Browsing by Author "Thornton, Christopher I., committee member"

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Open Access
Hazard area mapping during extreme rainstorms in South Korean mountains
(Colorado State University. Libraries, 2012) Kim, Jaehoon, author; Julien, Pierre Y., advisor; Watson, Chester C., committee member; Thornton, Christopher I., committee member; MacDonald, Lee H., committee member
The concern for climate change has increased worldwide. Localized rain storms with high intensity and short duration have been observed in the United States, Europe, Australia, and China. South Korea is one of the countries that have also been impacted by extreme rainfall events during typhoons. Extreme rainstorms have caused major damage from landslides and debris flows in the South Korean mountains. The Duksan Creek watershed in South Korea was selected to simulate surface runoff using TREX during the extreme rainstorm precipitation event from July 14 to July 16, 2006. The maximum hourly rainfall was 62 mm on July 15 in 2006. The three hour rainfall from 08:00 AM to 11:00AM on this day was 168 mm. This rainstorm triggered 518 landslides and caused major infrastructure damage from debris flows. The three hour rainfall precipitation has a 100 year return period. The TREX model was calibrated in two mountainous regions of South Korea. The relative percent difference of time to peak and peak discharge on the Naerin Stream and the Naesung Stream were 6.25 %, -2.58 % and 1.90 %, -0.25 %, respectively. The TREX simulation at the Duksan Creek was performed at a 30 m resolution with distributed data on topography (DEM), soil type, and land use. The peak discharge from the TREX simulation at the Duksan Creek watershed was 452 m3/s. This value was compared to the results of several other methods and the relative percent difference was -1.1 %. The peak discharge was also compared with specific peak discharge measurements and this value corresponds to the range of values for similar watersheds. The TREX model can calculate the distribution of infiltration depth. The infiltration depth calculation typically ranged from 0.2 m to 0.3m with maximum value of 1.2 m. Based on the infinite slope analysis, such infiltration depths correspond to a critical slope angle of 25° to 29°. This range of the critical slope angle was comparable to the angle of 26° from the field investigations and from the analysis of satellite images and aerial photographs at the Duksan Creek. Several different hazard mapping methods were compared including a landslide hazard map from the Korea Forest Institute (KFRI), SINMAP, and TREX. The result of the relative predictability of TREX was slightly better an improvement of 24.6 % than the result of SINMAP. The maximum shear stress could also be calculated by the TREX model. Values of shear stress typically ranged between 0.223 kPa to 0.895 kPa in the tributaries and 1.79 kPa to 17 kPa in the main channel. Based on a critical shear stress analysis, a 1 m diameter boulder reaches incipient motion at a shear stress of 0.895 kPa.
Open Access
Interflow dynamics and three-dimensional modeling of turbid density currents in Imha Reservoir, South Korea
(Colorado State University. Libraries, 2011) An, Sang Do, author; Julien, Pierre Y., advisor; Thornton, Christopher I., committee member; Venayagamoorthy, Subhas K., committee member; Wohl, Ellen E., committee member
This study reports a detailed research identifying the turbid density flow regimes and propagation dynamics of density currents in Imha Reservoir in South Korea during Typhoon Ewiniar. We employ a high resolution 3-D numerical model (FLOW-3D), based on nonhydrostatic Navier-Stokes equations, to investigate the propagation of density flows resulting from the complicated reservoir morphometry and various mixing processes. The 3-D numerical model was modified to simulate particle-driven density currents. The particle dynamics algorithm builds upon the original FLOW-3D code in two ways: (1) improve the original buoyant flow model to compute the changes in density via particle deposition; and (2) include multiple sediment sizes in mixtures as a function of particle size. The influences of inflow characteristics and seasonal changes of thermal structure of the reservoir on the turbid density currents intruding into Imha Reservoir are studied. A series of numerical simulations of lock-exchange are validated with laboratory experiments on: (1) gravity currents propagating into a two-layered fluid; (2) gravity currents propagating into a stratified fluid; and (3) particle-driven gravity currents. The model predictions of propagation speed compared very well with laboratory experiments and analytical solutions. Two numerical approaches (Reynolds Averaged Navier-Stokes model and large-eddy simulation) are equally effective and robust in predicting propagation speed and interfacial instability compared to the laboratory experiments. The simulation of gravity currents intruding into a stratified fluid matched the theoretical solution derived from an energy model. The modified FLOW-3D model successfully captured the decreasing propagation speed due to the different deposition rates of different particle sizes, compared to experimental measurements. We extended our simulations to include the effects of particle sizes on the propagation dynamics of gravity currents. The type of gravity currents depends on particle sizes and can be subdivided into three zones: (1) When ds, is less than about 10 μm, the particle-driven gravity currents behave like IGC (Intrusive Gravity Currents) and all sediments can remain in suspension. Thus the suspended sediments can increase the density of the currents enough to travel a longer distance; (2) When ds > 40 μm, particles will rapidly settle, resulting in a decrease in excess density of the gravity currents. So, such density currents lose their momentum quickly and rapidly vanish; and (3) When 10 μm ds 40 μm, some particles will settle quickly, but others remain suspended for a long time, affecting the propagation dynamics of the currents. Modeling gravity currents in this regime particle sizes must account for particle dynamics and settling. We applied the FLOW-3D coupled with the particle dynamics algorithm to Imha Reservoir in South Korea. The model application was validated against field measurements during Typhoon Ewiniar in 2006. In the field validation, absolute mean error (AME) and root mean squared error (RMSE) for the prediction in water temperature profiles were calculated to be 1.0 oC and 1.3 oC, respectively. For turbidity predictions, AME and RMSE were 37 and 47 NTU (nephelometric turbidity units) between the simulated and the measured turbidity at stations G3, G4, and G5. We showed the influence of inflow characteristics (discharge, temperature, sediment concentration, and particle size distribution) on the fate of density currents in Imha Reservoir. Two threshold values in particle size (10 μmand 40 μm ) were identified, consistent with previous findings from the simulations of Gladstone's experiments. The simulations indicate that when the particle sizes ds are less than 10 μm, most of the sediment inflows at the inlet point (G2) will be transported to Imha Dam (G4) in suspension by interflows. When the particle sizes ds are greater than 40 μm, they will rapidly settle before reaching the dam. Therefore, highly concentrated turbid interflows could only occur when ds is less than the threshold value of 10 μm. The numerical results also present three flow regimes determining the intrusion types of density currents: (1) river inflows will form interflows when the sediment concentration Ci is less than 2000 mg/l; (2) when Ci is between 2000 mg/l and 3000 mg/l, they will form multiple intrusions (i.e., interflows and underflows); and (3) when Ci is greater than 3000 mg/l, they will plunge and propagate as underflows. These threshold values (2000 mg/l and 3000mg/l) can be used to practically predict the formation of turbid density currents, flow type, and intrusion level in Imha Reservoir.
Open Access
Mechanics of sediment plug formation in the Middle Rio Grande, NM
(Colorado State University. Libraries, 2013) Park, Kiyoung, author; Julien, Pierre Y., advisor; Thornton, Christopher I., committee member; Venayagamoorthy, Subhas K., committee member; Wohl, Ellen E., committee member
The Rio Grande is a dynamic river system which has experienced significant hydraulic and geomorphic changes through recorded history from the early 1900's to the present. These changes stem, for the most part, from natural and human interventions to the river system, which experienced channel bed elevation changes, lateral migration, straightening, channel realignment, etc. Sediment plugs have formed in the Tiffany area near San Marcial in 1991, 1995, and 2005, and in the Bosque Reach 14 miles upstream from the Tiffany plug location in 2008. Many authors have investigated the cause of sediment plugs in the Middle Rio Grande but the previous studies do not provide a complete criteria for sediment plug formation. Better understanding of the complex mechanics of plug formation on the Middle Rio Grande is therefore pursed. Based on the historic flow and geometric characteristics of plug areas, seven parameters were identified as major causing factors of sediment plug formation in the Middle Rio Grande: (1) two geometric factors: variability of channel widths and roughness; (2) two water and sediment loss factors: perching/overbanking and sediment concentration distribution profiles; and (3) three backwater effect factors: backwater effects from a reservoir, a bridge, and sharp bends. The purpose of this research is to analyze possible sediment plug parameters and to assess the primary causing factors. The specific objectives are to: (1) investigate the mechanics of sedimentation effect due to each factor; (2) simulate the historic sediment plugs using a numerical aggradation/degradation program; and (3) determine which factors contribute the most to the formation of sediment plugs. Geometric factors show that the channel has narrowed 40% between 1962 and 2002 and channel capacity has 77% decreased over time. The representative composite roughness increased 50 % between 1992 and 2002. Accordingly sediment transport capacity has decreased 45%. The narrowing (40%) with increase in roughness (50%) causes considerable loss of sediment transport capacity (45%). Therefore geometric factors induce more overbank flows and channel bed aggradation. Sedimentation factors show that the perching ratio increased from 13% to 87% between 1992 and 2002. Bank depth has decreased 51% between 1992 and 2002. The perching and lower bank depth facilitated more overbank flows and 13 ~ 20% loss of water. As particle sizes have coarsened (0.2mm in 1992 → 0.25mm in 2002) and width/depth ratios have increased (129 in 1992 → 229 in 2002), leading to higher rouse numbers and more near-bed concentration profiles. High Rouse number (Ro >1.2) and near-bed sediment concentration profile speed up the aggradation rates (4 ~ 7 times faster) than for a uniform-concentration profile. The high near-bed concentrations shorten the plug formation time from 90 to 20 days. Since snowmelt floods exceed bankfull discharges less than 2 months, the acceleration factors are essential for sediment plugs to form. Backwater effects from the Elephant Butte Reservoir influenced the upstream channel bed elevation over time. At an average flow discharge (1,550cfs), the aggradation (up to 7ft) time to fill the 25.5 mile long channel is roughly 10 years. The historic Tiffany plug area has been influenced by the reservoir levels, but with a lag time of several years. Around the San Marcial Railroad Bridge, channel bed elevation has aggraded consistently (12ft increased between 1979 and 1987). The pier contraction and congested abutments generate about a 1ft high backwater propagating to the Tiffany plug area. Sharp bends caused a 1.6ft high backwater which propagates roughly 1 mile upstream. As the beginning point of the Bosque plug is located 0.6 mile upstream of the sharp bends, backwater does influence the channel aggradation of the Bosque plug. The time to fill the main channel up to the bank crest was estimated as approximately 17 days. In terms of significance, perching/overbank flow and sediment concentration profiles can be evaluated as the primary causing factors of sediment plugs, followed by the backwater effects from bridge and sharp bends. Backwater effect from the reservoir has influenced the upstream channel elevation on a long-term basis (7 ft / 10 years). Channel narrowing and higher roughness promote overbank flows and decrease of sediment transport capacity. Owing to the increase of overbank flows, sediment concentration profiles speed up the rate of channel aggradation, causing a sediment plug within a matter of weeks, thus these two factors are the most significant factors (1.2 ft / 20 days). Two other factors, the backwater effect from the railroad bridge and sharp bends, explain why the historic sediment plugs formed at particular areas, therefore these two parameters can be classified as local triggering factors (1~1.6 ft / 20 days). On the other hand, causal factors can be divided into two groups depending on the plug location. The Tiffany plugs have been more affected by the backwater effect from the reservoir and railroad bridge. On the other hand, the Bosque plug was more influenced by the decrease of channel width/channel capacity, roughness, and sharp bends.
Open Access
Numerical analysis of river spanning rock U-weirs: evaluating effects of structure geometry on local hydraulics
(Colorado State University. Libraries, 2011) Holmquist-Johnson, Christopher Lee, author; Watson, Chester C., advisor; Abt, Steven R., committee member; Thornton, Christopher I., committee member; Doe, William, committee member
River spanning rock weirs are being constructed for water delivery as well as to enable fish passage at barriers and provide or improve the aquatic habitat for endangered fish species. Many design methods are based upon anecdotal information applicable to narrow ranges of channel conditions and rely heavily on field experience and engineering judgment. Without an accurate understanding of physical processes associated with river spanning rock weirs, designers cannot address the failure mechanisms of these structures. This research examined the applicability of a Computational Fluid Dynamics (CFD) model, U2RANS, to simulate the complex flow patterns associated with numerous U-weir configurations. 3D numerical model simulations were used to examine the effects of variations in U-weir geometry on local hydraulics (upstream water surface elevations and downstream velocity and bed shear stress). Variations in structure geometry included: arm angle, arm slope, drop height, and throat width. Various combinations of each of these parameters were modeled at five flow rates: 1/10 bankfull discharge, 1/5 bankfull discharge, 1/3 bankfull discharge, 2/3 bankfull discharge and bankfull discharge. Numerical modeling results duplicated both field observations and laboratory results by quantifying high shear stress magnification near field and lab scour areas and low shear stress magnification near field and lab depositional areas. The results clearly showed that by altering the structure geometry associated with U-weirs, local flow patterns such as upstream flow depth, downstream velocity, and bed shear stress distributions could be altered significantly. With the range of parameters tested, the maximum increase in channel velocity ranged from 1.24 to 4.04 times the reference velocity in the channel with no structure present. Similarly, the maximum increase in bed shear stress caused by altering structure geometry ranged from 1.57 to 7.59 times the critical bed shear stress in the channel for a given bed material size. For the range of structure parameters and channel characteristics modeled, stage-discharge relationships were also developed utilizing output from the numerical model simulations. These relationships are useful in the design process when estimating the backwater effect from a structure for irrigation diversion as well as determining the spacing between structures when multiple structures are used in series. Recommendations were also made, based on the analysis and conclusions gathered from the current study, for further research. The analysis and results of the current study as well as laboratory studies conducted by Colorado State University and field reconnaissance by the Bureau of Reclamation provide a process-based method for understanding how structure geometry affects flow characteristics, scour development, fish passage, water delivery, and overall structure stability. Results of the numerical modeling allow designers to utilize the methods and results of the analysis to determine the appropriate U-weir geometry for generating desirable flow parameters (i.e. upstream flow depth and downstream velocity and bed shear stress magnification) to meet project specific goals. The end product of this research provides tools and guidelines for more robust structure design or retrofits based upon predictable engineering and hydraulic performance criteria.
Open Access
Numerical modeling of reservoir sedimentation and flushing processes
(Colorado State University. Libraries, 2011) Ahn, Jungkyu, author; Yang, Chih Ted, advisor; Julien, Pierre Y., committee member; Thornton, Christopher I., committee member; Wohl, Ellen E., committee member
As rivers flow into reservoirs, part of the transported sediment will be deposited. Sedimentation in the reservoir may significantly reduce reservoir storage capacity. Reservoir capacity can be recovered by removing deposited sediment by dredging or flushing. Generally speaking, the latter is preferable to the former. An accurate estimation sedimentation volume and its removal are required for the development of a long term operation plan in the design stage. One-dimensional, 1D, models are more suitable for a long term simulation of channel cross section change of a long study reach than two or three dimensional models. A 1D model, GSTARS3, was considered, because this study focuses on sedimentation and flushing in the entire reservoir over several years and GSTARS3 can predict channel geometry in a semi-two dimensional manner by using the stream tube concept. However, like all 1D numerical models, GSTARS3 is based on some simplified assumptions. One of the major assumptions made for GSTARS3 is steady or quasi-steady flow condition, which is valid for most reservoir operation. If there is no significant flow change in a reservoir, such as rapid water surface drop during flushing, steady model can be applied. However, unsteady effect due to the flushing may not be ignored and should be considered for the numerical modeling of flushing processes. Not only flow characteristics but also properties of bed materials in reservoir regime may be different from those in a river regime. Both reservoir and river regimes should be considered for a drawdown flushing study. Flow in the upper part of a reservoir may become river flow during a drawdown flushing operation. A new model, GSTARS4 (Yang and Ahn, 2011) was developed for reservoir sedimentation and flushing simulations in this study. It has the capabilities of simulating unsteady flow and coexistence of river and reservoir regimes in the study area. GSTARS4 was applied to the Xiaolangdi Reservoir, located on the main stream of the Yellow River. The sediment concentration in the reservoir is very high, 10 ~ 100 kg/m3 for common operation and 100 ~ 300 kg/m3 for flushing operation, with very fine materials about 20 ~ 70 % of clay. Stability criteria for computing sediment transport and channel geometric changes by using GSTARS4 model was derived and verified for the Xiaolangdi Reservoir sedimentation and flushing computations. Han's (1980) non-equilibrium sediment transport equation and the modified unit stream power equation for hyper-concentrated sediment flows by Yang et al. (1996) were used. Both unsteady and quasi-steady simulations were conducted for 3.5 years with calibrated site-specific coefficients of the Xiaolangdi Reservoir. The computed thalweg elevation, channel cross section, bed material size, volume of reservoir sedimentation, and gradation of flushed sediments were compared with the measured results. The unsteady computation results are closer to the measurements than those of the steady flow simulation results.
Open Access
Optimization of Sangju weir operations to mitigate sedimentation problems
(Colorado State University. Libraries, 2016) Kim, Hwa Young, author; Julien, Pierre Y., advisor; Fontane, Darrell G., committee member; Thornton, Christopher I., committee member; Rathburn, Sara L., committee member
To view the abstract, please see the full text of the document.
Open Access
Prediction of selenium in Spring Creek and Fossil Creek, Colorado
(Colorado State University. Libraries, 2014) Pierce, Adam L., author; Stednick, John D., advisor; Boone, Randall B., committee member; Thornton, Christopher I., committee member
The role and importance of selenium as an environmental contaminant has gained widespread attention among research scientists, natural resource managers, and federal and state regulatory agencies during the last two decades. Selenium has been listed on Colorado's Clean Water Act Section 303(d) List of Impaired Waters for Spring Creek and Fossil Creek in the city of Fort Collins. Selenium is one of the most hazardous of the trace metals, following mercury, with a narrow range between dietary deficiency and toxicity. Identifying selenium sources and understanding the environmental processes controlling how selenium is introduced to streams is critical to managing and mitigating the effects of elevated concentrations. A modeling approach was used to predict selenium concentrations with exploratory variables including 15 geospatial landscape parameters, precipitation, and streamflow for 5 sub-watersheds within Spring Creek and Fossil Creek watersheds. A correlation analysis was applied with surface water selenium concentrations and the better exploratory variables identified. Selected variables were used in a multiple linear regression model. Various combinations of different variables determined the best performing model, and included the area of shale, area of moderate to strongly alkaline soils, and the length of streams with an adjusted R2 of 0.99, [Se µg/L = 24.038 + 9.516(ALK) - 0.782(STR) -1.039(SHL)]; where ALK = area (km2) of moderate to strongly alkaline soils; STR = length (km) of streams; SHL = area (km2) of shale. Additional multiple linear regression models were developed in ArcGIS® using Ordinary Least Squares (OLS) Regression, and Geographically Weighted Regression (GWR) with area weighted geospatial variables. The best performing OLS model used only area (km2) of wetlands, with an adjusted R2 of 0.98, [Se µg/L = -6.584 + 170.509(wetlands)]. Similarly, the best performing GWR model included area of wetlands, with an adjusted R2 of 0.98. The second best performing GWR model included area of shale, with an adjusted R2 of 0.66. Limitations of this model include a very small sample size of water quality sampling stations, which limits the statistical power of multiple regression models used. Additional techniques applied in basin delineations with landscape element coupling for identification of hydrologic and/or chemical response units can further develop the platform for future modeling efforts targeting unmonitored watersheds.