Page - 1498 - in Book of Full Papers - Symposium Hydro Engineering

The limit equilibrium stereonet presents a graphical solution for 3D stability of the block. The different regions, delineated by the solid black lines, correspond to the different potential kinematic failure modes of the block (e.g., “2” = 1- plane sliding on joint 2, “13” = 2-plane sliding on joints 1 and 3). The dashed lines correspond to contours (10 deg. increments) of the required friction angle on the joint planes to maintain the block at equilibrium. Initially (at 4 pm), the active resultant force vector (R) plots within the “stable” region at the center of the stereonet as loading is only due to the self weight of the block. As the block experiences additional loading from hydraulic forces from the spillway flow, R rotates outwards away from the “stable” region into region “12” corresponding to the kinematic mode of sliding on joints 1 and 2. Accordingly, the likely failure mode in which the block was eroded was 2-plane sliding on joints 1 and 2. Flow conditions (velocity and turbulence intensity) in the proximity of Block 2 were assessed using recorded video of the spill event. Particle Image Velocimetry (PIV) software was used to track turbulence structures within the flow field overtime to estimate the instantaneous velocity at the flow surface (Fig. 9). A simplistic free online code, Tracker [18], was used for the analysis. More robust codes are available but are often reserved for more controlled laboratory settings. An existing high-resolution LiDAR point cloud of the spillway was used to spatially scale spillway features in the video used in PIV analysis. Video for PIV analysis was taken at a discharge of 30 m3/s during a site visit on March 12. From the video a mean flow velocity, u = 10.3 m/s and turbulence intensity, Tu = 19 % were calculated. Turbulence intensity values compared well with those determined from pressure sensor measurements on the free surface face of the block. Although no video was available during the time the block was removed, the PIV results provided guidance on estimation of flow characteristics at the time of block erosion. Fig. 9 Screen capture of particle tracking during March 2016 spill event. 1498