Browsing by Author "Gates, Timothy K., committee member"

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Open Access
A combined field analysis and modeling approach for assessing the impact of groundwater pumping on streamflow
(Colorado State University. Libraries, 2018) Flores, Luke, author; Bailey, Ryan T., advisor; Gates, Timothy K., committee member; Sanford, William E., committee member
The magnitude of volumetric water exchange between streams and alluvial aquifers impacts contaminant transport rates, channel erosion and sedimentation, nutrient loading, and aquatic and riparian habitat. Quantifying the interactions between stream water and groundwater is also critically important in regions where surface water and tributary groundwater are jointly administered under a prior appropriation doctrine, such as in the western United States. Of particular concern is the effect of a nearby pumping well on streamflow. When the cone of influence of a pumping well reaches a nearby stream, the resulting hydraulic gradient can induce enhanced seepage of streamflow into the aquifer or decrease the rate of groundwater discharge to the stream. The change in these rates is often modeled using analytical or numerical solutions, or some combination of both. Analytical solutions, although simple to apply, can produce discrepancies between field data and model output due to assumptions regarding stream and aquifer geometry and homogeneity of hydraulic parameters. Furthermore, the accuracy of such models has not been investigated in detail due to the difficulty in measuring streamflow loss in the field. In the first part of this thesis, a field experiment was conducted along a reach of the South Platte River in Denver, Colorado to estimate pumping-induced streamflow loss and groundwater head drawdown, and compare data against analytical modeling results. The analytical solutions proved accurate if streamflow was low and constant, but performed poorly if streamflow was high and variable. In particular, the models are not capable of accurately simulating the effects of increasing stream width and bank storage due to rapid increases in streamflow. To better account for these effects a new analytical modeling framework is introduced which accounts for all major factors contributing to streamflow loss for a given site for both periods of pumping and periods between pumping. For the reach analyzed herein, the method illustrates that pumping wells often only caused half of the given streamflow loss occurring along the reach. This method can be used in other stream-aquifer systems impacted by nearby pumping. The U.S. Geological Survey's three-dimensional finite-difference groundwater flow model, MODFLOW, was also used to assess the impacts of pumping on streamflow. While MODFLOW removes many of the restrictive assumptions that define analytical solutions, certain limitations persist when the program is applied on local, fine scales with dynamic interactions between a stream and alluvium. In particular, when the average stream width is greater than the computational grid cell size, the model will return systematically biased, grid-dependent results. Moreover, simulated streamflow loss will be limited in the range of values that can be modeled. To address these limitations, a new stream module is presented which (1) allows for streams to dynamically span multiple computational grid cells over a cross section to allow for a finer mesh; (2) computes streamflow and backwater stage along a stream reach using the quasi-steady dynamic wave approximation to the St. Venant equations, which allows for more accurate stream stages when normal flow cannot be assumed or a rating curve is not available; and (3) incorporates a process for computing streamflow loss when an unsaturated zone develops under the streambed. Streamflow loss is not assumed constant along a cross section. It is shown that most streamflow loss occurs along stream banks and over newly inundated areas after increases in upstream streamflow. The new module is tested against streamflow and groundwater data collected in a stream-aquifer system along the South Platte River in Denver, Colorado and to estimate the impact of nearby pumping wells on streamflow. When compared with existing stream modules more accurate results are obtained from the new module. The new module can be applied to other small-scale stream-aquifer systems.
Open Access
Economic and environmental trade-offs of irrigation best management practices in the Lower Arkansas River Valley
(Colorado State University. Libraries, 2017) Orlando, Anthony, author; Hoag, Dana, advisor; Goemans, Christopher, committee member; Gates, Timothy K., committee member
The flows of the Arkansas River cascade through the Rocky Mountains and spill into Colorado's eastern plains. In the Lower Arkansas River Valley (LARV), these flows serve irrigators on over 250,000 acres, and are critical to the production of everything from corn to cantaloupes. Concurrent to the "goods" produced with this irrigation, are a series of "bads" occurring in the form of pollution. Elevated selenium, nitrate, and salinity concentrations have been related to high volumes of irrigation return flows, and threaten compliance with the Arkansas River Compact. Implementing a series of regional land and water Best Management Practices (BMPs) is thought to reduce the negative impacts ("bads") of irrigated agriculture in the region and in some cases, increase the productivity of land and water ("goods"). A deeper understanding of impacts of proposed BMPs is required. The specific question I hope to answer with this thesis is "What are the economic and environmental trade-offs face by Lower Arkansas River Valley producers when implementing a series a land and water best management practices?" To answer this question, an economic linear programming (LP) model is written to maximize regional net returns for a representative area within the LARV, using the General Algebraic Modeling System (GAMS). The LP is calibrated to match the physical characteristics of the region, historic water application volumes, and the historic crop mix. BMPs are tested by constraining various equations in the model, resulting in a series of economic measures. These economic measures are then compared to the output of a water flow, and reactive solute transport model to quantify the trade-offs that exist between regional net returns, in-stream selenium concentrations, in-stream nitrate concentrations, and yield losses to soil salinity. The results of this analysis suggest the existence of win-win scenarios, which increase net returns, and reduce pollution concentrations. No single BMP outperforms all others, supporting the notion that LARV producers and water policy makers face trade-offs in their efforts to control irrigation-induced pollution.
Open Access
Evaluation of flow and scalar transport characteristics of small public drinking water disinfection systems using computational fluid dynamics
(Colorado State University. Libraries, 2011) Wilson, Jordan M., author; Venayagamoorthy, S. Karan, advisor; Gates, Timothy K., committee member; Wickramasinghe, S. Ranil, committee member
This study focuses on the evaluation of flow and scalar transport characteristics of small disinfection systems, primarily through computational fluid dynamics (CFD) as well as physical conservative tracer studies. Original research was performed on a pipe loop, series of pressurized tanks, and two separate open surface tank contact systems and a case study was performed on a baffled tank system. The flow dynamics for each of these respective disinfection systems were evaluated using CFD. The flow dynamics govern the transport of any quantity (e.g., a passive scalar, conservative tracer, or chlorine-containing species) through the system visualized through plotting the effluent concentration (e.g., passive scalar for computational models and conservative tracer for physical experiments) through time forming what is commonly referred to as a residence time distribution (RTD), or flow-through, curve. Physical experiments provided validation for the CFD models that give a more complete view of hydraulic efficiency thus overcoming the common "black-box" approach to contact tank design using only the theoretical detention time (TDT) (defined as the system volume V divided by the volumetric flow rate Q). The differing geometries of contact tank systems yield significantly different flow paths with varying degrees of separation, recirculation, inlet and outlet effects, and wall effects prompting the need for the evaluation of hydraulic efficiency to be unique to the system. Yet current practice evaluates the hydraulic efficiency of disinfection contact tank systems based on the TDT and the rising limb of the RTD curve, designated by the United States Environmental Protection Agency (USEPA) as baffle factor (BF). Research presented in this study using CFD models and physical tracer studies shows that evaluation methods based upon TDT tend to overestimate, severely in some instances, the actual hydraulic efficiency as obtained from the systems' flow and scalar transport dynamics and subsequent RTD curves. The main objectives of this study were to determine the systems' respective hydraulic efficiencies and to analyze an alternative measure of hydraulic efficiency, the ratio t10/t90, where t10 and t90 are the time taken for 10 and 90 percent of the input concentration to be observed at the outlet of a system. The pipe loop system was dominated by advection and thus showed little variance in the values of BF and t10/t90. Analysis of the series of pressurized tank systems showed significant regions of turbulent mixing and recirculation corresponding to a system that was much less efficient than the pipe loop system. BF values for the pressurized tank systems were nearly 100 percent greater than t10/t90 values as a result of a system behavior further from plug flow. The open surface tank systems exhibited the most uneven flow paths and lowest efficiencies seen in this study with BF and t10/t90 values differing by at least 100 percent. These systems exhibited significant degrees of short-circuiting and recirculation largely due to their inlet and outlet configurations. Finally, the baffled tank system showed an increase in system efficiency with the number of baffles (e.g., increase in advective forces) and a corresponding decrease in the variance between BF and t10/t90 values. Overall, the research presented in this thesis provides an extensive evaluation for the flow and scalar characteristics of the described small public drinking water disinfection systems allowing for the development of t10/t90 as a more representative evaluation of hydraulic efficiency.
Open Access
Evaluation of the Kipp and Zonen large aperture scintillometer for estimation of sensible heat flux over irrigated and non-irrigated fields in southeastern Colorado
(Colorado State University. Libraries, 2012) Rambikur, Evan H., author; Chávez, José L., advisor; Andales, Allan A., committee member; Ham, Jay M., committee member; Gates, Timothy K., committee member
The aim of this work was to assess the performance of the Kipp and Zonen Large Aperture Scintillometer (LAS; Delft, Netherlands) to predict surface sensible heat flux (H). The LAS was introduced approximately 30 years ago and has been marketed as an indirect tool for the estimation of vegetation evapotranspiration (ET). Several tests have shown the LAS to be a fairly robust tool for prediction of H, both over homogeneous and heterogeneous surfaces. However, the Kipp and Zonen LAS has been criticized for overestimation of H and for significant inter-sensor deviation in H. Field experiments were performed in 2011 using three Kipp and Zonen LAS units over two different surfaces to assess the accuracy and inter-sensor variability. Accuracy was evaluated based on reference measurements from eddy covariance (EC) instrumentation, which provides direct measurement of sensible and latent heat fluxes. Notably the EC method has been criticized for systematic underestimation of the sensible and/or latent heat flux, but is nonetheless a common tool used to validate LAS data. The first experimental test site was predominantly dry and uniform grassland located near Timpas, CO. At this site, all three LAS units were deployed together for some time in order to assess inter-sensor variability and an EC system was installed for some duration of the LAS deployment. The EC system was subsequently moved to the second site, which was the Colorado State University (CSU) Arkansas Valley Research Center (AVRC) near Rocky Ford, CO. At the AVRC, one LAS unit was set up over irrigated alfalfa. Results from the inter-LAS comparison suggested that there may be some inherent variability between 6-11% in LAS-predicted H (HLAS) and that the physical alignment of the LAS is critical for maintaining good performance. Testing different methods for estimation of the friction velocity (u*) variable revealed bias between the logarithmic wind profile (LWP) result and the EC measurement. Linear regression slopes between 0.94 and 1.35 were found for HLAS with respect to EC-derived H (HEC) for the Timpas site - dependent on the LAS unit, the LAS alignment, and the u* method. The overall conclusion was that HLAS was reasonably accurate, partially due to the potential of HEC being underestimated on the basis of lack of energy balance closure. For the CSU AVRC (irrigated) site, HLAS was generally observed to be greater than HLAS by 20-30%. However, heat flux source area differences between the LAS and EC units may have contributed to some of the observed biases. Further, the overall conclusion of reasonable accuracy of HLAS was made, again partially due to potential for H underestimation by the EC system. It is recommended, nonetheless, for future applications to calibrate the Kipp and Zonen LAS to a reliable reference on the basis of observed inter-sensor variability. Further, the benefit of the LAS is judged to be higher for a scenario of limited or no irrigation than for one of full irrigation, since the contribution of H to the overall energy balance would be relatively small for a full irrigation scenario.
Open Access
Improved assessment of nitrogen and phosphorus fate and transport for intensively managed irrigated stream-aquifer systems
(Colorado State University. Libraries, 2019) Wei, Xiaolu, author; Bailey, Ryan T., advisor; Arabi, Mazdak, committee member; Gates, Timothy K., committee member; Covino, Timothy, committee member
Nitrogen (N) and Phosphorus (P) are essential elements for animal nutrition and plant growth. However, over the previous decades, excessive loading of fertilizers in agricultural activities has led to elevated concentrations of N and P contaminations in surface waters and groundwater worldwide and associated eutrophication. Therefore, precisely understanding and representation of water movement and fate and transport of N and P within a complex dynamic groundwater-surface water system affected by agricultural practices is of essential importance for sustaining ecological health of the stream-aquifer environment while maintaining high agricultural productivity. Modeling tools often are used to assess N and P contamination and evaluate the impact of management practices. Such models include land surface-based watershed models such SWAT, and aquifer-based models that simulate spatially-distributed groundwater flow. However, SWAT simulates groundwater flow in a simplistic fashion and therefore is not suited for watersheds with complex groundwater flow patterns and groundwater-surface interactions, whereas groundwater models do not simulate land surface processes. This dissertation establishes the modeling capacity for assessing the movement, transformation, and storage of nitrate (NO₃) and soluble P in intensively managed irrigated stream-aquifer systems. This is accomplished by (1) developing a method to apply the SWAT model to such a system, and includes: designating each cultivated field as an individual hydrologic response unit (HRU), crop rotations to simulate the impact of changing crop types for each cultivated field, including N and P mass in irrigation water, and seepage from earthen irrigation canals into the aquifer; (2) simulating land surface hydrology, groundwater flow, and groundwater-surface water interactions in the system using the coupled flow model SWAT-MODFLOW, with the enhanced capability of linkage between SWAT groundwater irrigation HRUs and MODFLOW pumping cells, and the use of MODFLOW's EVT package to simulate groundwater evapotranspiration; and (3) linking RT3D, a widely used groundwater reactive solute transport model, to SWAT-MODFLOW to credibly represent of NO₃-N and soluble P fate and transport processes in irrigated agroecosystems to evaluate best management practices for nutrient contamination. This last phase will also address the uncertainty in system output (in-stream nutrient loads and concentrations, groundwater nutrient concentrations model predictions). Each modeling phase is applied to a 734 km² study region in the Lower Arkansas River Valley (LARV), an alluvial valley in Colorado, USA, which has been intensively irrigated for over 130 years and is threatened by shallow water tables and nutrient contamination. Multiple best management practices (BMPs) are investigated to analyze the effectiveness in reducing NO₃-N and soluble P contamination in the LARV. These strategies are related to irrigation management, nutrient management, water conveyance efficiency, and tillage operations. The most effective individual BMP in most areas is to decrease fertilizer by 30%, resulting in average NO₃-N and soluble P concentrations within the region could be reduced by 14% and 9%, respectively. This individual BMP could lower the average NO₃-N concentrations by 19% and soluble P concentrations by 2%. Combinations of using 30% irrigation reduction, 30% fertilization reduction, 60% canal seepage, and conservation tillage are predicted to have the greatest overall impact that can not only provide a decrease of groundwater concentration in NO₃-N up to 41% and soluble P concentration up to 8%, but also reduce the median of the in-stream NO₃-N and soluble P to meet the Colorado interim standard. As nutrient conditions within the Lower Arkansas River Valley are typical of those in many other intensively irrigated regions, the results of this dissertation and the developed modeling tools can be applied to other watersheds worldwide.
Open Access
Insights and methodologies in wall-bounded turbulent channel flows
(Colorado State University. Libraries, 2024) Mishra, Harshit, author; Venayagamoorthy, Subhas Karan, advisor; Gates, Timothy K., committee member; Julien, Pierre Y., committee member; Barnes, Elizabeth, committee member
Wall-bounded channel flows are of massive interest to civil and environmental engineers due to their immense application for water supply and management. This dissertation addresses five key aspects of turbulent channel flows relevant to practicing engineers, laboratory researchers, fluid scientists, and consultants leveraging computational fluid dynamics for modeling turbulent flows. In the first study, a device was developed and tested to enable Particle Image Velocimetry (PIV) for free surface flows. Measuring flows reliably requires that illumination provided by the laser sheet remains undisturbed. In open channel flows, introducing the laser sheet from the free surface can be necessary as the bed may be optically opaque. An oscillating free surface can further complicate maintaining an undisturbed laser sheet. This research has shown that the disturbance of the laser sheet, when introduced from the free surface, can be mitigated by introducing an improvised device called an optical coupler. The effect of the coupler on the measured velocity field was systematically studied using independent Laser Doppler Anemometer (LDA) measurements. The effect of the coupler on the measured velocity field was confined to its vicinity near the surface of the flow. The mean flow profile remains largely unaffected. Additionally, appropriate material for fabricating the coupler has been recommended by studying the relative performance of a glass and acrylic coupler. While the glass coupler measurements were closer to the undisturbed flow profile, the durability and ease of handling an acrylic coupler make it a viable alternative. The second study is focused on ensuring fully developed flow in short laboratory flumes. Ensuring a fully developed flow is essential for any experimental or modeling study that involves wall-bounded flows. Flow development in pipes has been extensively studied, and empirical relationships have been widely published. Recently, similar studies on open channels have revealed that the entrance length in laboratory flume is ≈ 100h, where h is the depth of the flow. Such a prescription renders most laboratories unfit for experimental work. Further, the inlet configuration in the flume can also hamper flow development, even after the length requirements are met. In this study, we develop a methodology to obtain developed flow in short channels by modifying the inlet and tripping the boundary layer. Further, we also provide a robust, rapid test to confirm if the flow is fully developed using Direct Numerical Simulation (DNS) datasets. The proposed method is validated using flume experiments for flows with friction Reynolds number Reτ ∼ 1500−3000. Against the current prescription, we show that it is possible to obtain fully developed profiles within a distance of ≈ 20h from the inlet. In the next (third) study, we leverage the DNS data for closed channel flow for a range of friction Reynolds Number (Reτ ∼ 180 − 5000) to develop a new One Point Friction Velocity Method (OPFVM) to calculate friction velocity U∗ in terms of free-surface velocity Um, flow depth h and kinematic viscosity ν for smooth wall-bounded flows. In contrast to prevalent methods that require several cumbersome near-boundary measurements to obtain friction velocity, the OPFVM relies on a single easy-to-measure free-surface velocity measurement. The formulation obtains friction velocity for a closed channel flow (CCF) DNS regime with Reτ = 10049 and on four open channel flow (OCF) DNS regimes with Reτ ∼ 180 − 2000. The same formulation was then experimentally verified in our laboratory. To avoid being prescriptive, a sensitivity analysis was performed to determine the permissible variation in Um to restrict the error in estimated U∗ to 2%. The relationship between the depth-averaged velocity Ub and the maximum free-stream velocity Um is also explored using the DNS datasets and an approximate relationship between Ub and Um is proposed. With advances in remote sensing technology that enables free-stream velocity measurements, this method extends the potential to measure even the friction velocity remotely. Computational Fluid Dynamics (CFD) is an essential tool for analyzing fluid flows. The k − ϵ model is a turbulence model used in Raynold-Averaged Navier-Stokes simulations to close the Reynolds stress terms. The empirical constants used in k − ϵ model were obtained using experiments conducted at low Reynolds numbers several decades ago. In this study, we revisit the turbulent viscosity parameter Cµ, based on the stress-intensity ratio c2 = |uw|k. Here, |uw| and k are the absolute values of the Reynolds stress and turbulent kinetic energy, respectively. Through a-priori comparisons, we find that the widely accepted value of Cµ = 0.09, does not agree with the latest DNS and experimental datasets of wall-bounded turbulent planar flows. Therefore, a new value is suggested by averaging c2 in the equilibrium region, where the production (P) of k is within 10% of the dissipation rate(ϵ), and consequently, c4 ≈ Cµ. We evaluate flows up to friction Reynolds number Reτ ≈ 10000 and find that with increasing Reτ, Cµ approaches a value of 0.06, which is 50% lower than the prevalent value of 0.09. Finally, we perform an a-priori test with the new (proposed) value of Cµ = 0.06 to show that the estimated turbulent viscosity νT for wall-bounded flows is in much closer agreement with the exact (DNS) values than when νT is estimated using Cµ = 0.09. The final study develops a new scaling law for wall-bounded turbulent flows. This formulation eliminates all arbitrary constants and depends only on physical parameters, namely, the free-stream velocity Um, the friction velocity U∗, the kinematic viscosity ν, and the distance from the wall z. This is a significant step towards describing the velocity profile using these pertinent parameters.
Open Access
Investigating best management practices to reduce selenium and nitrate contamination in a regional scale irrigated agricultural groundwater system: Lower Arkansas River Valley, southeastern Colorado
(Colorado State University. Libraries, 2015) Tummalapenta, Ravi Kumar, author; Bailey, Ryan T., advisor; Gates, Timothy K., committee member; Venayagamoorthy, Subhas K., committee member; Bauder, Troy A., committee member
The Lower Arkansas River Valley (LARV) is well known for its rich agricultural production, with 109,000 ha of irrigated area. Due to agricultural production extending for more than 100 years, the LARV now faces challenges of soil salinity, water logging from shallow groundwater tables, and a high concentration of selenium (Se, both within the alluvial aquifer system and within the Arkansas River and its tributaries). Se originates primarily from bedrock and outcropped marine shale, released due to chemical oxidation in the presence of dissolved oxygen and nitrate. Se is a dynamic element that is biologically essential for plants, animals and humans. However, it is known that Se can be harmful at elevated concentrations. Therefore, elevated concentration levels in the surface water and groundwater in the LARV are considered problematic, and methods must be found to decrease groundwater concentrations and Se loadings from the aquifer to the Arkansas River. This thesis assesses plausible methods that will decrease Selenium (Se) contamination in groundwater and surface water in the LARV. Best management practices (BMPs) to reduce selenium and nitrate mass loadings to the River Arkansas in a 55,200 ha area downstream of John Martin reservoir in the LARV were explored and analyzed using 18 scenarios. The UZF-MODFLOW and UZF-RT3D numerical models, calibrated against extensive sets of field data in the region, were used to simulate groundwater flow and the physical and chemical processes governing the fate and transport of Se and N species. Specific BMPs include reduction in the seasonal application of N fertilizer; decrease in concentration of selenate (CSeO4) and nitrate (CNO3) in canal water, representing treatment of water before application as irrigation water; reduction in irrigation application volumes; and combinations of these practices, along with fallowing of irrigated land. These practices are applied for a long term period (40 years) to observe the effects of each BMP on groundwater CSeO4 and CNO3 and on mass loadings from the aquifer to the Arkansas River. The BMPs are applied at varying levels: less aggressive (20%) to very aggressive (40%) of each practice. Results indicate that the highest aggressive combined scenario 40% reduction in N fertilizer reduction, 40% reduction in canal concentration, and 35% reduction in irrigation volume, with 25% irrigated land fallowing result in the highest decrease of mass loadings of SeO4 into the Arkansas River with 22.7%, followed by the less aggressive and highest aggressive combined scenarios of N fertilizer reduction, canal concentration reduction, and irrigation volume reduction with land fallowing showing decrease of mass loadings from 15% to 21%. For individual scenarios: the irrigation volume reduction scenario (13.1% to 13.4%) is followed by the canal concentration reduction scenario (3% to 6%); whereas the N fertilizer reduction scenario shows a minimum percent reduction (1.5% to 2.7%) as compared to the Baseline (“do-nothing” scenario). Similarly for NO3, results show that the highest aggressive combined scenario 40% reduction in N fertilizer reduction, 40% reduction in canal concentration, and 35% reduction in irrigation volume, with 25% irrigated land fallowing result in the highest decrease of mass loadings of NO3 to the Arkansas River with 34.7% followed by the less aggressive and very aggressive combined scenarios of N fertilizer reduction, canal concentration reduction, and irrigation volume reduction with land fallowing showing reduction of mass loadings from 15.5% to 30%. The results of individual BMPs is as follows: 35% irrigation volume reduction scenario (14.9%) is followed by 40% N fertilizer reduction scenario (14.5%); 20% irrigation volume reduction scenario (12%); 20% N fertilizer reduction scenario (8.3%); whereas 20% and 40% canal concentration reduction scenarios show minimum percent reduction (0.6% to 1.1%). The results are compared with results from a similar study recently performed in the Upstream Study Region of the LARV to observe the differences in BMP practices and their reduction of Se contamination in the study areas.
Open Access
Radio frequency field strength fluctuation due to digital conversion of television signals: a pilot study
(Colorado State University. Libraries, 2010) Lane, P. Brian, author; Johnson, Thomas E., advisor; Gates, Timothy K., committee member; Zimbrick, John D., committee member
All television stations in the United States ceased broadcasting on analog airwaves June 12, 2009 and now only broadcast in a digital format. Prior to June 12th, most stations broadcast in both analog and digital signals. The focus of this study was to determine whether this change in broadcasting affected exposures to radio frequency energies in the vicinity of Lookout Mountain in Golden, Colorado. The site, which is approximately 10 miles west of the Denver metropolitan area, is unique because there are homes located at and above the elevation of the transmitting towers with some homes located within 100 yards of the towers. There is public concern that the digital transition resulted in a significant increase in radio frequency exposure to homes. Measurements of radio frequency field strengths were taken during daylight hours at 21 locations where highest exposures were expected using an electromagnetic radiation meter. Measurements taken at the same locations before and after June 12, 2009 did not indicate a statistically significant change in radio frequency exposures and all measurements were below the Maximum Permissible Exposure (MPE) limit for the general public.
Open Access
TREX-SMA: a multi-event hybrid hydrologic model applied at California Gulch, Colorado
(Colorado State University. Libraries, 2012) Halgren, James, author; Julien, Pierre Y., advisor; Kampf, Stephanie K., committee member; Gates, Timothy K., committee member; Venayagamoorthy, S. Karan, committee member
This dissertation describes a hydrologic model, Two-Dimensional Runoff Erosion and Export (TREX) Soil Moisture Accounting (SMA), created from adding the Sacramento Soil Moisture Accounting model (SAC-SMA) to the TREX surface hydrology model. TREX-SMA combines the capabilities of TREX as a distributed physical surface hydrology model with a conceptual rendering of infiltration and return flow as found in SAC-SMA. In order to form the hybrid, infiltrated water (computed as a distributed function on the surface) is aggregated as an input to a system of soil moisture accounting zones, underlying the entire watershed. In each model time step, TREX SMA releases baseflow from the accumulated infiltrated water according to simple transfer functions. Evapotranspiration (ET) losses from the soil moisture zones are computed based on potential ET demand and available water. As baseflow and ET are released between precipitation events, TREX SMA recovers capacity in the soil moisture zones. Based on the simulated recovery, the model then re-initializes the infiltration parameters of the surface model to prepare for the next event, allowing continuous simulation of multiple events. The capabilities of the TREX SMA model to continuously simulate soil moisture, infiltration, and rainfall-runoff are demonstrated with an application to multi-event modeling on the 30 km2 California Gulch watershed, near Leadville, Colorado, United States. Precipitation inputs are derived from measurements at a system of six precipitation and stream flow gauges providing ten-minute data for the summer of 2006. Eight major events were recorded during this time with runoff produced at all gauges. One additional event with partial watershed response was also evaluated for a total of 54 event hydrographs in the 50-day simulated series. Time steps in the simulation ranged between 2.0 and 4.0 seconds. Parameters for the surface hydrology were obtained from a prior calibration of TREX and were distributed across 34,000 grid cells based on the 30-meter United States Geological Survey (USGS) Digital Elevation Model (DEM). Parameters for the soil moisture zones were obtained from a-priori estimates used by the Arkansas Basin River Forecast Center of the National Weather Service (NWS) of the National Oceanographic and Atmospheric Administration (NOAA) in their real-time operational flood forecasting model for the Arkansas River. Using conceptual soil moisture states to re-initialize distributed infiltration parameters, the simulation results with TREX SMA improved relative to results from the unmodified TREX model with constant infiltration parameters. Model results are processed using gnuplot to create real-time hydrograph plots as the simulation progresses. Gnu R scripts produce real-time plots of simulated minus observed residual and statistical analyses as the simulation progresses. Statistics generated for each gauge include Nash-Sutcliffe, percent bias, absolute percent bias, Pearson correlation and modified Pearson correlation, and mean-squared error. These statistics were generated both for the entire simulation series and for each individual storm event. The gnuplot and R plots are produced using web-based technology for instantaneous sharing via the Internet. Model results such as surface and channel water depth are processed with GRASS GIS and KML scripts to create 2.5 dimensional, browseable animations overlaid on a Google Earth terrain. Statistical measures of the improvement of TREX SMA over TREX are presented in this dissertation. The overall accuracy, measured by the Nash-Sutcliffe coefficient, improved in four out of six gauges. Peak over-estimation was corrected in a majority of the 54 peaks evaluated. Implementation of the TREX SMA soil moisture accounting algorithm to re-initialize the infiltration parameters reduces the total absolute peak error from 180% to 135% of the observed peak flow rates. The Nash-Sutcliffe model efficiency improved over standard TREX simulations by 43%, 11%, 5%, and 10% at CG-1, CG-4, CG-6, and SHG09A.
Open Access
Use of innovative techniques to optimize the residence time distribution of drinking water contact tanks
(Colorado State University. Libraries, 2014) Kattnig, Justin J., author; Venayagamoorthy, S. Karan, advisor; Gates, Timothy K., committee member; Sakurai, Hiroshi, committee member
The focus of this study is to understand the complex nature of flow dynamics within water disinfection contact tanks and to use this understanding in the development of beneficial tank modifications. In particular this study focuses on systems classified as small by the United States Environmental Protection Agency (USEPA). Methods involved in this process included the use of computational fluid dynamics (CFD), physical tracer studies, and acoustic doppler velocimetry (ADV). Attempted tank alterations included the installation of baffles, inlet modification, and the use of industrial packing material. Tested modifications aimed at altering existing velocity fields in order to increase the hydraulic disinfection efficiency of a given system. Hydraulic disinfection efficiency was measured through the use of residence time distribution (RTD) curves and the well-known baffling factor (BF) (as defined by the USEPA). The principal system that was investigated was a 1500 gallon rectangular concrete tank with a sharp circular inlet. A physical prototype of this system currently resides at Colorado State University's (CSU) Engineering Research Center (ERC) and was used for all physical testing. CFD models were used to compute the average velocity fields within the tank and to produce modeled RTD curves. This was done for the empty tank and for 37 different baffled configurations. Baffles were placed parallel to the longest axis of the tank and varied in number and length. Optimal configurations yielded baffling factors between 0.70 and 0.8, which is more than thirteen times as efficient as the original system. Several configurations were selected and physically constructed in the existing tank in order to validate the applied numerical methodology. After CFD models were experimentally validated, random packing material was placed within the tank at areas of high velocity and flow separation (at the inlet and at baffle turns). An extensive parametric study was conducted in order to determine the effects of using packing material as an inlet modifier within the open tank. Packing material was placed in box-like structures and fastened over the inlet. Dimensions of these packing boxes were systematically varied and tested at different flow rates. Observed baffling factors were as high as 0.36, which represents an improvement over the basic system by a factor of six. Resulting findings from the inlet modification study were then used to design and test internal modifications for a baffled system. In addition to material being placed over the inlet, structures were placed over channel openings at baffle turns. Configurations were tested at a number of flow rates in order to determine relative effects on gains in efficiency. The most effective system obtained a baffling factor of 0.72, representing an increase from the base system by a factor of 13. ADV measurements were conducted within the baffled system in order to assess changes in the velocity field and explain observed increases in baffling factor. Packing material was not modeled due to complexity and high computational cost. Results from this study show that the innovative use of industrial packing material and other modifications can significantly increase the hydraulic disinfection efficiency of simple systems. It also shows that the use of CFD is an invaluable guide in this endeavor. The work summarized in this thesis aids in an ongoing effort to understand the hydraulic characteristics of small scale drinking water systems. The findings summarized here will help to shape the designs of the future.