A novel finite element double porosity model for multiphase flow through deformable fractured porous media

Based on the theory of double-porosity, a novel mathematical model for multiphase fluid flow in a deforming fractured reservoir is developed. The present formulation, consisting of both the equilibrium and continuity equations, accounts for the significant influence of coupling between fluid flow and solid deformation, usually ignored in the reservoir simulation literature. A Galerkin-based finite element method is applied to discretize the governing equations both in the space and time domain. Throughout the derived set of equations the solid displacements as well as the fluid pressure values are considered as the primary unknowns and may be used to determine other reservoir parameters such as stresses, saturations, etc. The final set of equations represents a highly non-linear system as the elements of the coefficient matrices are updated during each iteration in terms of the independent variables. The model is employed to solve a field scale example where the results are compared to those of ten other uncoupled models. The results illustrate a significantly different behaviour for the case of a reservoir where the impact of coupling is also considered. © 1997 by John Wiley & Sons, Ltd.     

Controls on the sexual and asexual regeneration of Salicaceae along a highly dynamic, braided river system

Salicaceae are key pioneer riparian tree species that have the ability to reproduce sexually and asexually. Recent research has suggested that Salicaceae act as ‘ecosystem engineers’, modifying hydrological and geomorphological processes, resulting in the stabilisation and growth of landforms. Understanding these interactions requires knowledge of the controls on Salicaceae regeneration. This paper describes a study of Salicaceae establishment and growth along a reach of a highly dynamic, island-braided river. The sexual and asexual regeneration of three species were investigated using experimental planting of cuttings and observation of seedlings. Plots were located at a range of elevations, in different habitats associated with the established riparian vegetation and in contrasting sediment types. Survival and growth were monitored over two growing seasons. Asexual regeneration was more successful than sexual regeneration, with cuttings demonstrating faster growth rates and tolerance of broader environmental conditions than seedlings. Cutting survival and growth was highest in sediments with a relatively high organic content and in plots located between patchy Salicaceae stands or in the lee of islands. Seedling mortality was extremely high due to fluvial disturbance, although seedlings in habitats that were protected from fluvial disturbance survived. Seedling growth showed preferences for particular sedimentary conditions, which varied between species. The major control on regeneration was the upstream presence of established Salicaceae, particularly on islands, which provided open sites that were protected from fluvial disturbance and suitable for regeneration. Thus, asexual regeneration facilitated sexual regeneration by rapidly colonising sites that provided habitats protected from fluvial disturbance for seedling establishment. This supports previous work suggesting that Salicaceae can act as ‘ecosystem engineers’.     

Timing of Late Quaternary glaciations in the Himalayas of northern Pakistan

Optically stimulated luminescence dating of Late Quaternary glaciogenic sediments was undertaken in critical areas of the Himalayas of northern Pakistan in order to examine the timing of glaciation. The dates demonstrate that several glaciations occurred during the last glacial cycle. In Swat, the Grabral 2 Stade and the Kalam I Stade were dated at ca. 77 ka and ca. 38 ka, respectively. The error on the former date is large and it is conceivable that the moraines may have formed during the early part of Oxygen Isotope Stage 3 rather than during Oxygen Isotope Stage 4. The Kalam I Stade, however, clearly represents a glaciation during Oxygen Isotope Stage 3. The oldest moraines and those at the lowest altitude in the Indus valley at Shatial have an age of ca. 60 ka. These also relate to a major glacial advance during Oxygen Isotope Stage 3. A younger series of moraines, the Jalipur Tillite, and glaciofluvial sands at Liachar in the Indus valley, and moraines at Rampur–Tarshing have ages of ca. 27 ka, ca. 21–23 ka and ca. 15 ka, respectively. These dates show that glaciers also occupied parts of the Indus valley during Oxygen Isotope Stage 2. These dates and the morphostratigraphy show that glaciation in the Pakistani Himalaya was more extensive during the early part of the last glacial cycle and that the local last glacial maximum in Pakistan was asynchronous with the maximum extent of Northern Hemisphere ice sheets. Copyright © 2000 John Wiley & Sons, Ltd.     

Hydroxyl Radical Concentrations Estimated from Measurements of Trichloroethylene during the EASE/ACSOE Campaign at Mace Head, Ireland during July 1996

During the EASE/OXICOA campaign of the NERC ACSOE programme, trichloroethylene and a wide range of man-made halocarbons and radiatively-active trace gases were monitored with high precision and high frequency throughout July 1996 at Mace Head on the Atlantic Ocean coast of Ireland. Trichloroethylene concentrations in concert with many other trace gases became elevated as regionally-polluted and photochemically-aged air masses reached Mace Head. However, as the anticyclonic air masses retreated during 19 and 20 July, trace gas concentrations remained elevated for a significant period. During this 2–4 day period, trichloroethylene concentrations decayed significantly, though the concentrations of the other more chemically-inert trace gases did not. A detailed interpretation of this behaviour using a Lagrangian dispersion model has allowed the estimation of average and peak OH radical concentrations of 3 and 9×10⁶ molecule cm^-3, respectively, during the travel from the source areas in the U.K. and the low countries out to Mace Head. Using a simple box model, the available Mace Head measurements, when combined into a detailed chemical mechanism, generated OH radical concentrations which peaked at 7×10⁶ molecule cm^-3, in close agreement with the estimates based on trichloroethylene decay.     

'Hell on earth and paradise all at the same time': the production of smallholding space in the British countryside

Summary This paper begins to develop an analysis of the geography of smallholding, as a contribution to developing interest in cultural geographies of farming and the nature of the 'post-productivist' countryside. The paper uses evidence from a survey of UK smallholders to suggest that they are in a paradoxical position in relation to ideas about 'real' farming, production and consumption, leisure and work. Smallholder identity is revealed as constituted through ideas about 'nature', food, family, community, work and rural space. The paper concludes by suggesting a framework of ideas for future intensive research into smallholding and its links with farming cultures and rural social and cultural change.     

Assessment of Impact of Storm on Point Source Pollutant Transport in Estuary by Dissolved Tracer Modeling

A continuously discharged dissolved conservative tracer was simulated with the Chesapeake Bay Estuary Model Package to study pollutant transport in the estuary in response to point source loads and the impact of the November, 1985 storm. A visualization technology is applied to show 3-dimensional concentration variations in a continuous daily time sequence. The differential responses of daily net transport during storms versus inter-storm periods can be observed from an MPEG movie. It may take 2–3 months for a tracer to travel from the fall-line to the mouth of a river during relatively dry seasons, only 2 weeks in some medium storms, and less than 5 days in a big storm. Plots of daily concentrations from eleven selected locations in the estuary provide quantitative information on the response of tracer concentration to flows. The magnitude and time of tracer peaks related with different weather events in these locations reflect the combined effects of flows from various directions to these locations. The lower tributaries (which are closer to the Bay mouth) are affected more than the upper tributaries by a source discharged at a mid-tributary. A storm can transport materials more effectively to the Bay and affect adjacent tributaries more severely.     

Reconciling theories of a mechanically driven meridional overturning circulation with thermohaline forcing and multiple equilibria

It has recently been suggested that the structure and strength of the meridional overturning circulation in the global ocean is governed by the input of mechanical energy to the system by winds and tides. However, it is not clear how this suggestion relates to the existence of multiple equilibria of the meridional overturning circulation, which depends on thermohaline feedbacks and is more consistent with a buoyancy-driven view of the circulation. Both theories have been illustrated by box models in the past (Stommel in Tellus 13:224–230, 1961; Gnanadesikan in Science 283:2077–2079, 1999). Here we incorporate these two theories into a single box model in an attempt to reconcile the roles of mechanical and buoyancy forcing in driving the meridional overturning circulation. The box model has two equilibrium solutions, one with sinking at high northern latitudes as in the present-day Atlantic, and one without. The circulation is mechanically driven, but the northern sinking can be thought of as a release valve which acts as a sink of potential energy when the surface water at high northern latitudes is dense enough to convect. While the source of energy comes from mechanical forcing, the presence or otherwise of multiple equilibria is therefore determined by thermohaline feedbacks. In some areas of parameter space an oscillation between the model’s two circulation regimes occurs, reminiscent of a bipolar seesaw.     

Isotopic variation (δN, δC,and δS) with body size in post-larval estuarine consumers

Stable isotope analyses were conducted on fish and crabs in Apalachicola Bay, Florida (USA) to determine whether δ¹⁵N values are correlated with length within these species. Our objective was to define the smallest trophic unit of the system as a prelude to further isotopic evaluations of consumer trophic level in Apalachicola Bay. In some cases, the smallest trophic unit is the individual species; however, previous gut content analysis on organisms in this system indicated that subdivision by size class within species might be required. However, over the size ranges observed, there was no systematic change in the δ¹⁵N values of individual consumers. Analysis of the δ¹³C and δ³⁴S values of these species reveal no obvious shifts in the utilization of organic matter sources with length that could be obscuring a trend in the δ¹⁵N signal. We find no isotopic evidence for a systematic trophic level increase within the species examined over the size ranges observed in this estuary. These results indicate that species rather than size class is the best predictor of trophic habit among these Apalachicola Bay consumers.