Spillway Gates Explained: Crest, Radial, and Bear Trap Gates in Water Infrastructure

In this Practical Engineering episode, Grady Hillhouse explains the main types of spillway gates used on large dams, including crest gates, radial (tainter) gates, and bear trap gates. The video compares their operating principles, maintenance challenges, and failure modes, and it features a 3D printed acrylic flume mount with model gates to demonstrate action, leakage, and seal design. Real world examples such as the Oroville incident, Folsom Dam retrofits, and a Dunlap Dam bear trap failure illustrate the critical role of gates in water infrastructure and dam safety. The presenter also discusses stop logs for dewatering during inspections and the ongoing importance of gate maintenance and reliability.

Introduction

The episode opens with a tour of the Upper and Lower Saint Anthony Falls Locks and Dams on the Mississippi River and shifts to a broader discussion of spillway gates. Grady emphasizes that spillways are among the most complex components of a dam due to high velocity water and the need to prevent structural damage. While many dams use uncontrolled spillways, gated spillways offer control and cost benefits by allowing release at specific levels.

Crest Gates: Simplicity and Challenges

The first gate type discussed is the crest gate, a hinged leaf at the top of the spillway. Crest gates pass debris and ice over the top and act like a weir, enabling upstream water level control with adjustable top openings. The presenter illustrates the advantages with examples from San Antonio and Tempe, where crest gates help maintain upstream levels within a narrow range. A major drawback highlighted is the heavy hydrostatic force that the gate must resist, requiring powerful hydraulic or electric actuation. The math behind crest gate forces is presented in simple terms, showing that even relatively modest gate dimensions can produce millions of pounds of force on the leaf.

Radial (Tainter) Gates: Reliability and Efficiency

The radial gate section explains the curved face, rigid trunnion, and hoist mechanism that raises the gate while water flows underneath. This design concentrates hydrostatic pressure through the trunnion, reducing the force transmitted to the hoist and yielding reliable operation. The gates can close under their own weight, which is beneficial for safety. The video notes that older radial gates avoid significant friction but often required later retrofits to address friction and wear in the trunnion assembly. Examples from large dams around the world illustrate how radial gates have become the predominant choice for gated spillways due to their reliability and efficiency.

Bear Trap Gates: Self Containing and Maintenance Hurdles

The bear trap gate design uses two overlapping leaves that create a pressurized chamber between them. Upstream water pressure can raise the bottom leaf, which, in turn, raises the top leaf. The system can be self supporting once opened, but maintaining and inspecting bear trap gates is notoriously difficult because the chamber is prone to sediment deposition and requires complete dewatering for any fixes. The Dunlap Dam collapse in 2019 provides a dramatic reminder of the maintenance challenges associated with this type of gate, including hinge and wear failures and the resulting legal and societal consequences.

Stop Logs, Seals, and Maintenance

Maintenance practices such as stop logs allow technicians to dewater a gate for inspection. Grady notes that while our models show stop logs leaking slightly, real gates leak as well, and rubber seals play a critical role in water tightness. The seals, including J bulb seals, are designed so upstream water pressure aids the seal against embedded plates, helping to prevent leaks that would render a gate ineffective. The discussion includes practical considerations for keeping seals in good condition and the importance of dry access for inspection and repainting.

Historical Lessons and Practical Takeaways

The video ties these concepts to real world incidents, including the Oroville Dam crisis of 2017, which highlighted the dangers of uncontrolled spillway releases and the need for reliable gate operation. It also recounts California flood control stories and Tempe Town Lake’s crest gates, illustrating how gate selection and maintenance decisions impact flood management and downstream safety. Through these case studies, viewers gain an appreciation for the engineering behind dam spillways and the ongoing importance of reliable gate design and proactive maintenance.