AASHTO Journal, 5 February 2016
The Idaho Department of Transportation found itself caught between requirements to inspect overhead suspension cable saddles on the 1,500-foot-long Dent Bridge north of Orofino and the need to safeguard federally protected osprey that had been nesting at points that would prevent inspections.
Old osprey nest.
Its answer was to build four new aluminum platforms during Jan. 25-29 at the highest points on the bridge’s suspension cable towers, while the migratory birds were away for the season, and transferring some of their old nesting material to draw them to the new platforms. The goal was to have the platforms installed before the ospreys return in the spring.
The project is an example of how state DOTs must incorporate environmental concerns into their role of maintaining critical infrastructure, and work with other agencies to assure habitat is sufficiently protected.
The Dent Bridge spans the Dworshak Reservoir on the North Fork of the Clearwater River. The agency said the structure is a complex bridge design, and the Federal Highway Administration requires inspections of it every two years.
“Inspectors needed to check the 45-year-old bridge, but could not inspect the top of the towers because of the nesting osprey,” said ITD Bridge Asset Management Engineer Dan Gorley. “Efforts to remove the nests to clear the area for inspection only resulted in the birds returning to rebuild the nests in the same spot.”
The birds were building nests over the cable saddles, obstructing the inspections. But as protected birds, the nesting was also protected, so the ITD came up with a plan that accommodates the birds and also the inspections.
The ITD said nationwide many raptor-nest platforms have been built on utility facilities, but this is a rare example of osprey platforms constructed on a bridge. This project, it said, is similar to one last year by the Connecticut DOT and partners.
Bridge with new nesting platforms.
For the Dent Bridge, biologists were uncertain if building adjacent nest platforms would be successful because the old nesting location was highly attractive to the birds due of its height and proximity to the water. So crews installed the new platforms three feet higher, at the bridge’s crown, and about 125 feet above the bridge’s driving surface.
Each platform is about five feet wide by five ft. long, and engineered to hold up to 1,000 lbs.
The new platforms are the work of Gorley and Melinda Lowe of the ITD’s Environmental Section in Boise, but the plan involved collaboration among various offices. Three ITD sections – Environmental, Bridge and District 2, which made the platforms – coordinated with the U.S. Fish & Wildlife Service for biological expertise and with Clearwater County to help funding and platform installation.