I-84 Sandy River Bridge Replacement Project (OR)
A hydraulic and scour evaluation for a replacement of the Sandy River – Eastbound and Westbound I-84 Bridges (No. 06875 & 06875A) was conducted. Five bridge replacement alternatives were evaluated. All of the alternatives consist of three spans founded on two interior bents and two pile supported spill-through abutments. The project included a site inspection, development of site hydrology, geomorphic analysis to meet the requirements of the ODOT Fluvial Performance Standard, bridge hydraulic design, scour analyses, and erosion protection design.

The Columbia River is located approximately 2 miles downstream of the bridges. A hydraulic model including backwater from the Columbia River was developed for the existing bridges, proposed replacement bridge alternatives, and “natural” conditions. Hydraulic models of the Sandy River excluding the downstream backwater effect of the Columbia River were also developed for the proposed replacement bridge alternatives for use in the scour and riprap analyses. The Jordan Road overpass is located approximately 400 feet east of the Sandy River Bridge. The overpass consists of two bridges that allow for the under crossing of Jordan Road. The hydraulic opening for the Jordan Road bridges was included in the hydraulic model developed for the Sandy River Bridges. All models except the natural conditions model include the Jordan Road Overpass structures as they convey water during large magnitude flood events.

Additional analysis was required to evaluate the impacts of dam removal projects upstream of the bridge site. Marmot Dam is located approximately 28 miles upstream of the project site on the Sandy River. A hydraulic evaluation was performed which considered the potential future increase in channel bed elevation resulting from sediment deposition associated with the planned Marmot dam removal.