both the Federal Energy Regulatory Commission (FERC) and the California Division of Safety of Dams (DSOD). In 2011, AMEC Geomatrix under contract to the SCVWD completed a Seismic Stability Evaluation of Anderson Dam. The findings of this evaluation indicated that portions of finer rock fill and alluvium left in place beneath the embankment shells during construction could experience seismically induced liquefaction and slope instability. The evaluation also identified a fault rupture hazard which indicated that the Coyote Creek- Range Front Fault Zone with traces that cross the dam cannot be proved inactive and is 1268 Dams and Extreme Events

5 therefore considered Conditionally Active using the DSOD fault activity classifications. The dam is underlayed by rock generally identified as Franciscan Complex and the Santa Clara Formation. Figure 2 shows the complex geology of the Anderson Dam Project. Figure 2. Anderson Dam Site Foundation Geology (from AMEC 2010) As a result of the finding of the Seismic stability, evaluation, the Anderson Dam seismic Retrofit was initiated by the SCVWD. A reservoir restriction of approximately 45 feet below the crest of the dam was voluntarily established by the SCVWD in lieu of a more stringent restriction being in forced by DSOD. In 2012 the SCVWD initiated the Anderson Dam Seismic Retrofit Project. Separate consultant teams have been retained by the SCVWD to provide project management, planning and environmental permitting, and design services. Currently the project is in the design phase and construction is estimated to begin in 2016 and complete in 2018 per the agreement with DSOD. PROJECT PLANNING The planning phase of the project consisted of the problem definition which included the identification of the existing conditions, additional and known deficiencies and project requirements, alternatives analysis and evaluation, and the selection of the recommended alternative. An updated PMF evaluation based current HMR 58/59 standards (2013a) showed that the current spillway was inadequate to meet the estimated peak PMF flows and is likely to overtop the dam if such an event occurs. The outlet works was determined to have inadequate emergency drawdown capacity per the current DSOD requirements (HDR 2013a). Anderson Dam Seismic Retrofit Project 1269

6 Alternative analysis and evaluation utilized various management tools including qualitative scoring, conventional cost estimation, and schedule estimation and risk modeling. The objectives of the alternatives analysis and evaluation were to select the best alternative that to remediate the following major dam safety technical deficiencies of: 1. Upstream and Downstream embankment instability due to seismically induced liquefaction 2. Outlet works damage and inoperability due to surface fault rupture 3. Inadequate emergency drawdown capacity 4. Inability to safely pass the PMF Thirty nine (39) pre conceptual and conceptual alternatives were developed to address the deficiencies. Qualitative Scoring. The process of analyzing and evaluating the (39), alternatives developed started with the initial screening of the alternatives on the basis of the project requirements which consisted of SCVWD identified project objectives, input provided by the project consultants, operational requirements, FERC requirements and DSOD requirements. Following the initial screening of the alternatives, the remaining alternatives were scored using an evaluation criteria and methodology developed by the SCVWD (Table 1). A total of seven criteria were selected and weighting factors assigned based on the important elements that could impact SCVWD ability to repair the dam, restore original storage functions as well as the recreational facility provision for the community. Criteria weights were assigned by the SCVWD staff, project management and the planning team using their professional and engineering experience, knowledge and judgment Dams and Extreme Events

7 Table 1. Conceptual Alternative Criteria and Weighting Factors Scoring values of 1-3 were then assigned to each evaluation criteria in order for the alternatives to be scored and ranked. The highest ranked six alternatives were selected using the qualitative scoring and ranking described above. The top six alternatives were all quite similar in design concepts. Each of the concepts included: Excavation of portions of the liquefiable soils beneath the upstream and downstream shells and replacement with rock fill shell and buttress materials Various alignments of a new outletworks constructed as an oversized steel pipe in a vaulted tunnel, such that fault offset would cause some pipe buckling, however the pipe would remain functional after fault rupture. Construction of a new high level outlet that discharges to the spillway to provide increased emergency drawdown capacity Raising the dam crest and increasing the spillway capacity to safety pass the PMF Construction of the embankment measures and upstream improvements in the dry The reservoir can only remain dewatered between April and October Conventional deterministic cost and schedule estimation. Baseline costs and construction schedule were estimated for each of the six top alternatives using conventional deterministic cost estimating (based on the Association for the Advancement of Costs Engineering (AACE) Class 4/ Class 3 Estimate) and scheduling approaches. Using conventional deterministic methods, values were estimated for each criteria for each alternative, for example, from the baseline cost estimate, cost were estimated for each alternative under the capital construction cost criteria, contingency cost estimates for Anderson Dam Seismic Retrofit Project 1271

8 each alternative for the construction risks and impacts criteria, the number of months that it will take to complete the construction of the project estimated for each alternative and used for the project schedule criteria, and so on. Table 2 shows the deterministic evaluation of the criteria for the 6 top alternatives. Table 2. Deterministic Evaluation of Top Alternatives (from HDR, 2013b) Because of the similar conceptual designs, differentiation between the alternatives using traditional deterministic estimates of cost and schedule was not immediately clear. Illustrating the difficulty of distinguishing alternatives, using a deterministic cost estimate, the top alternatives had a range of difference in the estimated capital construction costs of $146 million to $166 million, which is well within the range of uncertainty for a planning level cost estimate. Risk assessment performed during the planning phase of the Anderson Dam Project was initiated to differentiate alternatives and support the selection of a project alternative to carry into design. Planning Study Risk Assessment. Risk analysis is commonly performed for major engineering projects to both better predict project costs and schedule, and to assist project management in identifying and managing the most project risks throughout the design and construction process (USACE 2007). The planning consultant for the project was directed to perform a risk assessment to determine probabilistic estimates of total project costs and project completion dates. The risk assessment modeling process included; a workshop to identify potential project risks, an elicitation session to assign probabilities and consequences to the individual risks, and estimation of the project costs and schedule 1272 Dams and Extreme Events

9 based on a project construction schedule model, where risk and impacts could be realized at the appropriate stage of the project Risk Workshop. During the planning phase of the project a workshop was held with members of the SCVWD, planning team, and project management team, over 80 risks were identified for the project cost and schedule. Risks included a variety of topics including poor weather during construction, unanticipated geology during construction, real estate acquisition delays, legal challenges to the project EIS/EIR, FERC or DSOD approval delays, and insufficient borrow materials. The risks identified were grouped, and probabilities and consequences were assigned during the planning risk elicitation process. Risk Elicitations. For each risk estimates of the probability that the risk would be realized, and cost and schedule consequence distribution estimates were made by the planning team. Table 3 show the elicitations estimates for one project risk, as prepared by the project planners. Table 3. Estimated Risk Event Probabilities and Consequence Estimates (from HDR 2013b). Risk Modeling. Risk modeling in the planning study was evaluated with a monte-carlo simulation (HDR 2013b), where the various risks were queried for realization at the appropriate stage of the project to see if: 1) would the risk occur, and 2) select a consequence based on the estimated distribution to apply to the cost and the schedule model to determine the overall project schedule and cost. Schedule and cost models included dependent event sequencing, constrained construction windows, and other factors. Results. Using the risk analysis results to evaluate the top alternatives, it was clear that there were distinct differences in project cost and completion dates between the different Anderson Dam Seismic Retrofit Project 1273

10 projects alternatives. Although developed using the same schedule constraints, Alternative 15 (Figure 3) had the highest likelihood of completion on-time, and 90 percent chance of 1-year delay or less. One of the key aspects of the monte-carlo modeling indicated that even seemingly small risks could have a potential large schedule impact due to the seasonal construction window constraints. The cost exceedance probability curves for the various alternatives are shown on Figure 4. Figure 3. Alternative 15 (HDR 2013b) 1274 Dams and Extreme Events

11 Figure 4. Probabilistic cost estimate results (HDR 2013b) RISK ASSEMENT SUPPORTS MANAGEMENT As with any major project such as the Anderson Dam Seismic Retrofit Project, numerous people and agencies have interest related to the project. The project manager must understand and mange the project technical needs, budget, schedule, regulatory commitments, and community concerns and prioritize resources to address issues over the course of the project. Due to limited resources, and seemingly unlimited supply of challenges, risk management proves a useful tool to understand the importance of specific challenges to the project may encounter. Risk analysis helps clarify and understand the various risks importance and focus resources to reduce the project risks. Using tools such as risk management plans, a risk register, and risk matrix, project management is able to identify the top project risks and take appropriate management actions. For the examples presented in this paper, cost and schedule risk scoring is presented to illustrate the usefulness risk management tools to the ADSRP. Risk tools can be applied to other specific risks such as safety, environmental impacts, or other consequences of importance. Using the risk workshops and risk register, risk modeling, adverse event occurrence probabilities and consequences scales are determined in accordance with the ADSRP risk management plan. Event probability scales are determined using the criteria shown in Table 4. Table 5 shows how the cost consequences are scaled to determine an overall risk score for each potential risk. Anderson Dam Seismic Retrofit Project 1275

12 Table 4, Event Probability (P) Scale Probability of Occurrence 5 - Frequent Greater than 60% 4 - Likely 40% to 60% 3 - Possible 20% to 40% 2 - Unlikely 5% to 20% 1 Rare Less than 5% Table 5. Severity of Impact to Cost Scale Range 5 Catastrophic Greater than 3% 4 Major 1%-3% 3 Moderate 0.6% to 1% 2 Minor 0.3% to 0.6% 1 Insignificant Less than 0.3% Using the probability and consequence scales determined for each event, risk scores are determined using the Risk Scoring Matrix shown in Table 6. Risk is the product of event probability and the estimated consequences. For uncertain risks, the risk markers are larger, spanning a range of risk scores. The risks that plot to the upper right are high, and risks that plot in the lower left are the lowest risk. Once all the risks scores are determined for the consequences of interest (cost, schedule, environmental impacts, facility closure times, etc), project management can prioritize and manage the largest risks, to reduce the potential for adverse project outcomes. Estimated Probability Table 6. Risk Scoring Matrix Risk Scoring Matrix 5 II III IV V V 4 II II III IV V 3 I II II III IV 2 I II II III 1 I I I II II Low Estimated Consequences High Strategies for Mitigating Risks. Periodic risk reporting by project management facilitates the identification and deliberate management of project risks. Strategies of transfer, avoidance, mitigation, or acceptance are considered for each of the top risks, and in general, several actions are pursued to implement one or more of the management strategies for each top risk. Top Project Risks. For the ADSRP, risk that have been identified as the top risks to project costs are identified as 1) delays in cofferdam construction, 2) delay in reservoir drawdown, 3) unanticipated foundation conditions during construction, 4) Rock fill of required strength cannot be achieved from borrow, and 5) reservoir rim landslides occur during dewatering Dams and Extreme Events

13 The top schedule risks were identified as delays in 1)property acquisition, 2)delays due to lack of or inability to produce rock fill of required strength, 3)delays in cofferdam construction, 4) delays in environmental permitting and clearance, and 5) delays in dewatering the reservoir. Some of the specific management decisions made for the ADSRP include. Risk management activities have included: 1) supporting conservatism of various design assumptions, 2)researching the appropriate environmental and legal strategies for approaching permitting of the project, 3) informing dewatering strategy and 4) identifying pro-active engineering mitigation for possible physical risks, and/or 5) early engagement with the community responsibly address stakeholder concerns. Delay in Cofferdam Construction. During the alternatives analysis for the project several upstream cofferdam designs were considered. The cofferdam alternatives were known as the large cofferdam (about 60 ft tall) and the small cofferdam (about 10ft ). The large cofferdam, and subsequent excavation into the upstream dam toe was judged to have a significantly higher chance of causing project delays, which also have significant cost impacts to the project. The small cofferdam would be constructed faster, have less risk of catastrophic failure due to seepage or instability, and would have less chance of exceeding the planned construction schedule, and was selected in the recommended project alternative. Additional research into large cofferdam benefits, such as wintering through (or just extending the construction season a few months or weeks longer) and additional water detention capacity to facility dewatering discharge treatment requirements are underway to optimize cofferdam contracting and design strategies. Cofferdams will be sized considering the probability of overtopping storms to occur, the potential damage caused by overtopping, the ability to intervene in an overtopping event, and other factors. These evaluation criteria are consistent with other cofferdam risk analysis (Fitzgerald et. al, 2012, Marengo et all, 20013). The risk of cofferdam construction delay has potential to impact both costs and schedule significantly, in large part because the risk is not realized until the delay occurs when the construction has already commenced. Delay in Reservoir Drawdown. Because of the importance of on-schedule reservoir dewatering the SCVWD has undertaken evaluation of the reservoir drawdown in the planning phase, to allow for sufficient time to begin the dewatering process. Reservoir lowering is likely to begin more than 1-year before upstream construction to reduce the risk of not being dewatered in time for construction. Figure 5 shows one of the dewatering scenarios evaluated for the project. Because the contractor has to be ready to work as soon as the water is lowered, this risk has a significant cost and schedule impact. By planning ahead the probability of realizing this risk is reduced. Anderson Dam Seismic Retrofit Project 1277

14 70,000 FIGURE 2 - Anderson Reservoir Drawdown Anderson Dam Seismic Retrofit Project WET YEAR INFLOW, 20% EXCEEDENCE PROBABILITY ,000 50,000 Outlet capaciity (cfs) STORAGE (AF) 40,000 30,000 20,000 Releases (cfs) Reservoir Storage (AF) RELEASE (CFS) 10, Dead pool 0 MAY '16 JUN JUL AUG SEP OCT NOV Figure 5. Reservoir Dewatering Scenario Used to Assist in Planning Dewatering Schedule Unanticipated Foundation Conditions. Unanticipated ground conditions can occur for any project especially for geologically complex site such as the Anderson Dam Seismic Retrofit Project. Because the foundation conditions would not be discovered until construction, there are significant potential impacts to cost and schedule due to design changes, and potentially long regulator review time. If delays are long enough, the construction window could be missed, greatly increasing the delays. The project management team, in addition to understanding and encouraging thorough subsurface investigation is encouraging conservative design parameters to reduce the chance of required design changes and the potential review and design explanation times that might be required by regulators. By making conservative assumptions the probability that designs will need revision is reduced, reducing the potential consequences of the risk. Additionally, some design features could potentially be designed with backup ideas or sufficient robustness, such that the designs, might be quickly modified during construction to accommodate potential geologic discoveries and still meet the project needs. A thorough geotechnical investigation also helps reduce the probability of discovering an unanticipated condition during construction. DEC MONTH Rockfill Strength and Real Estate Acquisition: The estimation of the rockfill shear strength was an important factor in the planning of the ADSRP. Both the upstream and downstream buttresses and replaced embankment fill is planned to be constructed from rock fill quarried on-site. Shear strength testing of rockfill is quite difficult to complete, and to have confidence the samples will be representative of a constructed condition. For the ADSRP, of high strength of the rock fill is achievable the buttress size requirements are reduced, potentially reducing the property acquisition needs of the project, and potentially reducing the project schedule risks due to reduction in property acquisition delay risks. Additionally, less material used may lower construction costs. JAN '17 FEB MAR APR MAY JUN Dams and Extreme Events

15 However, the overall project schedule risk is not reduced due to the introduction of adverse events associated with, potential delays in actually producing high strength rock, and potential delays in reviewing rockfill with the regulators during design and construction. Regulators indicated that high strength assumptions would require significant support during design and construction, and schedule delay risks associated with evaluation, production, and quality control around high strength assumptions cause both schedule and cost risks that exceed potential real estate schedule risks, in large part because the rockfill produced and compaction will not be known with certainty until construction. Because the problems of rockfill strength may not be realized until construction, high of rock fill strength assumptions (even if correct) have the potential to cause significant cost and schedule impacts, and the project management team recommended conservative rock fill strength assumptions be carried forward in design, to reduce overall project risks. Reservoir Rim Landslides. The reservoir rim is relatively unstable. Dewatering of the reservoir has the potential to cause instability of the rim near development and near other infrastructure such as roads. At this time, it is not yet known how the historical landslides in the area will be impacted at the onset of the construction. If the impacts are not minimal, then risk management strategies will be developed to mitigate the impacts. Such strategies may include construction of retaining structures (costs of these are large) a schematic showing. potential repair scheme is shown in Figure 6 and estimated to be 5 10% of the total project cost), reconstruction of landslide induced damage, surveying and monitoring, provision of compensation to the affected structures or facilities, and emergency planning at distressed structures of facilities. Anderson Dam Seismic Retrofit Project 1279

16 Figure 6. Typical Reservoir Rim Slide Property Acquisition Delays: Because the project will require both permanent and temporary use of private property there are risk that may occur such that the access to these properties is delayed. Because the delay would be known about prior to construction beginning the major risks impact the project schedule, and not the project costs. Early determination of property needs and coordination with the landowners help reduce the schedule risks, but the risks largely have to be accepted unless an alternate design requiring less real estate is selected. Delays in Environmental Permitting and Clearance: Delays in obtaining environmental clearance may have significant project schedule impacts. Through study, appropriate environmental clearances can be well thought out and evaluated. Specifically, the project identified that clearances for subsurface exploration may be an issue and began planning and environmental clearance process even before the designer was retained, to minimize schedule impacts that could be associated with geotechnical exploration in areas with sensitive species. Constrained Schedule. Although not an event risk, the project risk analysis has highlighted the critical project constraint of the short window (April-October) for construction. Because of the short window and the already planned double shifts, even small delays sometimes require a 1-year delay to complete the required construction in the construction window. The project management and design teams are currently 1280 Dams and Extreme Events

17 evaluating different strategies that would allow for additional schedule buffer around the dry construction season to more likely complete the project construction on-time. CONCLUSION By performing a schedule and cost risk assessment of the top project alternatives, differentiation between project alternatives that had similar designs was made clear and supported the selection of a recommended project alternative. Competing risks, such as fill strength assumptions vs property acquisition risks could be objectively compared facilitating better management decisions. Additionally, risk matrix scoring of the project risks allowed the project management team to identify the most important risks to the project, and pursue appropriate risk mitigation strategies. Risk analysis as a project management tool, has been useful navigating this interesting project that has several difficult engineering, environmental and schedule hurdles to this point. Risk workshops and updated risk evaluations will continue as part of the management process as the project continues, to help keep the project on-track as new project risks are identified, realized, or expire. AKNOWLEDGEMENTS This paper was prepared from work jointly done by the Santa Clara Valley Water District (SCVWD) staff (Dam Owners), Black and Veatch (Project Management Consultant) and HDR (Planning Consultant). The authors would like to gratefully acknowledge the contributions of Hemang Desai and Mike Mooers (SCVWD) and the planning consultant staff, specifically, Les Harder (Project Manager, HDR), Marc Ryan (Embankment Lead, AMEC), John Parrish (Spillway Lead, HDR), and Shawn Spreng (Jacobs Associates, Intake/Outlet Works Lead). REFERENCES AMEC Geomatrix (2011a), Seismic Stability Evaluation Report (SSE-1A), Seismic Stability Evaluation of Anderson Dam, June AMEC Geomatrix (2011b), Technical Memorandum No. 10, Phase 2 Fault Rupture Hazard Evaluation, June 23, Fitzgerald, Thomas; Pittman, Jonathan; Shearin, Laura; Kees, Samuel (2012) Risk based approach to stream diversion for dam construction, Source: Association of State Dam Safety Officials Annual Conference 2012, Dam Safety 2012, v 1, p , 2012, Association of State Dam Safety Officials Annual Conference HDR/AMEC (2013), Surface Fault Rupture Evaluation, Anderson Dam SeismicRetrofit Project, Technical Memorandum, March 6, Anderson Dam Seismic Retrofit Project 1281

18 HDR Engineering, Inc. (2013a), Anderson Dam PMF Study Revision, Anderson Dam Seismic Retrofit Project, Technical Memorandum, March 21, HDR Engineering, Inc (2013b), Anderson Dam Feasible Alternatives Matrix, consulting report March 29, H. Humberto Marengo, Felipe I. Arreguin, Alvaro A. Aldama, Victor Morales (2013), Case study: Risk analysis by overtopping of diversion works during dam construction: The La Yesca hydroelectric project, Mexico, Journal of Structural Safety No. 42. US Army Corps of Engineers, Engineering Construction Bulletin , Application of Cost Risk Benefits to Develop Contingencies for Civil Works Total Project Costs, September Dams and Extreme Events