Located eight miles northwest of town, Phase 1 of the West Fork Eno River Reservoir has served as Hillsborough, North Carolina’s primary water supply since it was completed in 2000. An original Phase 2 expansion plan included the construction of an ogee weir on the original broad-crested weir. However, early in the preliminary engineering phase of the Phase 2 expansion, several issues were identified that made the original designers’ plan problematic.

The spillway configuration at West Fork Eno was similar to a side channel spillway. The design posed several unique challenges.

  • The spillway chute downstream of the control section bends 70 degrees, while the width of the chute simultaneously transitions from 215 feet to 60 feet, causing an increase in flow depths.
  • The spillway chute’s wall heights were insufficient to contain the spillway design flood.
  • The spillway chute slab was only eight inches thick, which is not considered adequate to resist stresses from the flow of a new weir that is 10 feet higher than the original broad crested weir.

To obtain data critical to design decisions, Schnabel developed a 3D computational fluid dynamics (CFD) model. The CFD model, which estimated spillway performance under various flood conditions, helped us to determine chute wall heights, identified zones of high hydraulic pressures and turbulent spillway flows, and informed the evaluation of spillway alternatives for the control section.

The alternatives analysis considered labyrinth, ogee, and piano key (PK) weirs. The labyrinth was eliminated due to size constraints. Both the ogee and the PK weirs were viable, but a more compact PK weir was selected to allow the control section to be narrowed by about 45 percent while still passing the spillway design flood.

In addition to constructing the PK weir, other modifications for dam safety included:

  • Revising the control section layout for stability and adding a steel sheet pile cutoff wall in the foundation
  • Increasing spillway chute wall heights
  • Constructing a concrete overlay on the spillway chute floor to reinforce the existing chute floor

The project re-used as much of the existing spillway structure as practical by developing a selective demolition plan. This resulted in a more complex project that preserved portions of the structure and reduced demolition debris by about 50%. Concrete demolition debris was re-used as riprap for slope protection systems on the lower portions of the upstream dam slope. This reduced the volume of materials disposed in local landfills, demand on local aggregate resources, and the required number of truck trips to deliver materials to and from the site.