Uplift and Crack Flow Resulting from High Velocity Discharges Over Open Offset Joints - Laboratory Studies

This report describes laboratory tests that were completed to extend the available data on uplift pressures generated by high velocity flows over offset joints. In addition, magnitudes of flows into the joints or cracks were measured for various configurations. Lab data were supplemented with numerical modeling using the computer program, Flow-3D.
Three joint geometries were tested: sharp (90-degree) edges, ⅛-inch chamfered (45-degree) edges, and edges with a ⅛-inch radius in combinations of joint/crack gaps ranging from ⅛-inch to ½-inch and offsets into the flow ranging from ⅛-inch to ¾-inch. Velocities ranging from about 10 ft/s up to 55 ft/s were tested. The effects of orientation of the joint to the flow was also tested; however due to geometry restrictions of the test channel, joints perpendicular to the flow received the majority of attention for this effort.
Mean pressure differentials across the plate downstream from the offset were measured for all cases. The cavity beneath the test section could be closed, generating the maximum uplift, or opened to allow flow to enter the cavity through the joint/crack. This test facility feature allowed for measurement of a slightly reduced uplift pressure and flow rates through the open joint/crack area. These flow rate data are representative of a fixed head loss, as the exit piping remained constant throughout the testing.
Uplift pressure and flow rate data from this study will be used to reduce the uncertainty in analyses used during risk assessments to provide improved estimates of the level of effort required to bring a spillway or outlet works into a safe operating condition for a variety of geometries.