Preliminary Safety Evaluation of Existing Dams, Volume I

This report describes a methodology to conduct a preliminary risk based analysis of existing dams. It is the first in a two part series entitled. "Preliminary Safety Evaluation of Existing Dams." The report provides a methodology to conduct simplified risk and engineering analyses of dams as part of a preliminary safety evaluation. The second part in the series is a user manual.

The purpose of the analysis is to provide a dam safety manager with a procedure to assign priorities to the dams in a jurisdiction for purposes of allocating funds for further study and upgrade of unsafe structures. The method, called a screening process, is a risk analysis approach to preliminary dam safety evaluation. The question of how to prioritize the dams in a jurisdiction is a problem in decision making when a number of uncertainties must be considered. In order to assign priorities. two steps must be taken. First. an assessment of the safety of each dam must be made. second. a rule or decision criterion must be selected by the dam safety manager to quantitatively rank each dam. relative to others under investigation. The purpose of a decision criterion is to reflect the preferences of the decision maker. These can include the desire to reduce the probability of dam failure to an acceptable level. to minimize the consequences of a dam break or to make cost-effective, safety-related modifications that meet regulatory requirements.

The screening process was developed as an aid to dam safety managers concerned with weighting the risks associated with the dams in their: jurisdiction and the constraints of limited fiscal resources. The procedure is an approximate method that can be applied using a Corps of Engineers Phase I Inspection Report or equivalent level of input. It is intended to be a preliminary. simplified approach designed to provide a relative. as opposed to an absolute assessment of risk. By adopting a risk analysis approach as the means to make dam safety assessments. a common basis is established to evaluate the integrity of a dam. A probabilistic approach provides a format to achieve consistency and aids in assessing the effectiveness of remedial measures to improve safety.

Many dam owners have not developed a program to conduct preliminary safety evaluations. Therefore, in addition to providing the risk analysis framework of the screening process, some attention is given to individual tools to carry out particular tasks of the analysis (e.g., flood routing, seismic stability analysis, etc.}, The purpose is to provide the owner who is not currently in a position to perform a certain task, with an alternative to do so. The recommended techniques were selected on the basis that they provide a reasonably accurate estimate of the result desired, without an excessive level of effort.