Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item UNDERSTANDING CLIMATIC FACTORS DRIVING WILDFIRES IN THE WESTERN U.S.(2024) ZHANG, LEI; Li, zhanqing; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wildfires have profound and catastrophic impacts on landscapes and human society and act as important agents in the transformation of ecosystems. Over the past decades, the western United States (WUS) has experienced a significant increase of large wildfires, with substantial rise of the economic and ecological costs. Considerable research efforts towards understanding climate change as a primary driver of larger and more severe wildfires, which exacerbates summer drought, reduces spring snowpack, etc. However, the physical relationships among wildfires in North America (NA) and regional feedback processes to changes in the large-scale circulation, global dryness, and linkages to global warming are still poorly understood. Our observational analyses of wildfire-climate relationships in North America were conducted using diverse independent observations and reanalysis data sets for the period 1984–2014. Results show that the WUS has experienced the most robust increase in burned area. In addition to warming, the WUS has been under the influence of multi-decadal trends in tropospheric relative humidity deficit, reduced cloudiness, increased surface net insolation, and enhanced adiabatic warming and drying from increased tropospheric subsidence, as well as drying from enhanced offshore low-level flow. These trends are found to be associated with a widening of the descending branch of the Hadley circulation, consistent with climate model projections under greenhouse gases warming. This work sheds new light on the underlying regional climate processes affecting wildfire trends in NA and linkages with climate change under global warming. My second work focuses on analyzing the causes of the exceptional 2020 fire season in the WUS. Our comprehensive examination shows this extraordinary year for fires in the WUS is the results of “perfect storm”, a combination of multiple climate and weather extremes events. Extreme fuel aridity in September serves as a compelling example of the critical significance of tropospheric subsidence to the surface and atmospheric RH deficit. The third study evaluates performance of the Canada and US fire indices over the various ecoregions of the WUS. My study also finds Haines index combined with current index further improves the performance of conditional frequency distribution and predictive skill of large fires, suggesting the importance and merit of input from atmosphere dryness and stability into current fire indices.Item CLIMATE CHANGE RELATED EXTREME EVENTS AND ADVERSE HEALTH OUTCOMES AMONG HEMODIALYSIS PATIENTS(2024) Song, Hyeonjin; Sapkota, Amir AS; Epidemiology and Biostatistics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The increased frequency and intensity of extreme heat events (EHEs) and wildfires due to climate change are posing significant threats to vulnerable communicates including end-stage kidney disease (ESKD) patients. The specific aims of this dissertation are to Aim 1) Examine the association between EHEs exposure and serum concentrations of sodium and potassium among hemodialysis patients in the Western U.S. (2008-2018), Aim 2) Quantify the mortality and hospitalization risk associated with exposure to 2023 Canadian wildfire-related air pollution in the Eastern U.S., and Aim 3) Investigate how EHEs modify the association between wildfire-related air pollution exposure and the risk of mortality and hospitalization among hemodialysis patients in the Western U.S. (2010-2018). We analyzed health records of patients who receiving hemodialysis treatment at Fresenius Kidney Care clinics. We used the 10°C increase in daily average temperature and daily extreme heat events (EHEs) of each county as the primary exposures. The presence of wildfire smoke plume and wildfire fine particulate matter (PM2.5) concentrations for each clinic were measured using satellite-derived smoke polygons (Hazard Mapping System) and ground-based PM2.5 monitors (Air Quality System). We estimated mean serum sodium and potassium change per 10 °C increase in daily average ambient temperature using random intercepts linear mixed-effects models. We employed a time-stratified case-crossover analysis with conditional quasi-Poisson model to investigate the risks of mortality and hospitalization associated with exposure to wildfire-related air pollution and EHEs. In the first study, a 10°C increase in daily average temperature was associated with 0.43 mEq/L (95% Confidence Interval [CI]: 0.47, 0.59) increase in serum sodium during July-August. The serum sodium was 0.15 mEq/L (95% CI: 0.10, 0.20) higher during EHE days compared to non-EHE days. The serum potassium level did not show a significant change. In the second study, during June-July 2023, the presence of wildfire smoke plume was associated with an 18% increase in all-cause mortality risk (Rate Ratio [RR]:1.18; 95% CI: 1.13, 1.24) and a 3% increase in all-cause hospitalization risk (RR:1.03; 95% CI: 1.00, 1.07). A 10-μg/m3 increase in wildfire-related PM2.5 was associated with a 139% increase in all-cause mortality (RR: 2.39; 95% CI: 1.79, 3.18) and a 33% increase in all-cause hospitalization (RR:1.33; 95% CI: 1.10, 1.62). In the third study, we observed significant interactions between EHEs and wildfire smoke plume for mortality RRs among the hemodialysis patients in the Western U.S. Mortality risk was considerably higher when hemodialysis patients were simultaneously exposed to wildfire smoke plume and EHE compared to wildfire smoke plume alone (RR: 1.52; 95% CI: 1.25, 1.86 vs. RR: 1.15; 95% CI: 1.08, 1.23). We did not observe a significant interaction for all-cause hospitalization. Our findings underscore the need to revise operational and care protocols to prepare for such potential join exposures to extreme events that are exacerbated by ongoing climate change. Future work should focus on developing early warning systems to enhance resilience against such threats.Item COMPARISON OF IGNITION AND COMBUSTION CHARACTERISTICS OF PRESSURE TREATED WOOD AND TREX EXPOSED TO THERMALLY CHARACTERIZED GLOWING FIREBRAND PILES(2023) Lauterbach, Alec; Stoliarov, Stanislov I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent decades, the intensity of wildfires worldwide has escalated, leading to a rise in the destruction of structures and loss of lives within the Wildland-Urban Interface (WUI). Firebrands are small fragments of ignited vegetation or structural material that are carried by the plume of a wildfire, traveling in advance of the main fire front. Firebrand exposure has been recognized as the primary mechanism for the propagation of wildfires as well as a source of ignition of structural elements. However, this complex ignition process of structural elements in the WUI has yet to be fully understood. The ignition and combustion characteristics of a thermoplastic-wood composite (Trex) and Pressure Treated Wood (PTW), two frequently used WUI decking materials, when exposed to glowing firebrand piles were studied using a bench scale wind tunnel. An inert insulation material, ii Kaowool PM, was also used as a deposition substrate to quantify the heat feedback and combustion characteristics of solely the firebrand pile. Firebrand pile densities of 0.16 g cm-2 and 0.06 g cm-2 were deposited on each substrate in rectangular 10 cm x 5 cm orientations and exposed to air flow velocities of 0.9 m s-1, 1.4 m s-1, 2.4 m s-1, and 2.7 m s-1. Infrared camera measurements were used to determine the back surface temperatures of Kaowool PM tests. Using DSLR cameras, surface ignitions of the decking material in front of the firebrand pile (preleading zone ignition events), ignitions on top of the firebrand pile (pile ignition events), and surface ignitions of the decking material behind the firebrand pile (downstream ignition events) were visually quantified via their probability of ignition, time to ignition, and burn duration at each testing condition. A gas analyzer was used to compare combustion characteristics of Trex, PTW, and Kaowool PM tests through heat release rate (HRR) and modified combustion efficiency (MCE). Peak back surface temperatures of the firebrand pile were found to increase with increased air flow up to 2.4 m s-1, and then plateau. The same trend was observed for the ignition probabilities of preleading zone and pile ignition events. The probability of downstream ignition events increased with increasing air flow velocity. Peak HRR increased with increasing air flow velocity. Trex exhibited significantly less smoldering combustion than PTW yet was prone to more intense flaming combustion. When the rectangular 5 cm x 10 cm firebrand pile (10 cm edge facing the airflow), of which the majority of tests were conducted on, was rotated 90 degrees so that the 5 cm edge faced the airflow, the result was a significant decrease in the probability of ignition for both Trex and PTW, along with notable reductions in their HRR and MCE profiles.Item Machine Learning Approaches to Archaeological Predictive Modeling in the Age of Wildfire, Lake Tahoe Basin Management Unit, California and Nevada(2023) van Rensselaer, Maximilian; Palus, Matthew; Art History and Archaeology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Machine learning is a powerful tool for archaeological prediction mapping. This thesis compares machine learning approaches to Middle and Late Archaic archaeological prediction in the Lake Tahoe Basin Management Unit, California and Nevada. Specifically, the analysis seeks to answer whether logistic regression, Random Forest, or Maximum Entropy models perform better at archaeological prediction. The explanatory variables used to predict site presence include elevation, slope, aspect, distance to streams, land cover, soil, and geology. Of all three models, Maximum Entropy produced the most accurate predictive models based on combined diagnostic metrics. Predictive modeling is a valuable tool in preventative archaeology, where identifying and mitigating adverse effects to archaeological sites in a time-efficient manner is critical. Environmental challenges such as uncontrolled wildfires provide an impetus for indigenous communities, management agencies, and researchers to employ predictive modeling approaches in preventative cultural and heritage resource management applications.Item Firebrand Pile Thermal Characterization and Ignition Study of Firebrand Exposed Western Red Cedar(2021) Alascio, Joseph Anthony; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past several decades, the severity of wildfires across the world has grown, resulting in increased number of structures in the Wildland–Urban Interface being destroyed, and lives lost. An ignition pathway that has been identified to contribute to most structures destroyed during a wildland fire is that of firebrand ignition. Firebrands are small burning pieces of vegetative material that are lofted ahead of the fire front. This study seeks to quantify thermal conditions experienced by building materials exposed to accumulated firebrands and to identify conditions that lead to ignition of these materials. A bench scale wind tunnel was used to house a decking material, western red cedar, on which the firebrands were deposited, which allowed for testing at different air flow velocities, while simultaneously analyzing the temperature of the solid substrate and gaseous exhaust flow constituents to identify trends in flaming and smoldering combustion. Higher peak temperatures and larger heating rates were found with the exposure of a higher air flow velocity. An increased air flow velocity also allowed for quicker, more frequent, and longer sustained flaming of the firebrand pile. A Modified Combustion Efficiency (MCE) value of 0.81 ± 0.02 for the firebrand pile across all testing conditions was quantified, which is indicative of a hybrid–smoldering/flaming combustion mode.Item UNDERSTANDING THE INFLUENCE OF WIND AND SLOPE ON FLAMES IN WILDLAND FIRES(2019) Sluder, Evan; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wildland fire spread is typically described as a function of fuel, weather, and topography. An understanding of how these parameters are interrelated can help to close the gap on our understanding of the flame spread process. Fire spread over steep slopes is unique because the flame dramatically transitions from a detached, “plume” mode to an attached “boundary layer” mode at what appears to be a critical angle. The change in flame shape significantly increases the rate of spread by increasing the length and magnitude of heating ahead of the burning region. This attachment behavior has been observed in the literature, however the correlation between fire intensity, slope, wind, and flame shape is not yet well described or in a form which allows for prediction of fire behavior. A series of experiments using stationary gas burners have been undertaken to describe the behavior of a steady flame at multiple slopes under both wind and non-wind conditions. A stationary gas burner has been used to emulate flames under various fire intensities, burner aspect ratios, slopes, and wind conditions. A small-scale apparatus was first used to image hot gases using a shadowgraph technique coupled with downstream temperature measurements. Later a larger-scale apparatus was used with downstream temperature measurements to determine instantaneous downstream heating lengths. Both steady and time-dependent analysis of the attachment process is presented, along with its relation to fire spread. Based on the observed trends from these relationships non-dimensional parameters are introduced to relate the effects of inclination, wind, fire size and aspect ratio to the length of the heating region ahead of the burner. It is proposed that this value may be useful as a simple way to incorporate these effects in a wildland fire spread model.Item Critical Ignition Conditions of Structural Materials by Cylindrical Firebrands(2019) Salehizadeh, Hamed; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Smoldering firebrands (embers) are a major cause of ignition and eventual structural damage during wildfires at the wildland-urban interface (WUI). These small pieces of wood can loft several kilometers ahead of the main flame front and ignitespot fires directly on structural elements such as decks. In this research, dense structural materials such as wood and engineered wood will be studied with a focus on capturing the critical thermal conditions necessary for ignition. Unique to this study will be a configuration where whole piles of firebrands are placed on the recipient material, emulating observations from WUI fires. In order to design appropriate fire safety standards at the WUI and, someday, to model the propagation of these fires, the conditions leading to ignition of common WUI materials by piles of lofted firebrands must be quantified. Firebrands were modeled using small cylindrical wooden dowels which were ignited and placed in a small-scale wind tunnel. Two tests were performed for each loading condition of firebrands, one studying ignition of wooden structural elements such as decking and marine-grade plywood and another measuring temperatures and heat fluxes over an inert piece of ceramic insulation. A single-point water-cooled heat flux gauge was used for time-resolved measurements of heat flux at the center of the inert setup surrounded by thin-skin calorimeters and K-type thermocouples which allowed for a spatial characterization of heating. The wind speed was the main quantity of interest changed during the test to determine the effects of wind speed on the heat flux released from the glowing dowels to recipient fuels. The results showed a drastic increase in heating from piles of firebrands vs. individual brands. The piles also produced higher heat fluxes under increasing winds. This is due, for the most part, to higher surface temperatures resulting from increased surface oxidation under higher wind speeds. Both smoldering and flaming ignition of wood was found to be similarly dependent on wind speed. Larger piles also produced higher peak heat fluxes at the center of the pile, highlighting the role of re-radiation within the pile influencing heat fluxes to recipient fuels. Critical heat flux and firebrand loading conditions required to achieve smoldering and flaming ignition of structural materialsused in the WUI are determined by comparing tests with inert and flammable fuels. These critical conditions can be used to model the propagation of WUI fires over structural elements to design appropriate fire safety standards at the WUI. A non-dimensional relationship incorporating fuel type, geometry, and ambient conditions is also proposed to describe the results.Item Thermal Characterization of Firebrand Piles(2017) Hakes, Raquel Sara Pilar; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past several decades, the severity of wildland-urban interface (WUI) fires has increased drastically, resulting in thousands of structures lost globally each year. The cause of the majority of structure losses is ignition via firebrands, small pieces of burning material which are generated from burning vegetation and structures. In this thesis, a methodology for studying the heating to recipient fuels by firebrands is developed. Small-scale experiments designed to capture heating from firebrand piles and the process of ignition were conducted using laboratory-fabricated cylindrical wooden firebrands. The methodology compares two heat flux measurement methods. Experimental results compare the effects of varying firebrand diameter, pile mass, and wind speed. An ignition condition is described using temperature and heat flux.