This work is a study into the unsteady behaviour of aeolian sediment transport. A one-dimensional and a two-dimensional numerical model were developed in order to investigate the temporal behaviour of transport rate as well its spatial distribution. The numerical model of McEwan (1991) for steady state saltation served as a starting point in the development phase. Both models presented in this thesis are capable of simulating temporary varying winds. In addition, the two-dimensional model allows the relaxation of the assumption of streamwise homogeneous flow. The one-dimensional model was tested against results for steady state predicted by previous models. Further tests showed that the discretisation time step size Δ<I>t</I> has an influence on the model's temporal behaviour. The reason for this is the better coupling of the wind-sand system when a smaller Δ<I>t</I> is used. The implications of bed area choice on the statistical accuracy of predicted transport rate was demonstrated. In the one-dimensional case the grain cloud's total forward momentum equals transport rate, which is independent of model geometry. The initial over-shoot reported by previous investigators was found not to appear for simulation heights small than 50 to 60cm. This is due to the fast propagation of the grains' influence (momentum exchange) upward in the flow and the immediate deceleration of the wind. Confirmation of these findings comes from reports of experiments conducted in differently sized wind tunnels. Different types of wind velocity variations were investigated. The transport rate's response depends on the amplitude and frequency of the wind fluctuations. At frequencies higher than <I>f </I>≈ 0.5Hz the transport rate was found not to respond to the wind changes.

The aims of this work are to improve the understanding of the way in which a river system approaches equilibrium during and after sediment transporting events and by using experimental data and hypothetical models to gain an insight into mixed grain sized transport and its connection with local bed morphology. Large scale flume experiments were undertaken in a trapezoidal channel where a mixed grain sized river bed is subjected to a variety of imposed sediment loads. The river system was allowed to adjust naturally to the imposed conditions while flow and bed conditions were frequently monitored. Changes to both flow and bed occur at many scales and the superimposition of these changes adds to the complexity of the interpretation of hydraulic and bed data. Three experiments are reported in which the sediment influx differ. One in which no material is added. One in which a feed rate is established equal in magnitude to 50% of the initial transport rate for the equivalent non-fed case. In the third experiment the feed rate is doubled to 100% of the initial transport rate in the degradation experiment. The imposed load is continued for approximately 75% of the duration of the run or until a dynamic equilibrium transport rate is evident within the reach. The river channel is analysed at two distinct scales; grain scale and reach scale. Conclusions about the relative influence of grain and form effects are drawn on the basis of detailed surface texture information and reach scale measurements of the bed surface. A hypothetical model is proposed to show that the transport activity during a degradation experiment can be qualitatively linked to a wave like translation of grain dislodgement through the channel.

The steep continental slopes of the southern Celtic shelf have long been thought to be a major export region for the flux of sediment from the south west approaches to the deep ocean (47-49°). Recent studies have suggested that the transfer of energy from the large barotropic tidal currents to internal tides, and higher frequency internal waves is locally enhanced, and provides a mechanism for the re-suspension and downslope transport of bed material on the upper slope region. This material is thought to be preferentially transported at the head of the many submarine canyons that exist along these ocean margins, where the barotropic tidal currents are locally amplified and internal wave energy focused. A unique 23 day deployment of the benthic lander STABLE (Sediment Transport and Boundary Layer Equipment) in July 1990, was at a depth of 388m on La Chapelle Bank continental slope. The site was at the head of a canyon, and at a depth thought to be critical for the generation of internal tidal energy. It was also at the deep water end of a transect of two current meter moorings across the slope. These measurements have shown that during summer stratified conditions the barotropic and baroclinic tides are sufficiently energetic near to the bed to mobilise the sand/gravel sized sediment on the upper slopes and at the shelf break. Eulerian residual bottom currents and maximum tidal currents are orientated cross-slope and this has important implications for sediment transport. The shelf break is predicted to be a region of bed load parting with bedload transport shelfwards at the shelf break and downslope immediately oceanward. On the critical slope region peaks in suspended sediment concentration occur at times of locally enhanced maximum down-slope flow (40cmsˉ¹) and maximum current shear. This suggests that sediment is being eroded from the bed locally and confirms a net flux of material downslope. Above the boundary layer suspended particulate matter will be transported with the net flow of water which is predominantly along-shelf and polewards. A second 10.4 day deployment of STABLE (II) in January 1994 was at a depth of 879m on the Goban Spur. These observations demonstrated the variability of continental slope processes on the margins of the Celtic shelf. During the deployment, weaker tidal currents (maximum of 24cmsˉ¹) were orientated along-slope and there was no evidence to suggest that the fine cohesive sand/mud sized sediment was mobile. Current meter measurements show that any suspended material will be predominantly transported along-slope and poleward. This will be periodically reversed and the net flux will be equatorward. The two studies have highlighted the temporal and alongslope variability of geological and hydrodynamical conditions near to the bed and highlight the difficulties in estimating shelf-ocean fluxes of material across the whole of the north-west European continental margin.

No description available.

Graduation date: 1997

The formation of aeolian ripples has been modeled, quite successfully, using discrete approaches like cellular automaton models. Numerical analysis of continuum models to obtain similar success in modeling ripple evolution, however, has not been studied extensively. A numerical model based on continuum theories expedites calculations, as opposed to discrete approaches which model trajectory of each and every sand grain, and are hence relatively more economical. The numerical analysis strives to contribute to the field of study of aeolian ripple migration by an extensive comparison and discussion of modeled ripple evolution results with those of a particular laboratory based wind-tunnel experiment. This research also endeavors to under- stand the physics behind ripple generation and what parameters to be modified to account for multiple grain sizes. Incorporation of multiple grain sizes would enable us to study the stratigraphy of the generated bed-forms. To obtain smoother and realistic ripple surfaces, a sixth-order compact finite difference numerical scheme is used for spatial derivates and fourth-order Runge-Kutta scheme for time derivates. The boundary conditions incorporated are periodic and the initial condition employed to generate ripple is a rough sand surface. The numerical model is applied to study the effect of varying the angle, at which the sand bed gets impacted by sand grains, on the evolution of ripples. Ripples are analyzed qualitatively and quantitatively by considering the contribution of processes involved in the evolution process. The ripple profiles and the time taken to reach equilibrium state, obtained by numerical experiments, are in close agreement with the ones obtained by the wind-tunnel experiment.

I seek to reconstruct the balance between sediment storage and yield across multiple drainage basin scales in a large (1 230 km2) watershed in the Northern Cascade range, British Columbia and Washington. Chilliwack Valley and surrounding area has been the site of numerous studies that have detailed much of its Quaternary sedimentary history. In the present study this information is supplemented by reconstruction of the morphodynamic trajectory of the river valley though the Holocene Epoch, and development of a sediment transfer model that describes the relaxation from the Fraser glaciation. The total Holocene sediment yield is estimated from basins across several scales using field and remotely sensed evidence to constrain the historical mass balance of delivery to higher order tributary basins. Rates of hillslope erosion are estimated using a diffusion-based relation for open slopes and delimitating the volume evacuated from major gully sources. Digital terrain models of paleo-surfaces are constructed to calculate total sediment erosion and deposition from tributary valleys and the mainstem. Chilliwack Lake has effectively trapped the entire post-glacial sediment load from the upper catchment (area = 334 km2), allowing to compare this "nested" system with the larger catchment. Rates of lake sediment accumulation are estimated using sediment cores and paleomagnetism. These are compared with accumulation rates in the terminal fan inferred from radiocarbon dating of fossil material, obtained by sonic drilling in the apex gravels. A sediment budget framework is then used to summarize the net transfer of weathered material and glacial sediments from the hillslope scale to the mainstem. The long-term average sediment yield from the upper basin is 62 +/- 9 t/km2/yr; contemporary yield is approximately 30 t/km2/yr. It is found that only 10-15% of the material eroded from the hillslopes is delivered to mouths of the major tributaries; the remaining material is stored at the base of footslopes and within the fluvial sedimentary system. Since the retreat of Fraser Ice from the mouth of the valley, Chilliwack River delivered over 1.8 +/- 0.21 km3 of gravel and sand to Vedder Fan in the Fraser Valley. In the sediment budget developed here, roughly 85% of that material is attributed to glacial sources, notably the Ryder Uplands and glacial valley fills deposited along the mainstem, upstream of Tamihi Creek. In tributary valleys, local base-level has fallen, leading to the evacuation of deep glacial sedimentary fills. Many of the lower reaches of major tributaries in upper Chilliwack Valley (e.g. Centre and Nesakwatch Creeks) remain primarily sediment sinks for slope-derived inputs, since base-level fall has not been initiated. In distal tributaries (Liumchen, Tamihi and Slesse creeks), paraglacial fans have been incised or completely eroded, entrained by laterally active channels. A transition from transport-limited to supply-limited conditions has been effected in many of these reaches. Slesse Creek has struck an intermediate balance, as it continues to remobilize its considerable sediment stores. It functions today as the sedimentary headwaters of Chilliwack Valley. Using grain size data and fine-sediment geochemical data gathered from Chilliwack River over the course of several field seasons, a simple finite-difference, surface-based sediment transport model is proposed. The aim of the model is to integrate the sediment-balance information, as inferred from estimates of hillslope erosion and valley storage, and physical principles of sediment transport dynamics to reproduce the key characteristics of a system undergoing base-level fall and reworking its considerable valley fill during degradation. Such characteristics include the river long profile, the river grain-size fining gradient, the percentage of substrate sand, and the diminution of headwater granite lithology in the active load. The model is able to reproduce many of the characteristics, but is not able to satisfy all criteria simultaneously. There is inevitably some ambiguity as to the set of parameters that produce the "right" result, however the model provides good insight into long-term interactions among parameters such as dominant discharge, grain size specifications, abrasion rates, initial topography, hiding functions, and hydraulic parameters.

Thesis (M.S. - Watershed Management)--University of Arizona. / Includes bibliographical references.

Thesis (M.S.)--Humboldt State University, 2007. / Includes bibliographical references (leaves 100-103). Also available via Humboldt Digital Scholar.

Thesis (M.S.)--West Virginia University, 2007. / Title from document title page. Document formatted into pages; contains viii, 122 p. : ill., maps. Includes abstract. Includes bibliographical references (p. 78-85).