The proposed crossing of the Willamette River at Lambert Bend involves a 2.3-mile-wide flood plain. Two of the three principal tangents of the crossing will include bridges that will span the main channel and an overflow channel of the river, as shown in figure 1, page 3. The Oregon State Highway Department wants to know what flow will result when the water-surface elevation upstream from the bridges is 100.0 feet (mean-sea-level datum). This design elevation will be referred to as Condition 1 in this report. Also required in Condition 1 is how much backwater is represented in the 100-foot elevation and how often did this flow occur before the river became regulated in 1941.
The Highway Department also wants to know what flow could be expected from a flood event equal to that of December 1964, but regulated by three additional reservoirs that have been completed since 1964. This design discharge will be referred to as Condition 2 in this report. Also required in Condition 2 is the amount of backwater caused by the two constrictions and how often the design flood occurred before regulation began in 1941.
