Using Piezometers to Predict Extreme Conditions at Dams

Industry-standard references on dam instrumentation (Reclamation 1987; Dunnicliff 1993; and USACE 1995) provide detailed information on instrument function, selection, and installation, but very little on how the data obtained from the various instruments are used in dam safety analyses. This paper explains how to use piezometer data collected at lower reservoir levels to predict piezometric levels during extreme events, such as the Probable Maximum Flood. These projected levels are used to analyze the safety of the dam and foundation under these conditions and to set alarm levels for each piezometer. The paper presents three methods for estimating piezometric levels for extreme storage events: 1) graphical extrapolation of piezometer readings collected during the first filling; 2) mathematical extrapolation using the percent of total pool head method, based on flow net theory; and 3) calibration of seepage models using piezometer data at normal pool. The limitations and advantages of each method are discussed. Actual data from two well-instrumented dams illustrate the application of the various methods.