The potential and extent of erosion due to plunging jets of water from flows overtopping dams need to be evaluated for many dams. Hydraulic scouring is a phenomenon that may cause erosion of material downstream, potentially causing dam failure. Methods to assess the effects of erosion have received considerable attention in the past few decades. Many of the current methods have been developed based on empirical data, carrying a significant level of uncertainty. A numerical simulation based on the Discontinuous Deformation Analysis (DDA) is used in this study to allow a physically based assessment that takes into account the actual geologic conditions. Conventional DDA models are improved to allow the application of time-dependent and heightdependent loads from water overtopping the dam. An abutment slope just downstream of a dam is analyzed considering the joints in the rock mass. The results show that rock erosion is affected by the duration and intensity of the water jet hitting the rock. Moreover, rock stability is influenced by the strength of the joints. Failure phenomena are observed in some cases analyzed. The purpose of this paper is to present a DDA model that incorporates preliminary hydro-mechanical coupling effects and to present some preliminary results of analyses using this DDA model to assess dam abutment erosion.
In recent years, many dams have experienced significantly larger flood events than they were designed for, resulting in dam overtopping. When dam overtopping occurs, the hydraulic power of the impinging jet might cause foundation and downstream scouring/erosion, which potentially could lead to failures resulting in significant consequences. Significant scouring/erosion has been reported at several dam sites worldwide. As future flood events are expected to be more severe due to climate change, dam erodibility is becoming a significant risk concern.
Dam erodibility assessment typically consists of four steps: (1) selection of the extreme precipitation event to be considered, such as the probable maximum precipitation (PMP); (2) hydrologic rainfall-runoff analysis to estimate the extreme discharge rate, such as probable maximum flood (PMF), that a dam will experience under the extreme precipitation event; (3) hydraulic analysis to compute the impinging water jet load on rock foundation under the extreme flood event; and (4) geomechnical stability/erodibility analysis of the rock foundation subject to the hydraulic load. For step (4), the empirical Erodibility Index (EI) method  and the Comprehensive Scour Model (CSM)  are the most common approaches used for assessing the potential for dam overtopping scour and erosion. This paper presents an alternative way to complete step (4) using a geomechanical stability analysis. In this geomechanical analysis, the rock mass is modeled as a blocky system and its behavior is analyzed using the Discontinuous Deformation Analysis (DDA) [3-6].