The scour experiments on the bare root-free and root-permeated soils indicated that for each soil, critical tractive force (CTF) linearly increases with both root density and vane shear strength. However, for both soilsq CTF was mainly related to vaneýshear strengthp indicating the potential importance of soil shear strength as an index of scour erodibility of cohesive channel bank materials. The analysis of the relative effects of the grass vegetation parameters on scour resistance confirmed the dominance of vegetation shoots relative to the roots in resisting scour in-non-bending vege- tation. The results showed that it is the initial introduction of vegetation into bare (root-free) bank conditions that produces the greatest increase in scour resistance and that subsequent increases in vegetation density bring about relatively lower increases in scour resistance. However, in all the vegetation densities studied, root- permeated so-ils contributed significantly to scour resistance in low flows especially through low vegetation densities. Compared to root- free soil conditions, sandy clay loam soils permeated with 1.8 g/CM3 of roots increased their scour resistance by more than 400%. Althgouh these results may only be indicative of the low flow depths as would exist in shallow grassed channels commonly used for agricultural run- off drainage, they nevertheless highlight the importance of root density in contributing to the total flow resistance of grassed channel banks. The bank stability analysis indicated that for low channel banks (1.5m high), grass roots can stabilise banks with even vertical slopes against toe and slope vailures. For high (2.25m) and vertical bank conditions, the results indi- cate that the effects of increases in root density may need to be com- plemented by bank shaping in order to achieve stability. The scour and bank stability findings indicate that the three most important charac- teristics for the selection of grass vegetation for bank protection are quick establishmentg the development of a stiff shoot system and a strong root mat.

The geomorphic and geochemical characteristics of landscapes impose a physical template on the establishment and development of ecosystems. Conversely, vegetation is a key geomorphic agent, actively involved both soil production and sediment transport. The evolution of hillslopes and the ecosystems that populate them, are thus intimately coupled; their co-dependence potentially has a profound impact on the way in which landscapes respond to environmental change. This thesis explores how rates of erosion, integrated over millennia, impact on the structural characteristics of the mixed conifer forest that presently mantles this landscape, the development of the underlying soils and emergence of bedrock. The focus for this investigation is the Feather River Region in the northern Sierra Nevada in California, a landscape characterised by a striking geomorphic gradient accompanied by spatial variations in erosion rate spanning over an order of magnitude, from 20 mm ka-1 to over 250 mm ka-1. Using LiDAR data to quantify forest structure, I demonstrate that increasing rates of erosion drive a reduction in canopy height and aboveground biomass. Subsequently, I exploit a novel method to map rock exposure, based on a metric of topographic roughness, to show that as erosion rates increase and soil thickness consequently decreases, the degree of bedrock exposed on hillsides increases. Importantly, this soil-bedrock transition is gradual, with rapidly eroding hillslopes frequently possessing a mosaic of bedrock outcrop and intermittent soil mantle. Both the ecological and geomorphic trends are shown to be impacted by the underlying bedrock, which provides an additional source of heterogeneity in the evolution of the Feather River landscape. The negative correlation between AGB and erosion rate has potential implications for soil production. Using a simple hillslope model I show that if this decrease in AGB is associated with a drop in biotic soil production, then feedbacks between soil thickness and biotic soil production are capable of generating a complex response to geomorphic forcing, such that hillslopes possess multiple stable states: for intermediate rates of erosion, equilibrium hillslopes may be either soil mantled or bedrock. Hillslope evolution in these simulations is path dependent; once exposed at the surface, significant patches of bedrock exposure may persist over a wide range of incision rates.

Soil erosion is a global problem, affecting much of the world’s agricultural land. As the world’s population increases, the pressures placed upon the land resource to provide space for food production, leisure, housing and industrial facilities also increases. Thus it is vital that the land resource is as productive as possible. As soil erosion is the major cause of soil degradation globally, it is vital that methods for accurately monitoring the degree of erosion from a site, and the effectiveness of any remediation attempts are available. Reported here is the development of a novel soil erosion particle tracer, based upon a lanthanide chelate complex doped silica particle. The luminescent lanthanide chelate complexes were comprised of 2-thenoyltrifluoroacetone (TTA) and 2-pyridinol-1-oxide (2PO) coordinated with either trivalent europium or terbium ions. These complexes were then doped into silica sol-gel particles using a core shell technique. This method resulted in the synthesis of two luminescent soil tracers, targeted to two of the key eroded soil fractions; fine silt (63-250 μm) and clay (< 1.2 μm). The behaviour of the tracers was analysed within three different soils obtained from the Glensaugh research station. They retained their luminescence when mixed with soil, and could be detected at concentrations of 10 mg tracer / kg soil using a standard benchtop fluorescence spectrometer (Perkin Elmer LS55B). Scanning electron micrographs indicated that the tracer particles interacted with the soil particles, whilst soil sedimentation experiments demonstrated that the tracer particles had a similar sedimentation pattern to natural soil particles. Soil microbial respiration studies were performed for the tracers and showed that the tracers did not significantly impact the soil microbial population. Studies of the luminescence stability of the tracer in soil over time showed that the tracer could be detected in the soil for one season (approximately 3 months). A prototype rainfall simulator, designed to simulate the kinetic energy of raindrops on the surface of the soil, was developed, and used during a series of rainfall simulation experiments. These simulations were performed at two different rainfall intensities (30 and 90 mm.h-1) and both of these conditions resulted in movement of the tracer particles within the plot. This movement was both horizontal, in overland flow over the plot surface, and vertical, through the plot. The pattern of tracer movement reflected that of the soil mass moved, and as such indicated that the tracers exhibited similar transport behaviour during the erosion simulations performed. These initial simulations demonstrated that the tracers can be detected at low concentrations within the soil using standard laboratory equipment, and that they move with the eroded soil particles during simulated soil erosion experiments. As such, these tracers are excellent candidates for further study in larger scale erosion events.

This study originated from an evaluation of the performance of a commercially available high concentration point focus concentrator PV system. The effect of module design flaws was studied by using current-voltage (I-V) curves obtained from each module in the array. The position of reverse bias steps revealed the severity of mismatch in a string of series-connected cells. By understanding the effects of the various types of mismatch, power losses and damage to the solar cells resulting from hot spot formation can be minimized and several recommendations for improving the basic performance of similar systems were made. Concern over the extent and type of defect failure of the concentrator photovoltaic (CPV) cells prompted an investigation into the use of a light beam induced current (LBIC) technique to investigate the spatial distribution of defects. An overview of current and developing LBIC techniques revealed that the original standard LBIC techniques have found widespread application, and that far-reaching and important developments of the technique have taken place over the years. These developments are driven by natural progression as well as the availability of newly developed advanced measurement equipment. Several techniques such as Lock-in Thermography and the use of infrared cameras have developed as complementary techniques to advanced LBIC techniques. As an accurate contactless evaluation tool that is able to image spatially distributed defects in cell material, the basis of this method seemed promising for the evaluation of concentrator cells.

An evaluation of headwater erosion and sediment dynamics was carried out to assess the health of the Middle Mountain agricultural system in Nepal. Controversial statements predicting this system's imminent demise and identifying Middle Mountain farming practices as major contributors to downstream sedimentation and flooding have long been promoted and have suggested the following research hypothesis: soil and sediment dynamics and the indigenous management techniques within headwater Middle Mountains basins do not indicate a deterioration in the health of the agricultural system. Three questions were addressed in this research. What are the main controls on normal-regime erosion? How effective is the system of indigenous management at modifying sediment dynamics? What do headwater sediment budgets (erosion, storage, and yield) reveal about the health of the agricultural system? Answers to these questions are suggested and development initiatives proposed. Intensive monitoring was carried out during 1992-1994 within nested basins ranging in size from 72 to 11 141 ha. Variation of storm-period variables in time and space was assessed using five recording rain gauges and a network of up to fifty 24-hour gauges. Surface erosion was measured from five erosion plots on steep bari (rainfed cultivated land). Suspended sediment behaviour was examined through event sampling at seven hydrometric stations. Basin sediment yield was determined for three of these nested basins. Sediment storage was assessed using accumulation pins in khet fields (irrigated cultivated land), khet canals, and bari ditches and through erosion and channel surveys. An annual average of 77 storms were identified over the three-year period with 3.5% of these delivering more than 30 mm total rainfall and a peak 10-minute rainfall intensity of more than 50 mm/h. About 1/3 of all storms regardless of magnitude occurred during the pre-monsoon season. Pre-monsoon and monsoon storms delivered equivalent high-intensity short-term rainfall disputing the hypothesis that it is a higher rainfall intensity in the pre-monsoon season which causes an elevated sediment regime during that season. Total storm rainfall was significantly higher during the monsoon season whereas the period without rain before a storm begins was longer for pre-monsoon storms. The source of suspended sediment was found to vary with season and spatial scale. During the pre-monsoon season, surface erosion from bari was severe when high-intensity rain fell on bare ground. Indigenous farming practices were found to be effective at limiting surface erosion except during the pre-monsoon season when targeted intervention may be useful. During the pre-monsoon season, nutrient loss from headwater basins due to sediment export was at its highest. Severely degraded land remained bare throughout the rainy season, producing sediment at an elevated rate and in relation to total rainfall. The onset of the monsoon season reduced this bari source markedly due to the complete development of a vegetative cover under conventional management. The pre-monsoon-season surface-erosion mechanism of sediment production was replaced with scale-dependent mechanisms resulting from the higher total rainfall of monsoon-season storms. Within the steep terraced hillslopes, the capacity of runoff ditches was more often exceeded resulting in episodic-regime rilling, gullying, and in some instances, terrace failure. When sufficiently heavy and widespread, monsoon storm rainfall led also to stream discharge high enough to damage riparian areas and the system of irrigation dams. The farmers alter the sediment regimes profoundly and their management activities reduce soil loss collectively over all spatial scales. Sediment budgets reveal that a significant component of the sediment produced in the study basin (5.3 km2) was recaptured (35% to 50%) because of these indigenous farming practices. Objective calibration of indigenous knowledge showed it to be well founded but inconsistent. Farmers practise techniques which are well adapted to this environment reflecting their stated receptiveness to innovation and outside support. The detailed measurements show that the important controls on erosion are variable temporally and spatially over scales too small to be considered by conventional monitoring programs in these environments. Spatial differences in rainfall delivery, hysteresis effects, variability in land-surface response, and management activities conspire to yield sediment dynamics which are difficult or impossible to quantify with typical limited monitoring. Site-specific opportunities for investigation should be exploited and a high degree of uncertainty be anticipated. Management recommendations focus on two topics. An improved vegetative cover during the pre-monsoon season is required to reduce soil erosion during that period. Greater retention of these nutrient-rich soils would directly benefit the upland farmer. Rehabilitation of degraded lands and the halting or reversing of further degradation would benefit all farmers by providing a greater land base for biomass production especially in light of an increasing population. Both strategies would benefit hydropower developments by limiting reservoir sedimentation. Above all, proposed changes should enhance - not undermine - indigenous management. Current soil dynamics may be sustainable but it is unlikely that they can remain so in the future under the increased landuse intensification that may be necessary with projected population increases unless support is provided strategically from outside sources. Working with the farmers to develop techniques to improve their ability to recapture previously-eroded soil is a useful area of applied research. The high degree of skill and adaptability of the farmers within this environment suggest that carefully designed intervention which targets vulnerable aspects of the agricultural system while not undermining the present methods have a reasonable likelihood for success. / Graduate and Postdoctoral Studies / Graduate

Accelerated erosion is a major environmental, social and economical threat in South Africa. It is estimated that in excess of 400 million tones of soil is lost every year in South Africa, with much of this erosion being attributed to improper land management and little or no erosion control methods. This study was aimed to determine the efficiency of soil erosion structures, which were constructed in the Mabula Private Game Reserve (Limpopo Province), in reducing the amount of sediment eroded at the sites as well as to determine the amounts of sediments deposited due to their presence. Factors such as rainfall, slope, soil type and particle size were measured to determine their influence on sediment erosion and the ultimate deposition of this sediment. The geology of the area is characterised by Waterberg Sandstones and igneous intrusions of mostly granite. Rainfall in the area occurs between the October and April, with an average of 550 – 750 mm. The temperatures range from 18°C - 32°C during the summer months and 4°C to 22°C in the winter months. The study area falls within the Sour Bushveld and the Sourish Mixed Bushveld, according to the Acocks classification. These veld types are characterised by open an open savanna of tall trees and shrubs. Five sites affected by soil erosion were selected and at each site structures were constructed, the size and number of structures at each site was dependant on the extent of the sites area. The structures are made from a shade net product known as T65, a shade cloth developed by Alnet, which has not been UV treated allowing it to disintegrate in the open atmosphere after 2-3 years. Soil measurements and samples were taken before the rainfall season to establish baseline data to compare the results with once the filed work has been concluded. Thereafter samples and measurements were taken every month during the rainfall season, which extended from October 2004 to April 2005. In conjunction with the measurements taken at each structure the slope of each site was determined and the profiles of the sediment deposited at the structures after the rainfall season were studied. From the measurements it was clear that as the rainfall increased so the amount of sediment being deposited at the structures increased. There were structures that experienced more iii sediment deposition than others and theses differences could be attributed to factors such as parent material, soil texture, slope angle and soil type. Due to the deposition of sediment and the reduction in surface flow caused by the structures, vegetation established itself and began to flourish in the newly deposited soil. Through the establishment of vegetation at the sites an element of success was achieved. With a total of 2 101 mm of sediment being deposited at the structures and the establishment of vegetation in areas where there was none previously it can be concluded that the soil erosion structures are successfully curbing soil erosion in Mabula Private Game Reserve. / Professor J.T. Harmse

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Digital satellite remote sensing and Geographic Information Systems (GIS) have been used in conjunction with the Universal Soil Loss Equation (USLE) to model soil erosion potential within watersheds. This study compared erosion estimates calculated by the remote sensing method to results obtained in the field by soil conservationists using conventional methods.

No description available.

No description available.