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Changes in floodplain occupance Portland, Oregon 1957-1986 Public Deposited

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  • Portland, Oregon Is in the northern end of the Willamette River Basin and Valley. The Willamette River caries the runoff discharge from approximately 12,000 square miles of basin area, and flows northward where it bisects the Portland metropolitan area and enters the confluence with the Columbia River. However, high discharge rates In the Willamette are seldom the cause of flooding In this area. Elevated flood crests in the Columbia create backwaters in the confluence area, which are the prevalent cause of most severe flooding events in the Portland area Willamette floodplalns. Upstream storage has reduced the flood crests of both rivers. The Columbia's reduction contributes most to Portland area flood-related problems, while the Willamette's upstream storage contributes mostly to a significant improvement in water quality In the Portland area. In 1957, Gilbert White conducted a study of the urban flood plain occupance in Portland to determine if any occupance patterns emerged as a result of the 1936 flood control legislation. Some of White's data, methods, and occupance classification guidelines are used (with some modifications) to account for some of the changes that have occurred since 1957. The purpose of this paper is to conduct a similar study centered around the era of legislation enacted from 1968 to 1973; specifically the National Flood Insurance Protection Act of 1968, and the Flood Disaster Protection Act of 1973. The 100-year regulatory elevation was established in the 1973 legislation to use as a floodplain management tool by local governments. This study investigates the hypothesis that noticeable floodplain occupance patterns will emerge as a result of those floodplain regulations. The basis of comparison Is the occupance data during the time period 1957-1972, compared with changes occurring from 1972-1986.
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