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RATTLESNAKE RESOLUTION 8-25-14 LMS

A course of action has been decided regarding what directions to take towards resolving the unsafe conditions of the Rattlesnake Bridge and providing access to properties on the west side of the Salmon River.

The decisions were made Monday, August 25, after Lemhi County Commissioners Rick Snyder and John Jakovac heard an updated report on the extent of the bridge’s deteriorated condition and the five alternative solutions researched by Northwest Engineering. Northwest Engineering Services President Dan Sharp presented the result of the company’s analysis which was commissioned last May by the county.

The 180 foot Rattlesnake Bridge has a posted three ton load limit and is currently closed to traffic. He said there is evidence the bridge is still being used which means the people using it are at risk and are risking the bridge as well.

In addition to bridge research, the study was commissioned to evaluate potential west riverside access options from the Shoup Bridge to the Rattlesnake Bridge and on up south to the Iron Creek Bridge. The study was done primarily with safety in mind along with environmental impacts and ultimate financial costs.

As far as a need for a bridge Sharp said the Rattlesnake Bridge does access a lot of private property as well as a good deal of public land. It is felt that even with a bridge installation a secondary access is also needed and will continue to be needed. The access has currently become vital for the stranded west side residents. The access road will also be necessary in the actual bridge construction and afterward as an emergency-backup alternative. Right now there is no emergency access to the properties on the river’s west side therefore, whichever of the five alternatives is chosen as outlined by Northwest, the priority is to initiate access work as soon as possible due to the safety issues related to the bridge closure.

Northwest’s fourth and fifth alternatives have to do with the road access options. The company reviewed the possibilities of a west side road from Rattle Snake Creek to Iron Creek and from the existing bridge to the Lake Creek subdivision which was home to Salmon’s first golf course and to long timers is still known as ‘the old golf course.’

Sharp said that even though the route from Rattlesnake to Iron Creek is two miles shorter, topographically it presents major challenges that would require blasting and a series of switch backs due to one portion of steep incline. Carving a road in that area would involve moving around 251,000 cubic yards of material and would cost around $1,939,331. The six mile route between Rattlesnake and Lake Creek is by far the easiest and most cost effective option.

He said the old trail route could be opened with a blade rather than by blasting and that a 20 foot wide pathway, that would not impact scenic views, could be built to accommodate 20 mile per hour traffic. First steps would include a survey followed by an engineered road design. Permission to survey would have to be obtained from the Bureau of Land Management and then a road easement agreement would be required. It would also have to be determined whether or not there are any archeological issues to mitigate. The numerous property owners along the route will be contacted and easement discussions of rights-of-way will be initiated. Cost of the project is estimated at $160,000. If the roadway is to be kept open year around it would cost the county approximately $25,000 to maintain.

Northwest Engineering assessed three bridge options.

Alternative #1 is to repair the existing bridge. Due to the extent of deterioration there are over 45,000 component points on the bridge that would have to be analyzed at a cost of around $250,000. Every inch of the structure would have to be sand blasted followed by an aggressive inspection program and a yearly maintenance cost of $2,000. The bottom line was it will cost a minimum of $750,000 to get the existing structure up to a three ton load limit which would still not support large fire trucks or bulldozers. The work would extend the bridge’s life span another 20 years. Engineer Sharp said that from a life safety aspect he is not totally comfortable with the rebuild alternative.

Alternative #2 is to replace the 180 foot bridge with a new bridge right next to the old one. The abutments would be extended to 220 feet to avoid the increased water velocity being caused by pushing the river through the old bridge’s more narrow abutments. A parallel bridge would be the preferred alternative of bridge building crews. Sharp recommended a prefabricated metal bridge that could be installed fairly quickly and would support heavy equipment loads. It would have a 50 year life span and with proper maintenance would probably last longer than that. He recommended a ‘weathered’ steel bridge. The technique is to allow some rust to form, then seal coat the rust in place as opposed to a painted bridge that is vulnerable to scratching and chipping. The cost would be approximately $1.9 million. Time for completion would be about a year.

Alternative #3 is to install a new bridge at Lime Creek which is located one mile south of Rattlesnake Creek and just north of Deer Creek. He said shore elevations from one side of the river to the other are even, the river channel is 130 feet wide, there is adequate visibility for traffic entering Highway 93 South, good accessibility to the river on both sides and is the alternative recommended by Northwest Engineering. Estimated cost is about $1.5 million. He said there are some easement rights-of-way that would have to be acquired.

Sharp recommended the County Commissioners pursue a three pronged approach; look into building a bridge at Lime Creek, do what is necessary to open a road north to Lake Creek as soon as possible and begin interim Rattlesnake Bridge repairs to make it passable for light traffic. Then, make sure the load restrictions are strictly enforced. Sharp said he is so uncomfortable about the bridge’s condition that even with the repairs he will be inspecting it every two weeks to make sure it is safe. The cost of the limited bridge repairs, which should last up to two years, is an estimated $250,000. A county search for potential funding options will go into high gear.

The commissioners will be hosting a pre-announced public presentation of the bridge and road plans. Meeting notifications of the presentation will be sent to each land owner involved.

Research has shown the bridge was originally acquired by the county from the state in 1956 to replace a Rattlesnake Creek bridge that had washed out. The present structure had previously been used on the Snake River and as of now is approximately 100 years old. Sharp said it must be remembered what average vehicular weights were when the bridge was designed and built and that when it was installed at its present location the county’s largest dump truck weighed around four tons.

Before the original bridge, access to properties on the west side of the river was provided by an old north/south wagon trail. Remnants of the trail are still visible but not passable to vehicular traffic. Before any excavation begins the wagon trail will be surveyed to establish property boundaries.




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