The Inflow Design Flood for High Hazard Dams

The accepted practice is to use the PWIPW concept to develop an inflow design flood* for high hazard dams. However, the approach has been criticized as too conservative, with the result that excessive costs are incurred when building or modifying a dam. This paper discusses the history of dam design leading up to the development of PNV amounts as the basis for designing high hazard dams. Observed precipitation amounts are compared to PW values to demonstrate that such rains are rare but not impossible. Evolution of the PNV and its present development is presented. Attempts to assign probabilities to rare floods, approaching a PW, are discussed leading to the conclusion that such estimates must be viewed with considerable skepticism. Even floods with low probabilities pose risks of being exceeded when numerous sites, dams, are involved over a period of time. The many good reasons for not using economic analysis to identify a least cost, or national economic development plan, for sizing a dam's spillway, outlet works, and setting a top of dam elevation are presented. In conclusion the present practice of using a PNE as the basis for establishing an inflow design flood for the design of high hazard dams is appropriate and should be continued. 12 pp., 15 references.

*An inflow design flood may also be called a spillway design flood or just design flood.