Clearwater River bridge project fixes erosion problem

LEWISTON - How would the Idaho Transportation Department save a critical bridge over the Clearwater River in Lewiston while also enhancing fish habitat? Over the course of 29 days early this fall, that's the question ITD engineer-in-training Janet Zarate needed to answer.

The majestic Clearwater is a 75-mile-long river flowing west from the Bitterroot Mountains at the Idaho-Montana border until it converges with the Snake River at Lewiston. It passes through four of the five counties in north-central Idaho - the department's District 2 - and is the largest tributary of the Snake.

The recent bridge project in District 2 was a unique effort to combat erosion around the Spalding Bridge's piers. It also provided the perfect project for Zarate to hone her engineering skills.

Zarate (rhymes with karate), who is on the district's project development team, is currently designing left-turn bays and right-turn lanes for a project on Idaho State Highway 8.

She is completing environmental documents, right-of-way plans and other measures. Until recently, Zarate and Robert Gordon were the project coordinators wrapping up the department's construction following West Company's completion of the Spalding Bridge (Clearwater River Bridge) project under budget and ahead of schedule.

The concrete-deck, steel-girder bridge was built in 1962, is more than 1,200 feet long, and approximately 33 feet wide.

The project was completed approximately 4 percent less than the original contract amount, and was finished 14 days before the Sept. 30 deadline.

The project had unique challenges from the beginning. The bridge spans the Clearwater, a body of water vital to the area's recreation and commerce, so minimizing impact on the river was critical. The bridge was one of the state's 10 most scour-critical bridges - at risk of failure due to erosion - so repairs could not wait.

Department officials anticipate that the "sufficiency rating" of the bridge will rise significantly pending a bridge inspection next spring.

The project was complex, requiring ingenuity. The main concerns addressed in design were "constructability," and reducing the effects of construction on endangered fish such as bull trout, Snake River Basin steelhead and Snake River fall Chinook salmon.

Also unique were the nearly 2,700 pounds of articulated riprap per mat placed at the river's bottom to shield the piers against further erosion from water flow. After work was done, nearly 400 of the heavy concrete A-Jacks mats were placed on the riverbed to continue to provide armor against erosion. Research indicates that voids in the mats provide a support system for the river gravels and help to promote fish habitat. The mats, made of more than 30 individual units, were secured with corrosion-resistant steel cables.

Several different options for construction were investigated. Lowering the bundles from the Spalding bridge deck would have affected the flow of traffic on the narrow bridge. After numerous discussions with barge suppliers, the department's District 2 staff determined that using a light-duty barge would be a feasible option for transporting the heavy A-Jacks bundles.

"This is the only project that I am aware of in about the past 10 years in this area that did not affect the flow of traffic in some way," said Curtis Arnzen, District 2 project development engineer.

The project had a mobility-focused design that maintained an open roadway for commuting, commerce, recreation, and tourism.

Approximately 5,700 vehicles cross the bridge each day.

Collaboration with participating agencies was vital to the project's ultimate success.

"Communication was key," Arnzen said. "The permitting agencies responsible for the endangered fish were very good to work with, understood the urgency of the project and helped the project to succeed."

The project contractor was only allowed to start once the flow rate of the Clearwater River had reached 25,000 cubic feet per second and was falling.

Work began Aug. 4. and was completed 29 days later.

This allowed the contractor to have all equipment removed from the site so as not to interfere with the river's catch-and-keep fishing season. This was especially important to anglers who started camping in September in anticipation of an expected record Steelhead run this year.

Although the A-Jacks articulated riprap approach was not the first use of the technology - the department used it in 2002 on the Tilden Bridge in Blackfoot and on the Snake River Bridge west of Shelley, and also on the Carmen Bridge on the Salmon River last year - it was the first time it was used in a fast-moving water situation and to this magnitude.

More than 14,000 individual A-Jacks units were proposed. This dynamic created a unique set of challenges. First, a diver typically provides real-time guidance for successful placement of the A-Jacks mats, according to Joe Schacher, the resident engineer on the project.

"In our case, the water was too swift to safely allow a diver to be involved," he said. "We met with the A-Jacks supplier-manufacturer early on and received insight into how the units react to the river morphology to successfully armor the piers."

Another significant obstacle was changing river conditions. When the contractor began work, the river was running at more than 12,900 cubic feet per second and the flow direction turned crosswise to six of the seven piers, making it very difficult to place the barge and equipment.

Zarate said the high volume and discharge rate of the river led to formidable safety challenges navigating the barge and placing mats.

In addition, for approximately a week, the flow dropped rapidly to 7,980 cubic feet per second, which restricted the barge from accessing the piers downriver when the dropping water level exposed gravel bars below the bridge and caused concerns that areas of the river would become inaccessible to the barge and tugboat.

The contractor and department found innovative methods to efficiently place the mats while maintaining public access for boaters. With each issue, the teams met to discuss procedures.

"It was unique, complex and ultimately, a very successful project," Zarate said. "It met our needs, and most importantly, the needs of the users — the citizens of this part of the state."