In Gila County, Ariz., Tonto Creek may be so dry during most of the year that residents can cross it on foot or in a car. But when it rains, the water can spread to a width of 1,200 feet and flow at 20,000 cubic feet per second.
On average, Tonto Creek is impassable 25 days each year. When this happens, residents of Tonto Basin are cut off from emergency services, schools and businesses. To avoid flooded areas, people drive an unpaved for hours, which is impractical and dangerous.
Eight people have died trying to cross the creek in the last 25 years, including three children in 2019.
Gila County built the Tonto Creek Bridge to improve safety and quality of life for residents in Tonto Basin. It also helps tourism to Tonto National Forest. For its success, the project has earned a spot on our list of top bridges of 2024.
Gila County, which owns the bridge, kickstarted the project when it received $21 million in federal funding from the Better Utilizing Investments to Leverage Development (BUILD) grant in 2020. Kimley-Horn was brought on as a designer with Ames Construction as the contractor.
Construction began in 2022, and the project was split into three phases. The first two phases were constructed simultaneously to minimize stockpiling and re-handling of soil for the abutment embankments.
Five locations were evaluated for the crossing. The location that was decided on provided the most direct access to Tonto Basin Elementary School and access to State Route 188.
The final alignment and length of the bridge was set using two criteria. The first was bridge hydraulics and floodplain impacts. The length and span minimized the rise in water surface elevation. The second was Section 404 impacts to Waters of the U.S. (WOTUS). The length was set to keep abutment and roadway embankment fill out of the WOTUS.
These two criteria resulted in a 14-span bridge measuring in at 1,981 feet and 8 inches.
The bridge design used semi-integral abutments, which moved expansion joints from the abutment to the end of the approach slabs. It used gabions (rock filled baskets) for scour protection at each abutment.
Screening and stockpiling cobble for gabion construction was not part of the environmental clearance. This process was halted until a new option could be implemented, setting it back three months.
Thanks to the quick thinking and brave actions of Ames Water Truck Driver Steve Oubre and Project Engineer Ben Husband, a potential tragedy was averted. On June 8, 2023, the pair noticed a plume of smoke outside of the project limits. Taking the water truck to the location where they saw smoke, Oubre was the first person on the site of a brush fire that was spreading quickly. He extinguished the fire using the water truck’s spray bar, saving property and possibly lives, according to Tonto Basin Fire District Chief John D. Wisner.
Crews faced challenging elements of their own as the creek bed was not passable for approximately 70 days during the first winter, which halted construction from January through March of 2023.
In the summer, crews faced temperatures exceeding 100 degrees Fahrenheit. During the winter, the project site temperature dropped to negative 40-degrees due to the elevation. Winter temperatures meant that the project needed to implement cold-weather concrete procedures.
Environmental constraints made it so clearing limits had to be approved by the U.S. Forest Service. The presence of endangered species meant a full-time biological monitor was present during all construction activities.
Clearing activities at the site were restricted from Oct. 1 to April 15 to ensure that construction would not disturb breeding attempts of endangered species. The bridge was completed for the summer 2024 tourist season.
Project: Tonto Creek Bridge
Location: Tonto Basin, Az.
Owners: Gila County
Designer: Kimley-Horn
Contractor: Ames Construction
Cost: $21,574,803
Length: 6,218 feet
