Geology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2774
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Item EFFECTS OF BAR FORMATION ON CHANNEL STABILITY AND SEDIMENT LOADS IN AN URBAN WATERSHED(2009) Blanchet, Zachary; Prestegaard, Karen L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study investigates channel adjustment due to urbanization in the Little Paint Branch creek of the Anacostia River watershed. In the past 15 years, large gravel bars have formed in the channels, more than doubling the active channel width of some reaches. Field data was collected to analyze downstream hydraulic geometry and the effects of gravel bars on shear stress, turbidity, and morphological change. The watershed was gauged at three locations to document the contributions of discharge and sediment to the downstream Anacostia Estuary. The results indicate that Little Paint Branch Creek generates proportionally more runoff per basin area than the watershed does as a whole, even though the impervious surface area is lower in the upstream tributaries, like Little Paint Branch Creek. Bar formation induces channel widening, which decreases flow depth and thus shear stress for bankfull and higher stages. This shoaling limits bed transport and will eventually limit bank erosion.Item Scales of Bank Roughness and Their Relationship to Bank Erosion Processes(2009) Hankin, Erik Ravnholt; Prestegaard, Karen L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Stream bank erosion rates and the stabilization of channel width are poorly understood processes. There have been two distinct approaches to the study and prediction of bank erosion rates in natural streams. In order to predict bank shear stresses, scientists either define a reach as being meandering or straight, even though most river channels are neither meandering nor straight but a combination of the two. This thesis aims to determine if river segments can be divided into straight reaches and curved reaches with different bank erosion prediction approaches applied to each as well as investigating the role of bank roughness element size and spacing in bank erosion. The results show that straight reaches are affected by upstream curvature and that large isolated bank protrusions that are widely spaced generate erosion-causing, stable, macroturbulent eddies. The thesis has implications for stream restoration practices regarding bank stability and erosion.Item Floods, Hurricanes and Climate: Influences on the Potomac River Basin(2008) Rhodes, Gwendolyn; Prestegaard, Karen L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The suggestion from preliminary analysis of data derived from stream gauges in the Potomac River Basin is that tropical storm types generate many of the high magnitude flood events in the Potomac River Basin. The goals of this study are to 1) determine whether or not tropical storms are, in fact the primary cause of extreme floods, 2) determine whether tropical-storm characteristics, e.g., landfall location and wind speed, or climatic conditions can be used to predict extreme floods in the Potomac River Basin; and 3) evaluate the flood potential of sequential tropical storms by comparing the flood response to a single September tropical storm event (Isabel, 2003) with the flood response to a series of four September tropical storm events in 2004. All assessments utilized thirty-seven stream gauge discharge records and archival data on named tropical storm characteristics for the period 1950 to 2004. The data were used in logistical regression to establish the importance of tropical storms in flood generation. Models of the relationship between tropical storm characteristics, climatic factors, including the Atlantic Multi-decadal Oscillation and Pacific Decadal Oscillation; and annual peak discharge were developed using generalized linear modeling. Western Potomac River Basin flood responses to Hurricanes Isabel (2003), Ivan and Jeanne (all in 2004) were evaluated based on hydrograph and runoff characteristics. Antecedent moisture and soil moisture storage capacity, during the four storms, were also assessed using discharge data. Tropical storms and frontal storms generate most of the floods in the upper 10% of flood distributions for study. Generalized linear modeling indicates the Pacific Oscillation Index and atmospheric pressure associated with the tropical storm play a key role in the storm's ability to generate a flood. It was also determined that a single intense storm, such as Hurricane Isabel (2003), is a better flood generator than a series of closely spaced storms, such as the series of tropical storms in 2004.Item A CHEMICAL AND ISOTOPIC COMPARISON OF TWO 1ST-ORDER AGRICULTURAL TRIBUTARIES, KENT COUNTY, MARYLAND(2005-05-27) Teerlink, Jennifer; Candela, Philip; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nitrate derived predominantly from agricultural fertilizers results in algal blooms and wide-spread anoxia in estuarine environments. Roughly half the nitrate delivered to the Chesapeake Bay is derived from groundwater (residence time up to 40 years). Two 1st-order tributaries in eastern Maryland were sampled on five dates. Over the study interval, average nitrate, alkalinity, and delta 13C were 207 µM, 212 µM, and -12.1, respectively, in the ditched tributary, and 106 µM, 451 µM, and -9.7, respectively, in the unaltered tributary. Ditching of the western tributary results in discharge of less anthropogenically-altered groundwater. Nitrate, calcium, and magnesium concentrations decrease and 13C abundance of DIC becomes enriched along the reach. The unaltered tributary is stagnant in the headwaters, resulting in consumption of dissolved oxygen and denitrification. Alkalinity correspondingly increases suggesting reduced carbon as the electron source for this microbial process. Alteration of 1st-order tributaries influences the processing and delivery of nutrients.Item Characterization of the spatial differences in hydrological functioning in a tidal marsh, Patuxent River, MD: A framework for understanding nutrient dynamics(2004-12-08) Phemister, Karen; Prestegaard, Karen L.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study investigates spatial variations in sediment hydraulic conductivity (K), network channel shape and horizontal groundwater flux magnitude toward tidal network channels in a freshwater tidal marsh. Results showed the average value of K at zero meters from the creekbank was significantly higher than the K at both 5 and 15 meters from the network channel creekbank. Creekbank gradient did increase with increasing distance from the main channel and some data indicated that channel width-to-depth ratio (F), which is inversely related to creekbank gradient, correlates well with K. In addition, horizontal groundwater flux magnitude at a depth of 11 cm was significantly greater than flux magnitude at 22 cm below the ground surface at the first-order network channel location. Horizontal flux magnitude was also significantly higher from 5 to 0 meters than from 15 to 5 meters from the network channel creekbank at both the first- and second-order channel locations.