Page - 1499 - in Book of Full Papers - Symposium Hydro Engineering

4. DISCUSSION AND CONCLUSIONS The instrumentation of two artificial rock blocks within an unlined spillway channel provided first-of-a-kind, high resolution insight into the rock scour process in a prototype setting. The observations and measurements recorded from the March 2016 spill event that resulted in removal of one of the instrumented blocks allowed assessment of applied hydrodynamic loads and subsequent block response precursory to failure. The gradual displacement of the block from it’s mold even under highly turbulent flow conditions agrees with other measurements made on 3D block response from laboratory flume experiments [16]. In these experiments, block kinematics (related to the orientation of the bounding joint planes on the block with respect to the flow direction) played a significant factor in the observed block response. Blocks with limited kinematic resistance to movement exhibited gradual displacement prior to removal whereas blocks with higher kinematic resistance showed a more dynamic response with larger, impulse-type movements. The distinction between low and high kinematic resistance is thought to occur when the active resultant force vector (R) must be rotated outwards by more than approximately 60 deg. from it’s initial vertical orientation (due to the self weight of the block). Block geometries requiring rotation of R less than 60 deg. are considered to have a low kinematic resistance [16]. At the field site, the rotation of R required for instability of Block 2 (assuming a friction angle on joints1 and 2 equal to 20 deg.) is 58 deg., suggesting Block 2 has a low kinematic resistance. Other near-prototype laboratory experiments on 3D block response using a simplified cubic block geometry showed block response was highly dynamic when subject to loading from turbulent jet impact (e.g., [11,12]). In light of the observations from the field study presented herein and the laboratory results presented by [16], this response can be expected given the minimum rotation of R required for uplift of the cubic block in the above experiments would be approximately 110 deg. (assuming a joint friction angle of 20 deg.). Accordingly extrapolating data obtained from idealized block geometries for analysis of rock scour for sites with highly irregular 3D geologic structure can be challenging. A framework based on block theory principles [19] has been developed in order to assess 3D rock block erodibility. This has been shown to reasonably predicted block erodibility thresholds obtained from the laboratory flume experiments on 3D tetrahedral blocks [16] as well as in the back calculation of the erodibility threshold of instrumented field block presented herein [20]. Accordingly, this framework can allow practitioners to incorporate site-specific 3D geologic structure into analysis rock erodibility. 1499