| OTIA III State Bridge Delivery Program |
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| Web Brief (Mar 06) |
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Flooding traps debris and blocks water flow.
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Bridge redesign takes the long view for communities and the environment
When recent high-water levels exceeded the 100-year flood marks on two Interstate 5 bridges south of Albany, ODOT knew it would have to design the replacement bridges to meet the changed hydraulic conditions. So OTIA III State Bridge Delivery Program staff collaborated with Region 2 staff to improve the current situation and leave room for future adjustments.
Each winter and spring, rain swells the Calapooia River and carries large driftwood debris downstream toward I-5. In the past few decades, changes in the river have resulted in greater amounts of water overflowing from the river and being diverted down Sodom Ditch. As a result, much more debris reaches the bridges that carry I-5 over Sodom Ditch. This debris accumulates against the bridges’ eight piers and causes regular flooding, threatening to overtop the highway or damage the bridge.
In response to local concerns about future inundations and the threat of flood waters to the highway and bridge structure, the Bridge Delivery Unit and Region 2 determined that the replacement bridges should be designed with increased clearance for debris and improved water flow.
One way to accomplish this was to reduce the number of piers in the water from eight to three. But when the number of piers is reduced, larger beams are required to support the bridge deck, the bottom of the bridge structure gets closer to the water, and a new vulnerability to flooding is introduced by the reduced clearance.
BDU and Region 2 decided that the best solution was to also raise the elevation of the bridge and the roadway approaching the bridge by about two feet. By reducing the number of piers without lowering the bottom of the bridge, the designer can improve flow capacity and the passage of debris.
“The bottom line was to protect the structure, the investment and public safety from a sustainable point of view,” said Vivian Payne, Area 4 manager. “This is absolutely the right thing to do, in the short term and the long term.”
Additionally, bridge program engineers designed the bridges so they can be raised in the future without being totally reconstructed. If the river continues to change its flow and winter water levels continue to rise, it will be a relatively straightforward design and construction process to raise the bridges and accommodate that higher water level.
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