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Browsing by Subject "physical geography"

Now showing 1 - 2 of 2
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    Holocene Paleofloods And Their Relevance To Flood Mitigation, Risk Assessment, And Policy.
    (2020-04) Hefner, Amanda
    Paleofloods are past floods that occurred without being recorded by direct hydrological measurement. Paleoflood evidence can tell us about older, and often much larger historic flooding events than those of the instrumental record. For the past 40 years, paleoflood hydrologists have made the argument that geological or botanical flood evidence can help us better understand the true risks of extreme floods. However, because paleoflood studies are typically presented as individual works for single rivers, it is not known how often this kind of work produces information about floods that are equal or larger than the flood-of-record in conventional hydrological gage records. In my research, I created a new synthesis of all available paleoflood case studies on river systems in the U.S. spanning all available proxy records during the Holocene. My synthesis compared the largest paleoflood event in an area against the flood of record reported by the nearest stream gage, and also evaluated the relevance of paleoflood hydrology to natural hazards research. In order to be included in my analysis, studies had to have taken place on a river system in the continental U.S. during the Holocene, and also had to report a variable of flood magnitude. In the strong majority of cases (70%), Holocene paleoflood events are larger than the largest flood as recorded by instrumental river gages. In the most extreme case, a paleoflood event on Elk Creek, SD had a magnitude 12 times larger than the flood of record. While there is great potential for using data from paleoflood studies in flood frequency analyses, paleoflood hydrologists do not have a standard approach to reporting their results. In my review, I will present a list of suggestions for reporting the location, date, and magnitude in future paleoflood studies to comply with new reporting standards outlined in USGS Bulletin 17C.
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    Wind in the Upper Midwest: Assessing Wind Resource Variability and Representation in Reanalyses
    (2020-08) Coburn, Jacob
    Wind has become an important element of climate for consideration due to the its growing presence as a renewable source of energy. Variations in wind create uncertainty which adversely impacts investment and planning decisions and can lead to structural damage to equipment when extreme events occur which are not able to be adequately planned for. Long-term wind variations are related to climate variability over periods ranging from months to years, with the so-called teleconnections possibly driving significant changes in power output. The goal of this study was to assess the potential impacts of modes of variability within the climate system on wind energy output in the Upper Midwest (UMW: 40-52°N, 87-105°W), a North American region rich in wind resources and experiencing rapid turbine deployment. First, to facilitate this goal, the representation of wind resources by reanalysis models was tested, as were methods of extrapolating 10-meter wind speeds to heights more common of wind turbines (hub-height, often around 80-100 meters). Reanalyzed wind fields were found to capture many of the mean, variational and distributional characteristics of wind speeds at 10-meters as measured by weather stations, though declining trends in the observations were not found to be accurately replicated in the reanalysis models. Next, four methods of wind speed extrapolation commonly used in the literature were tested for their capacity to capture the mean and variations in wind speeds from tall towers measuring at heights ranging from 39-100 meters above ground level. Each method was applied to four reanalyses and results compared against tall tower data. All of the method-reanalysis combinations produced wind speeds which were too slow than observed and less variable than those measured at the tall towers, though the variable exponent power rule applied to MERRA was able to achieve relatively close results with small mean biases. 80-meter wind fields were generated from MERRA using the variable exponent power rule for application in the final section of this study. The 80-meter wind fields were utilized to derive wind energy output. This power output data was then used in a multiple linear regression model to assess the influence of several teleconnections important to the UMW, as well as potential effects of solar forcing variations. This model was applied to each grid cell and season, allowing for spatial and temporal variations in the relationships between the modes of variability and power production to manifest. The magnitude and significance of the teleconnections and solar forcing vary throughout the year and across the region. These influences are shown to fit with expectations of flow set by sea level pressure anomaly patterns. Extreme monthly wind energy anomalies are explored, with the strongest extremes affecting most of the region simultaneously and negative power anomalies found to persist for periods of several months to a year. Negative power output episodes are shown to follow from a combination of synoptic and teleconnection-driven factors while strong, positive output episodes are mostly short-lived and the result of synoptic factors (favorable positions of high and low pressure and strong pressure gradients). These findings have important implications for long-term energy planning and have the potential to improve seasonal and interannual predictions for the industry.