What Happens when Spillway Gates Can't be Opened

Hydraulic analyses of spillway gates typically focus on discharge capacity when the gates are open. However, for many projects the impact of a closed gate on discharge capacity can be an important dam safety consideration. Many gates are designed such that the top of the gate is not significantly higher than the normal operating pool, resulting in some potential discharge capacity over the gate during high flow conditions, with the gate closed. The hydraulic capacity of a closed gate is of particular concern where sudden operations or a failure of an upstream dam may produce a significant flow increase, without sufficient time to operate. The discharge capacity of closed gates is also of interest in evaluating the area of inundation for Emergency Action Plan (EAP) mapping of dams where the passage or attenuation of flow may be impacted by operations at downstream dams. However, due to the configuration of structures necessary to operate, access and maintain gates, analysis of the discharge capacity over closed gates is not as straight forward as a typical gate rating curve. Potential complications with the analysis of flow over closed gates, including head losses due to overhanging structures and the potential for structures to modify an approaching flood wave will be presented. Methods for analyzing the complex hydraulics of the closed gates will be presented, including the application of two‐dimensional and three‐dimensional Computational Fluid Dynamics (CFD) modeling. Results of a case study for a series of vertical lift gates where the support beams of the operating deck extend to the normal pool elevation of the reservoir will be presented. Of particular concern in the case study was the potential for the operating deck structure to reflect the momentum of an approaching flood wave back upstream, increasing inundation upstream of the dam.