Graduate Thesis Or Dissertation
 

Characterizing the spatial distribution of short duration, high intensity rainfall in the central Oregon Coast Range

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1g05fd67s

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  • Shallow, translational landslides occur naturally and are the dominant form of erosion in the Pacific Northwest and the Oregon Coast Range. These landslides are triggered during large, infrequent storms. Forest management activities, such as timber harvesting, can exacerbate the occurrence of these landslides. Understanding the relationship between the occurrence of shallow, translational landslides and the temporal and spatial variability of short duration, high intensity rainfall is an important step to understanding how to manage landslide-prone forested terrain. Rainfall intensity data collected during the last 14 years from a network of 13 tipping bucket rain gauges were analyzed to characterize the rainfall regime of the central Oregon Coast Range. Partial duration analysis was carried out to determine the 2-, 5-, 10-, 25-, 50-, and 100-year return period antecedent precipitation index (API) and rainfall intensities for the 30-minute, 1-, 2-, 6-, 12-, and 24-hour rainfall durations. The Spearman Rank Correlation method was used to determine if patterns existed in the relative rank of rainfall intensities across the study area. Isohyetal maps of rainfall intensity for each unique combination of rainfall duration and return period were also developed. Analysis of the rainfall intensity data showed that rainfall intensity was highly variable across the study area, although the variability across the study area was fairly constant. The coefficient of variation in average rainfall intensity across the study area was approximately 20 percent. The Spearman Rank Correlation method showed that patterns in rainfall occurred across the study area and that these patterns were consistent for all rainfall durations and return periods. In other words, the rain gauge locations that got the most rainfall were the wettest regardless of rainfall duration or return period and the same is true for the rain gauge locations that received the least amount of rainfall. Examination of the isohyetal maps showed that all of the variability in rainfall intensity occurred within the minimum distances between rain gauge locations, which is 10 to 15 km. Analysis of the API data showed similar trends.
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