Safety of Existing Dams Evaluation and Improvement (1983) / Chapter Skim
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3. Risk-Based Decision Analysis
3. Risk-Based Decision Analysis
Pages 41-70
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From page 41... ...
However, there is a largely unexplored potential for extension of risk analysis into other aspects of dam safety. The role of risk assessment is to provide a formal, consistent approach to evaluate the likelihood of occurrence of various adverse outcomes.
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From page 42... ...
than to initiate extensive engineering studies, including formal risk analysis. In prioritizing dams for safety evaluation, it is appropriate to use an approximate risk-based screening process.
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From page 43... ...
Increasingly, therefore, dam safety program managers, owners, and their technical staffs must be prepared to support and justify engineering decisions by the use of an analysis of the trade-off between cost and risk. For an owner of a large number of dams, such as a large utility, city, or water district, the problem may be to prioritize the dams for remedial measures and to budget appropriate funds in order to achieve the greatest safety for the least money.
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From page 44... ...
In the context of the U.S. Army Corps of Engineers' Dam Safety Program, decisions taken during "Phase I" dam inspection are nonterminal, while "Phase II" decisions involving repair and rehabilitation may be thought of as one-step or terminal decisions.
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From page 45... ...
In a formal risk assessment one estimates occurrence frequencies, relative likelihoods of different levels of response and damage, and the various components of cost and consequences. Although an actual value of risk cost is determined, this value often need not be considered in absolute terms but as a number suitable for comparison among alternative risk reduction
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From page 46... ...
· Calculation of the risk costs, i.e., the summation of expected losses (economic and social) from potential dam failure.
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From page 47... ...
The total economic risk cost is obtained by summing the product of the likelihood of the loading condition, the likelihood of dam failure in different modes given the loading condition, and the cost of the damages resulting from that failure mode over the entire range of load levels and failure modes. Similar calculations can be made for the expected losses of life or the "social risk cost." These require expressing the life loss consequences for each potential failure mode in Step 4.
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From page 48... ...
In several MIT Research Reports (1982) an analogous framework is presented for decision analysis of hazard mitigation measures for existing dams.
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From page 49... ...
of the U.S. Army Corps of Engineers and the Safety Evaluation of Existing Dams (SEED)
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From page 50... ...
METHOD OF RISK ASSESSMENT FOR SPECIFIC CONDITIONS It has been noted in previous discussions that an effective risk-based decision analysis must incorporate site-specific conditions related to the most likely failure modes, hazard conditions, and possible remedial measures. The technical elements that are common to different procedures of risk-based dam safety as
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From page 51... ...
The approach to the solution of these technical problems is best examined in the context of the major loading conditions to which the dam is exposed: static loading, hydrologic loading, and earthquake-induced dynamic loading. Risk of Dam Failure Due to Static Loading To determine the risk of dam failure due to static loading, each of the failure modes relevant to a particular dam needs to be identified.
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From page 52... ...
Some important potential failure modes do not lend themselves to a factor of safety formulation as is common in stability analysis. Also, the calculated probability of failure is very sensitive to the tails of probability functions describing the various parameters, and these are not well known at all.
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From page 53... ...
This means that specific flood magnitudes have an assigned probability of being exceeded during any given year. The use of exceedance probability for conveying the risk of the design flood being exceeded during the useful life of a project is extremely important in judging inflow design floods for sizing spillways and establishing crest elevations.
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From page 54... ...
A PMF is defined as the largest flood considered reasonably possible for a specific location and its probability of exceedance should be close to zero. The function describing annual exceedance probability could be extended in a smooth curve from the limit of the relation obtained by historical records of flood events until the curve becomes asymptotic to the PMF value.
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From page 55... ...
For example, slope instability of the upstream face of an earth embankment and liquefaction of the foundation may be postulated as potential failure modes when the dam is subjected to strong earthquake ground shaking. Once the hazards and corresponding modes of dam failure have been identified in a preliminary way, the risk analysis proceeds to the evaluation of the probabilities in each step of load-response-failure sequence.
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From page 56... ...
Discussed below are the most common alternatives available to an engineer or owner facing the decision of how to remedy problems with existing dams. The emphasis in the discussion of each alternative is on its impact on failure risk and consequences.
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From page 57... ...
In dam safety the objective is to avoid the sudden release of the reservoir. Damage to the spillway and to the dam are usually anticipated during the inflow design flood; however, maintenance of spillway crest is necessary to reduce the likelihood of sudden release of water.
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From page 58... ...
Other Remedial Measures. The above listing gives some common possible remedial measures.
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From page 59... ...
A case history of an example of the use of riskbased decision making to reduce risk by reservoir restriction is presented in the section Examples of Risk Assessment and Decision Analysis. The cost of this alternative is generally low.
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From page 60... ...
EXAMPLES OF RISK ASSESSMENT AND DECISION ANALYSIS In this section a number of case studies involving the application of risk analysis to dams will be described. The emphasis in each write-up is on the problem formulation and the results.
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From page 61... ...
Because it was anticipated that it would be several years for any permanent remedial measures to be completed, temporary restrictions on the water level at Jackson Lake were considered. A risk analysis was performed to assess the probability of an overtopping condition as a function of the restriction level and to assess the level of downstream damage as a function of various modes of failure and restriction levels.
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From page 62... ...
Although the data were limited, the risk analysis clearly showed a marked decrease in the risk of overtopping with decreasing reservoir elevation. Analysis of the flood hydrograph for the most likely hypothetical failure mode likewise showed a significant decrease in potential hazard with decreasing reservoir elevation.
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From page 63... ...
Island Park Dam is a zoned earthfill structure approximately 91 feet in height. The exterior slopes are 4: 1 upstream and 2: 1 downstream.
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From page 64... ...
2. Continue use of the existing structures and provide adequate freeboard for the flood events and possible plugging of the existing service spillway.
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From page 65... ...
6 Sensitivity Analysis Before making any conclusions on the least-cost alternative, a sensitivity study was conducted to determine the impact of different probabilities of occurrence (for the flood events and spillway inlet flow restriction) on the computed risk costs.
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From page 66... ...
(A second sensitivity analysis was made by assuming that the peak and volume IDFs have a recurrence interval of 1,000 years rather than 10,000 years.) Conclusions Notwithstanding the "other factors" presented for consideration by the decision maker, the results of the risk-based decision analysis for Island Park Dam (hydrologic aspects)
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From page 67... ...
A flood in 1964 caused a small flow, approximately 30 cubic feet/second, through the grass-lined natural-channel emergency spillway at Willow Creek Reservoir and resulted in an erosion phenomenon known as headoutting. This occurrence for a minimum flow caused concern that spillway flows expected from the PMFs could result in erosion cutting back to and failing the spillway crest wall, releasing the reservoir into the Sun River Valley.
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From page 68... ...
The final decision between designing for the PMF and a lesser event would consider the cost comparison between the two schemes as well as other factors that are not incorporated into the cost analysis (agency credibility, funding, public acceptability, etc.~. Risk Assessment The decision analysis study for Willow Creek spillway modification alternatives requires as input the risk cost associated with alternatives that provide for an IDF' less than the PMF.
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From page 69... ...
(1982) "Decision Analysis for Prioritizing Dams for Remedial Measures: A Case Study," MIT Department of Civil Engineering Research Report R82-12.
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From page 70... ...
(1983) , Application of Risk Analysis to the Assessment of Dam Safety, Department of Civil Engineering, Stanford University, Stanford, Calif.
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