Spillway Gates Explained: Crest Gates, Radial Gates, Bear Trap and Gate Maintenance

Overview

Practical Engineering host Grady Hillhouse explains how spillway gates regulate dam releases, comparing crest gates, radial gates, bear trap gates, and the use of stop logs. He ties the concept to real world structures on the Mississippi, notably the Upper Saint Anthony Falls and its downstream dam, illustrating how gates control flood flows, how they interact with weirs, and why seals and maintenance matter for reliability. The video also shows how flood events like Oroville and the 2019 Dunlap Dam incident illustrate what can go wrong when gates are neglected. Throughout, Grady uses scale models and simple calculations to reveal the physics behind water control and the engineering choices behind dam design.

Introduction and Context

In this episode, Practical Engineering dives into spillway gates, the devices that allow dams to release excess water safely. The host uses the Upper Saint Anthony Falls Lock and Dam in Minneapolis as a context, then contrasts it with the downstream Lower Saint Anthony Falls Dam. The key observation is that even on the same stretch of river with similar floods, the spillway widths and gate configurations differ markedly, largely because downstream gates are present and operable in the lower dam.

Gate Types and Operation

The video surveys several common gate types that engineers rely on to regulate spillway flow. Crest gates, which are hinged leaves on top of a spillway, provide strong control over upstream water levels and allow debris or ice to pass over the top. Radial gates, also known as tainter gates, feature a curved face connected to struts that pivot on a trunnion, with water flowing beneath the gate. A hoist raises and lowers radial gates, and because water pressure acts perpendicularly to the gate surface, the resultant force is carried through the trunnion rather than the hoist, reducing friction and increasing reliability.

Bear Trap Gates and Maintenance Challenges

Bear trap gates are more unusual. They consist of two overlapping leaves that form a sealed chamber between them. When upstream water is introduced into the chamber, the pressure can float the bottom leaf and raise the upper leaf, creating a self-opening mechanism. While appealing for not requiring large external forces, bear trap gates present significant maintenance challenges. They tend to trap silt, are difficult to inspect when dewatered, and historically required custom components rather than off the shelf parts. The video recounts a dramatic 2019 failure at Dunlap Dam in Texas where a gate hinge collapsed, draining the lake and triggering legal disputes. The episode emphasizes that even seemingly elegant ideas can be hard to maintain in practice, and reliable operation depends on regular inspection and robust design choices.

Stop Logs, Seals, and Leakage

Maintenance practices such as stop logs are discussed. Stop logs allow sections of a gate to be dewatered for inspection and repair, while leaks at seals are common but must be controlled to keep gates effective. The video explains that most spillway gates use rubber seals, such as J bulbs, that seal against embedded plates, leveraging upstream water pressure to improve the seal. The emphasis is on keeping water tightness consistent while allowing the gates to move as needed.

Real World Case Studies

Historic events illustrate why gate design and maintenance matter. The Oroville Dam incident in 2017 underscored the dangers of uncontrolled spillways during extreme floods. The Folsom Dam story from a prior video highlighted the risks of gate failures if retrofits are neglected. The Dunlap Dam collapse in 2019 served as a sobering reminder of how a single failed hinge can trigger rapid downstream consequences and long legal battles. The episode uses these cases to argue for robust design, regular inspection, and appropriate maintenance strategies for all gate types.

Takeaways

The episode concludes that water infrastructure and gate design sit at the intersection of physics, reliability, and public safety. Crest gates are simple and debuggable, radial gates dominate large gated spillways due to their reliability, and bear trap gates offer potential savings at the cost of maintenance complexity. Stop logs and sealing technologies are essential for safe operations. Across all types, ongoing maintenance and a deep understanding of hydrostatics are crucial to prevent downstream disasters when river flows rise or fall unexpectedly.