Browsing by Author "Wohl, Ellen E., advisor"
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Item Open Access Island dynamics and their role in regulating sediment flux in the Middle Snake River, Idaho(Colorado State University. Libraries, 2014) Thomas, Dai B., author; Wohl, Ellen E., advisor; Anthony, Deborah J., committee member; Rathburn, Sara L., committee member; Bledsoe, Brian P., committee memberThis study was conducted to provide an improved understanding of the dynamics of river islands and to investigate the role of islands in regulating sediment flux within the fluvial system. The study showed that the islands in entrenched geomorphic subreaches of the MSR form, erode, and reform in locations controlled by lateral constrictions. The geometry of the islands adjust on a decadal or even longer time scale in response to a disturbance or changes in water and sediment supply, and thus, the islands form part of a temporal and spatial continuum of bedforms. The formation of the islands regulates sediment flux through the reach. The study reach of the Middle Snake River (MSR) in Idaho contains over 300 islands within approximately 200 km between Swan Falls Dam and Brownlie Reservoir. The hydrology of the study reach has been significantly altered by upstream dams on the mainstem and dams on tributaries within the study reach. Data used in the study include: (1) historical aerial photos (1938/1939) and topographic maps (c1894-1906), (2) topographic and bathymetric survey data collected in 1997 through 1999, (3) flow measurements from 1911 to present, (4) bed material samples, (5) morphostratigraphic mapping of 194 islands and (6) stratigraphic soil profile data collected on 95 islands. The soil profile data included soil stratigraphy, soil samples (used for sediment gradations and pollen analyses), pedological descriptions, historical artifacts and charcoal fragments (used for carbon dating). A previously developed 1-dimensional hydraulic model of the study reach was used to evaluate the hydraulic conditions along MSR and to calculate the overtopping discharges of the islands. Comparison of historical aerial and topographic data with 2012 aerial photography showed evidence of the growth and erosion of islands and reworking of island chains to form new configurations, illustrating the dynamic nature of the islands over the last approximately 100 years. The historical document review also showed that the location of almost all larger islands and island groups are controlled by lateral constrictions such as tributary fans. Soil profile data, pollen analyses, historical artifacts and radiocarbon dating of soil charcoal were used to determine the approximate age of islands and to evaluate the erosional and depositional activity of the islands. The soil profile data showed an extreme range in age at some islands where the gravel platform of the islands is old (circa 7,000 years), but the overlying sediments are young (on the order of hundreds of years). Two-dimensional sediment-transport models were developed to evaluate the baseline conditions and simulate island development. Baseline conditions modeling showed the gravel- to cobble-sized material forming the core of the islands is not mobilized under the current hydrology. The islands formed in response to more recent floods from silt-sand sized sediment supply, which explains the relatively young soils overlying older gravel cores. Modeling results showed that: (1) the island geometry adjusts to a disturbance or a change in sediment supply, (2) the formation of islands regulates sediment flux, and (3) the islands form, erode, and reform in the same general locations, which supports the study hypotheses that islands form part of a temporal and spatial continuum of bedforms.Item Open Access Physical modeling of jointed bedrock erosion by block quarrying(Colorado State University. Libraries, 2009) Dubinski, Ian Michael, author; Wohl, Ellen E., advisorThe primary objective of this dissertation is to provide insight in erosional processes, types of channel geometry, and relative rates of incision and knickpoint retreat of channels formed on jointed, resistant rock in a controlled experimental flume setting. Jointed, resistant rock occurs primarily in crystalline lithologies such as granite, gneiss, quartzite, and basalt. These lithologies can be found in a wide range of climatic and tectonic settings. Channels in jointed bedrock may have distinctive erosional processes and geometry relative to channels formed in unjointed bedrock. Joints, fractures, and other discontinuities such as bedding planes in the bedrock are locally weakly resistant zones in contrast to the resistance of unjointed bedrock. These areas may be preferentially weathered to form weakly or completely detached blocks that may be mobilized by flows in the channel. Channels in jointed bedrock commonly have abrupt lateral or downstream discontinuities in bed elevation including steps and knickpoints. A physical model of jointed bedrock using concrete divided into discrete blocks was constructed in a flume and allowed to erode over time by primarily block quarrying. Experimental controls examined in the flume included discharge, channel width, and joint spacing. Observed changes in planform geometry were retreat of the downstream knickpoint with no development of anabranching channels. Erosion by block quarrying occurred with variation between runs of differing control variables. A force analysis of block quarrying combined with a statistical analysis of the erosion results in conjunction with the control variables, including joint spacing and stream power, provided insight into the process of block quarrying. Wider vertical joint spacing produced more easily eroded blocks than a narrower joint spacing with equal block height in each case when friction forces along the side of the blocks are considered. As blocks loosen over time, the side forces diminish. Without side forces resisting motion, blocks formed by the wider vertical joint spacing are less easily mobilized than the smaller blocks. The other important element in defining block erosion is the key block concept. Erosion of blocks occurred as either a few blocks at one time or a mass movement of blocks at roughly the same time. Mass movements sometimes occurred after removal of a few blocks. These movements of a few blocks were termed key block movements and formed a bimodal population in terms of event magnitude with the mass movements as the other sub-population. Comparison with joint spacing field data from observed anabranching, inner channel, and transitional reaches along the Orange River in South Africa generally concurred with the conclusions drawn from analysis of the model results. Block quarrying is controlled by the balance between block mobility and hydraulic conditions that change over time with periods of little block movement punctuated by mass movements.Item Open Access Seasonal precipitation and soil moisture dynamics of a hyperarid wash in the Sonoran Desert, U.S.A.(Colorado State University. Libraries, 2013) Howe, Susan R., author; Wohl, Ellen E., advisor; Laituri, Melinda J., committee member; Kelly, Eugene F., committee member; Rathburn, Sara L., committee memberPrecipitation and runoff in arid and hyperarid landscapes is infrequent and both spatially and temporally variable, and the relationship between these hydrologic components and vegetation, soils, and geomorphology in these environments is complex and not well understood. In this study, precipitation and soil moisture were monitored beneath three cover types in three locations across two geomorphic surfaces in the Yuma Wash watershed, located in the Lower Colorado River Valley of the Sonoran Desert, on the US Army Yuma Proving Grounds in Yuma, Arizona. Monitoring, sampling, and characterization occurred from July 2006 to February 2010. Six tipping bucket rain gages and sixty time domain reflectometry soil moisture sensors recorded moisture inputs and storage on a middle to late Pleistocene age alluvial terrace, and a younger, Holocene age alluvial wash. Sensors were spatially distributed in the lower, middle and upper locations of the watershed, beneath bare ground at 2.5, 25, 50, and 100 cm, and beneath the dripline radius of Olneya tesota and Parkinsonia microphylla, at 25, 50, and 100 cm depths. These data suggest that precipitation is highly variable in space and time, and is generally greater than the surrounding valley bottoms of Yuma Proving Grounds. Findings also suggest that soils beneath the dripline radius of these plant species on terraces are wetted more frequently and to greater depths in response to smaller magnitude and lower intensity storm events relative to soils beneath the same species on washes, and relative to bare ground soils. Threshold precipitation conditions necessary to generate changes in soil moisture were compared across surfaces, and illustrate that the vesicular structure in the A (Av) horizons beneath desert pavement plays a key role in redistribution of moisture as runon to O. tesota and P. microphylla on terraces, and that soils beneath the dripline radius of both species on washes receive moisture only during rainfall events exceeding 30 mm. There is also some evidence to suggest precipitation and near surface soil moisture may be greater in the upper basin relative to the mid- and lower basin on both surfaces, but at depths of 25-100 cm, soil moisture responses were difficult to interpret due to local soil properties not quantified in this study. The influence of soil temperature on the imaginary permittivity component of soil moisture readings due to high soluble salt content, the presence of enriched clay layers, soil compaction and induration is discussed. Findings highlight the need to quantify these age-dependent soil pedogenic and hydrologic properties when assessing soil moisture response to spatially variable precipitation in these water-limited environments. Implications for management of military lands are discussed.