As the dam safety community moves further into risk assessments, and our knowledge of the climate continues to grow, accurate understanding of spillway capacity is increasingly critical. When hydroelectric projects were developed and constructed decades ago, discharge rating curves were estimated with the best tools available; analytical equations and sometimes physical scaled models. Through the continued development of Computational Fluid Dynamics, we are learning more about the hydraulic conditions in complex settings.
We will present a recently completed project using FLOW3D to calculate the discharge rating curve of a confidential hydroelectric project in the upper Midwest of the United States. The powerhouse includes a service spillway consisting of six gated spill tubes (conduits) with complex geometries, discharging into the space below two turbine draft tubes, into the tailrace, and ultimately into the downstream river. Discharge capacity through a fully-flowing pressurized conduit is complex on its own. However, the discharge capacity through the spill tubes is further complicated as the water level in the reservoir rises and activates the emergency spillway, raising tailwater levels at the tailrace and reducing net effective head. This condition often leads to discharge rating curves bending backwards on themselves, allowing less flow through spill tubes as the reservoir level rises, rather than more flow.
Throughout the modeling work using FLOW3D, the model produced expected results, at least initially. A few of the conditions that were modeled, however, yielded surprising results. In some conditions, even with tailwater levels rising by upwards of 10 feet (with supposed 14% reduction in the net effective head), discharge capacity was unaffected. As hydraulic engineers work on complex problems using the best available tools, we can continue to improve our understanding of service spillways and our ability to calculate discharge rating curves and share these findings with the broader dam safety community.
The outcomes of risk assessments based on discharge rating curves can be the difference in millions of dollars of repairs or upgrades to hydroelectric projects, depending on how they affect decisions. Ensuring that discharge rating curves are accurate is important so that risk assessments can properly determine whether or not costly repairs are needed to ensure dam safety.
