Extreme weather occurrences are leading to an increase in the frequency and intensity of flooding events. In June 2013, heavy rainfall on the melting snowpack in the Rocky Mountains triggered in a catastrophic flood that resulted in approximately $6 billion in financial losses and property damage. The historical flood caused extensive damage to the Calgary Zoo which is partially situated on St. George’s Island in the middle of the Bow River. Quick thinking by a dedicated staff allowed the zoo to save most of the animals before the island flooded. The flood caused $50 million in direct damages to the zoo and demonstrated the need for flood protection above and below the ground surface to protect the zoo from overland flooding and rising groundwater.
To help the zoo achieve its mission of becoming Canada’s leader in wildlife conservation, we were hired to complete the Calgary Zoo Flood Mitigation project. The project consisted of building a flood wall around the perimeter of the island to protect the zoo from future historic floods. The mitigation system provided complete overland and subsurface isolation of St. George’s Island by combining a sheet pile cofferdam perimeter wall with a dual-purpose dewatering system. The system basically built a giant bathtub around the island and dewatering wells within the island that can remove any water that’s inside the wall.
A hydrogeological study carried out at the confluence of the Bow and Elbow Rivers informed the design and advanced The City’s understanding of groundwater flow and underground flooding, which will enhance future flood mitigation planning throughout Calgary. Before resources were fully mobilized, we conducted a pilot program that tested the limitations and advantages of the proposed sheet pile system. The program confirmed that the team would be able to drive the steel sheets through the coarse river cobbles and boulders and get a watertight seal at the level of the bedrock. The use of sheet piles for the structure minimized loss of essential riparian habitat, floodway encroachment and zoo property at the edge of the island. Flood mitigation was completed within an extremely small footprint that ensured uninterrupted continuity for zoo access and operations and minimal disruption to neighbors, zoo visitors and animal residents.
As the existing inflatable rubber dam system at Tempe Town Lake started to fail, the City of Tempe needed a safe, reliable, cost-effective and durable replacement that allowed the lake to transition into a river during major water flow events. We were contracted to construct a new dam to replace the existing dam. The replacement is one of the world’s largest hydraulically controlled crest-gate dams with eight steel gates supported by seven, nine-foot-wide, 25-foot-high concrete piers and two abutment walls. Each gate is 106 feet wide, 17 feet high and weighs 260,000 pounds.
The complex project required many creative solutions for elements including dewatering, demolition, earthwork, roller-compacted concrete, electrical work, cast-in-place concrete, coatings, cement-bentonite walls, scour walls, gate fabrication and erection, hydraulic systems and geosynthetic clay linings.
From the start, the project team knew that the dewatering work was going to be challenging. Once crews began the excavation process the level of difficulty became crystal clear. “We had a lot of water coming from the north slope, which was expected but not at the rate it was flowing,” says Special Projects Manager, Adam Gordon. “This project is also 100 feet downstream of an existing dam that holds approximately one billion gallons of water. Water was coming from every direction.”
The project team worked collaboratively with the City of Tempe, providing multiple cost models and value engineering solutions to meet the client’s project goals. One value engineering option that saved the City a substantial amount of money was the construction of the downstream scour wall. This 1000-foot-long, 25-foot-tall wall was originally proposed as a reinforced cast-in-place wall that required an extensive amount of excavation, forming, backfilling and dewatering efforts. Working with trade contractors, our team developed a plan to install the wall using bentonite, an absorbent aluminum phyllosilicate clay, to keep the excavation stable so the downstream scour wall could be placed in an excavation instead of installing it by traditional methods. Forty-foot-long panels were dug and filled with a bentonite slurry to keep the excavation stable during construction. Upon completion of the panel excavation, custom end-stop beams, and the reinforcing cage were installed. Finally, a high-slump concrete mixture, used to maximize flow without sacrificing strength was pumped up from the bottom of the panel while the bentonite was displaced into a holding tank so it could be recycled and used on a future panel. This process was repeated for a total of 22 panels.
The completed dam can manage the flow of infrequent river flood events and is prepared to handle a 100-year-flood rate of 210,000 cubic feet per second. Extensive coordination with the City of Tempe and many outside agencies including the Arizona Department of Water Quality, the Army Corps of Engineers and the Flood Control District of Maricopa County allowed us to successfully deliver a dam that meets Tempe’s long-term water management needs.