An experimental study on the ripple–dune transition

Flume experiments were conducted on different bed stages across the ripple–dune transition. As flow velocity increases, an initially flat bed surface (made of fairly uniform sandy material) is gradually transformed into a two‐dimensional rippled bed. With further increase in velocity, two‐dimensional ripples are replaced by irregular, linguoid ripples. As the average velocity necessary for the ripple–dune transition to occur is imposed on the bed surface, these non‐equilibrium linguoid ripples are further transformed into larger, two‐dimensional dunes. For each of these stages across the transition, a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated. An acoustic Doppler velocimeter was used to study the flow characteristics above each bed surface. Detailed profiles were used along a transect located in the middle of the channel. Results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields (section view). They clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition. Turbulence intensity and Reynolds stresses gradually increase throughout the transition. Two‐dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed. Linguoid ripples induce three‐dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region. A very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes. The turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability (when compared with the rippled beds). Further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis, together with an emphasis on quadrant analysis. Copyright © 2001 John Wiley & Sons, Ltd.     

An experimental investigation of water level effects on the dynamic behaviour of a large arch dam

The need for full‐scale dynamic tests, which are recognized as the most reliable method to evaluate a structure's vibration properties, is increasing as new analysis techniques are developed that take into account the complex interaction phenomenons that occur in dam–reservoir–foundation systems. They are extremely useful to obtain reliable data for the calibration of newly developed numerical methods. The Earthquake Engineering and Structural Dynamics Research Center (CRGP) at the University of Sherbrooke has been developing and applying dynamic testing methods for large structures in the past 10 years. This paper presents the experimental evaluation of the effects of the varying water level on the dynamic response of the 180 m Emosson arch dam in Switzerland. Repeated forced‐vibration tests were carried out on the dam during four different periods of the reservoir's filling cycle during a one‐year span. Acceleration and hydrodynamic pressure frequency responses were obtained at several locations while the dam was subjected to horizontal harmonic loading. The variation of the resonant frequencies as a function of the reservoir level is investigated. A summary of the ongoing numerical correlation phase with a three‐dimensional finite element model for the dam–reservoir–foundation system is also presented. Copyright © 2001 John Wiley & Sons, Ltd.     

Lithospheric thickness beneath the southern Kenya Rift: implications from basalt geochemistry

Geochemical data are reported for samples from the flanks and floor of the southern Kenya Rift Valley in the Lake Magadi area, and from two central volcanoes located within the rift valley. Rift lavas include samples of Singaraini and Ol Tepesi basalts on the eastern flank, Kirikiti basalts from the western flank, and plateau trachytes from the rift valley floor. Central volcano samples are from Ol Esayeiti and Lenderut located on the eastern flank. The rift basalts are mildly ne-normative, moderately evolved (Mg#=0.39-0.62) alkali basalts and show an overall range in differentiation. Incompatible trace element abundances are moderately elevated (Nb=17-51; Zr=93-274; La=17-55 ppm) and show strongly coherent variations and constant inter-element ratios (e.g. Zr/Nb=4.2-5.5; Nb/Ta=17.5ǂ.4; (La/Sm)_n=7.3ǃ.1); isotope ratios are restricted in range (⁸⁷Sr/⁸⁶Sr=0.70393-0.70436; ¹⁴³Nd/¹⁴⁴Nd=0.51272-0.51280; ²⁰⁶Pb/²⁰⁴Pb=19.87-19.92; ²⁰⁷Pb/²⁰⁴Pb=15.68-15.70; ²⁰⁸Pb/²⁰⁴Pb=39.56-39.71). Central volcano lavas are more alkaline in character and include basanite (Ol Esayeiti; Mg# >60) and hawaiite to benmoreite (Lenderut; Mg#=0.48-0.38). Incompatible element ratio are similar to those of the rift basalts, although the chondrite normalised REE patterns are steeper (La/Sm)_n=17.4ǃ.2). ⁸⁷Sr/⁸⁶Sr (0.70358, 0.70391), ¹⁴³Nd/¹⁴⁴Nd (0.51280, 0.51267), ²⁰⁶Pb/²⁰⁴Pb (19.96,20.17), ²⁰⁷Pb/²⁰⁴Pb (15.66,15.76) and ²⁰⁸Pb/²⁰⁴Pb (39.80,40.00) ratios of Ol Esayeiti basanites are similar to the rift basalts, whereas the Lenderut lavas have unusually low¹⁴³Nd/¹⁴⁴Nd (0.512388-0.512453) ratios for their ⁸⁷Sr/⁸⁶Sr (0.70370-0.70481) ratios, and distinctly less radiogenic and variable Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb=17.93-19.01; ²⁰⁷Pb/²⁰⁴Pb=15.43-15.58; ²⁰⁸Pb/²⁰⁴Pb=37.91-39.14). An integrated model is developed in which the geochemical signature of the lavas is attributed to variable degrees of melting to depths within the garnet stability field, and in the presence of residual amphibole. The stability fields of these phases in P-T space indicates that the lavas must have formed within the sub-continental lithosphere rather than within the underlying ambient asthenosphere or a rising mantle plume. The subcontinental lithospheric mantle must therefore extend to a depth of at least 75 km beneath the Lake Magadi area, which contrasts with recent gravity models for the area, which infer that lithospheric mantle is absent beneath this section of the southern Kenya Rift.     

Lithological and spatial controls on the distribution of quartz veins in andesite- and rhyolite-hosted epithermal Au–Ag deposits of the Hauraki Goldfield, New Zealand

Vein distributions in line samples from four epithermal Au–Ag deposits of the Hauraki Goldfield were logged and quantified by vein spacing, vein density, vein thickness and percentage of vein extension. One deposit is hosted in andesite lavas (Martha Hill), one in andesite lavas and dacite porphyry, dacitic tuffs and pyroclastic breccias (Golden Cross), and two in rhyolite lavas and rhyolitic tuffs with minor andesite lavas or andesite dikes (Ohui and Wharekirauponga). The vein systems in these deposits form fault-controlled arrays of extensional veins. Vein spacing distributions are non-fractal over two to three orders of magnitude (1 mm to 5 m), and therefore fractal dimension statistics are not applicable. The coefficient of variation (C_v) of vein spacing was used as a measure of the degree of vein clustering. Rock type has a marked influence on vein spacing distributions, with veining in rhyolite lava having lower average thickness and percentage extension, but a generally higher degree of vein clustering compared with veining in andesite lava in the same deposit. Vein spacing distributions in well-jointed lithologies, mainly andesite lava, have C_v values (0.8–1.2) that are indicative of anticlustered to weakly clustered patterns, particularly in the vein stockwork of the upper part of the Golden Cross deposit. These C_v values are consistent with field observations that joints are a major control on vein spacing. In the poorly jointed dacitic and rhyolitic rocks, the veins are weakly to strongly clustered as shown by higher C_v values (1.2–2.4), and are commonly associated with normal faults. Overall, andesite lava and dacite porphyry and pyroclastics host thicker and more persistent veins than rhyolite lava and tuff. These larger veins contain significant volumes of high-grade gold mineralisation. The higher chemical reactivity to hydrothermal fluids of andesite and dacite compared with rhyolite may have aided propagation and thickening of the veins in andesite-hosted deposits. Within an individual epithermal deposit, location close to thick veins, representing major fluid conduits, commonly overrides the effect of different lithologies. Sites that are deeper and located within or adjacent to major vein structures have higher average vein thickness, percentage extension and degree of vein clustering. Systematic collection and analysis of vein spacing, thickness and density data can be used to define trends that are useful in the exploration of gold-bearing epithermal vein deposits. Received: 25 August 1998 / Accepted: 23 December 1999     

Ecological conflicts in Poland

Transformation of the political system, observed in Poland for 10 years in connection with economic restructuring and basic socio-economic reforms to harmonise with European Union structures, is also evident in terms of environmental management. Pollution has been reduced in many ways but the rise in living standards, linked with the rapid growth of cars and other forms of consumption, has had a negative effect and environmental conditions have deteriorated in the central areas of large cities. The growth of waste water and refuse is a considerable problem. More areas are now officially protected and the three levels of local government established at the beginning of January 1999 (voivodeship, district and community) all have certain defined environmental protection responsibilities which are cumulatively greater than before. Meanwhile there is a greater awareness in society of environmental issues, although it does not yet lead to a high level of participation in protection work. However, there are strongly-expressed opinions on the ecological acceptability of certain economic developments and conflicts are frequently arising within communities and local governments and also between investors on the one side and ecological organisations and environmental protection institutions on the other. Most controversial are new roads, power stations (like the Zarnowiec nuclear project) and reservoirs like the one on the Dunajec river at Czorsztyn. Local communities frequently resist plans for waste dumps, incinerating plants and other developments which are seen as injurious to health or the quality of life. The planned reprivatisation of the state-owned forests is also criticised by foresters and ecological movements. Conflicts are likely to grow as economic development accelerates but better information and greater public understanding of the need for sustainability should make for consensus. This revised version was published online in July 2006 with corrections to the Cover Date.     

Organic matter, putative nannobacteria and the formation of ooids and hardgrounds

Modern ooids from Joulters Cay, when baked at 500 °C, turn various shades of black depending upon the organic content. Mucus-rich laminae occur at quasi-regular intervals of a few micrometres within the cortex. When mucus is still present, it turns black; when it is absent, there is a gap. The cortex consists of two types of aragonite: (1) 'batons' of circular cross-section capped by a single 0·1-μm (100-nm) ball, which can be interpreted as a single nannobacteria cell that precipitated the baton; (2) elongate crystals made of multiple rows of minute balls of about 0·03 μm (30 nm), which may or may not have been small organisms in the size range of viruses. There are also some crystals that show no evidence of organic precipitation. Hardground cementation begins with the formation of a terminal mucus-rich ring on the ooid that bakes black and is crowded with 0·1-μm (100-nm) balls. Some ooids are then joined by a meniscus also made of mucus with aragonite crystals. The final, most abundant hardground cement forms a fur of inorganic aragonite crystals often shaped like plywood sheets, although some 'organic', elongate crystals composed of ≈0·03 μm (≈30 nm) balls are also found in the later cement. For a century, ooids have been known to be closely associated with organic matter; this paper goes further and proposes that the bulk of the ooid may be precipitated by nannobacterial processes. Hardground formation, in the beginning, may also be a microbiological process, but most cementation is accomplished inorganically.     

Chemical assembly of Archean anorthosites from amphibolite- and granulite-facies terranes, SW Greenland

We report whole-rock, major- and trace-element compositions (obtained by XRF and INA methods) for the amphibolite-facies Buksefjorden and granulite-facies Nordland anorthosites, SW Greenland. In a previous petrologic study on the same sample suite, we documented differences in texture, mineralogy, and mineral compositions between these two anorthosite bodies. Chemical analyses confirm differences in composition between the two bodies, but these differences cannot be explained by variations in metamorphic conditions, and point towards differences in the nature of their protoliths. Analyzed Nordland samples are anorthosites and leucogabbros with 88-98% normative plagioclase, whereas those from Buksefjorden include anorthosite, leucogabbro, and gabbro with ~55-95% normative plagioclase. Two or more compositional groupings can be recognized at each site, which correspond to differences in color and mineralogy of the hand samples. Samples from Buksefjorden are mainly quartz-normative, whereas those from Nordland are olivine (- nepheline) normative. Other differences include higher Ni/Co ratio and REE contents in the granulite-facies anorthosites from Nordland. REE pattern shapes are similar, however, being moderately fractionated at ~0.5-102 chondrites with positive Eu-anomalies. Calculated equilibrium melt patterns are similar for both anorthosites, being relatively flat at ~50-1502 chondrites, suggesting unfractionated (but evolved) parental magmas. Olivine must have been present in the protoliths of the Nordland rocks compared with Buksefjorden. Otherwise, the protoliths contained plagioclase with variable An-content (~An₆₂-An₉₂) and a mafic component with variable Fe/Mg (mg ~0.3-0.8). This mafic component was either hornblende or a combination of ortho- and clinopyroxene in fixed proportions, plus a small amount of magnetite. Mixing calculations demonstrate that some Buksefjorden anorthosites contain two varieties of plagioclase: a calcic type that may correspond to cumulus crystals, and a sodic-type that may correspond to a trapped-melt component. On plots of normative whole-rock An versus mg, compositions of the Buksefjorden and Nordland anorthosites form crude negative arrays that differ from the generally positive trends of mafic layered intrusions (Kiglapait, Skaergaard) and from the generally flats trends of plagioclase-rich cumulate rocks (St. Urbain and Stillwater anorthosites). This difference provides further evidence for the distinctive nature of Archean calcic-anorthosite complexes compared with other types of mafic intrusions. Moreover, this distribution of data points is consistent with the assembly of the protolith of the SW Greenland anorthosites mainly as mixtures of plagioclase and hornblende. Finally, the field for the Buksefjorden and Nordland anorthosites overlaps only slightly with that for the Fiskenaesset Complex, thus extending the known range of compositions for Archean anorthosites in West Greenland.     

Results from the Big Spring basin water quality monitoring and demonstration projects,Iowa, USA

Agricultural practices, hydrology, and water quality of the 267-km² Big Spring groundwater drainage basin in Clayton County, Iowa, have been monitored since 1981. Land use is agricultural; nitrate-nitrogen (-N) and herbicides are the resulting contaminants in groundwater and surface water. Ordovician Galena Group carbonate rocks comprise the main aquifer in the basin. Recharge to this karstic aquifer is by infiltration, augmented by sinkhole-captured runoff. Groundwater is discharged at Big Spring, where quantity and quality of the discharge are monitored. Monitoring has shown a threefold increase in groundwater nitrate-N concentrations from the 1960s to the early 1980s. The nitrate-N discharged from the basin typically is equivalent to over one-third of the nitrogen fertilizer applied, with larger losses during wetter years. Atrazine is present in groundwater all year; however, contaminant concentrations in the groundwater respond directly to recharge events, and unique chemical signatures of infiltration versus runoff recharge are detectable in the discharge from Big Spring. Education and demonstration efforts have reduced nitrogen fertilizer application rates by one-third since 1981. Relating declines in nitrate and pesticide concentrations to inputs of nitrogen fertilizer and pesticides at Big Spring is problematic. Annual recharge has varied five-fold during monitoring, overshadowing any water-quality improvements resulting from incrementally decreased inputs. Electronic Publication     

Fine sediment transport and accumulations at the mouth of the Seine estuary (France)

A comprehensive study of fine sediment transport in the macrotidal Seine estuary has been conducted, including observations of suspended particulate matter (SPM), surficial sediment, and bathymetric data, as well as use of a three dimensional mathematical model. Tide, river regime, wind, and wave forcings are accounted. The simulated turbidity maximum (TM) is described in terms of concentration and location according to tidal amplitude and the discharge of the Seine River. The TM is mainly generated by tidal pumping, but can be concentrated or stretched by the salinity front. The computed deposition patterns depend on the TM location and are seasonally dependent. The agreement with observations is reasonable, although resuspension by waves may be overestimated. Although wave resuspension is likely to increase the TM mass, it generally occurs simultaneously with westerly winds that induce a transverse circulation at the mouth of the estuary and then disperse the suspended material. The resulting effect is an output of material related to wind and wave events, more than to high river discharge. The mass of the computed TM remains stable over 6 months and independent of the river regime, depending mainly on the spring tide amplitude. Computed fluxes at different cross-sections of the lower estuary show the shift to the TM according to the river flow and point out the rapidity of the TM adjustment to any change of river discharge. The time for renewing the TM by riverine particles has been estimated to be one year.     

Geographical signatures of middle Atlantic estuaries: Historical layers

Estuaries of the middle Atlantic region can be characterized and viewed broadly against the backdrop of their geomorphologic features. While geomorphology is literally at the base of every estuary, these features do not necessarily yield regional signatures. A conceptual model, with layering in time and space, is proposed as an alternative to simplistic geomorphologic characterization. Humans have altered virtually every physical, chemical, and biological feature of middle Atlantic estuaries. A basic model premise is that middle Atlantic estuaries have a base of fundamental geomorphology features. Layered, in GIS fashion, on this base are the estuaries' components: climate, nutrients, watershed soils and vegetation, producers, and consumers. These components have been so strongly influenced by humans in time and space that the signature is anthropogenic. As a consequence, best management practices, stock assessment, and restoration have replaced concepts such as ecosystem integrity and stability. The focus of the layered model is the Chesapeake Bay watershed, and although middle Atlantic estuaries differ along climatic and latitudinal gradients, all reflect the detrimental effects of a massive human presence. The ability or inability of middle Atlantic estuaries to absorb human perturbation over the last 10,000 years gives them their signatures. From the Hudson-Raritan to the Pamlico-Albemarle estuaries, we have made some progress in curbing our impacts. Nearly everything we do affects our estuaries, and our actions are proportional to the number of humans living in the watersheds. Continued population growth on our coasts and many years of abuse may be irreversible as our estuaries lose their ability to be self-regulating, biological systems.