Improved water table dynamics in MODFLOW

The standard formulation of a block-centered finite-difference model, such as MODFLOW, uses the center of the cell as the location of a cell node. Simulations of a dynamic water table can be improved if the node of a cell containing the water table is located at the water table rather than at the center of the cell. The LPF package of MOD-FLOW-2000 was changed to position a cell's node at the water table in convertible cells with a water table. Improved accuracy in the upper regions of an unconfined aquifer is demonstrated for pumping from a partially penetrating well. The change introduces a nonlinearity into the solution of the flow equations that results in slightly slower convergence of the flow solution, 7% slower in the presented demonstration. Accuracy of simulations is improved where vertical flow is dominated by a moving water table, but not when a large water table gradient dominates over the water table movement.     

Urban disasters and megacities in a Risk society

Cities like London, New York and Los Angeles possess complex urban environments that, despite their technological sophistication, present their citizens with a multiplicity of risks. This paper presents a discussion of the effects of physical and social trends on the creation of risk and vulnerability to hazard in such cities. The interplay between physical hazards and perceived risks in late modern, or Risk, urban societies is considered, and the impact of this process on adjustment to hazard.     

Observations of wave pump efficiency

Currently available data on wave pump efficiency is reviewed. The obtainable efficiency is an important consideration in the design of practical devices for the extraction of wave energy and the analysis of natural systems (e.g., coral flats and rip currents). We find that the peak efficiency is 0.5 for very steep (∼ 40–45°) ramps where the waves break over the top of the ramp. For flatter (< 30°) ramps, the breaking process is more gradual and the peak efficiency is less than 0.1. We have identified natural atoll lagoon systems where the flushing is wave driven and successfully modeled it as driven by a wave pump. The same is the case for rip currents. For both of these natural systems, the pump efficiency is around 0.035. In addition a numerical swash model is used to estimate wave pump efficiency and is seen to match the experimental results for natural systems or breaking wave scenario.     

Modelling magnetization processes in single-and multi-domain rock samples using the preisach diagram

Summary The paper presents the results of identifying a model of the Preisach type for haematite grains of the single-domain as well as multi-domain size. Due to the phenomenological conception of the model, the relations between the parameters of the model and actual parameters are very complex, however, impressive accuracy and stability of the modified model indicate the need to resume studies of this problem. The model parameters also confirm the adequate properties of the vibrating-sample magnetometr (VSM) used and developed in our laboratories.     

Tidal Marsh Restoration Design Affects Feedbacks Between Inundation and Elevation Change

Tidal marsh (re)creation on formerly embanked land is increasingly executed along estuaries and coasts in Europe and the USA, either by restoring complete or by reduced tidal exchange. Ecosystem functioning and services are largely affected by the hydro-geomorphologic development of these areas. For natural marshes, the latter is known to be steered by feedbacks between tidal inundation and sediment accretion, allowing marshes to reach and maintain an equilibrium elevation relative to the mean sea level. However, for marsh restoration sites, these feedbacks may be disturbed depending on the restoration design. This was investigated by comparing the inundation-elevation change feedbacks in a natural versus restoration site with reduced tidal exchange in the Scheldt estuary (Belgium). This study analyzes long-term (9 years) datasets on elevation change and tidal inundation properties to disentangle the different mechanisms behind this elevation-inundation feedback. Moreover, subsequent changes in sediment properties that may affect this feedback were explored. In the restoration area with reduced tidal exchange, we found a different elevation-inundation feedback than on natural marshes, which is a positive feedback on initially high sites (i.e., sediment accretion leads to increasing inundation, hence causing accelerating sediment accretion rates) and a gradual silting up of the whole area. Furthermore, there is evidence for the presence of a relict consolidated sediment layer. Consequently, shallow subsidence is less likely to occur. Although short-term ecological development of the tidal marsh was not impeded, long-term habitat development may be affected by the differences in hydro-geomorphological interactions. An increase of inundation frequency on the initially high sites may cause inhibition of habitat succession or even reversed succession. Over time, the climax state of the restoration area may be different compared to natural marshes. Moreover, sediment-related ecosystem services, such as nutrient and carbon burial, may be positively influenced because of continuing sedimentation, although flood water storage potential will decrease with increasing elevation. Depending on the restoration goals, ecosystem trajectories and delivery of ecosystem services can be controlled by adaptive management of the tidal volume entering the restoration area.     

Timing and mechanisms controlling evaporite diapirism on Ellef Ringnes Island,Canadian Arctic Archipelago

The Permo‐Carboniferous to Eocene Sverdrup Basin in Canada's Arctic Archipelago is strongly influenced by evaporite diapirism. However, salt structures within the basin have not been extensively investigated recently due to their remote location. This study includes the interpretation of legacy seismic reflection and borehole data to characterize the geometry of selected evaporite domes, and 1D backstripping of wells to investigate tectonic and sedimentary influences on diapirism. Extensional rift‐structures appear to have played a significant role in the formation of evaporite domes by triggering and directing salt movement. Diapirism was initiated by at least the Middle Triassic and continued to develop during the Mesozoic. Differential loading of salt on opposing east–west dome margins led to their present day asymmetric geometries. Diapir growth rates in the Mesozoic were closely linked to the rate of sedimentation and influenced by regional tectonism.     

Book reviews

REGIONAL GEOGRAPHY: Current Developments and Future Prospects edited by R. J. Johnston, J Hauer and G. A. Hoekveld. 14 × 22 cm, xiii and 216 pages. Routledge: London 1990 (ISBN 0 415 05247 5) $A89.95 (hard).

THE MAKING OF THE AMERICAN LANDSCAPE edited by M. P. Conzen. 19 × 25 cm, xvi and 433 pages. Unwin Hyman: Boston 1990 (ISBN 0 04 917010 4) $A59.95 (soft).

EASTERN EUROPE: An Historical Geography 1815–1945 by D. Turnock. 14 × 22 cm, ix and 357 pages. Routledge: London 1989 (ISBN 0 415 01269 4) $A115.00 (hard), distributed by the Law Book Co.

THE HUMAN GEOGRAPHY OF EASTERN EUROPE by D. Turnock. 14 × 22 cm, xii and 345 pages. Routledge: London 1989 (ISBN 0 415 00469 1) $A105.00 (hard), distributed by the Law Book Co.

A HISTORY OF THE PACIFIC ISLANDS by I. C. Campbell. 14 × 21 cm, 239 pages. University of Queensland Press: St Lucia 1990 (ISBN 0 7022 2291 7) $A34.95 (soft).

THE CORPORATE FIRM IN A CHANGING WORLD ECONOMY: Case Studies in the Geography of Enterprise edited by M. de Smidt and E. Wev‐er. 14 × 23 cm, xiv and 247 pages. Routledge: London 1990 (ISBN 0 415 03497 3) $A105.00 (hard).

THE CAPITALIST CITY: Global Restructuring and Community Politics by M. P. Smith and J. R. Feagin. 15 × 23 cm, 393 pages. Basil Black‐well: Oxford 1988 (ISBN 0 631 15182 6) £35 (hard).

CONFLICT AND CHANGE IN THE COUNTRYSIDE: Rural Society, Economy and Planning in the Developed World by G. M. Robinson. 15 × 23 cm, xxvi and 482 pages. Belhaven Press: London 1990 (ISBN 1 85293 043 8) £40.00 (hard); (ISBN 1 85293 044 6) £14.95 (soft).

ENVIRONMENTAL SCIENCE (Collins Reference Dictionary) by G. Jones, A. Robertson, J. Forbes and G. Hollier. 13 × 20 cm, vi and 473 pages. Collins: London 1990 (ISBN 0 00 434348 4) £5.95 (soft).

THE HUMAN IMPACT ON THE NATURAL ENVIRONMENT (Third Edition) by A. Goudie. 17 × 24 cm, xi and 388 pages. Blackwell: Oxford 1990 (ISBN 0 631 16164 3) $A39.95 (soft).

PATTERNS OF LIFE: Biogeography of a Changing World by H. W. Mielke. 19 × 24 cm, xiv and 370 pages. Unwin Hyman: London 1989 (ISBN 0 04 574032 1).     

Petrology of the Skien basaltic rocks,southwestern Oslo Region,Norway

The pile of Permian continental basaltic rocks near Skien has a minimum thickness of 1 500 m, dipping 40^g towards NE, consisting of several thin lava flows. Most flows are porphyric, though both tuffs and aphyric flows occur to a lesser extent. Amygdules are abundant and a characteristic feature. The flows in the lower 2/3 of the basalt pile are mainly classified as melanite-ankaramite; some in the middle part as melanite-nephelinite; and the upper 1/3 of the flows as basanite. The basaltic rocks evolved from an assumed olivine-nephelinitic magma at great depths principally by clinopyroxene crystal fractionation accompanied by later olivine and maybe melilite reactions at moderate pressure in minor shallower magma chambers.