Strong finish to a very challenging dam design project
Dam closure system engineering design at Broken Bow and Hugo Lakes nears completion
Earlier this month, we submitted the draft final 100 percent design for the replacement of the bulkheads at Broken Bow and Hugo Lakes. It’s been a challenging project, but one where my team and I have learned a lot and really honed our skills.
Last year, I wrote “Challenging bulkhead design” about the work we’d done for Tulsa District of the U.S. Army Corps of Engineers at Broken Bow and Hugo Lakes. These bulkheads block water from flowing through the tainter gate bays when the tainter gates are serviced or inspected.
The project kicked off in 2015, when the Mead & Hunt team analyzed the feasibility of multiple gate closure systems and provided a life-cycle cost for each system. My team did a great job, demonstrating our dam safety expertise on a large USACE project. The final report recommended that the Tulsa District procure a single segmental bulkhead to be used at both lakes.
Segmental bulkhead designs are widely used as a closure system. They have the advantage of being fully shop-fabricated with relatively light sections that can be removed from the water with a medium-sized crane. Assembly of a segmental bulkhead in the water requires proper alignment of bulkhead sections, followed by fastening the sections together in a safe and reliable manner.
In March of this year, the district issued a task order to our Mason & Hanger and Mead & Hunt Joint Venture. We were asked to complete the preliminary design and cost estimate for a single segmental bulkhead system to be used at both lakes. Our design had to include storage locations at each lake and launch locations for the bulkhead system.
Our joint venture team designed the segmental bulkhead system, a storage facility and launch sites. We faced numerous constraints associated with this design. These are a few of the issues we addressed:
- Two different spillway structures with varying geometry, design heads, spans and support conditions
- Provide for a six-foot variation of reservoir level operating range at each project
- No divers assemble or place the bulkhead
- Bulkhead loading, unloading and assembly accomplished using only the District’s 75-ton crane to lift, limiting the maximum weight of each segment
- Minimize the number of USACE personnel required to deploy the system
- Maintain marine stability of individual segments and 10 different bulkhead assemblies during assembly, towing and movement between bays
- Provide safe access to the interior of each bulkhead segment for inspection
- Design bulkhead in accordance with USACE Engineer Technical Letters on hydraulic steel structures
- Challenging launch site conditions requiring construction of crane pad jetties to enable deployment and retrieval of segments
Our design team used an iterative approach to identify segment geometries and skin thicknesses that satisfied the design constraints. Beginning with the general constraints and requirements of size, weight and required hydrostatic head capacity, the structural engineer would design the most challenging segment – the bottom (deepest) segment at Hugo Dam – to fit in this box. That segment would then be checked for the required load cases using both hand calculations and finite element modeling. If satisfactory, the mechanical engineer would check it for buoyancy and stability. After each individual section was designed, then the various bulkhead configurations for both Broken Bow and Hugo Dam were checked in the same manner. Final design included adding all appurtenant equipment (piping, valves, lifting eyes, etc.) and then rechecking to make sure it met the requirements.
In this manner, the design team satisfied each of the often-competing design constraints to produce a usable, durable and cost-effective system. The resulting segmental bulkhead system has individual segments that, when joined, create a bulkhead floating in a horizontal orientation. Different segment combinations can be assembled to produce a bulkhead for both lakes and various water levels.
It felt great to submit the draft final 100 percent design. We’re working through the final comments with Tulsa District now, and I’m looking forward to seeing this system constructed and placed into operation. This exacting project has helped me hone my skills as a project manager and engineer.
I would love to discuss this very interesting dam closure system design project with you. I’ll be at the Society of American Military Engineers Federal Small Business Conference in New Orleans from November 1-2. Come visit me at Mead & Hunt booth #121. I’ll also be discussing this project at the U.S. Society on Dams Conference in Chicago from April 8-12, 2019.
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