Steady state behaviour of the black ven mudslide: The application of archival analytical photogrammetry to studies of landform change

The concept of dynamic equilibrium has provided geomorphologists with a challenging paradigm for studying landform evolution but quantitative evidence for its existence has proved illusive, particularly for complex geomorphological systems. The authors believe that the principle has now been verified through the application of the ‘archival photogrammetric technique’ to a sequence of historical photographs spanning 50 years of process at the Black Ven mudslide complex in Dorset, U.K. The principles and limitations of the archival photogrammetric technique are described. The method is applied to oblique and vertical aerial photographs of Black Ven at five epochs, commencing in 1946, continuing at approximately 10 year intervals until 1988. The technique is used to generate plans/contours/sections and a dense and accurate digital elevation model (DEM) of the whole site at each epoch. This is used to generate ‘DEMs of difference’ and a ‘distribution of slope angle’ which suggest that the mudslides are in equilibrium despite the removal of 200 000 m³ of sediment between 1958 and 1988. Extrapolation of the slope distribution through time suggests that the frequency of an episodic landform change model at Black Ven may be approximately 60 years.     

Spectra and gust factors for gale force marine winds

Wind turbulence measurements obtained at a stable floating platform 10 km off the coast of Nova Scotia are unique in the range of wind speeds covered, and in freedom from influences of platform motion and flow distortion. Spectra of wind velocity components are presented according to Monin-Obukhov similarity theory. Gust factors for downwind, lateral and vertical components are reported for a range of gust durations and sampling intervals. These results are compared with published values, confirming the concept of similarity theory for spectra in gale force winds, and resolving a discrepancy among published gust factors.     

The Stress Field in the Upper Crust as Determined from In Situ Measurements

This paper provides a synopsis of the state of stress in the upper parts of the earth's crust based uponin situ rock stress determinations. Despite the large scatter of the data, two dominant trends can be detected in the variations of average horizontal stress with depth in various geological environments. Basement rocks in ancient shields and deformed rocks in fold belts usually show horizontal stresses larger than the theoretical overburden pressure. Sedimentary cover rocks and fissured massive rocks show horizontal stresses smaller than the overburden pressure. The ratio of the maximum to the minimum horizontal stress exhibits a clear stress anisotropy in most cases. Directions of maximum horizontal compression are fairly consistent in areas where sufficient measurements are available (North America and Fennoscandia), although in many instances they do not conform to any simple predicted stress pattern. Many factors complicate the interpretation ofin situ stress determinations. There is no simple relationship between the stress trajectories and the free surface. Topographic features and erosional processes may cause horizontal stress concentrations. Remanent stresses of great age can be superimposed on current tectonic stresses, while sometimes current stresses no longer coincide with the stress systems that caused observable faulting and folding. Observation of remanent stresses in ancient rocks shows that rocks in the upper crust have finite strength even under geological time intervals. From the viewpoint of global tectonics,in situ stress determinations ought to be used with great caution, and in conjunction with focal mechanism solutions of earthquakes. Although the state of stress is everywhere compressive, the fact that undeformed sedimentary cover rocks often show no excess horizontal stress would seem to indicate that no active global horizontal compression is required. Horizontal stresses larger than the overburden pressure in regions of intense palaeodeformation may be due to remanent stress effects and to the influence of the local structure. It is premature to advance any general statement on tectogenesis on the basis ofin situ stress determinations. More measurements, and a quantitative evaluation of the factors affecting them, are required before further progress can be made.     

Assessing carbon emission control strategies: The case of China

Scientists are now being asked to recommend measures to reduce the risks of climatic change due to anthropogenic greenhouse gases. Considerably less effort, however, has been allotted to understanding the efficacy of controlling these gases than to their effects. This paper briefly describes and applies an energy-economic model to assess the effectiveness of carbon dioxide control policies that theoretically could be enacted in China, a large, developing nation with an energy inefficient and carbon-intensive economy. The paper also assesses the effectiveness of similar international efforts, as well as the effect of each initiative on Chinese income levels. Carbon dioxide control measures are contained in scenarios drawn to the year 2075 and include family planning, fossil fuel taxes, mandatory or technical energy efficiency improvements, and a combination of these.The results suggest, not surprisingly, that no nation alone, not even China, can decisively affect the global CO₂ problem. More importantly, however, the potential for energy efficiency improvements in China is found to be both very large and economically attractive. Scenario analysis suggests that energy efficiency measures could both reduce carbon emissions significantly and increase Chinese per capita incomes. Similar conclusions are drawn regarding worldwide energy-efficiency measures. Thus, appropriate public policy measures to capture the existing energy-efficiency potential might both reduce the risk of climatic change and improve economic standards of living.     

The small-scale outflow structure of embedded sources in taurus

High resolution interferometric COJ=1–0 observations of the outflows from two young embedded sources, TMC1 and TMC1A, show the high-velocity gas to have a conical structure, with a constant opening angle of 45° extending to within 1000 AU of the central stars. The correspondence of near-infrared reflection nebulosity atK band with blueshifted CO emission in both objects suggests the lobes are partially evacuated, as do position-velocity diagrams from single-dish COJ=2–1 data. We suggest that the outflows are driven by jets which impart momentum to the ambient medium through shocks, rather than through the entrainment of molecular material along the edges of the jet.The NRAO is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation     

Changes in extreme wind speeds in NW Europe simulated by generalized linear models

Summary We investigate the capability of generalized linear models (GLMs) to simulate sequences of daily maximum wind speed (DMWS), at a selection of locations in NW Europe. Models involving both the gamma and Weibull distributions have been fitted to the NCEP reanalysis data for the period 1958–1998. In simulations, these models successfully reproduce the observed increasing trends up to 0.3 m/s per decade in coastal or oceanic locations for the wintertime and the decreasing trends down to –0.2 m/s per decade in inland Europe for the summertime. Annually extreme winds exhibit an increasing tendency (with median estimates up to 0.6 m/s per decade) at the studied locations. The gamma model slightly overestimates the upper percentiles of the wind speed distribution, but reproduces trends better than the Weibull model. In both the NCEP data and GLM simulations, local extreme DMWS events (defined in terms of threshold exceedances) have increased dramatically in frequency during winter; decreasing trends are more common in summer. The NCEP data indicate similar trends in the frequencies of large-scale windy events (defined via simultaneous exceedances at 2 or more locations). Overall, these events have increased in number; at the scale of the North Sea basin, their number may have changed from 3–5 days per year during the earlier decades, to 5–7 days per year during later decades based on observational estimates. An increase in the frequency of large-scale extreme winter storms is implied. The GLMs underestimate these large-scale event frequencies, and provide imprecise estimates of the corresponding secular trends. These problems could be rectified by using a better representation of spatial dependence. The present results suggest that GLMs offer a useful tool to study local climate extremes in the context of changing climate distributions; they also provide some pointers towards improving the representation of extremes at a regional scale.