Concurrent Session 02C: Evaluation of Whittier Narrows Overtopping Protection Construction and Material Requirements

Abstract Description: Overtopping of embankment dams is attributed to a large portion of historic dam failures. Changes in the understanding of extreme hydrologic events have resulted in a large number of existing embankment dams identified as hydrologically deficient, requiring remediation. The use of Roller-Compacted Concrete (RCC) and Soil Cement (SC) stepped overtopping protection has become a popular remediation alternative due to the energy dissipation provided by the steps and the economy of the construction methods and materials. The typical performance requirements of the cemented materials used for stepped overtopping protection alternatives have mostly been based on rules-of-thumb of unconfined compressive strength. Many projects have included a "sacrificial" outer edge of less competent materials that may not have the required compaction effort to meet strength requirements; but are not considered to be part of the critical section. Often the estimate of energy dissipation provided by the stepped geometry is based on idealized conditions and do not reflect the actual as-built condition nor consider the range of potentially eroded geometric conditions of the steps. This presentation explores the issue of non-idealized step geometry through a design case history for a large USACE Dam Safety Modification Project. This case history includes the construction of multiple prototype demonstration sections, the evaluation of erosion characteristics by means of a large-scale submerged jet test, and evaluation of the resulting impacts on energy dissipation by non-idealized step edge geometry.