Performance of geologic material under hydraulic stress

Abstract

Greater than 20,000 small and medium sized dams use excavated emergency spillway channels to handle flood flows. More than 95% of these dams have never experienced a spillway flow. However, because a great number are less than 15 years old, the probability of spillway flow increases each year. Little information concerning the performance of rock material subjected to hydraulic stresses exists. In this study twenty-seven spillways with different geologic material and erosion damage were analyzed to identify important variables controlling the erosion process. Field observations indicate geology is the major control on erosion and can be defined by the following geologic erosion equation: Gr = f(L, SP, G, ST, MP) where Gr = Geologic Resistance to hydraulic erosion, L = Lithology, SP = rock Substance Properties, G = Genesis of material, ST = Structure and Tectonic history, MP = rock Mass Properties. These values can be further used in a material performance classification to delineate stable non-erosive flow conditions. In this classification system material is divided into four classes from the most resistant AAAA to the least resistant A. Limits for each of the geologic erosion equation factors are established for each class. Clastic sedimentary rocks were found to be the most variable in erosion resistance and the most severe erosion events observed occurred in this rock type. Density, cementation, and strength characteristics determine the resistance of soils and severely weathered rock; conversely, other substance properties such as permeability were not found to be as important. Rock genesis determines both the lateral and vertical continuity of rock units as well as their erodibility. For example, it determines if an erosion resistant sandstone unit disappears along the length of a spillway channel due to a facies change. It also determines if an erodible unit is interbedded with non-erodible units thereby creating a potentially unstable situation. The deformation history is also an important factor because, depending on their orientation, the dip of bedding planes may concentrate erosion toward the dam, away from the dam, or minimize erosion entirely if the rock units dip upstream. The rock mass properties of fracture density and cementation are critical factors as they provide the detachment surfaces for rock particles.