Morony Dam Spillway Gate Rehabilitation

Conceptual design, Engineering services, Final design, Self-performed construction, Engineering during construction, Testing & commissioning
Cascade County, MT
NorthWestern Energy
why it matters

NorthWestern Energy (NWE) is responsible for maintaining and upgrading hydroelectric facilities that have been powering the state of Montana for over a century. Among these facilities is Morony Dam, located in Cascade County, Montana. The existing Tainter gates at Morony became vulnerable to ice buildup, hindering their use during the winter. To overcome this challenge, NWE determined that the most effective solution was to replace the gates with new heated vertical lift roller gates, specifically designed to prevent ice accumulation. Under a "Design Assist" contract, McMillen refined the initial 10% concept and finalized the design, ensuring it met the highest performance standards, completed the construction, and startup and commissioning.

No items found.

McMillen designed and installed nine new vertical-wheeled lift spillway gates, each measuring 36 feet wide x 26 feet high and weighing 60kips each. The gates are powered and controlled from an overhead gatehouse building that spanned the length of the gated spillway portion of the dam.  

The scope of work included selective demolition, construction, and installation of new guide slots into the existing concrete piers. Additionally, the top 5 feet of each pier was demolished, strengthened, and reconstructed to support the new 40-foot-tall hoist house columns. McMillen also managed the fabrication, delivery, and deployment of four sets of stackable dewatering bulkheads (stoplogs) to isolate and dewater each of the nine gated spillway sections.

Our engineers designed W-shaped steel stoplog slots anchored to the pier noses with custom stoplog bearing seats and guide pins. These stoplogs were capable of spanning 41 feet between piers while holding back up to 30 feet of reservoir pool elevation. McMillen also designed the distribution and control systems for the new gate hoists and hoist building, including motor controls, gate heating systems, HVAC electrical support, and more.

In addition to design and engineering, McMillen was responsible for site assembly, installation, testing, and commissioning of each gate. We also handled the removal and disposal of the existing Tainter gates, pier modifications for installing new embedded gate components, and the construction of a new environmentally controlled gate hoist house. The project also included temporary access roads, environmental support, and permitting.

Our team worked closely with the selected AISC-certified gate manufacturer to ensure compliance with all contract requirements. This included factory inspections, complete hoist and gate system testing, and delivery and offloading at the project site. A temporary barge launch bulkhead structure was engineered and constructed at the reservoir edge to facilitate the assembly and transfer of gate components.

McMillen’s meticulous design, efficient installation, and close coordination with manufacturers ensured the successful replacement of the inefficient Tainter gates, enhancing the dam's functionality and resilience for years to come.

McMillen thoughtfully developed an engineered solution for fabricating, anchoring, and installing the bulkhead system at the dam’s upstream face to allow for dewatering of each spillway bay and the subsequent pier modification and gate removal/gate installation work. McMillen specialists, along with our extensive in-house engineering department, led by mechanical and hydraulic steel structure subject matter experts, devised the stackable stoplog system, bulkhead guides, and underwater sills which were ultimately used for dewatering the gate bays. NWE’s approach of allowing the contractor to develop the dewatering system allowed our team the flexibility to arrive at a cost-effective solution and devise a system that could be installed using the construction equipment we already planned to have onsite.


Related projects.