Evaluation of Safety Against Erosion and Piping at Whittier Narrows Dam, Los Angeles County California

Whittier Narrows Dam is located approximately 10 miles east of downtown Los Angeles, adjacent to the densely populated communities of Montebello and Pico Rivera. Condominiums and single family residences line the downstream toe, less than 150 feet from the embankment. Figure 1 shows an aerial of the west embankment and the development immediately downstream.
During a storm in March 1983, pool elevation behind the west embankment rose to elevation 207 feet MSL, equivalent to approximately 23 feet of head. A boil occurred downstream of the embankment and residents immediately downstream reported flooding from high groundwater. The design pool for the 100-year event is elevation 229, or approximately 45 feet of head. While subsequent to 1983 pool elevation 207 has not been reached, ponding has been observed downstream of the embankment during periods of lower pools.
Initial attempts to prevent similar or worse performance focused on repairing damages to the relief well system caused by wear and vandalism. However, continued occurrence of ponding downstream during very low pools, the difficulties of maintaining the relief wells, and questions regarding the effectiveness of those wells raised concerns over the performance of the seepage control system during major events and ultimately led to the reevaluation of that system. This paper summarizes that reevaluation and serves to illustrate the usefulness of reliability analysis in evaluating such multivariable problems.