Hatch completed a comprehensive dam safety assessment of the Conestogo dam in 1997. The dam was classified as a “high” hazard structure and was assigned an inflow design flood (IDF) equal to the probable maximum flood (PMF). One of the issues identified during the dam safety assessment was the stilling basin’s inability to contain the maximum discharge from the service spillway. The service spillway was estimated to have a capacity of approximately 1400 m³/s but the stilling basin could only safely contain (in the form of a hydraulic jump) a flow of 700 m³/s. A flow release from the service spillway greater than 700 m³/s would force the hydraulic jump downstream where the dissipation of its energy could jeopardize the safety of the dam.

In 2007, Hatch identified and evaluated options to increase Conestogo dam’s overall spill conveyance capacity to 1400 m³/s (i.e., the combined capacity of all four service spillway gates). Following detailed three-dimensional computational fluid dynamics (CFD) modeling using a state-of-the-art software package called FLOW-3D, a remediation concept was developed which involved:

• The addition of four baffle blocks on the floor of the stilling basin
• The increase of the end sill height to 4 m
• Vertical extension of the wingwalls to minimize splash and spray on the adjacent embankment
• Adding erosion protection downstream of the stilling basin

The objective of this project was to conduct a detailed design of the proposed modifications and draft tender documents for contractor bids.

Scope of Services
Hatch completed the detailed design for stilling basin modifications at the Conestogo Dam.

Major tasks included:

• Establish design criteria
• Hydrotechnical assessment
• Structural design
• Develop drawings
• Estimate construction schedule and costs
• Submit design report
• Prepare tender package
• Evaluate tenders
• Recommend award