WOODY DEBRIS ON THE MAINLAND SHELF OFF VENTURA, SOUTHERN CALIFORNIA

Land plant debris occurs in appreciable quantities in the shelf sediments off Ventura, southern California. Concentrations range from less than 2 g (wet weight)/1 (less than 250 g/m²) to 53 g/1 (10,662 g/m²). The highest values are found near the outer edge of the shelf.     

REVISITING RIO DE JANEIRO AND SÃO PAULO*

ABSTRACT. Contrasting models of urban development characterize the historical-geographical evolution of Brazil's two leading cities, viewed in an updating of the seminal 1933 article by Preston James. Native to Rio de Janeiro is a distinctive Luso-Brazilian style of irregular coastal urbanization, whereas SÃo Paulo displays a more uniform, modern type of inland commercial-industrial expansion. Even as Rio de Janeiro and SÃo Paulo sprawl today to form a virtual megalopolis in southeastern Brazil, they retain distinguishing roles in the national city system. Both metropolises are experiencing increasing functional decentralization and socioeconomic polarization, but in their own characteristic fashions. Generalized models of “Latin American city structure” are of limited value, unless they take into account such significant historical-geographical variations in urban form.     

GLOBAL AND LOCAL CHANGE ON THE PORT‐CITY WATERFRONT*

ABSTRACT. Successful waterfront redevelopment requires an understanding of global processes and an appreciation of the distinctiveness of port‐city locations. Waterfront revitalization occurs at the problematic and controversial interface between port function and the broader urban environment. It reflects varied forces and trends, involves community attitudes and environmental sensitivities, and influences transport evolution and urban change. The revitalization phenomenon is examined using community attitudes in Canada and urban regeneration in East Africa to illustrate retrospective and prospective dimensions.     

High-temperature 'clockwise'P-T paths and melting in the development of regional migmatites: an example from southern Brittany, France

The Southern Brittany Migmatite Belt (SBMB), which evolved through the metamorphic peak between c. 400 Ma and c. . 370 Ma ago, consists of a heterogeneous suite of high-grade gneisses and anatectic migmatites, both metatexites and diatexites. Rare garnet-cordierite gneiss layers record evidence of an early prograde P-T path. In these rocks, growth-zoned garnet cores and a sequence of included mineral assemblages in garnet, from core to rim, of Qtz + Ilm + Ky, Pl + Ky + St + Rt + Bt and Pl + Sil + St + Rt + Bt constrain a prograde evolution during which the reactions Ilm + Ky + Qtz→ Aim + Rt, Ms + Chl→ St + Bt + Qtz + V and St + Qtz→ Grt + Sil + V were crossed. Parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types. In all rock types, garnet has reverse zoned rims, and garnet replacement by cordierite and/or biotite and plagioclase suggests the following reactions have occurred: Grt + Sil + Qtz→ Crd → Hc ± Ilm, Bt + Sil + Qtz → Crd ± Hc → Ilm → Kfs + V and (Na + Ca + K + Ti) + Grt → Bt + Pl + Qtz. Microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained 'clockwise' P–T path for the SBMB. The high-temperature part of the path has a steep dT/dP slope characteristic of near isothermal decompression. It is proposed that the P-T path followed by the SBMB is the result of the inversion, by overthrusting, of a back-arc basin and that such a tectonic setting may be applicable to other high-temperature migmatite terranes. The near isothermal decompression is at least partly driven by the upward (diapiric) movement of the diatexite/anatectic granite core of the SBMB.     

PIPEFLOW CONTRIBUTING AREAS AND RUNOFF RESPONSE

Links that have been established between the characteristic responses of hillslope drainage processes and the size of drainage catchments enable valuable extrapolations and comparisons to be made, both within and beyond the range of the field data. To date, flow through natural soil pipes has not been included in these investigations. A methodology is offered for achieving this and the first such analyses of field data on pipeflow. The results indicate that pipeflow responses follow trends which are broadly similar to other hillslope drainage processes and are intermediate between throughflow and saturation overland flow. © 1997 by John Wiley & Sons, Ltd.     

Short-wave aerosol radiative efficiency over the global oceans derived from satellite data

Using 5 yr (December 2000–November 2005) of satellite data from the clouds and the earths radiant energy system (CERES) and moderate resolution imaging spectroradiometer (MODIS), we examine the instantaneous short-wave radiative efficiency ( E_τ ) of aerosols during the morning Terra satellite overpass time over the global oceans (60°N–60°S). We calculate E_τ using two commonly used methods. The first method uses the MODIS aerosol optical thickness (AOT) at 0.55 μm with radiative transfer calculations, whereas the second method utilizes the same AOT values along with a new generation of aerosol angular distribution models to convert the CERES-measured broad-band radiances to fluxes. Over the 5 yr, the global mean instantaneous E_τ between the methods is remarkably consistent and within 5 W m⁻²τ⁻¹ with a mean value of –70 W m⁻²τ⁻¹. The largest differences between the methods occur in high-latitude regions, primarily in the Southern Hemisphere, where AOT is low. In dust dominated regions, there is an excellent agreement between the methods with differences of <3 W m⁻²τ⁻¹. These differences are largely due to assumptions in aerosol models and definition of clear sky backgrounds. Independent assessments of aerosol radiative effects from different satellite sensors and methods are extremely valuable and should be used to verify numerical modelling simulations.     

Risk assessment and drought management in the Thames basin

Abstract

A risk assessment procedure is described for use in managing a system of pumped-storage reservoirs in the Thames basin during a drought. Historical daily rainfall sequences are used as equi-probable scenarios of future rainfall. These are transformed to flow, reservoir level, and demand restriction sequences through the use of rainfall-runoff and water resource system models. The risk assessment information required is then obtained through a statistical analysis of these sequences. A novel technique is presented for incorporating monthly rainfall forecasts, presented as probabilities of rainfall being above average, average, or below average, into the risk assessment scheme. Information on current hydrological conditions is incorporated in the procedure through the use of recently observed natural flows to adjust the internal state variables of a conceptual rainfall-runoff model to achieve agreement between observed and model flow. The overall procedure is accommodated within a decision support system for drought management which is implemented on a microcomputer and makes extensive use of interactive menus, forms and colour graphic displays. A key feature of the system is the maintenance of an up-to-date archive of hydrometric data which is achieved through a real-time communication link with a second computer dedicated to real-time data acquisition via telemetry. Monitoring the reliability of the water resource system during droughts is made a quick and easy task, and the effect of a change in the operating policy on system reliability can be readily assessed. The information obtained provides valuable support for tactical decision-making within the overall long-term operating strategy.     

A box model for non-entraining, suspension-driven gravity surges on horizontal surfaces

The propagation of and the deposition from a turbulent gravity current generated by the release of a finite volume of a dense particle suspension is described by a box model. The approximate model consists of a set of simple equations, a predetermined, depth-dependent leading boundary condition and one experimentally determined parameter describing the trailing boundary condition. It yields predictions that agree well with existing laboratory observations and more complex theoretical models of non-eroding, non-entraining, suspension-driven flows on horizontal surfaces. The essential features of gravity-surge behaviour have been observed and are captured accurately by the box model. These include the increased rate of downstream loss of flow momentum with increased particle setting velocity, the existence of maxima in the thickness of proximal deposits, and the downstream thinning of distal deposits. Our approximation for the final run-out distance, x_r, of a surge in deep water is given by x_r3(g'_oq³_o/w²_s)^1/5, where g'_o is the initial reduced gravity of the surge, q_o the initial two-dimensional volume, and w_s the average settling velocity of the particles in the suspension. A characteristic thickness of the resulting deposit is given by φ_oq_o/x_r'where ø_o is the initial volumetric fraction of sediment suspended in the surge. Our analysis provides additional insight into other features of gravity-surge dynamics and deposits, including the potential for the thickening of currents with time, the maintenance of inertial conditions and the potential for strong feedback in the sorting of particle sizes in the downstream direction at travel distances approaching x_r. Box-model approximations for the evolution of gravity surges thus provide a useful starting point for analyses of some naturally occurring turbidity surges and their deposits.