Experimental field assessment of suspended sediment pathways for characterizing hydraulic habitat     

Gemma L. Harvey  Nicholas J. Clifford 《地球表面变化过程与地形》2010,35(5):600-610

The distribution of particulate matter within river channels, including sediments, nutrients and pollutants, is fundamental to the survival of aquatic organisms. However, the interactions between flow and sediment transport at the patch scale of river systems represents an under‐researched component of physical habitat studies, particularly those concerning the characterization of ‘physical biotopes’ (riffles, runs, pools, glides). This paper describes a field methodology for exploring the transfer of particulate matter at small scales within river channels, which may be used to aid hydraulic habitat characterization. The field protocol combines field measurement of high frequency flow properties, to characterize hydraulic habitat units, and deployment of spatial arrays of turbidity probes, to detect the passage of artificially‐induced sediment plumes through different biotope units. Sediment plumes recorded by the probes are analysed quantitatively in the manner of the flood hydrograph, and qualitative inferences are made on the dominant mixing processes operating within different parts of the channel. Relationships between the nature of spatio‐temporal hydraulic variations within glide, riffle and pool biotopes, and the character and mixing behaviour of sediment plumes within these habitat units are identified. Results from these preliminary experiments suggest that investigating and characterizing the transfer and storage of sediments, nutrients and pollutants within and between different biotopes is a viable avenue for further research, with potential to contribute to improved physical habitat characterization for river management and habitat restoration. The experiments are also an illustration of the value of neglected synergies between process geomorphology, ecology and river hydraulics. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Landscape response to tipping points in granite weathering: The case of stepped topography in the Southern Sierra Critical Zone Observatory     

Barbara S. Jessup  W. Jesse Hahm  Scott N. Miller  James W. Kirchner  Clifford S. Riebe 《Applied Geochemistry》2011

The dynamics of granitic landscapes are modulated by bimodal weathering, which produces patchy granular soils and expanses of bare rock ranging from meter-scale boulders to mountain-scale domes. We used terrain analysis and with cosmogenic nuclide measurements of erosion rates to quantitatively explore Wahrhaftig’s decades-old hypothesis for the development of “stepped topography” by differential weathering of bare and soil-mantled granite. According to Wahrhaftig’s hypothesis, bare granite weathers slower than soil-mantled granite; thus random erosional exposure of bare rock leads to an alternating sequence of steep, slowly weathering bedrock “steps” and gently sloped, but rapidly weathering, soil-mantled “treads.” Our investigation focused on the terrain surrounding the Southern Sierra Critical Zone Observatory (CZO), which is underlain by granitic bedrock and lies outside the limits of recent glaciation, in the heart of the stepped topography described by Wahrhaftig. Our digital terrain analysis confirms that steep steps often grade into gentle treads, consistent with Wahrhaftig’s hypothesis. However, we observe a mix-and-match of soil and bare rock on treads and steps, contrary to one of the hypothesis’ major underpinnings – that bare rock should be much more common on steps than on treads. Moreover, the data show that bare rock is not as common as expected at step tops; Wahrhaftig’s hypothesis dictates that step tops should act as slowly eroding base levels for the treads above them. The data indicate that, within each landscape class (i.e., the steps and treads), bare rock erodes more slowly than surrounding soil. This suggests that the coupling between soil production and denudation in granitic landscapes harbors a tipping point wherein erosion rates decrease when soils are stripped to bedrock. Although broadly consistent with the differential weathering invoked by Wahrhaftig, the data also show that steps are eroding faster than treads, undermining Wahrhaftig’s explanation for the origins of the steps. The revised interpretation proposed here is that the landscape evolves by back-wearing of steps in addition to differential erosion due to differences in weathering of bare and soil-mantled granite.  相似文献   

Enrichment of Rare Earth Elements during magmatic and post-magmatic processes: a case study from the Loch Loyal Syenite Complex, northern Scotland     

A. S. Walters  K. M. Goodenough  H. S. R. Hughes  N. M. W. Roberts  A. G. Gunn  J. Rushton  A. Lacinska 《Contributions to Mineralogy and Petrology》2013,166(4):1177-1202

Concern about security of supply of critical elements used in new technologies, such as the Rare Earth Elements (REE), means that it is increasingly important to understand the processes by which they are enriched in crustal settings. High REE contents are found in syenite-dominated alkaline complexes intruded along the Moine Thrust Zone, a major collisional zone in north-west Scotland. The most northerly of these is the Loch Loyal Syenite Complex, which comprises three separate intrusions. One of these, the Cnoc nan Cuilean intrusion, contains two mappable zones: a Mixed Syenite Zone in which mafic melasyenite is mixed and mingled with leucosyenite and a Massive Leucosyenite Zone. Within the Mixed Syenite Zone, hydrothermal activity is evident in the form of narrow altered veins dominated by biotite and magnetite; these are poorly exposed and their lateral extent is uncertain. The REE mineral allanite is relatively abundant in the melasyenite and is extremely enriched in the biotite–magnetite veins, which have up to 2 % total rare earth oxides in bulk rock analyses. An overall model for development of this intrusion can be divided into three episodes: (1) generation of a Light Rare Earth Element (LREE)-enriched parental magma due to enrichment of the mantle source by subduction of pelagic carbonates; (2) early crystallisation of allanite in melasyenite, due to the saturation of the magma in the LREE; and (3) hydrothermal alteration, in three different episodes identified by petrography and mineral chemistry, generating the intense enrichment of REE in the biotite–magnetite veins. Dating of allanite and titanite in the biotite–magnetite veins gives ages of c. 426 Ma, overlapping with previously published crystallisation ages for zircon in the syenite.  相似文献   

Permafrost thaw in a nested groundwater-flow system   总被引：3，自引：2，他引：1  

Jeffrey M. McKenzie  Clifford I. Voss 《Hydrogeology Journal》2013,21(1):299-316

Groundwater flow in cold regions containing permafrost accelerates climate-warming-driven thaw and changes thaw patterns. Simulation analyses of groundwater flow and heat transport with freeze/thaw in typical cold-regions terrain with nested flow indicate that early thaw rate is particularly enhanced by flow, the time when adverse environmental impacts of climate-warming-induced permafrost loss may be severest. For the slowest climate-warming rate predicted by the Intergovernmental Panel on Climate Change (IPCC), once significant groundwater flow begins, thick permafrost layers can vanish in several hundred years, but survive over 1,000 years where flow is minimal. Large-scale thaw depends mostly on the balance of heat advection and conduction in the supra-permafrost zone. Surface-water bodies underlain by open taliks allow slow sub-permafrost flow, with lesser influence on regional thaw. Advection dominance over conduction depends on permeability and topography. Groundwater flow around permafrost and flow through permafrost impact thaw differently; the latter enhances early thaw rate. Air-temperature seasonality also increases early thaw. Hydrogeologic heterogeneity and topography strongly affect thaw rates/patterns. Permafrost controls the groundwater/surface-water-geomorphology system; hence, prediction and mitigation of impacts of thaw on ecology, chemical exports and infrastructure require improved hydrogeology/permafrost characterization and understanding.  相似文献   

Impacts of climate, lake size, and supra- and sub-permafrost groundwater flow on lake-talik evolution, Yukon Flats, Alaska (USA)   总被引：3，自引：2，他引：1  

Tristan P. Wellman  Clifford I. Voss  Michelle A. Walvoord 《Hydrogeology Journal》2013,21(1):281-298

In cold regions, hydrologic systems possess seasonal and perennial ice-free zones (taliks) within areas of permafrost that control and are enhanced by groundwater flow. Simulation of talik development that follows lake formation in watersheds modeled after those in the Yukon Flats of interior Alaska (USA) provides insight on the coupled interaction between groundwater flow and ice distribution. The SUTRA groundwater simulator with freeze–thaw physics is used to examine the effect of climate, lake size, and lake–groundwater relations on talik formation. Considering a range of these factors, simulated times for a through-going sub-lake talik to form through 90 m of permafrost range from ～200 to >?1,000  years (vertical thaw rates <?0.1–0.5  m?yr^?1). Seasonal temperature cycles along lake margins impact supra-permafrost flow and late-stage cryologic processes. Warmer climate accelerates complete permafrost thaw and enhances seasonal flow within the supra-permafrost layer. Prior to open talik formation, sub-lake permafrost thaw is dominated by heat conduction. When hydraulic conditions induce upward or downward flow between the lake and sub-permafrost aquifer, thaw rates are greatly increased. The complexity of ground-ice and water-flow interplay, together with anticipated warming in the arctic, underscores the utility of coupled groundwater-energy transport models in evaluating hydrologic systems impacted by permafrost.  相似文献   

An evaluation of the engineering properties of some Nigerian limestones as construction materials for highway pavements     

S. Clifford Teme 《Engineering Geology》1991,31(3-4):315-326

Several hundred limestone samples representing eight different deposits within Nigeria have been evaluated and results indicate that some of the deposi highway pavement construction materials. The evaluation included tests on index properties, strength, chemical, petrographic and resistance-to-attritio

The suitability or adverse features of the eight limestone deposits have been evaluated and the various engineering characteristics are discussed. base recommendations are made for the use of selected limestones as pavement materials in Nigeria.  相似文献   

Geotechnical characteristics of some clays from south—western Nigeria: implications for industrial utilization     

S. Clifford Teme 《Engineering Geology》1991,30(3-4):305-324

Clays in adequate quantity and of quality for industrial use occur in the Ifon area of Ondo State, Nigeria, in the form of alluvial deposits at Sobe, as sedimentary deposits at Non and Arimogija and as residual clays at Omialafara.

The alluvial clays from Sobe and the sedimentary clays from Ifon are highly plastic while the sedimentary clays from Arimogija are moderately plastic and the Omialafara residual clays only slightly plastic. The clays in general are kaolinitic in composition with minor amounts of illite.

The unconfined compressive strength characteristics of fired pellets of these various clays indicate that the reddish to greyish-brown alluvial clays at Sobe tend to have higher strength values than the dark grey clays of Arimogija. Porosity, firing shrinkage, clay mineralogy and colour after firing all indicate that the clays are very suitable for the manufacture of ceramic wares.  相似文献   

Descriptive statistics forN-dimensional closed arrays: A spherical coordinate approach     

Clifford R. Stanley 《Mathematical Geosciences》1990,22(8):933-956

Recent publications have demonstrated that the composition of a system can be described by the angles which define the vector resulting from the projection of the composition onto the unit hypersphere. Although published trigonometric relationships allow determination of the angles in any number of dimensions, no general hyperdimensional recursive formulae exist for the calculation of these quantities and their variances and covariances. A general methodology for calculating the angles which describe the compositions of systems in any number of dimensions is presented. These angles can be used to calculate statistics (central moments and resultant vectors) describing the central tendency and dispersion among rock compositions, as well as to quantify the angular differences between compositions. Equations that relate the variances and covariances of these angular variables to the variances and covariances of the actual component proportion variables are presented, allowing projection of these measurements from one reference frame to another (cartesian to spherical and vice versa).  相似文献   

Bursting bubbles and their effect on the sea-to-air transport of Fe, Cu and Zn     

Stephen R. Piotrowicz  Robert A. Duce  James L. Fasching  Clifford P. Weisel 《Marine Chemistry》1979,7(4):307-324

The effects of the bubble breaking process on the atmospheric geochemical cycles of the elements Fe, Cu and Zn were investigated, in situ, in the estuarine waters of Narragansett Bay, Rhode Island. Enrichment, as defined by the metal-to-sodium ratios in the aerosols produced compared to their ratio in bulk water, occurred for the three metals investigated. The extent and potential geochemical importance of the process were different for each element. Iron enrichment was quite low (enrichment factor (EF) <100) and constant, and scavenging of iron from the water column and subsequent enrichment on the aerosols produced did not appear to occur. Copper enrichment on the aerosols was 200 and appeared to be influenced by both microlayer and scavenging effects. In addition, copper enrichment appears to be correlated to biological processes. Zinc enrichment was approximately the same as Cu; however, a strong scavening effect appeared to occur, suggesting scavenging of Zn by rising bubbles. Scavenging effects suggest that open-ocean enrichments for Cu may be slightly higher than observed here and a great deal higher for Zn.Geochemical implications of the data, together with other existing data, indicate that the sea is an insignificant source of Fe to the atmosphere. The sea may be a significant source (contributing on the order of 10% or more) of the total annual quantity of Cu and Zn to the atmosphere.  相似文献   

Marsh vertical accretion via vegetative growth   总被引：1，自引：0，他引：1  

John A. Nyman  Russel J. Walters  Ronald D. Delaune  William H. Patrick  Jr. 《Estuarine, Coastal and Shelf Science》2006,69(3-4):370

Coastal marshes accrete vertically in response to sea-level rise and subsidence. Inadequate accretion and subsequent conversion of coastal marshes to open water generally is attributed to inadequate mineral sedimentation because mineral sedimentation is widely assumed to control accretion. Using ¹³⁷Cs dating to determine vertical accretion, mineral sedimentation, and organic matter accumulation, we found that accretion varied with organic accumulation rather than mineral sedimentation across a wide range of conditions in coastal Louisiana, including stable marshes where soil was 80% mineral matter. These results agreed with previous research, but no mechanism had been proposed to explain accretion via vegetative growth. In an exploratory greenhouse experiment, we found that flooding stimulated root growth above the marsh surface. These results indicated the need for additional work to determine if flooding controls accretion in some marshes by stimulating root growth on the marsh surface, rather than by mineral accumulation on the marsh surface. Restoration or management that focus on mineral sedimentation may be ineffective where a relationship between accretion and mineral sedimentation is assumed rather than tested.  相似文献