No description available.

No description available.

No description available.

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.

<p> A sediment core from Clear Lake, CA that was drilled in 2012 is the centerpiece of a current interdisciplinary paleoclimate investigation in northern California. Two similar research projects were conducted in Clear Lake in 1973 and 1980, but inconsistent core recovery and a possible hiatus in sedimentation introduced uncertainty about the length of time represented by recovered sediments. Two cores recovered in 2012 were drilled in effort to create a continuous splice and attain a complete paleoenvironmental record for the Clear Lake area. Until now, the core has been radiocarbon dated to about 55 meters below lake floor (mblf) and one ash has been identified at 63 mblf, but the lower 90 meters of sediment remained undated. </p><p> Paleomagnetic research provides a chronostratigraphic framework for the collaborative project and reveals variations in sediment flux and environmental conditions over time. In Clear Lake sediments, magnetic minerals are ubiquitous and demonstrate reliable magnetic behavior for magnetostratigraphic interpretation, such as relative paleointensity (RPI) correlation. RPI correlation provides continuous chronostratigraphy for the core to 108 mblf, which corresponds to more than 200 ka. </p><p> Magnetic mineral concentration and grain domain size of Clear Lake sediments reflect zones of varying environmental conditions within the watershed that correlate with previous pollen research. These zones can be correlated to select Marine Isotope Stages (MIS), providing a direct link between paleoclimatic conditions in the marine and terrestrial environment in California. The boundary of MIS 1 and MIS 2, known to occur at 14 ka, is clearly shown as an increase in magnetic mineral concentrations responding to wetter conditions in MIS 2. These results are consistent with previous palynological interpretations from Clear Lake [Adam and Robinson, 1988]. Magnetic mineral concentrations reflect five zones of distinct environmental conditions and also suggests that sedimentation rate increases dramatically in the lower 30-40 meters of the core. </p><p> <b>The environmental magnetic response of Clear Lake sediments, in conjunction with a magnetostratigraphic interpretation, provides a unique opportunity to investigate the timing of environmental change in the lake region.</b> California&rsquo;s current drought may be a new norm of extreme weather in response to higher average temperatures associated with climate change, and paleoenvironmental research like this provides insight on whole-system responses to rapid environmental change.</p>

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.

No description available.