Poudre River Drop Structure Replacement

Alden was retained by the Northern Water Conservancy District (Northern Water) for the Hansen Supply Canal Poudre River Drop Structure Replacement Project. Alden was the prime consultant for the Project and provided complete structural and hydraulic engineering services, including hydraulic modeling for the design of the replacement drop structure and overall site improvements. The new structure included a stepped spillway, stilling basin, and retaining walls.

Alden performed Computational Fluid Dynamics (CFD) modeling of the existing structure and the proposed structure. The new structure is designed for a peak discharge of 1,500 cfs. Alden also performed 3D finite element modeling for the structural design. Alden collaborated with Northern Water staff to identify maintenance needs and preferences for the new structure. The stilling basin includes an access bridge, jib cranes, and stoplogs to isolate the structure for future maintenance and inspections. The existing site featured extremely steep slopes at 1V:1H with poor surface drainage. The final grading established stable slopes and improved surface drainage to keep water away from the new structure.

Details of the project included:

• Concrete structure demolition, removal, and repair
• New concrete stepped spillway, concrete stilling basin, and concrete retaining walls
• Foot bridge with access stairs
• Subsurface drain system and surface drainage improvements
• Temporary construction cofferdam and dewatering
• Site grading and access road
• Trapezoidal channel restoration and riprap sizing

The project also required coordination regarding the environmental impacts to the Preble's mouse habitat and an adjacent eagle's nest. Design was completed in July 2019. Construction was completed in March 2020 with the first water flowing through the structure on April 1, 2020.