No description available.

No description available.

Hydrology is recognised as an important component of the glacial system in alpine environments. In particular, the subglacial drainage of surface meltwaters is known to exert a strong influence on the motion of glaciers and on their capacity to erode the underlying bedrock. This thesis examines the more poorly understood drainage system of the Greenland Ice Sheet, with specific focus on Leverett Glacier, a landterminating outlet glacier on the ice sheet’s western margin. Because of the vast size of the ice sheet, the influence of the drainage system could have wide ranging implications, most notably for sea level rise and continental scale landscape evolution. The thesis commences with an investigation into the morphology of the drainage system of the lower 14 km of Leverett Glacier. This is undertaken using a variety of field methods, including dye tracing and the monitoring of proglacial discharge, englacial water levels, surface melt rates and glacier motion. The data reveal that the drainage system of the glacier closely resembles that of alpine glaciers, undergoing an evolution from distributed to channelised drainage morphologies as the melt season progresses. Another aspect of the field data, the suspended sediment load evacuated from the subglacial system in the emerging proglacial river, is then examined to investigate the impact that this drainage system morphology has on the interaction between the glacier and the underlying bedrock or substrate. This demonstrates that the presence of large, efficient subglacial drainage channels allows for the removal of vast quantities of basal debris during much of the melt season, facilitating an erosion rate 1-2 orders of magnitude greater than previously proposed for ice sheet settings. The thesis then focuses on the relationship between discharge, water pressure and ice motion. Observations from Greenlandic and alpine glaciers demonstrate that glaciers generally decelerate through the melt season following a maximum velocity induced by the onset of melt in the spring. The data indicate that the evolution of the drainage system from a distributed to a channelised morphology occurs rapidly and so can only explain this trend in ice velocity during the early part of the melt season. Beyond this period, ice velocity patterns can instead be explained primarily by transient fluctuations in water pressure within the channelised drainage system. These transient pressure fluctuations result from the lag between changes to the rate of meltwater input to the glacier and the subsequent adjustment of channel cross section. This indicates that it is crucial to consider temporal variability in melt rate when seeking to link climate with the dynamics of ice sheets and glaciers. This process can be simulated, which is demonstrated by using the proglacial discharge record to model subglacial water pressure and ice velocity. In the following chapter, this model is built upon by considering how these variations in water pressure, originating in discrete subglacial channels, control sliding velocities across large areas of the glacier. Detailed examination of high-resolution ice velocity records from Leverett Glacier reveals that, in keeping with theory, horizontal ice velocity is dependent on both the volume of subglacial cavities and the rate-of-change of this volume. A simple model of subglacial water movement is then used to demonstrate how these changes in the cavity system could be driven by the pressure fluctuations predicted within the channelised drainage system. This enables a system scale model of glacier hydrology to be developed, which is presented in the final chapter, linking variations in surface melt rate to channel pressure, cavity volume and ultimately ice motion. In summary, this research has helped to illuminate the morphology and functioning of the drainage system of Leverett Glacier. This has improved our understanding of how hydrology influences both the motion of the Greenland Ice Sheet and its impact on the underlying topography, and enabled better prediction of how these processes are influenced by changes in climate.

This study reviews the assumptions, the assembled data, and the considerations involved in calibrating a preliminary steady-state model of the City of Tucson's Central Well Field. This study introduces a hydrostratigraphic characterization of basin sediments and develops a new set of aquifer parameter data. The results of two preliminary steady-state calibrations are presented based on assumed steady-state conditions in the early 1900s and quasi-steady-state conditions in 1940. Transient stresses over a 50-year period are imposed onto the results of the 1940 steady-state simulation to identify major deficiencies in the preliminary calibration.

Most investigations of river meanders have concentrated on classical bends that have a helical flow structure through the pool, which is carried through into the entrance of the next bend. However, it is known that flow separation can occur at the outer bank before the bend apex or at the inner bank after the apex and that this results in the breakdown of the classical flow model. Although some of the controls and the implications of flow separation are now known, the frequency of occurrence, threedimensional (3-D) flow structure, sediment dynamics, controls on separation presence, and full geomorphological implications are poorly understood. This thesis uses a combination of fieldwork and 3-D numerical flow modelling (computational fluid dynamics, CFD) to investigate meander bends where flow separation and recirculation zones are present. An initial reconnaissance survey of over 600 bends revealed that flow separation is common, being present in approximately 50% of bends. The survey also identified high bend angles of tum and the high angles of flow impingement onto the outer bank as important controls on the presence of separation. Investigations in a 22 bend sub-set indicated that expansions in channel width, breaks in bank-line curvature and the angle of inflow, as governed by upstream planform, were important in generating flow separation. Detailed fieldwork combined with validated and verified time-averaged CFD modelling in three bends with separation zones enabled identification and examination of the 3-D flow fields. The simulations reveal that the flow structure in bends with separation differs considerably from the classical flow model, with the helical motion found in classical bends being very intense in the bend entrance but dissipating at or just after the bend apex. The turbulent flow structures produced by the shear layer between the downstream and recirculating flow are also investigated in the field bends and attempts at modelling transient flow structures with large eddy simulation (LES) in a simple open channel flow expansion are detailed. The geomorphological, sedimentological, and ecological implications of both the time-averaged and the transient flow structures within the bends are examined and discussed.

No description available.

A water balance model is derived for Sitka spruce in which transpiration is estimated with a diffusion equation, and interception is predicted with the Gash model. A model drought stress index is defined. It assumes that transpiration and growth are maintained at maximum rates until the predicted soil moisture deficit equals 80% of the available water in the root zone. The model has been tested at Kirkhill forest, near Aberdeen, and at Monaughty forest, near Elgin. The drought stress index is validated by field investigations into the effects of drought on tree water status and growth at the Kirkhill podsol P60 site. At the Kirkhill podsol P49 site, the model accounts for 86% of the variation in measured stand annual volume increment (discounting suppressed trees). The model only accounts for 19-35% of the year to year variation in the measured growth of 15 trees at the Monaughty podsol P29 site. The poorer fit of the model at Monaughty is ascribed to periodic thinnings and lower quality growth data. Growth data for two Kirkhill peaty gleys (P57 and P59) demonstrates that drought stress is an unlikely occurrence on such sites. Model predictions of drought stress and evaporative demand are given for several locations across Scotland. It is shown that drought stress is rare in regions such as the West coast and Southern Uplands; in contrast to the East coast, rainfall is high and evaporation low. The model is generalised so that the effects of drought can be assessed simply as a function of annual rainfall and available water. On the basis of the limited number of sites investigated in this study, it is tentatively concluded that, given moderate available water (100-200 mm), reasonable yield classes (16-20) can be expected even when annual rainfall is as low as 700-800 mm.

This study was concerned with the submerged bottom type of river intake, which consists, essentially, of a filter located in the river bed, through which water can be pumped. This type of intake has been used recently, for example, in salmon rivers, to minimise disruption of fish movement. The problem of assessing the change in bed shear stress over the intake was studied with a view to estimating the depth of the scour hole, which was known to be a feature associated with abstraction. The literature on local scour in alluvial channels was studied, several different types of scour being covered by the survey. Although most of the methods described were limited in application to a certain type of scour, some general principles were observed. In particular, it became clear that it was important to obtain an adequate description of the modification of the flow field giving rise to the scour. Several different turbulence models were assessed before a decision was made on the approach to be adopted to the problem. The more complex models were considered unsuitable, mainly because excessive attention to detail in the flow field would be unwarranted when other aspects of the problem, such as the suction force on the grains and the relationship between grain movement and boundary shear stress, were not known with sufficient accuracy. In addition, there was some uncertainty in the modifications to such models that would be required for their application to the intake problem. The extended law of the wall for suction flows was rejected because it is valid only for suction velocities up to about 1% of the external flow velocity, whereas the suction velocity in the intake flows studied ranged from 2% to 10% of the mean flume velocity. Solutions based on the mean flow momentum equation were studied, and a model was developed which was based on the hypothesis that the increase in shear stress at the bed was related directly to the momentum given up by the abstracted fluid at the bed. This model gave significantly improved predictions compared with a previous model, in respect of both the magnitude and pattern of shear stress distribution in the suction zone. It also had an advantage over the earlier model in that the results did not depend significantly on the assumption of a hydrostatic, or any other, pressure distribution in the main flow. Measurements of the modification of the velocity field associated with abstraction were made over a suction zone in a wind tunnel. The velocity profiles at a number of different points along the suction zone were measured using a hot-wire anemometer and these provided confirmation that the profiles assumed for the purposes of calculation gave an adequate description of the flow. Shear stress measurements over a model intake in a laboratory flume were made by observing the combinations of suction and flume flow which induced threshold conditions for sand grains placed on an impermeable disc in the suction zone, whose threshold shear stress in uniform flow was known. The measurements of shear stress using this method gave results which were between five and ten times lower than the predicted values, although they were comparable to estimates of shear stress based on the near-bottom velocities measured in the wind tunnel. It was deduced that the absence of suction at the disc itself was responsible for this large difference, since tests with dye showed that the disc did not interfere noticeably with the main flow. Measurements were also made of the bed profile over the model intake in the flume at various stages in the development of the scour hole, and the bed shear stress estimated from these measurements. These estimates showed a reasonable correlation with the predictions of the new mathematical model, but it was clear that there was a need for a better understanding, especially with regard to the interpretation of the experimental data. This study has provided a means of estimating the boundary shear stress associated with abstraction, and the resulting grain dislodgement rate and scour. These estimates are approximate in nature, but ways of obtaining improved predictions have been indicated, and these should provide a sound basis for an extension of this work to the study of three-dimensional scour which is important in the application of the results to intakes in practice.

A programme of field sampling was undertaken to quantify annual organic carbon fluxes at a range of sites on the River Dee and River Don in NE Scotland. The annual fluxes of both DOC and POC in the R. Dee and R. Don were found to increase cumulatively with distance down the river system. In headwater and tributary catchments of the R. Dee, annual DOC fluxes were positively related to the coverage of peat in the catchment area. This work as supplemented by a desk study in which organic carbon exports in 1993 were calculated for 85 large British rivers, using archive information. The resulting estimates, along with data on the soil carbon content of 17 river catchments were used to develop a predictive model of British riverine DOC fluxes. Calculated and predicted DOC fluxes and estimates of POC export based on suspended solids data, were combined to produce estimates of the annual British riverine organic carbon export during 1993. The 'non-storm' flux of DOC in British rivers in tidal waters during 1993 was estimated at 0.69 Mt +/- 0.28 Mt; the corresponding POC flux estimate was 0.2 Mt. A direct method for measuring dissolved CO2 in river and lake waters was developed. Preliminary studies of rivers in NE Scotland and lakes in Northern Wisconsin, USA, suggested that lakes may act as conduits for CO 2 transport, particularly in catchments containing substantial wetland. In conclusion, soil carbon content appears to be a useful integrative measurement for predicting annual DOC fluxes in British rivers. The establishment of a link between soil carbon stores and riverine fluxes, should help to improve future modelling of the carbon cycle. These findings suggests that rivers (and lakes) may regulate increases in soil carbon pools induced by climate change.

The River Don discharges into the North Sea by way of a small estuary on the northern outskirts of the city of Aberdeen. This thesis aims to establish the patterns of mixing of river and seawater within the estuary and its associated coastal waters, and to interpret those factors that affect these patterns. Fieldwork consisted mainly of boat borne surveys of salinity, temperature and currents. This programme was augmented with air based visual observations, and a photogrammetric survey of surface water movement using fluorescent dyes. River flow data was acquired from the North East River Purification Board, and wind records from Total Oil Marine Ltd., Altens. Estuarine mouth configuration was monitored via bimonthly tacheometric surveys. The estuary was found to be highly stratified under mean conditions, a distinct saltwedge penetrating upstream beneath seaward flowing freshwater during the flooding tide. The extent of this saline intrusion depends upon the freshwater head, as represented by river flow; and the tidal head, as represented by the height of high water. During the ebbing tide the saline waters are completely flushed from the estuary, except where retained within the depths of 'potholes'. Under particularly high river flows saline penetration is prevented at all stages of the tide. Mixing during the flooding tide was by entrainment of freshwater down into the advancing salt wedge. However, only during the ebbing tide does a high level of mixing occur, as a result of increased turbulent diffusion, causing a rise in surface salinity. Under most conditions, therefore, fresh or brackish water spreads seawards from the estuary mouth, forming a thin, highly buoyant plume. As a result of buoyant spread and inertia induced turbulence, mixing of these waters is intense in the zone immediately seaward of the mouth. However, mixing is not complete and the waters form a thin surface layer which may preserve its identity for several kilometres beyond this initial zone. The direction of spread of this buoyant discharge is chiefly determined by the tidal stream, although initially by the geometry of the estuary mouth. During the ebbing tide the plume forms to the north of the mouth, and during the flood, to the south. Wind and wave induced currents may enhance or restrict the spread of these surface waters. Storm wave activity, causing mixing of estuarine discharge as soon as it leaves the outlet, precludes the formation of a buoyant plume. Restricted estuarine discharge during the flood tide, and enhanced discharge during the ebb, result in the ebb plume being a considerably more extensive feature. The plume was often demarcated by a sharp thermohaline front marked by a distinct colour change. The front was a zone of strong surface convergence, and, consequently, was often demarcated by a collection of foam and floating debris.