No description available.

No description available.

Studies have documented spatially and temporally variable rates of surface subsidence across the Mississippi River delta plain of Louisiana. Variations in patterns and rates of delta plain subsidence may reflect subsurface distribution of compactionprone lithosomes. This research investigates historical changes in the surface geomorphology of the Caillou Bay headland in relation to the distribution of subsurface lithosomes. The stratigraphic framework was developed for the headland, and lithosomes were identified to establish the distribution of different sedimentary units. The geomorphic evolution as indicated by maps was then evaluated in order to locate patterns of shoreline change and wetland loss for the headland. Land loss maps developed were overlain on lithosome contour maps to calculate amounts of land loss overlying each lithosome contour interval. Analysis of results revealed that land loss was not uniform throughout the headland and that land loss patterns for several time periods varied as a function of the thickness of compactionprone lithosomes.

Field and modelling investigations of eight failed or removed dams have been undertaken to examine the upstream effects of low head dam decommissioning on channel morphology. Failed or decommissioned sites were selected such that no upstream interventions or channel mitigation had been applied since the time of decommissioning resulting in a physically-based analog consistent with the passive dam removal restoration approach. Field surveys of the sites, which failed between 2 years and 70 years ago, included longitudinal profiles, cross-sections and bed material pavement sampling on each riffle, run, and headcut. Findings demonstrate that vertical disturbances typically in the form of headcuts frequently extend well beyond the backwater limits of most reservoirs. Although in most cases, critical velocity and shear stress thresholds were exceeded, the localized increases in friction slope where headcuts occurred demonstrated that the velocities associated with larger flows exceeded critical thresholds more often than critical shear stress thresholds. Findings show that if the grain size distributions of the underlying alluvial geologic units are close to that of critical velocity thresholds, when headcuts are initiated (with their resulting increase in friction slope), they can result in continued channel degradation upstream of impoundment regions.

Field and modelling investigations of eight failed or removed dams have been undertaken to examine the upstream effects of low head dam decommissioning on channel morphology. Failed or decommissioned sites were selected such that no upstream interventions or channel mitigation had been applied since the time of decommissioning resulting in a physically-based analog consistent with the passive dam removal restoration approach. Field surveys of the sites, which failed between 2 years and 70 years ago, included longitudinal profiles, cross-sections and bed material pavement sampling on each riffle, run, and headcut. Findings demonstrate that vertical disturbances typically in the form of headcuts frequently extend well beyond the backwater limits of most reservoirs. Although in most cases, critical velocity and shear stress thresholds were exceeded, the localized increases in friction slope where headcuts occurred demonstrated that the velocities associated with larger flows exceeded critical thresholds more often than critical shear stress thresholds. Findings show that if the grain size distributions of the underlying alluvial geologic units are close to that of critical velocity thresholds, when headcuts are initiated (with their resulting increase in friction slope), they can result in continued channel degradation upstream of impoundment regions.

Doctor Educationis / Recent research has highlighted the importance of cut-and-fill cycles in valley-bottom wetlands. This study considers the impact of longitudinal and lateral sediment connectivity on the natural recovery potential of valley-bottom wetlands. Pietersielieskloof is a Prionium serratum (commonly known as palmiet) dominated discontinuous valley-bottom wetland. P. serratum is considered to be a peat-forming ecosystem engineer that enhances sediment infill in valleybottom wetlands. Due to its ecological importance and potential as a carbon store, this wetland has been earmarked for rehabilitation by Working for Wetlands. The study ascertains the importance of including sedimentological and geomorphological input in wetland rehabilitation and management strategies. A study of wetland geomorphology was conducted to develop an understanding of the natural dynamic of cut-and-fill processes as context for recent erosion and deposition events. Sediment samples from gully walls and cores were collected for organic content and particle size analysis and five sediment samples predating the current phase of erosion were radiocarbon dated. The valley form was surveyed using cross-sections and long profiles, and historical change was digitised using 30 m – 5 to 30 mm resolution aerial imagery from 1938-2016 in ArcMap.

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Coastal wetlands are dynamic ecosystems that are highly susceptible to change due to natural and human factors. The study area, located within the Native Dog Creek sub-catchment of the Logan River - which drains into Moreton Bay, south east Queensland - holds a detailed history of environmental change spanning most of the Holocene epoch. This history is preserved in the estuarine sedimentary record and is a valuable indicator of natural environmental change. More recently, human-induced changes within the study area have been superimposed on the natural process of environmental change. In order to develop a conceptual bio-geomorphic model of the coastal wetlands of Native Dog Creek, this thesis examined - on an integrated catchment basis - the evolution and connectivity of four coastal wetland community types (Melaleuca, Casuarina, saltmarsh and mangroves). The research consisted of four discrete studies within the study area: a geomorphic investigation that provided a framework for understanding how the wetlands evolved during the Holocene epoch; an acid sulfate soil (ASS) study that surveyed the distribution and concentration of sulfides; a palynological study that examined the natural directions of ecosystem change; and an investigation of the impact of specific human activities on these ecosystems. Detailed stratigraphic modelling found that the Logan River system (and its Native Dog Creek sub-catchment) has evolved from an infilling estuary since the peak of the Holocene transgression 6500 years before present. Recognition of the major controls that influenced geomorphic coastal development during the Holocene, provided important insights into the distribution and genesis of estuarine pyritic sediments which strongly influence the soils within the study area. In general, the estuarine central basin and fluvial delta sediments posed the greatest risk to the environment from acidification if disturbed. The major focus of the ASS study was to survey the distribution of ASS and to identify other areas most vulnerable to acidification. A predictive approach that combined chemical and stratigraphic analysis was used. Results showed that these areas are intrinsically related to their environment of deposition. The study found, for example, that the alternation of excessively wet and dry conditions - combined with high organic carbon levels and variations in microtopography - provided ideal conditions for the re-formation of pyrite in the stream channel within the Melaleuca wetlands. The palaeo-environmental study reconstructed the evolution of Holocene coastal wetland vegetation during the marine transgression and subsequent shoreline progradation. Pollen records from the four representative wetland communities (previously mentioned) were examined. The results found the mid-late Holocene vegetation history was controlled by the development of geomorphic features that have affected freshwater input, drainage and salinity. In response to the progradation of the shoreline after sea level stabilised, changes in fossil pollen from mangroves and saltmarsh taxa during the early-mid Holocene, to freshwater taxa during the late Holocene, are estimated to have taken 800 years. Thus, pollen analysis when used in combination with stratigraphic modelling, provided an important point of reference for rates of natural ecological change in response to evolutionary changes to the physical environment. The wetlands within the study area have suffered varying degrees of disturbance since European settlement in the 1820s. The most significant changes occurred during early European settlement, when vast areas of coastal lowlands were cleared for timber, sheep and cattle grazing and for agricultural purposes. A second period of change occurred from 1989 to 1995, when the Melaleuca community suffered dieback in response to hydrological modifications to Native Dog Creek for the development of a golf course. Results indicate that human-induced changes over the past 170 years have occurred at a rate far beyond the ability of the natural ecosystem to adapt or move to a more ecologically sustainable state, at least in the short-term. Hence the current environment is experiencing degradation through both decline in health and loss of indigenous species. The development of a conceptual bio-geomorphic model was based on the integration of results from all four studies, in an effort to provide a holistic understanding of the coastal wetland environment and of the impact of human-induced changes upon that environment. If these vulnerable ecosystems are to be maintained, successful and sustainable coastal management strategies must rely on a sound scientific understanding of the response of a coastal ecosystem to both human and environmental changes.

Debris-flows pose serious hazards to communities in mountainous regions of the world and are often responsible for loss of life and damages to infrastructure. Characterised by high flow velocity, large impact forces and long runout, debris-flows have potential discharges several times greater than clear water flood discharges and possess much greater erosive and destructive potential. In combination with poor temporal predictability, they present a significant hazard to settlements, transport routes and other infrastructure located at the drainage points (fan-heads) of watersheds. Thus, it is important that areas vulnerable to debris-flows are identified in order to aid decisions on appropriate land-uses for alluvial fans. This research has developed and tested a new GIS-based procedure for identifying areas prone to debris-flow hazards in the Coromandel/Kaimai region, North Island, New Zealand. The procedure was developed using ESRI Arc View software, utilising the NZ 25 x 25 m DEM as the primary input. When run, it enabled watersheds and their associated morphometric parameters to be derived for selected streams in the study area. Two specific parameters, Melton ratio (R) and watershed length were then correlated against field evidence for debris-flows, debris-floods and fluvial processes at stream watershed locations in the study area. Overall, strong relationships were observed to exist between the evidence observed for these phenomena and the parameters, thus confirming the utility of the GIS procedure for the preliminary identification of hydrogeomorphic hazards such as debris-flow in the Coromandel/Kaimai region study area. In consideration of the results, the procedure could prove a useful tool for regional councils and CDEM groups in regional debris-flow hazard assessment for the identification of existing developments at risk of debris-flow disaster. Furthermore, the procedure could be used to provide justification for subsequent, more intensive local investigations to fully quantify the risk to people and property at stream fan and watershed locations in such areas.

Earlier researchers have produced conceptual models of Mississippi River delta plain development which divide the deltaic plain into upper and lower reaches. The upper deltaic plain has been described as an area composed mainly of lacustrine, lacustrine delta, backswamp, and crevasse channels, with minimal distributary development. The lower deltaic plain is characterized by numerous distributaries forming distributary systems and lobes. Detailed geomorphic mapping and chronologic reconstruction within the Atchafalaya Backwater Area of the upper deltaic plain of the Mississippi River has led to the recognition of a complex network of distributary development related to three distinct distributary systems that formed in the upper deltaic plain over the past 2500 years. These systems do not fit previous models of upper deltaic plain development. The East Atchafalaya Basin Protection Levee blocked Atchafalaya River water and sediment from entering the study area and burying these older distributary systems, preserving their surface expression and allowing their identification. Results show that distributary systems can be a major contributor to upper deltaic plain development and that these systems are not always related to the lower delta plain delta switching process. A stable Mississippi River position and a favorable gradient in the study area over the past 4,000 years appear to be responsible for the geomorphic development of the study area.