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Soil erosion is recognized as a global threat against the sustainability of the natural ecosystem and the environment because of its severe effects in agricultural productivity, damage to infrastructure and pollution of water bodies. Adverse impacts due to human activities resulting in accelerated soil erosion process have been well documented. Much more attention has been given to study the mechanisms associated with the process of soil erosion in the second half of the 20th century. Different mathematical models have been developed to simulate soil erosion processes and incorporate the result in different options of erosion controls. Modelling soil erosion is a complex process that involves numerous parameters. It is for this reason that even highly sophisticated and advanced erosion prediction models like Water Erosion Prediction Project (WEPP) do not incorporate all mechanisms of the soil erosion process. An obvious gap is the satisfactory explanation and incorporation of soil erosion mechanism associated with the initial portion of microchannels where both inter-rill and rill erosion exist. This study attempts to fill this gap through extension of knowledge in the area of soil erosion mechanism, specifically within the initial portions of rill where both splash erosion and erosion due to shear stress exist. Detachment of soil particles from the soil surface depends on the kinetic energy imparted by raindrops. Therefore, it is essential to estimate kinetic energy as accurately as possible to enable study of soil erosion and infiltration mechanisms. Rainfall simulation is widely used to generate rainfall of desired intensities and durations to study soil erosion, infiltration and other dynamic behaviours of soil. Kinetic energy of a rainfall event is often estimated from its intensity. The actual kinetic energy imparted on a soil surface is generally less than the total value of kinetic energy of a rainfall event. This is because of the cushioning effect of the overland flow. Therefore, there is a potential risk of over prediction of splash erosion by an erosion prediction model that does not account for this cushioning effect. In this study, experiments were carried out to estimate the kinetic energy of three different simulated rainfall events produced by three different combinations of pressures and nozzle sizes. The equipment consisted of a multipurpose hydraulic flume, 2m long and 1.4m wide. Five highly sensitive force transducers were mounted on the surface of the flume to measure the impact of raindrops. Different slopes were represented in the experiment by tilting the flume in four different angles from 0 to 15 degrees. Two tipping bucket rain gauges were used to measure rainfall intensity. The nozzles were placed at a height sufficient to produce terminal velocity by the falling rain drops before they hit the flume surface. Overland flow was generated by continuously supplying water to the inlet tank constructed at the upstream of the hydraulic flume. Responses received from the transducers (in the form of voltage) and from the tipping bucket (in the form of pulses) were recorded at regular intervals. Based on this experimental study, a logarithmic energy loss model that accounts for the depth of shallow overland flow, rainfall intensity and bed slope to estimate potential loss of kinetic energy is proposed. Analysis of the results from the study indicated a significant reduction in kinetic energy when the surface flow starts to build up. The analysis also indicated that a significant portion of the energy is lost even though the flow depth is small. This implies that while splash erosion initially contributes to the total amount of soil erosion, most of the erosion after the initial phase is due to the flow induced shear stress. Another important conclusion of this study is that steeper the slope, the lesser the expected overland flow depth and hence more potential for splash erosion and sheet erosion. The Nash Sutcliffe model efficiency statistic of 90% obtained from this study signifies that the model could be used as a useful predictive tool to estimate rainfall kinetic energy loss. The energy loss model developed as a result of this study can be incorporated in process-based soil erosion models to accurately estimate splash erosion and improve the predictive power of these models. In Addition, the model can be used to estimate the critical depth of overland flow when the kinetic energy approaching the soil surface is practically nil. This critical depth can be used to define the transition zone and explicitly define the term “Rill”. The multipurpose hydraulic flume designed and developed for this study can be used for further studies in area of hydraulic and soil erosion research. The methodology developed in this research will be helpful in carrying out further experiments and improve the proposed energy loss model. The potential Future improvements to the model include the followings: i) incorporating the effect of sediment concentration, ii) using wider ranges of intensities, and iii) using an actual soil bed. / Doctor of Philosopy (PhD)

Stepped chutes have been used as hydraulic structures for more than 3.5 millennia for different purposes: For example, to dissipate energy, to enhance aeration rate in the flow and to comply with aesthetical functions. They can be found acting as spillways in dams and weirs, as energy dissipators in artificial channels, gutters and rivers, and as aeration enhancers in water treatment plants and fountains. Spillways are used to prevent dam overtopping caused by floodwaters. Their design has changed through the centuries. In ancient times, some civilizations used steps to dissipate energy in open channels and dam over-falls in a similar fashion as natural cascades. However, in the first half of the twentieth century, the use of concrete became popular and the hydraulic jump was introduced as an efficient energy dissipator. In turn, the use of a stepped geometry became obsolete and was replaced with smooth chutes followed by hydraulic jump stilling basins. In recent years, new construction techniques and materials (Roller Compacted Concrete RCC, rip-rap gabions, wire-meshed gabions, etc.) together with the development of new applications (e.g. re-aeration cascades, fish ladders and embankment overtopping protection or secondary spillways) have allowed cheaper construction of stepped chutes, increasing the interest in stepped chute design. During the last three decades, research in the hydraulics of stepped spillways has been very active. However, studies prior to 1993 neglected the effect of free-surface aeration. A number of studies since this time have focused on air-water flows in steep chutes (θ ≈ 50o). But experimental data is still scarce, and the hydraulic performance of stepped cascades with moderate slope is not yet understood. This study details an experimental investigation of physical air-water flow characteristics down a stepped spillway conducted in two laboratory models with moderate slopes: the first model was a 3.15 m long stepped chute with a 15.9o slope comprising two interchangeable-height steps (h = 0.1 m and h = 0.05 m); the second model was a 2.5 m long, stepped channel with a 21.8o slope comprising 10 steps (h = 0.1 m). Different arrangements of turbulence manipulators (vanes) were also placed throughout the chute in the second model. A broad range of discharges within transition and skimming flow regimes was investigated to obtain a reliable representation of the air-water flow properties. Measurements were conducted using single and double tip conductivity probes at multiple span wise locations and at streamwise distances along the cavity between step edges to obtain a complete three-dimensional representation of the flow. Although the present study was conducted for two moderate slope chutes (θ = 15.9º & 21.8o), it is believed that the outcomes are valid for a wider range of chute geometry and flow conditions. The purpose of this study is to improve the understanding of turbulent air-water flows cascading down moderate slope stepped chutes, and gain new understandings of the interactions between aeration rate, flow turbulence and energy dissipation; scale effects are also investigated. The study provides new, original insights into air-water turbulent flows cascading down moderate slope stepped spillways not foreseen in prior studies, thus contributing to improve criterion designs. It also presents an extensive experimental database (available in a CD-ROM attached at the end of this thesis) and a new design criterion that can be used by designers and researchers to improve the operation of stepped chutes with moderate slopes. The present thesis work included a twofold approach. Firstly, the study provided a detailed investigation of the energy dissipative properties of a stepped channel, based upon detailed airwater flow characteristics measurements conducted with sub-millimetric conductivity probes. Secondly, the study focused on the microscopic scale properties of the airwater flow, using the experimental data to quantify the microscopic scale physical processes (e.g. momentum transfer, shear layer development, vertical mixing, airbubbles/ water-droplets break-up and coalescence etc.) that are believed to increase the flow resistance in stepped canals. The study highlighted the tridimensionality of skimming flows and hinted new means of enhancing flow resistance by manipulating turbulence in the stepped chute. Basic dimensional analysis results emphasized that physical modelling of stepped chutes is more sensitive to scale effects than classical smooth-invert chute studies and thus suggested that the extrapolation of results obtained from heavily scaled experimental models should be avoided. The present study also demonstrated that alterations of flow recirculation and fluid exchanges between free-stream and cavity flow affects drastically form losses and in turn the rate of energy dissipation. The introduction of vanes demonstrated simple turbulence manipulation and form drag modification that could lead to more efficient designs in terms of energy rate dissipation without significant structural load on the stepped chute.

Thesis(M.S.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.

Thesis (M. Sc.)--University of Alberta, 2009. / Title from pdf file main screen (viewed on Sept. 1, 2009). "A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering, Department of Chemical and Materials Engineering, University of Alberta." Includes bibliographical references.

Ulm, Univ. Diss., 2006.

Konstanz, Univ., Diplomarbeit, 2006.

Thesis (Ph. D.)--University of Texas at Austin, 2009. / Title from PDF title page (University of Texas Digital Repository, viewed on Aug. 6, 2009). Vita. Includes bibliographical references.

Thesis (M.S.)--University of Wyoming, 2008. / Title from PDF title page (viewed on June 24, 2009). Includes bibliographical references.

abstract: Gallium Nitride (GaN) based microelectronics technology is a fast growing and most exciting semiconductor technology in the fields of high power and high frequency electronics. Excellent electrical properties of GaN such as high carrier concentration and high carrier motility makes GaN based high electron mobility transistors (HEMTs) a preferred choice for RF applications. However, a very high temperature in the active region of the GaN HEMT leads to a significant degradation of the device performance by effecting carrier mobility and concentration. Thus, thermal management in GaN HEMT in an effective manner is key to this technology to reach its full potential. In this thesis, an electro-thermal model of an AlGaN/GaN HEMT on a SiC substrate is simulated using Silvaco (Atlas) TCAD tools. Output characteristics, current density and heat flow at the GaN-SiC interface are key areas of analysis in this work. The electrical characteristics show a sharp drop in drain currents for higher drain voltages. Temperature profile across the device is observed. At the interface of GaN-SiC, there is a sharp drop in temperature indicating a thermal resistance at this interface. Adding to the existing heat in the device, this difference heat is reflected back into the device, further increasing the temperatures in the active region. Structural changes such as GaN micropits, were introduced at the GaN-SiC interface along the length of the device, to make the heat flow smooth rather than discontinuous. With changing dimensions of these micropits, various combinations were tried to reduce the temperature and enhance the device performance. These GaN micropits gave effective results by reducing heat in active region, by spreading out the heat on to the sides of the device rather than just concentrating right below the hot spot. It also helped by allowing a smooth flow of heat at the GaN-SiC interface. There was an increased peak current density in the active region of the device contributing to improved electrical characteristics. In the end, importance of thermal management in these high temperature devices is discussed along with future prospects and a conclusion of this thesis. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2016