Laboratory soil piping and internal erosion experiments: evaluation of a soil piping model for low‐compacted soils          下载免费PDF全文

Garey A. Fox  Rachel G. Felice  Taber L. Midgley  Glenn V. Wilson  Abdul‐Sahib T. Al‐Madhhachi 《地球表面变化过程与地形》2014,39(9):1137-1145

Mechanistic models have been proposed for soil piping and internal erosion on well‐compacted levees and dams, but limited research has evaluated these models in less compacted (more erodible) soils typical of hillslopes and streambanks. This study utilized a soil box (50 cm long, 50 cm wide and 20 cm tall) to conduct constant‐head, soil pipe and internal erosion experiments for two soils (clay loam from Dry Creek and sandy loam from Cow Creek streambanks) packed at uniform bulk densities. Initial gravimetric moisture contents prior to packing were 10, 12 and 14% for Dry Creek soil and 8, 12, and 14% for Cow Creek soil. A 1‐cm diameter rod was placed horizontally along the length of the soil bed during packing and carefully removed after packing to create a continuous soil pipe. A constant head was maintained at the inflow end. Flow rates and sediment concentrations were measured from the pipe outlet. Replicate submerged jet erosion tests (JETs) were conducted to derive erodibility parameters for repacked samples at the same moisture contents. Flow rates from the box experiments were used to calibrate the mechanistic model. The influence of the initial moisture content was apparent, with some pipes (8% moisture content) expanding so fast that limited data was collected. The mechanistic model was able to estimate equivalent flow rates to those observed in the experiments, but had difficulty matching observed sediment concentrations when the pipes rapidly expanded. The JETs predicted similar erodibility coefficients compared to the mechanistic model for the more erodible cases but not for the less erodible cases (14% moisture content). Improved models are needed that better define the changing soil pipe cross‐section during supply‐ and transport‐limited internal erosion, especially for piping through lower compacted (more erodible) soils as opposed to more well‐compacted soils resulting from constructing levees and dams. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Lower/middle tropospheric ozone variability in Senegal during pre-monsoon and monsoon periods of summer 2008: observations and model results     

Gregory S. Jenkins  Seydi Ndiaye  Moussa Gueye  Rachel Fitzhugh  Jonathan W. Smith  Abou Kebe 《Journal of Atmospheric Chemistry》2012,69(4):273-302

During the summer (8 June through 3 September) of 2008, 9 ozone profiles are examined from Dakar, Senegal (14.75°N, 17.49°W) to investigate ozone (O₃) variability in the lower/middle troposphere during the pre-monsoon and monsoon periods. Results during June 2008 (pre-monsoon period) show a reduction in O₃ concentrations, especially in the 850–700 hPa layer with Saharan Air Layer (SAL) events. However, O₃ concentrations are increased in the 950–900 hPa layer where the peak of the inversion is found and presumably the highest dust concentrations. We also use the WRF-CHEM model to gain greater insights for observations of reduced O₃ concentrations during the monsoon periods. In the transition period between 26 June and 2 July in the lower troposphere (925–600 hPa), a significant increase in O₃ concentrations (10–20 ppb) occur which we suggest is caused by enhanced biogenic NO_X emissions from Sahelian soils following rain events on 28 June and 1 July. The results suggest that during the pre-monsoon period ozone concentrations in the lower troposphere are controlled by the SAL, reducing ozone concentrations through heterogeneous chemical processes. At the base of the SAL we also find elevated levels of ozone, which we attribute to biogenic sources of NO_X from Saharan dust that are released in the presence of moist conditions. Once the monsoon period commences, lower ozone concentrations are observed and modeled which we attribute to the dry deposition of ozone and episodes of ozone poor air that is horizontally transported into the Sahel from low latitudes by African Easterly Waves (AEWs).  相似文献   

Facilitating adaptation of biodiversity to climate change: a conceptual framework applied to the world’s largest Mediterranean-climate woodland     

Suzanne M. Prober  Kevin R. Thiele  Philip W. Rundel  Colin J. Yates  Sandra L. Berry  Margaret Byrne  Les Christidis  Carl R. Gosper  Pauline F. Grierson  Kristina Lemson  Tom Lyons  Craig Macfarlane  Michael H. O’Connor  John K. Scott  Rachel J. Standish  William D. Stock  Eddie J. B. van Etten  Grant W. Wardell-Johnson  Alexander Watson 《Climatic change》2012,110(1-2):227-248

The importance of ecological management for reducing the vulnerability of biodiversity to climate change is increasingly recognized, yet frameworks to facilitate a structured approach to climate adaptation management are lacking. We developed a conceptual framework that can guide identification of climate change impacts and adaptive management options in a given region or biome. The framework focuses on potential points of early climate change impact, and organizes these along two main axes. First, it recognizes that climate change can act at a range of ecological scales. Secondly, it emphasizes that outcomes are dependent on two potentially interacting and countervailing forces: (1) changes to environmental parameters and ecological processes brought about by climate change, and (2) responses of component systems as determined by attributes of resistance and resilience. Through this structure, the framework draws together a broad range of ecological concepts, with a novel emphasis on attributes of resistance and resilience that can temper the response of species, ecosystems and landscapes to climate change. We applied the framework to the world’s largest remaining Mediterranean-climate woodland, the ‘Great Western Woodlands’ of south-western Australia. In this relatively intact region, maintaining inherent resistance and resilience by preventing anthropogenic degradation is of highest priority and lowest risk. Limited, higher risk options such as fire management, protection of refugia and translocation of adaptive genes may be justifiable under more extreme change, hence our capacity to predict the extent of change strongly impinges on such management decisions. These conclusions may contrast with similar analyses in degraded landscapes, where natural integrity is already compromised, and existing investment in restoration may facilitate experimentation with higher risk?options.  相似文献   

Geochemistry of a spring-dense karst watershed located in a complex structural setting, Appalachian Great Valley, West Virginia, USA     

Dorothy J. Vesper  Rachel V. Grand  Kristen Ward  Joseph J. Donovan 《Environmental Geology》2009,58(3):667-678

The distribution and chemistry of the springs in the Tuscarora Creek watershed is controlled by both geologic structure and karst dissolution. The watershed is located in eastern West Virginia in the structurally complex Great Valley of the Appalachian Valley and Ridge province. The upper portion of the stream parallels strike along a mapped fault zone and is bordered by clastic rocks that comprise North Mountain. The lower reaches of the stream flow cross-strike through Cambro-Ordovician carbonates. The controlling chemical signature in the spring water is carbonate dissolution. Little evidence was seen for the recharge from adjacent clastic rocks although differences in the Ca/Mg molar ratio between springs indicated the presence of localized spring basins in headwater reaches. Na, Cl and Ca generally increased from upstream to downstream in the cross-strike reaches. Comparison of stream and cumulative spring discharge was consistent with significant groundwater base-flow contribution directly to the creek, particularly in the strike-parallel region. The largest spring in the watershed (>162 L/s) was sampled during and after a large storm event along with the adjacent creek. The creek displayed a typical dilution response with each flood pulse, whereas the spring had only a limited or delayed response. The overall chemical and thermal stability of the spring, relative to the creek, indicated the lack of significant direct hydraulic connection between the two. The conceptual model for the area includes localized flowpaths in the headwater region where the stream flow is parallel to strike and a thrust fault. In addition to the shallow localized flowpaths, a deeper, more regional flowpath likely exists for a large spring further downstream.  相似文献   

Flow fields in tightly curving meander bends of low width‐depth ratio     

Rachel A. Nanson 《地球表面变化过程与地形》2010,35(2):119-135

Upland swamp channels with low width/depth ratios (w/d), armoured beds, minimal sediment loads, tightly curving bends and an absence of point bars provide a striking contrast to the flow characteristics of larger channels with higher w/d ratios. Two subsets of these bends were examined in relation to their patterns of cross‐stream flow relative to the channel boundary. The first, with mean w/d = 2·0 and gentle barforms, exhibited even velocity distributions at bend entrances but developed vertically stacked pairs of maximum velocity filaments (MVFs). Cross‐stream circulation increased with decreasing curvature before essentially ceasing in the tightest bend due to the conservation of angular momentum and reduced vertical velocity differentials; bed friction has more limited influence in narrow deep channels relative to bank friction. In the second subset of bends, with larger w/d (mean 4·8) and much steeper barforms, the MVFs were laterally paired and strongly helical flow was partly driven by the vertical confinement of flow due to large, stable barforms at the bend entrances. In one bend, the velocity profile became inverted immediately past the apex and caused helical flow to abruptly reverse. Point bars in relatively wide bedload channels appear to greatly distort secondary flow patterns. In narrow, deep, sediment‐starved channels, separation zones against the convex and/or the concave bank deliver the flow confinement that would otherwise be provided by point bars or concave‐bank benches. In these channels, separation zones are important for protecting both the channel bed and banks from scour. Three‐dimensional near bankfull flow fields are presented for one bend with a meander pool; inward shifting of the MVF and limited sediment supply are proposed as mechanisms for the development and maintenance of these features. These flow data in narrow and deep peatland channels demonstrate very different flow patterns and morphological characteristics relative to the more commonly studied wide, shallow channels with more abundant sediment. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Combining Organic Matter Source and Relative Trophic Position Determinations to Explore Trophic Structure     

Rachel Marie Wilson  Jeffrey Chanton  Graham Lewis  Douglas Nowacek 《Estuaries and Coasts》2009,32(5):999-1010

Stable isotope ratios of carbon and sulfur were used to assess organic matter utilization of numerically abundant consumers present in Apalachicola Bay, FL, USA. These results were used to infer nitrogen isotopic enrichment of organic matter sources in an effort to establish baseline δ ¹⁵N enrichment for trophic evaluations. We compared results from concentration-independent and concentration-dependent mixing models and found that the two methods resulted in widely different conclusions about the importance of organic matter sources that varied ninefold in sulfur concentrations. Nitrogen isotopic enrichment was used to determine relative trophic positions of consumer organisms. Source elemental concentrations of nitrogen were also considered in the calculation of relative trophic levels in a concentration-dependent approach. Concentration-independent and concentration-dependent methods of calculating trophic results were compared. While relative trophic levels of individual species varied continuously from approximately 1.7 to 3.5, comparisons of trophic level among consumers indicated four possible trophic groupings. Filter feeders (mussels and oysters) made up the lowest trophic tier while teleost fishes made up the highest trophic tier. Invertebrates sampled were assigned intermediate nondiscrete trophic levels. Because δ ¹⁵N values of important organic matter sources in the system were similar, the concentration-independent and concentration-dependent methods did not result in significantly different conclusions about trophic level for any of the consumers examined. However, a comparison of the two methods applied to a hypothetical case found that differences in base δ ¹⁵N values ranging approximately 4‰ resulted in significantly different trophic-level assignments when comparing the concentration-dependent and concentration-independent methods of trophic-level calculations. Our results confirm that consideration of the elemental concentrations of the base organisms is an important factor in determining source contributions and may affect trophic-level calculations in systems with a sufficient range of base nitrogen enrichment. However, this result depended on the relative isotopic signatures of the chosen sources and their elemental concentrations and should be considered individually for each system.  相似文献   

Weathering and the mobility of phosphorus in the catchments and forefields of the Rhône and Oberaar glaciers, central Switzerland: Implications for the global phosphorus cycle on glacial-interglacial timescales     

Karl B. Föllmi  Rachel Hosein  Philipp Steinmann 《Geochimica et cosmochimica acta》2009,73(8):2252-117

In this study we evaluate the dynamics of the biophile element phosphorus (P) in the catchment and proglacial areas of the Rhône and Oberaar glaciers (central Switzerland). We analysed erosion and dissolution rates of P-containing minerals in the subglacial environment by sampling water and suspended sediment in glacier outlets during three ablation and two accumulation seasons. We also quantified biogeochemical weathering rates of detrital P in proglacial sedimentary deposits using two chronosequences of samples of fresh, suspended, material obtained from the Oberaar and Rhône water outlets, Little-Ice-Age (LIA) moraines and Younger Dryas (YD) tills in each catchment. Subglacial P weathering is mainly a physical process and detrital P represents more than 99% of the precipitation-corrected total P denudation flux (234 and 540 kg km⁻² yr⁻¹ for the Rhône and Oberaar catchments, respectively). The calculated detrital P flux rates are three to almost five times higher than the world average flux. The precipitation-corrected soluble reactive P (SRP) flux corresponds to 1.88-1.99 kg km⁻² yr⁻¹ (Rhône) and 2.12-2.44 kg km⁻² yr⁻¹ (Oberaar), respectively. These fluxes are comparable to those of tropical rivers draining transport-limited, tectonically inactive weathering areas.In order to evaluate the efficiency of detrital P weathering in the Rhône and Oberaar proglacial areas, we systematically graded apatite grains extracted from the chronosequence in each catchment relative to weathering-induced changes in their surface morphologies (grades 1-4). Fresh apatite grains are heavily indented and dissolution rounded (grade 1). LIA grains from two 0-10 cm deep moraine samples show extensive dissolution etching, similar to surface grains from the YD profile (mean grades 2.7, 3.5 and 3.5, respectively). In these proglacial deposits, the weathering front deepens progressively as a function of time due to biocorrosion in the evolving acidic pedosphere, with mechanical indentations on grains acting as sites of preferential dissolution. We also measured iron-bound, organic and detrital P concentrations in the chronosequence and show that organic and iron-bound P has almost completely replaced detrital P in the top layers of the YD profiles. Detrital P weathering rates are calculated as 310 and 280 kg km⁻² yr⁻¹ for LIA moraines and 10 kg km⁻² yr⁻¹ for YD tills. During the first 300 years of glacial sediment exposure P dissolution rates are shown to be approximately 70 times higher than the mean global dissolved P flux from ice-free continents. After 11.6 kyr the flux is 2.5 times the global mean. These data strengthen the argument for substantial changes in the global dissolved P flux on glacial-interglacial timescales. A crude extrapolation from the data described here suggests that the global dissolved P flux may increase by 40-45% during the first few hundred years of a deglaciation phase.  相似文献   

Local and regional variation of MORB parent magmas: evidence from melt inclusions from the Endeavour Segment of the Juan de Fuca Ridge     

Rachel Sours-Page  Kevin T. M. Johnson  Roger L. Nielsen  Jill L. Karsten 《Contributions to Mineralogy and Petrology》1999,134(4):342-363

The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K₂O: 0.01 to 0.4 oxide wt%, P₂O₅: <0.01 to 0.2 oxide wt%, La_N: 7 to 35, Yb_N: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K₂O: 0.32 to 0.9 oxide wt %, P₂O₅: 0.02 to 0.35 oxide wt%, La_N: 11 to 60, Yb_N: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust. Received: 2 December 1997 / Accepted: 27 August 1998  相似文献   

Semi-analytic modelling of galaxy formation: the local Universe     

Rachel S. Somerville †  Joel R. Primack 《Monthly notices of the Royal Astronomical Society》1999,310(4):1087-1110

Using semi-analytic models of galaxy formation, we investigate galaxy properties such as the Tully–Fisher relation, the B - and K -band LFs, cold gas contents, sizes, metallicities and colours, and compare our results with observations of local galaxies. We investigate several different recipes for star formation and supernova feedback, including choices that are similar to the treatment by Kauffmann, White & Guiderdoni and Cole et al., as well as some new recipes. We obtain good agreement with all of the key local observations mentioned above. In particular, in our best models, we simultaneously produce good agreement with both the observed B - and K -band LFs and the I -band Tully–Fisher relation. Improved cooling and supernova feedback modelling, inclusion of dust extinction and an improved Press–Schechter model all contribute to this success. We present results for several variants of the CDM family of cosmologies, and find that models with values of Ω₀≃0.3–0.5 give the best agreement with observations.  相似文献   

The neighborhood council: Where ‘top-down’ engages with ‘bottom-up’     

Einat?Yinon-Amoyal  Rachel?KallusEmail author 《GeoJournal》2005,64(2):91-104

The paper examines the neighborhood council from the perspective of its contribution to town planning. Within the context of planning in Israel, two distinct models of neighborhood council and their relationship to the municipal planning administration in two different towns are considered. Their potential as a new arena of planning at the neighborhood level is evaluated, including the ability to mediate between ‘bottom-up’ and ‘top-down’ planning processes, and to offer a planning framework that combines local with professional knowledge.  相似文献