SR-1/96-97 : Procedure for risk based hydrological design

Abstract

Risk is inherent in nature and society, while uncertainty depends on investigations. Decision making in water resources is most often a deterministic selection of dimensions which divide the probability distribution of the controlling variable into two regions: performing well and failing to perform. Interpretation of nature is either deterministic, stochastic, or mixed. The various deterministic hydrologic concepts have been used particularly for design of major hydraulic structure. The need exists for bridging the deterministic and stochastic approaches to solving various water resources problems. The large number of sources of risk and uncertainty, and their complexities, explain why they have not yet been fully introduced into the practice of decision making. However, there is a good outlook for significantly improving the decision process in the future by using information on risk.

The decision-making process-which is based on four attributes: benefit, cost, selected technology, and risk and uncertainty-is expected to play a significant role in the future development of water resources. Risk analysis is an alalytical process which complements and aids planning evaluation and design as well as regulatory cost effectiveness analysis. At present, for most of the hydrological designs risk analysis is not being paid attention. Looking at the enormous cost of the construction of the hydraulic structures, and other social, legal, environmental and engineering, considerations as well as safety aspects, there is an urgent need for adoption of risk based hydrologic design procedures. Risk analysis must fit within a broader evaluation framework, whether it be benefit cost analysis, multiobjective analysis or standard based cost effectiveness analysis. Therefore, risk analysis should be embedded in formal approaches which deal with risk cost trade-offs.

The objective of this report is to present the status of risk based hydrologic design which has been reported in literature. Based on this, a computational procedure has been identified for developing hydrologic design bases for dams and spillways using a risk-based methodology. In order to demonstrate the impact of the risk based analysis on current design practices a few illustrative examples are presented. A brief overview of the hydrologic design criteria adopted by some of the government agencies in India and abroad for the safety evaluation of dams is presented. It is shown that the hydrologic design basis resulting from a risk-based analysis may not always be in conformity with applicable hydrologic design criteria. In addition to that, some of the limitations of the performance of an accurate risk-based analysis are presented.